中国生态农业学报(中英文)

Mitigation of nitrogen and phosphorus leaching from cropland in northern China

MA Lin, WANG Hongyuan, LIU Gang, HU Kelin, LIANG Chao, DU Lianfeng, GUO Shengli, BAI Zhaohai, WANG Fenghua, LI Xiaoxin, WANG Shiqin, HU Chunsheng

2021, 29(1): 1-10. doi: 10.13930/j.cnki.cjea.200910

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Abstract:
Understanding on nitrogen (N) and phosphorus (P) leaching from root-vadose-groundwater system and mitigation options is research gaps in the world. We presented the main research progresses of a project 'Mitigation of Nitrogen and Phosphorus Leaching from Cropland in China' in National Key Research and Development Program in this paper. Four research contents are included: 1) clarifying temporal and spatial variation of N and P leaching out of root zone; 2) analyzing nitrogen and phosphorus leaching in the 'root-vadose-groundwater' system; 3) exploring mitigation options of leaching in cereals and vegetables fields of three soil types, namely black soil, cinnamon soil and fluvo-aquic soil; and 4) exploring regional strategies for decreasing N and P leaching in the 'root-vadose-groundwater' system. Main scientific findings are as follows: 1) the N leaching in root zone shows different trends with exceeding standard rates of nitrate in groundwater in three soil type areas, due to their differences in land use, groundwater depth, lithologies of vadose zones and hydrogeological conditions etc. In black soil areas, although the N leaching is not high in root zones, the interactions between groundwater quality and N leaching in root zone are more sensitive because of the topography. Therefore, more researches are needed to explore the interaction mechanism between groundwater quality and leaching N in black soil areas. In cinnamon and fluvo-aquic soil areas in the North China Plain, vadose zones are deep and can buffer N leaching from root zone to groundwater. Thus, it is also necessary to further explore the N leaching mitigation mechanism of vadose zones. 2) Based on the long-term fertilization experiments and observation in 12 m deep borehole, we analyzed the accumulation characteristics of N surplus in vadose zones of farmland. The results show that the safe N application rate in the North China Plain is about 200 kg(N)·hm^-2·a^-1. If the threshold is exceeded, 51% of the N would leach to out of the root areas (1 m). The unreasonable irrigation, heavy rainfall, macrovoids and crack are the main causes of soil nitrate leaching. It can lead accumulated nitrate in vadose zone to be leached to below 6 m. 3) Combined deep sampling and biological method, we analyzed denitrification activity and floristic composition of soil microbe in 0-10.5 m of vadose zones. The results show that surface soil is main site for microbial denitrification, while in deep soil layer the denitrification weaken significantly, which indicates that "Carbon Starvation" is the key factor for limiting the abundance and activity of denitrifying microbes in the bottom soil. Furthermore, the indoor incubation experiment proved that adding carbon could effectively activate the soil denitrifying microbes, which explained the mechanism of "the nitrogen interception in root areas and denitrification in vadose zones". 4) Using the data from N and P leaching mitigation experiments, national agricultural non-point source pollution monitoring network, groundwater nitrate monitoring network in northern China and NUFER (NUtrient flows in Food chains, Environment and Resources use) model, we compartmented nutrient losses vulnerable zones. Based on this, we explored regional mitigation strategies of N and P leaching, which could provide scientific support for non-point sources pollution.

Nitrogen and phosphorus leaching characteristics and temporal and spatial distribution patterns in northern China farmlands

WANG Hongyuan, LI Jungai, FAN Bingqian, LUO Xiaosheng, PENG Chang, ZHAI Limei, LI Hu, MA Lin, LIU Hongbin

2021, 29(1): 11-18. doi: 10.13930/j.cnki.cjea.200572

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Abstract:
The main agricultural production areas in northern China are the black soil area, fluvo-aquic soil area, and cinnamon soil area. In the area N and P leaching is a common cause of groundwater pollution, but the leaching characteristics and distribution patterns (temporal and spatial) are unclear. The in situ monitoring of field leakage ponds and literature data analysis were used to analyze N and P leaching characteristics at 285 monitoring sites using the four main planting patterns (spring maize, winter wheat-summer maize rotation, open-field vegetables, and greenhouse vegetables). The results showed that the average N and P leaching rates were 30.7 kg(N)·hm^-2 and 0.09 kg(P)·hm^-2 for spring maize, 49.9 kg(N)·hm^-2 and 0.07 kg(P)·hm^-2 for winter wheat–summer maize rotation, 51.7 kg(N)·hm^-2 and 0.10 kg(P)·hm^-2 for open-field vegetables, and 117.5 kg(N)·hm^-2 and 0.74 kg(P)·hm^-2 for greenhouse vegetables. Fertilizer application and irrigation, often determined by the planting pattern, were positively correlated with N leaching. Therefore, high fertilizer and water amounts used in vegetable fields resulted in more N and P leaching than observed in grain fields. In fields using the same planting pattern, the fluvo-aquic soil area had the greatest total N loss intensity, followed by the cinnamon soil area; the black soil area had the least intensity. Different soil textures resulted in different leached N amounts when the same fertilization and irrigation practices were used. Fields using the same planting pattern also had different leaching amounts because of regional differences in fertilization and irrigation practices. Annual N and P leaching was mainly affected by rainfall intensity, and total N leaching was positively correlated with rainfall intensity. If no leaching events occurred in the previous year, a sharp increase in leaching was observed in the following year. Spatially, the cinnamon and fluvo-aquic soil areas were the primary N leaching risk areas, especially, some provinces with large vegetable planting areas (particularly those with large greenhouse areas)showed high N and P leaching risks. Northern Chinese agricultural areas are primarily at risk for N leaching, but also P leaching; cinnamon and fluvo-aquic soils are the highest risk areas. Regionally, N and P leach mainly from grain fields, but as vegetable field size increases, so does the risk of N and P leaching.

Effects of rainfall on nitrogen and phosphorus leaching in rainfed spring maize black soil farmland in Jilin Province, China

JIAO Yunfei, LI Qiang, GAO Hongjun, WANG Zhou, ZHANG Xiuzhi, ZHU Ping, PENG Chang

2021, 29(1): 19-28. doi: 10.13930/j.cnki.cjea.200571

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Abstract:
The black soil area of Jilin Province is important for maize production in China, where agricultural development and nonpoint source pollution risks are intensifying. Understanding the effects of nitrogen and phosphorus leaching from rainfed spring maize farmland is important for the sustainable development of the region. This study investigated rainfall and leaching amounts, nitrogen and phosphorus leachate concentrations, and leaching intensity from 2016 to 2019 at four nonpoint source pollution monitoring stations in Jilin Province, China, and analyzed the relationship between rainfall and farmland nitrogen and phosphorus leaching. The results showed that the inter-annual and inter-regional rainfall differences were large, ranging from 424 to 554 mm. The average rainfall during spring maize growth season was 475 mm. Tonghua monitoring station had the most rain (593–785 mm), followed by Gongzhuling station (512–699 mm) and Lishu station (305–434 mm); Nong'an station had the least rain (197–342 mm). Tonghua and Nong'an growth seasons had primarily light and moderate rain, and Gongzhuling and Lishu had moderate and heavy rain and thunderstorms. There was a significant positive correlation between the leaching amount and rain intensity (P < 0.01). For every 10 mm·(24h)^-1 increase in rain intensity, the leaching amount increased by 1.81 mm. Rainfall during the spring maize growth season (April to October) was also significantly correlated with the leaching amount (P < 0.05). For every 100 mm rain increase, the leached sample number and the leaching probability increased (3 times and 6%, respectively). When the growing season rainfall exceeded 74 mm, the leaching probability increased, and when it exceeded 217 mm, leaching could occur. Leaching occurred when rain levels were 10.0–24.9 mm (moderate rain) and 25.0–49.9 mm (heavy rain). There was a significant positive correlation between leaching amount and total nitrogen concentration, but no correlation with total phosphorus concentration. The total nitrogen leaching intensity had a strong positive correlation (P < 0.01) with rain intensity; for every 10 mm·(24h)^-1 increase in rain intensity, the total nitrogen leaching intensity increased by 0.73 kg·hm^-2. The total phosphorus leaching intensity did not correlate with rain intensity. Nitrogen primarily leached from black soil area farmland in Jilin Province during the rainfed spring maize growth season and was correlated with rainfall. Agronomic measures should be adopted to prevent agricultural nonpoint source pollution at the source.

Effects of nitrogen application on nitrogen and phosphorus leaching in fluvo-aquic soil on a winter wheat-summer maize rotation farmland

LUO Xiaosheng, KOU Changlin, WANG Xiaofei, LI Taikui, WANG Hongyuan

2021, 29(1): 29-37. doi: 10.13930/j.cnki.cjea.200548

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Abstract:
Fluvo-aquic soil is predominant in the North China Plain, where a large amount of wheat and corn are grown in China. Understanding the relationship between nitrogen and phosphorus leaching and nitrogen application is important for preventing and controlling nonpoint source pollution in this area. The field seepage pool method was used to explore nitrate nitrogen and total phosphorus leaching in fluvo-aquic soil on a winter wheat and summer maize rotation farmland. Three fertilization treatments were tested: traditional nitrogen application (CON), optimized nitrogen application (OPT), and optimized nitrogen plus nitrogen reduction (OPTJ). The results showed that from 2016 to 2018, the annual leachate volume from the 90 cm soil layer was between 79.0 and 102.5 mm (all treatments). The leached nitrate nitrogen concentrations were 18.9-208.7 (average 72.7) mg·L^-1 (CON), 9.0-99.2 (average 33.8) mg·L^-1 (OPT), and 4.7-55.5 (average 15.4) mg·L^-1 (OPTJ), fluctuating among leaching events. The nitrogen leaching risk was higher, and the phosphorus leaching risk was lower in the fluvo-aquic soil area. The average leaching amount was 66.4 kg·hm^-2 and apparent leaching loss coefficient was 10.3% for nitrate nitrogen; these values for total phosphorus were 0.06 kg·hm^-2 and 0.04%, respectively. Reducing nitrogen fertilizer decreased nitrogen leaching by 56.3% (OPT) and 78.9% (OPTJ), and the apparent leaching coefficients were 10.3% (CON), 6.2% (OPT), and 4.9% (OPTJ), indicating that nitrate nitrogen leaching increased as nitrogen fertilizer increased. Nitrogen leaching had interannual variation; 2018 had high rainfall, and the leaching amount was 57.0% higher than in 2017, which had low rainfall. During the sampling years, the total phosphorus leached was 0.06 kg·hm^-2 (CON), 0.06 kg·hm^-2 (OPT), and 0.08 kg·hm^-2 (OPTJ). A moderate nitrogen fertilizer reduction increased crop yield; the OPT yield was 1.08 times higher than the CON yield. However, excessive fertilizer reduction decreased yield. OPTJ had 56% less nitrogen than CON, and the yield decreased by 2.0%-8.1%. The partial factor productivities were 25.3 kg·hm^-2 (CON), 35.7 kg·hm^-2 (OPT), and 57.4 kg·hm^-2 (OPTJ) for winter wheat and 28.5 kg·hm^-2 (CON), 44.8 kg·hm^-2 (OPT), and 62.7 kg·hm^-2 (OPTJ) for summer maize. The nitrogen fertilizer partial factor productivities of OPT and OPTJ were significantly higher than that of CON. These results showed that the nitrate nitrogen leaching potential was high in fluvo-aquic soil, and reducing nitrogen fertilizer could significantly reduce nitrogen loss without decreasing crop yield. Considering crop yield and nitrate nitrogen leaching risk together, the optimal nitrogen amount for winter wheat and summer maize farmland in the study area was 465 kg·hm^-2. When nitrogen decreased to 285 kg·hm^-2, nitrogen leaching sharply decreased, but the crop yield decreased slightly.

Modeling nitrogen transport and leaching process in a greenhouse vegetable field

LEI Haojie, LI Guichun, DING Wuhan, XU Chi, WANG Hongyuan, LI Hu

2021, 29(1): 38-52. doi: 10.13930/j.cnki.cjea.200570

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Abstract:
Nitrogen (N) leaching is caused by the mismanagement of water and fertilizer in greenhouse vegetable fields. Understanding N movement and leaching process is important for achieving high crop yields at low environmental costs. A field experiment was conducted for a greenhouse cucumber–tomato rotation system in the suburbs of Beijing, China. The DeNitrification-DeComposition (DNDC) model was used to quantitatively evaluate the soil N transport and leaching loss in the facility vegetable field after considering factors obtained from field experiments, such as soil temperature, humidity, and nitrate nitrogen (NO₃^--N) content. Conventional practices were selected as the baseline scenario, and the modeled scenarios, such as changes in soil properties, irrigation, and N application, were set according to the baseline. The results showed that the DNDC model can better simulate the vegetable yield, 5 cm soil temperature, 0–20 cm soil water-filled pore space, and NO₃^--N migration process, indicating that it is an effective tool for simulating and evaluating N transport and leaching in vegetable field soil. The modeling scenarios showed that the accumulation of NO₃^--N in the 0–60 cm soil was primarily affected by the irrigation amount and N application; soil pH and organic carbon were also important factors affecting NO₃^--N migration. Increasing irrigation amount significantly accelerated the downward movement of NO₃^--N, and increasing N application promoted the accumulation of NO₃^--N at the surface and a depth of 20 cm. Increasing soil pH lessened NO₃^--N surface accumulation; and to a certain extent, increasing soil organic carbon delayed the downward movement of NO₃^--N.Controlling water and fertilizer was the most effective method for mitigating N leaching. Compared with conventional measures, reducing irrigation and N application simultaneously by 20% significantly reduced NO₃^--N leaching by 59.04%. Changing irrigation method and increasing soil organic carbon content by 20% (to save water and fertilizer) further reduced NO₃^--N leaching by 69.04%. The DNDC model is a useful method for evaluating and controlling NO₃^--N leaching in vegetable fields. Changing management practices, such as N and water amounts as the soil quality improves, may be an effective way to reduce N leaching in vegetable fields.

Mechanism of nitrogen leaching in fluvo-aquic soil and deep vadose zone
in the North China Plain

NIU Xinsheng, ZHANG Chong, JU Xiaotang

2021, 29(1): 53-65. doi: 10.13930/j.cnki.cjea.200644

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Abstract:
Rationally managing nitrogen (N) and water results in high crop yield and quality, maintains (or improves) soil fertility, and reduces environmental pollution. However, since the 1990s, excessive use of N fertilizer and flood irrigation has created problems in Chinese croplands, causing agricultural nonpoint source pollution and groundwater nitrate contamination. Data integration and literature review of winter-wheat summer-maize farmlands in fluvo-aquic soil in the North China Plain was used to investigate the temporal and spatial variation of N leaching, the contribution of cracks and macropores to N leaching, and N movement through soil (along the surface to groundwater continuum). The results showed that the N surplus was very high (299-358 kg·hm^-2·a^-1) when conventional management was used, resulting in high nitrate accumulation in the root and deep vadose zones. Nitrate movement in the winter wheat season was primarily caused by unsaturated flow and affected by irrigation; the nitrate movement distance was short. Water and nitrate loss from the root zone was negligible if the irrigation amount was lower than 60 mm. In the winter wheat season, tillage- and irrigation-induced cracks contributed minimally to nitrate and water movement out of the root zone. In the wet and hot summer maize season, the soil was frequently water-saturated, and small precipitation amounts lead to nitrate leaching, accounting for 81% of the annual leaching events and 80% of the annual nitrate leaching. In the summer maize season, the leached nitrate amounts were much higher than in the winter wheat season, and 71% of the total nitrate leaching was preferential flow caused by macropores. Nitrate from the root zone could be partially removed by denitrification in the deep vadose zone. In the North China Plain, avoiding high nitrate accumulation after winter wheat harvest was effective at decreasing nitrate leaching in the summer maize season. Matching the N fertilizer supply with crop demand and controlled release fertilizer in summer maize season (to avoid costly topdressing N fertilizer) may also play important roles in leaching reduction. Frequent and heavy rainfall accelerates the movement of nitrate via saturated and preferential flow to groundwater. Therefore, rational water and N management is key to reducing nitrate movement to the deep vadose zone and groundwater.

Effect of macropore preferential flow on nitrogen leaching in a North China Plain farmland

ZENG Hui, WEN Na, ZHANG Jianfeng, ZHANG Jie, HU Kelin, LIU Gang

2021, 29(1): 66-75. doi: 10.13930/j.cnki.cjea.200508

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Preferential flow is an important mechanism that relies on macropores for moisture to infiltrate into soil. Understanding this process affects the study of soil moisture, solute transport, and environmental protections for field management practices. In this study, a brilliant blue staining tracer field experiment and the soil water heat carbon nitrogen simulator (WHCNS) model were used to explore the effects of preferential flow of macropores on soil water transport and nitrate nitrogen leaching. The WHCNS model was used to simulate soil water and nitrogen migration through macropores in a North China Plain winter wheat-summer maize rotation field with heavy rainfall, fertilization, and irrigation. A dyeing tracer was used to follow water infiltration into no-tillage and rotary-tillage soil, and Pearson correlation coefficient analysis was performed on the stained area and the no-tillage soil stable infiltration rate. The results showed that the no-tillage soil infiltration depth and dyeing area were higher than that of the rotary-tillage soil. The no-tillage soil had a deeper dyeing depth, reaching 80–100 cm, while that of rotary-tillage was shallow, reaching only 15–20 cm. The no-tillage soil had a high degree of preferential flow and transported moisture to the deep-soil. There was no correlation between the no-tillage soil dyeing area and the stable infiltration rate (P = 0.68). Therefore, dye tracers cannot quantify the soil stable infiltration rate. At the same time, the WHCNS simulation results of nitrate nitrogen leaching in 0–100 cm soil layer showed that the presence of macropores increased the nitrate nitrogen leaching in both traditional and optimal fertilization modes, compared with no macropores. On the other hand, in the presence of macropores, optimized fertilization reduced nitrate nitrogen leaching by 46.0% compared with that in traditional fertilization. The sprinkler irrigation reduced leaching by 15.6% compared with that in conventional flood irrigation, and heavy rainfall increased leaching by 119.4%. If the farmland has macropores, organic fertilizer and sprinkler irrigation may be used to save water and reduce nitrate nitrogen leaching; however, increased leaching is expected during heavy rainfall. Therefore, climatic conditions should be considered when fertilizing to determine suitable irrigation amounts. This study used a field tracing experiment and WHCNS model simulation to demonstrate that preferential flow can increase soil water infiltration and nitrate nitrogen downward movement and provides guidance for optimizing farmland water and fertilizer management with macropores in the North China Plain.

The effects of farmland cracks on nitrate leaching in the North China Plain

A Liman, ZHANG Jie, ZENG Hui, DING Tianyu, LUO Zhiying, LI Lili, HU Kelin, LIU Gang

2021, 29(1): 76-84. doi: 10.13930/j.cnki.cjea.200506

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Desiccation cracking is a common soil natural phenomenon. Research on desiccation cracking has mainly focused on morphological characteristics in lab-based experiments. In this study, three-dimensional (3D) geometric crack structures were extracted using paraffin casting in the field and transient image processing. The influence of cracks on farmland water and nitrogen leaching was quantified using the Water Heat Carbon Nitrogen Simulator (WHCNS) model. The 3D structural characteristics of the cracks obtained by the laser scanner were as follows: average length per square meter = 4.58 m, average surface width = 5.72 mm, and average depth = 9.06 cm. WHCNS analysis showed that cracks increased nitrogen leaching (97.40%, traditional fertilizer; 256.43%, optimized fertilizer), and that traditional fertilizer application had a greater nitrate nitrogen leaching risk. Irrigation type did not affect nitrate leaching, but heavy rainfall increased the risk and led to an 83.61% annual leaching volume increase. Additionally, the WHCNS model was used to simulate the effects of fracture, fertilization, irrigation, and rainfall intensity on nitrate leaching. The results showed that cracks had a notable influence on nitrate nitrogen leaching using optimal and traditional fertilization methods, and optimized fertilization reduced nitrate nitrogen leaching. Precipitation intensity was a key factor affecting nitrogen leaching. In this study, the simulation only calculated the nitrate nitrogen leached to underground and ignored nitrogen runoff from heavy precipitation, reducing the effect of precipitation on nitrogen leaching; however, the timing and amount of fertilizer and precipitation should be considered together when managing fields, especially in the summer, when rainfall is concentrated. The simulation showed that irrigation did not affect nitrate nitrogen leaching, which may be related to irrigation intensity and the leaching soil layer (100 cm) set by the model. However, the basin irrigation amount simulated in this study could not leach nitrate nitrogen below the 100 cm soil layer, which may have contributed to the small differences between methods. Thus, sprinkling irrigation and optimized fertilization should be adopted in combination to take full advantage of the water and fertilizer saving methods.

Phosphorus adsorption onto ferrihydrite and predicting colloidal phosphorus transport

MA Jie, MA Yuling, QIAN Xiaoyan, WANG Zhiqiao, CHEN Yali, WENG Liping, LI Yongtao

2021, 29(1): 85-93. doi: 10.13930/j.cnki.cjea.200461

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Losing soil phosphorus to aquatic environments often causes eutrophication; however, the stability and transport of colloidal phosphorus, especially amorphous iron colloid-bound phosphorus in porous soil, is poorly understood. In this study, adsorption experiments were conducted to investigate phosphorus adsorption onto ferrihydrite. The effects of pH, ionic strength, and humic acid (HA) on the dissolved phosphorus distribution and colloidal and solid ferrihydrite adsorbed phosphorus were explored. Ferrihydrite colloid-bound phosphorus stability was calculated using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to predict the transport of colloidal complexes. The results showed that the pseudo-second-order kinetic model (R² = 0.964) best described the phosphorus onto ferrihydrite adsorption process. Adsorption was controlled by liquid film diffusion as well as internal diffusion, and chemisorption. The Freundlich model (R² = 0.970) was a better fit for isothermal adsorption than the Langmuir model (R² = 0.842), indicating that phosphorus onto ferrihydrite adsorption was multi-layer; however, the parameters of Langmuir model revealed that the maximum theoretical phosphorus onto ferrihydrite adsorption capacity was 22.55 mg∙g^-1. Phosphorus adsorption onto ferrihydrite decreased with increasing pH and decreasing ionic strength; low ionic strength and high pH were considered beneficial for releasing ferrihydrite colloids. Approximately 5%–20% phosphorus bound to ferrihydrite colloids in alkaline and low ionic strength conditions (1–10 mmol∙L^-1) regardless of HA, and the electrostatic repulsion between ferrihydrite-phosphorus colloids was notably. According to the DLVO theory, the colloids were stable and easily transported in the soil pores due to their negative surface charge. Negatively charged ferrihydrite colloids can transport long distances in negatively charged water-bearing media, such as soil or aquifer. In agricultural activities, excessive phosphate fertilizer application may cause large amounts of phosphate ion loading onto iron minerals and promote the formation of stable, negatively charged iron mineral colloids. Ferrihydrite-phosphorus colloid transport is likely to become another form of phosphorus leaching. Thus, this study investigated ferrihydrite-phosphorus colloid generation and stability in variable pH, ionic strength, and HA conditions, qualitatively predicted their transport, and assessed colloid-facilitated phosphorus loss risk. However, in complex soil systems, chelation and precipitation of co-existing ions may alter the fate of ferrihydrite-phosphorus colloids, which needs further investigation.

Isolation, identification, and functional characterization of denitrifiers from the deep vadose zone and aquifer in the North China Plain

ZHAO Huicheng, LIU Mengshuai, CHEN Shuaimin, WANG Xinzhen, WANG Shiqin, HU Chunsheng, LIU Binbin, WANG Fenghua

2021, 29(1): 94-101. doi: 10.13930/j.cnki.cjea.200519

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Abstract:
The long-term excessive use of nitrogen (N) fertilizer in agricultural regions has increased soil nitrate and residual N accumulation and poses a threat to groundwater quality. Nitrate leaching into the vadose zone is becoming a global concern as the N stock in this habitat comprises a significant portion of N budgets. The vadose zone is also an essential channel for the conversion and reduction of nitrate. Therefore, the elimination of nitrate accumulation in the vadose zone is significant for maintaining groundwater safety. Microbial denitrification is the reduction of nitrogen nitrate (NO₃^--N) to gaseous nitric oxide (NO), nitrous oxide (N₂O), or dinitrogen (N₂). This mechanism is essential for removing excess nitrate in the subsoil before it leaches into the groundwater and saturates deep soil zones or discharges into ground aquifers through subsurface drainage. Therefore, isolating and screening bacteria with strong denitrification abilities may strengthen the vadose zone and aquifer microbial denitrification process, preventing groundwater nitrate pollution. In this study, 62 microbial denitrifiers were isolated from the 0-150 m vadose zone in a position experiment of long-term nitrogen application in the Agricultural Ecosystem Experimental Station of Luancheng, Chinese Academy of Sciences, located the North China Plain. 16S ribosomal RNA (16S rRNA) gene sequence analysis showed that the isolated denitrifiers had high homology with nine genera, belonging to the phyla Proteobacteria, Actinobacteria, and Firmicutes. Of the 62 denitrifiers, seven strains (L37, L71, L96, L103, L104, L133, and L13) were selected for denitrification potential experiments based on the phylogenetic tree results. Gas kinetics under anoxic incubations showed that three strains (L71, L13, and L103) could reduce nitrate substrates to nitrous oxides, such as N₂O and N₂, in anaerobic conditions. Electron microscopy showed that the denitrifying strains were 1.0 μm (L71), 1.5 μm (L13), and 1.5 μm (L103) long rod-shaped bacteria. Strain L103 had motile and complete denitrification abilities, and the denitrification rate was between 1.62 and 2.36 g(KNO₃)∙d^-1∙L^-1, indicating a high potential for use in agricultural practices. Furthermore, the denitrification ability of strain L103 was inhibited in acidic conditions, suggesting that pH also affects the microbial denitrification potential. Bacterial denitrifiers that reduce nitrate to N₂ in hypoxic/anoxic conditions exist in the deep vadose zone of the North China Plain. The denitrification potential of these strains is important for understanding how microorganisms contribute to the soil nitrate accumulation self-remediation process.

Nitrogen leaching risks and control mechanisms of spring maize fields in black soil

YUAN Lei, CHEN Xin, LYU Liping, MA Jian, SHI Yi, JIA Jingchao, XIE Hongtu, ZHANG Xudong, HE Hongbo, LIANG Chao, LU Caiyan

2021, 29(1): 102-112. doi: 10.13930/j.cnki.cjea.200499

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Nitrogen (N) availability and retention in soil-crop systems are important for increasing crop productivity, improving N use efficiency (NUE), and minimizing environmental pollution from N losses. In the black soil region of Northeast China, it is unclear how agricultural management practices affect soil mineral N accumulation and leaching. In this study, an in-situ ¹⁵N-labeled tracer field experiment was performed to quantify the transformation characteristics, migration, and soil N fate when long-term no-till with maize stover mulching was used. The soil profile was investigated under three treatments: conventional ridge tillage (RT), no-till with no maize stover mulching (NT0), and no-till with 100% maize stover mulching (NT100; 7500 kg·hm^-2 maize stover). The accumulated mineral N [primarily as nitrate nitrogen (NO₃^--N)] in the 300 cm soil profiles were 461.6 kg(N)·hm^-2 (RT), 450.7 kg(N)·hm^-2 (NT0), and 439.7 kg(N)·hm^-2 (NT100) when traditional fertilizer applications were used, suggesting a N leaching risk. In all 0–40 cm soil layers, the percentage of fertilizer-derived NO₃^--N to total NO₃^--N was on average 60.9% (maize seedling stage) and 58.0% (maize tasseling stage), indicating a high N leaching risk in the seasonally applied fertilizer. NT100 decreased the transformation of fertilizer N into mineral N pools by 20.8% in 0–40 cm soil layers but accelerated the conversion into fixed ammonium NO₃^--N and organic N pools by 39.4% and 30.5%, respectively, compared with that by RT. The clay mineral to fertilizer-derived NO₃^--N fixation capacity was the same as the soil microorganism to fertilizer-derived mineral N immobilization capability at a depth of 0–20 cm, but the fixation capacity was higher than the immobilization capability at 20–40 cm. These findings suggest that the immobilization potential of soil microorganism to fertilizer-derived mineral N is dependent on the maize straw mulch quantity and maize straw accessibility to soil microorganisms. No-till with maize stover mulching reduced the soil mineral N accumulation in black soil spring maize fields, increased the fertilizer nitrogen use efficiency and maize yield by 9.7%, decreased the fertilizer N gaseous loss by 27.7%, and delayed fertilizer N leaching to deeper soil.

Mitigation of nitrogen and phosphorus leaching from black soil croplands in Northeast China

ZHANG Wei, WANG Rui, LI Siqi, LU Caiyan, XIE Hongtu, SUI Yueyu, ZHANG Xiuzhi

2021, 29(1): 113-118. doi: 10.13930/j.cnki.cjea.200496

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Black soil (Mollisol) is a fertile and productive soil type found in Northeast China and is important for China's maize production. Large amounts of synthetic fertilizers are applied to meet the increasing cereal production demands but have low efficiency, leaving excessive nitrogen and phosphorus in the soil. This excess increases the risk of agricultural nonpoint pollution, black soil degradation, and surface/underground water pollution, threatening drinking water security. Studies conducted in the black soil region indicate that nitrogen and phosphorus leaching intensities are lower in the cereal croplands than in the other regions, especially those in the North China Plain. However, residual nitrogen and phosphorus remaining in soils owing to high fertilizer application levels increase the leaching potential, especially with intense precipitation. Environmental factors and field management practices were analyzed to identify effective control measures for nitrogen and phosphorus leaching and propose leaching reduction strategies for rain-fed maize and vegetation fields in the black soil region. Fertilization and precipitation are the primary drivers of nitrogen and phosphorus leaching in cultivated black soils, and irrigation is correlated to leaching intensity in vegetation fields. New strategies should be adopted to mitigate leaching, such as setting maximum fertilizer thresholds based on crop demands, adjusting fertilization timing to avoid high precipitation seasons, and using water-saving irrigation. Replacing synthetic fertilizer with manure, combining inorganic and organic fertilizers, and using no-tillage with maize stover mulching, crop rotations, and biochar are other field management practices to reduce the nitrogen and phosphorus pollution risk. In the maize croplands, only basal fertilizer should be applied during the growing season, an organic and synthetic fertilizer combination (50%-70% organic) should be used, and no-tillage with maize stover mulching should be considered to control the nitrogen and phosphorus leaching intensities. Reducing fertilizer amounts and irrigation water use by 20% and deeply burying the crushed straw after the autumn harvest are also recommended for vegetation fields. Practical strategies for nitrogen and phosphorus pollution prevention are important for sustainable agricultural development and maintenance in the black soil region.

The effect of reduced irrigation and chemical fertilizers on phosphorus accumulation and leaching in Mollisol vegetable fields

CHEN Yimin, XU Xin, JIAO Xiaoguang, QU Hongyun, HOU Meng, SUI Yueyu

2021, 29(1): 119-127. doi: 10.13930/j.cnki.cjea.200462

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Abstract:
Excessive fertilization and irrigation have led to phosphorus leaching in Mollisol vegetable fields, and optimization of these practices is critical for reducing phosphorus pollution. A leaching monitoring experiment was performed in a Mollisol eggplant field using the following three treatments: standard irrigation and chemical fertilizer amounts (WF), standard irrigation + 80% chemical fertilizer (W80%F), and 80% irrigation + standard chemical fertilizer (80%WF). The soil phosphorus storage, available phosphorus dynamics, and phosphorus leaching amounts were analyzed to determine the effects of irrigation and fertilization treatments on phosphorus leaching. After one growing season, phosphorus storage in the 0–100 cm soil layers were 9.69 t·hm^-2 (WF), 9.36 t·hm^-2 (W80%F), and 8.84 t·hm^-2 (80%WF), which were 26.5%, 27.5%, and 7.1% higher than before planting, respectively. These results showed that phosphorous accumulation occurred, which increased the leaching risk. During the extended eggplant growing period, the available phosphorus in the 0–20 cm soil layer increased and then decreased, and was highest in the 80%WF treatment, ranging between 145.17–224.55 mg·kg^-1. The available phosphorus in the 20–40 cm soil layer did not change under WF treatment and increased under 80%WF treatment. The available phosphorus fluctuated with W80%F but was significantly higher than that in the other treatments, except during the full fruit period. The phosphorus leaching amounts were 17.84 kg·hm^-2 (WF), 17.47 kg·hm^-2 (W80%F), and 9.02 kg·hm^-2 (80%WF). Organic phosphorus leaching was more than 90% of the total phosphorus leaching, differing from other soil types. There were significant positive correlations between phosphorus leaching and increased phosphorus storage, available phosphorus in the 0–40 cm layer at the full fruit stage and in the 0–20 cm layer at the withering stage (P < 0.05). This indicates that changes in phosphorus storage and the available phosphorus content may help predict phosphorus leaching in Mollisols. After one growing season, phosphorus storage in the 0–100 cm layer increased in all treatments; the smallest increase was in the W80%F treatment, indicating that reduced irrigation lowers the phosphorous leaching risk. Reducing chemical fertilizers did not affect phosphorus leaching or the leaching risk. These results provide information for preventing phosphorus leaching, which may be used to develop new techniques for Mollisol vegetable fields.

A review of the freeze-thaw cycling effect on arable soil nitrogen and phosphorus leaching

DENG Fangbo, BAO Xuelian, LIANG Chao, XIE Hongtu

2021, 29(1): 128-140. doi: 10.13930/j.cnki.cjea.200494

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Abstract:
Excessive agricultural fertilization has caused nutrient leaching and severe surface and groundwater pollution in recent years. Soil freeze-thaw cycling (FTC) is common at middle and high latitudes, high altitudes, and partial temperate regions. FTC plays an important role in soil biogeochemical processes in cold regions and may be complicated by climate change. Understanding the effects of FTC on soil nitrogen (N) and phosphorus (P) leaching is critical for effective mitigation. This study reviewed the involvement of FTC on soil nutrient leaching based on soil physical, chemical, and biological properties and found that FTC affects soil nutrient concentrations, leachate forms, and nutrient leaching pathways. FTC damages soil aggregates, microbial cells, and plant root residues, leading to the release of organic matter and various N and P forms into the soil, subsequently stimulating soil mineralization and increasing the mineral nutrient concentrations. Soil hydrothermal regime variations and soil structure changes during the FTC period promote preferential flow, thereby increasing the nutrient leaching potential. FTC affects the soil microbial biomass and the microbial community composition and structure, which changes the nutrient cycling processes. Soil chemical properties, including organic matter, pH, and cation exchange capacity, indirectly influencing soil aggregate stability, microbial resistance, and nutrient holding capacity changed during the FTC period. Soil properties (e.g., soil texture, organic matter content, and soil moisture) and climate (e.g., air temperature and snowpack) determine the nutrient leaching degree during the FTC period. The relationships between nutrient leaching and existing agricultural practices were also analyzed. Mineral fertilizer application is the primary source of nutrient leaching on farmlands. Therefore, fertilizing for the efficient use of nutrients by plants is crucial for mitigating nutrient leaching. Other practices, such as biochars, cover crops, no-tillage with straw mulching, may have a role in reducing nutrient leaching. Biochars have a high sorption capacity and may increase the soil water and nutrient holding capacity, cover crop implementation may absorb excess fertilizer nutrients from the soil and reduce leachable N and P, and no-tillage with straw mulching may change FTC by avoiding exposed soil and influencing soil physicochemical and microbial properties, thereby increasing fertilizer efficiency. However, these measures have shortcomings; cover crops and crop residues are the nutrient leaching sources during FTC. Further research is needed to understand the nutrient leaching mechanisms of these practices and to establish a complete evaluation system.

Nitrogen leaching mitigation in fluvo-aquic soil in the North China Plain

MENG Fanqiao, WANG Kun, XIAO Guangmin, WANG Kaiyong, HU Zhengjiang, ZHANG Haixia, XU Xiuchun, ZHANG Wei, YANG Xuan

2021, 29(1): 141-153. doi: 10.13930/j.cnki.cjea.200523

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The North China Plain is a grain production region with fluvo-aquic soil and has seen rapid agricultural development over the past four decades. Excessive fertilization and frequent irrigation have increased nitrogen (N) leaching and nonpoint source pollution since the 1990s. This study screened published nitrogen leaching data on the North China Plain grain farmlands to identify the relationship between fertilization and irrigation with N leaching and to evaluate the primary N leaching mitigation measures. The results showed that regional groundwater during the 1970s was shallow and then deeper. During the 2010s, the regional cropping system changed from one to two crops per annum, and the annual N fertilizer rapidly increased to 600 kg(N)·hm^-2·a^-1 but then slowly decreased to 500–550 kg(N)·hm^-2·a^-1. Since the 1990s, irrigation increased from zero (rainfed during the 1980s) to 150–400 mm per annum, crop straw had gradually been incorporated into farmlands, and the fertilizer synergist technology had been accepted. The soil organic matter and total N improved by 38%–47%, pH decreased by 0.5 units, and available potassium decreased slightly. Fertilization and irrigation were the main influencing factors of N leaching, and the exponential relationship between N leaching and the N fertilizer balance (N fertilizer rate - crop above-ground N uptake) was better than the relationship between N leaching and N fertilizer rate. Random forest (RF) regression modeling based on machine learning was used to determine the relationship between N leaching and impacting factors such as irrigation, soil properties, and climate; the prediction results were satisfactory. At the same rate of N fertilization, organic fertilization combined with chemical fertilization significantly decreased N leaching because the N supply and crop demand were synchronized. Fertilizer synergists, such as control-release fertilizers, ureases, and nitrification inhibitors, mitigated N leaching by 1/3 and should be used in the North China Plain. Crop straw incorporation microbially improved N fertilizer in the short term and increased the long-term soil total N stock and inorganic N buffering capacity and reducing N leaching by 10%. The no-tillage mitigation effects were low and variable among farmlands. Fallow farmland and rotation/intercropping of deep root and shallow root crops, leguminous crops with cereal crops, and grains with vegetable crops were effective at reducing N leaching, but the crop yields also reduced. Therefore, these techniques required careful examination during technical dissemination. Governmental support, technical training, and proper planning should be implemented during the 14^th Five-Year Plan of China to prevent and mitigate N pollution. Ecological compensation and an agricultural sector water use charge could also be used to encourage farmer participation.

Effects of groundwater fluctuation on nitrate nitrogen transport after nitrogen application in cropland soil

LIU Jing, ZHU Xinyu, LI Shunjiang, KANG Lingyun, MA Maoting, DU Lianfeng

2021, 29(1): 154-162. doi: 10.13930/j.cnki.cjea.200707

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Understanding the effects of groundwater fluctuations on nitrate nitrogen (NO₃^--N) transport in the soil vertical profile is important for reducing nitrogen (N) leaching and nitrate pollution in croplands with shallow groundwater. This study investigated effects of groundwater fluctuations and nitrogen application rate in a greenhouse cabbage with large soil column experimental devices. Two groundwater fluctuation levels (W0, stable; W1, 20 cm fluctuation per 10 days) and three levels of nitrogen application (N0, 0 kg(N)·hm^-2; N1, 225 kg(N)·hm^-2; N2, 450 kg(N)·hm^-2) were tested to determine water contents and NO₃^--N concentration changes at different soil depths, NO₃^--N concentration in groundwater, and crop yield. The results showed that fluctuating groundwater affected NO₃^--N transport depending on the soil depth and nitrogen application rate. At 0–20 cm depth (part of the vadose zone), excessive N application led to NO₃^--N accumulation in soil, but there was no correlation with groundwater fluctuation. At 20–60 cm depth (groundwater fluctuation zone), increased N fertilizer increased the soil NO₃^--N, and groundwater level changes promoted NO₃^--N migration into deeper soil, increasing the NO₃^--N groundwater pollution risk (especially in soil with already high NO₃^--N concentrations). At 60–80 cm depth (the flooding area), there was less NO₃^--N in soil, mainly due to denitrification. Crop yield did not significantly correlate with groundwater level changes. Groundwater fluctuations affected = NO₃^--N transport and should be considered in agricultural areas with shallow groundwater levels to prevent and control nitrogen pollution.

Effects of field management practices on nitrogen and phosphate leaching in the cinnamon soil area of China

GUO Shengli, ZHANG Shulan, DANG Tinghui, GUO Liping, LI Lijun, GAO Pengcheng, WANG Rui

2021, 29(1): 163-175. doi: 10.13930/j.cnki.cjea.200576

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Excessive application of nitrogen (N) and phosphate (P) fertilizers have increasingly caused agricultural nonpoint source pollution and groundwater contamination in China since the 1990s. Understanding N and P leaching is critical for reducing groundwater contamination. Field survey data, long-term experimental data, and recent experimental results were used to investigate N and P leaching from arable cinnamon soil across northern China. The results suggested that wheat and corn yield decreased with increasing N or P fertilization in a wheat-corn rotation system. Large amounts of nitrate accumulated in the soil across the region, and the accumulation amount and its downward movement was a potential risk to groundwater. The Olsen-P surplus (>20 mg·kg^-1) accounted for 80% of the arable soils in the region. There were strong relationships between NP fertilization rates, crop yield, and residual NP amounts and were into three phases: efficient NP-environmentally friendly phase, low NP efficiency-environmentally low risk phase, and inefficient NP-environmentally harmful phase. Optimized water and fertilizer use ensured crop yield, improved nitrogen use efficiency, and reduced nitrogen leaching losses, but the effects of biochar application and straw incorporation were inconsistent. The nitrate leaching-preventing effects of crop straw incorporation was resulted from soil microbial biomass increase, nitrification potential decreased or denitrification potential increase. Other issues also require investigation, such as tracing regional sources of underground water nitrate pollution, the effects of legacy NP accumulation from excess river anthropogenic inputs, and the environmental consequences of legacy NP accumulation in crop-fruit ecological agriculture.

The effects of water and fertilizer practices on nitrogen leaching in open-field vegetable soil in North China

YANG Rongquan, XIE Liyong, ZHENG Yimin, LI Ming, WEI Na, LI Yingchun, JU Xiaotang, GUO Liping

2021, 29(1): 176-186. doi: 10.13930/j.cnki.cjea.200533

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Groundwater nitrate pollution is a concern for the government and scientific community. During the growth period of open-field vegetables in North China, water and nitrogen (N) are often excessively used, resulting in a lower efficiency rate, which threatens groundwater quality. A field experiment was conducted in cinnamon soil on cucumber and Chinese cabbage crop rotation farmland to evaluate the effects of water and fertilizer on crop yields, N leaching, and N balance. Four standard N treatments were used [conventional N application in each vegetable season, 550 kg(N)·hm^-2·a^-1, N3; 20% less N, N2; 50% less N, N1; no nitrogen, CK], and leaching was monitored using a lysimeter. Five additional treatments were tested that combined a 20% N reduction with an alternative management practice: urease and nitrification inhibitors (N2I), biochar (N2B), straw incorporation (N2S), 15% irrigation reduction (N2W1), and 30% irrigation reduction (N2W2). The results showed that deep soil nitrate accumulation and root zone nitrogen leaching were primarily nitrogen loss. Using conventional N (N3), 10.0% of the applied N leached from the 80 cm soil layer, and the leached amount decreased by 23.8% and 45.6% by using 20% less N (N2) and 50% less N (N1), respectively, compared with that of N3. A 20% N reduction did not affect vegetable yield, but a 50% reduction decreased the cucumber yield by 19.6%. The combined practices (inhibitors, biochar, and straw) decreased the total leached N by 40.7% (N2I), 43.0% (N2B), and 34.3% (N2S) without affecting yields. Reducing irrigation decreased the total leached N by 43.1% (N2W1) and 50.5% (N2W2) compared with N3, but N2W2 decreased the cucumber yield by 13.9%. After three years (six continuous seasons), large amounts of nitrate accumulated and then moved to deeper soil. Nitrate accumulation in the 0-80 cm soil layer after conventional fertilization accounted for 38.2%-50.7% of the 0-200 cm soil layer, which was high compared to other management practices. Decreasing water and N fertilizer use combined with urease and nitrification inhibitors may reduce N leaching and cost. These results provide solutions for improving water and nitrogen management, thereby decreasing soil nitrate accumulation and deep soil leaching, reducing vegetable production and groundwater quality threats.

Effects of water and nutrient management and biochar application on crop yield, phosphorus use efficiency, and phosphorus leaching

LU Huiyu, DU Wenting, ZHANG Hongtao, XU Jiaxing, HAN Yan, ZHENG Jingrui, WANG Renjie, YANG Xueyun, ZHANG Shulan

2021, 29(1): 187-196. doi: 10.13930/j.cnki.cjea.200513

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Excessive water and chemical fertilizer application is often reported in North China winter wheat-summer maize rotation systems, resulting in economic losses and environmental issues. Therefore, optimizing water and nutrients (e.g., P) for crop yield is important. A 3-year field lysimeter experiment was performed on Lou soil (loess-derived and developed on natural cinnamon soil) in Yangling, Shaanxi Province, Northwest China, to investigate P use efficiency and P leaching of winter wheat-summer maize rotation systems in cinnamon soil. Seven treatments were used to investigate crop yield, P partial productivity (PFP_P), and P leaching: conventional practices (CP1, lysimeter depth = 120–150 cm; CP2, lysimeter depth = 100 cm), CP1 plus reduced water supply (CP1-W), CP1 plus reduced nutrient supply (CP1-F), CP1 plus reduced water and nutrient supplies (OPT), CP2 plus biochar application (CP2+B), and OPT plus biochar application (OPT+B). The results showed that the mean wheat, maize, and total wheat + maize yields were similar among CP1, CP1-W, CP1-F, and OPT. Compared with CP1, CP1-F and OPT significantly increased PFP_P by an average of 69.3% and 56.4%, respectively. CP1-W and CP1-F did not affect P leaching, but annual particulate phosphorus leaching decreased significantly under OPT treatment (by 58.4%). Biochar use did not affect the mean annual crop yield, but CP2+B significantly increased PFP_P (by 43.6%) compared with CP2. OPT-B did not affect PFP_P compared with OPT. Each year, all forms of leached P were similar between CP2 and CP2+B. In the first treatment year, OPT+B significantly decreased (compared with OPT) the dissolved organic phosphorus, particulate phosphorus, and total phosphorus (TP) leaching losses by 60.0%, 57.1%, and 62.4%, respectively, but TP leaching increased significantly in the following 2 years. The 3-year average showed that biochar application did not affect P loss. Therefore, only reducing water and fertilizer applications to cinnamon soil may improve P use efficiency and reduce P leaching while maintaining crop yield. Applying wheat straw biochar did not affect crop yield or P leaching, and the effect on PFP_P was inconsistent. Further studies are needed to clarify the effectiveness of biochar application.

Nitrogen and phosphorus leaching differs among cinnamon soil layers

MA Linjie, HUO Xiaolan, JIN Dongsheng, LIU Ping, HUO Chen, HUI Wei, LI Lijun

2021, 29(1): 197-207. doi: 10.13930/j.cnki.cjea.200585

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Abstract:
Leached agricultural N and P are the most prominent groundwater pollutants. Five cinnamon soil layers (cultivation, leaching, calcium, clay, and parent) were analyzed via leaching tests to investigate N and P migrations. After five tests, the amount of soluble total N in the leaching solutions were 2412.63 mg·L^-1 in cultivation layer, 3028.94 mg·L^-1 in leaching layer, 244.16 mg·L^-1 in calcium layer, 3648.99 mg·L^-1 in clay layer, and 3356.51 mg·L^-1 in parent layer. The amount of soluble total N in the leaching, clay, and parent layers was significantly higher than that in the cultivation layer but that of soluble total N in the calcium layer was significantly lower than that in the cultivation layer. The amount of soluble total P in the cultivation layer leaching solution was 0.52 mg·L^-1, which was significantly higher than that in all other layers. In the 1st to 3rd leaching time, the leached amounts of nitrate nitrogen, soluble total N, and orthophosphate in the cultivation and leaching layers were significantly higher than those in the clay and parent layers. However, in the 4th and 5th leaching time, the leached amounts of nitrate nitrogen and soluble total N in the clay and parent layers were significantly higher than those in the other layers, and the leached orthophosphate amount did not differ among layers. The amount of leached ammonium nitrogen in the clay and parent layers was significantly higher than that in the other layers after each test, and that of soluble total P in the cultivation layer was always significantly higher than that in the other layers. Nitrate nitrogen was the primary form of leached N in the cultivation and calcium layers, accounting for 69.0% and 85.4% of the total amount of N, respectively; the nitrate nitrogen percentages in the other layers were 41.3% (leaching layer), 5.1% (clay layer), and 4.6% (parent layer). Inorganic orthophosphate was the primary form of soluble P, accounting for 75.9% of the total amount of soluble P. The soil organic matter content, cation exchange capacity (CEC), and clay content affected the migration and transformation of soil N and P. There was a significant positive correlation between organic matter and N and P leaching, and more organic matter content increased the leaching risk in the initial leaching stage. The CEC and clay content were negatively correlated with N and P leaching, and increased CEC and soil clay particles reduced the leaching risk. The physical and chemical properties and N and P leaching characteristics differed among soil layers, and leaching was affected by soil CEC, clay and organic matter contents.

Spatial distribution and changes of nitrate in the vadose zone and underground water in northern China

LI Xiaoxin, WANG Shiqin, CHEN Xiaoru, LEI Yuping, GAO Pengcheng, HU Chunsheng, MA Lin

2021, 29(1): 208-216. doi: 10.13930/j.cnki.cjea.200862

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Nitrate leaching from Chinese farmland causes non-point source pollution and is an increasingly serious issue. The vadose zone is an important place for nitrate nitrogen accumulation and storage and a common way for nitrate to leach into the groundwater. Nitrate spatial-temporal changes in the underground water and vadose zones were analyzed in this study. Farmlands in black soil, fluvo-aquic soil, and cinnamon soil in northern China were investigated by monitoring underground water nitrate and water level changes to determine the underground water nitrate contents. The results showed that the black soil region (Northeast China) had the highest groundwater nitrate content with excess standard rate of 39.6%, followed by the fluvo-aquic soil region (North China) (19.3%); the cinnamon soil region (Northwest China) had the lowest rate (14.9%). In the North China Plain, the excess standard rates of nitrate in shallow underground water trended upward over the years; the groundwater nitrate excess standard rate was 11.8% in 1998 and 18.9% from 2016 to 2018. The underground water nitrate excess standard rate was higher in vegetable-planting areas than in grain crop-planting areas. Soil nitrate was distributed and accumulated in the vadose zone before being leached into the underground water. Nitrate accumulation increased with vadose zone thickness; the total nitrate-N storage in the North China Plain deep vadose zone was up to 18.54 million tons. Nitrate accumulated mainly at a depth of 0–6 m, and crop production contributed, on average, 78.3% toward the regional vadose zone nitrate storage.

Regional nitrogen and phosphorus leaching mitigation strategies based on nutrient losses vulnerable zones in China

JIN Xinpeng, BAI Zhaohai, MA Lin

2021, 29(1): 217-229. doi: 10.13930/j.cnki.cjea.200792

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Agricultural non-point source pollution control research has primarily focused on field-level technology development rather than regional mitigation options, reducing their large-scale effectiveness. This study proposed the mitigation strategies based on nutrient-loss risk at the regional level to achieve full potential of mitigation technologies. Nutrient-loss vulnerable zones (NLVZ) in China were designed from water quality monitoring and nutrient flow data and natural condition spatial characteristics. Based on the NLVZ, hierarchical management according to regional vulnerabilities were adopted, zone-specific mitigation methods based on natural and socio-economic conditions were selected, and regional mitigation strategies and nitrogen (N) and phosphorus (P) leaching technique lists were developed. Furthermore, using the NUtrient flows in Food chains, Environment and Resources model (NUFER), the effects of regional mitigation strategies were evaluated. The results indicated that NLVZ and potential NLVZ covered 52% of Chinese croplands and were widely distributed in major agricultural production areas. Regional mitigation strategies reduced potential NLVZ by 51%, particularly in the Northeast and Southwest China and the middle and lower reaches of the Yangtze River. Regional mitigation strategies reduced leaching by approximately 40% in cultivated areas with high N leaching (>22.6 kg(N)·hm^-2), from 3.1×10⁷ kg(N)·hm^-2 to 1.9×10⁷ kg(N)·hm^-2. Designing regional mitigation strategies based on NLVZ reduced nonpoint source pollution, promoting ecological agricultural development in China.

Regional characteristics of nitrate sources and distributions in the shallow groundwater of the Lake Baiyangdian watershed

WANG Shiqin, TAN Kangda, ZHENG Wenbo, MA Lin, SONG Xianfang, TANG Changyuan, HU Chunsheng

2021, 29(1): 230-240. doi: 10.13930/j.cnki.cjea.200672

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Lake Baiyangdian is located in Xiong'an New Area, China, where groundwater is the primary water supply. Groundwater nitrate (NO₃^-) contamination is common in the Baiyangdian Lake watershed because of industrial and domestic wastewater discharge and over-application of agricultural fertilizer. However, the source characteristics and NO₃^- distribution across the entire watershed are still unclear. In this study, NO₃^- samples collected from rivers and shallow groundwater over the past decade were analyzed. Samples were also collected in December 2016 from the Lake Baiyangdian watershed area, and the spatio-temporal NO₃^- distributions of groundwater and the effects of various sources on groundwater NO₃^- were analyzed using water chemical ions and stable nitrate nitrogen isotopes (δ¹⁵N-NO₃^-). The results showed that the NO₃^- concentration in shallow groundwater differed, and the nitrogen sources had variable effects, particularly from the hills to the plains. In the hilly area, high NO₃^- concentrations measured in the alluvial valley groundwater were attributed to local rural sewage, with the highest NO₃^- concentration of 313 mg·L^-1; while the regional farmland manure application over several decades was the main cause of commonly high groundwater NO₃^- concentration in recent years. Rainy season leaching led to NO₃^- concentrations two times greater than that during the dry season, which exceeded the World Health Organization's (WHO) standard (50 mg·L^-1) and threatened downstream water quality safety. Of the shallow groundwater samples collected in the plains in December 2016, 21.5% had NO₃^- concentrations exceeding the WHO standard. The median groundwater nitrate concentrations trended downward from upstream to downstream in geomorphological type (proluvial fan: 42.4 mg·L^-1 > alluvial-proluvial fan: 24.1 mg·L^-1 > alluvial-proluvial plain: 6.0 mg·L^-1 and river zone: 6.2 mg·L^-1), but the median δ¹⁵N-NO₃ isotopes trended upward (proluvial fan: 12.8‰ and alluvial-proluvial fan: 11.3‰ < alluvial-proluvial plain: 16.7‰ < river zone: 20.9‰), indicating that denitrification increased from upstream to downstream. High aquifer sediment permeability in the proluvial fan and alluvial-proluvial fan regions increase the risk of nitrate leaching into the aquifer. Sewage (33.3%) and manure (34.0%) were primary sources of groundwater nitrate and caused the deviation from the WHO standard rate. In regions with lakes and depressions, groundwater nitrate was affected by industrial and domestic sources and fertilization, and, compared to other regions, groundwater nitrate was higher near the domestic and industrial wastewater river (but also had drastically different surface pollution control measures). However, the reduced conditions in other lake and depression regions lowered the groundwater nitrate concentration (< 10 mg·L^-1). This study provides suggestions for managing nonpoint source pollution in the Lake Baiyangdian watershed shallow groundwater based on the regional source characteristics and nitrate distribution, particularly for vulnerable places, such as hilly areas, the proluvial/alluvial-proluvial fan region of the piedmont plain, and wastewater influence areas.

2021 Vol. 29, No. 1