Nitrogen Runoff Losses Research Articles

Contributions of Volume and Concentration on Runoff Nitrogen Losses from Intensive Vegetable in China

Contributions of Volume and Concentration on Runoff Nitrogen Losses from Intensive Vegetable in China

Runoff nitrogen losses under confluence and diverging drainage systems in the sloped plot scale: A comparative study

The drainage system is a key measure for regulating runoff nutrient losses on sloping farmlands. Confluence and diverging drainage systems are two drainage layouts representing natural water network systems and are widely distributed in sloping farmlands; however, the effects of these drainage systems on runoff nutrient losses in the sloped plots remain unclear. This study investigated the effects of different drainage systems on the characteristics of runoff nitrogen (N) losses in sloped plots using laboratory rainfall simulations. Three treatments, including bare slope (without drainage system, CK), confluence drainage system (T1), and diverging drainage system (T2), were used to compare the changes in concentrations and losses of total nitrogen (TN), dissolved nitrogen (DN), and particulate nitrogen (PN), and the DN:TN ratio in runoff under a combination of 1.8 mm min−1 rainfall intensity and three slope gradients (5°, 10°, and 15°). The results showed that the time to runoff was significantly delayed in T2 compared with that in CK and T1 across all slopes (p < 0.05). Accumulated runoff depth was considerably lower in T1 and T2 than in CK across all slopes (p < 0.05). The TN and PN concentrations in T1 were markedly lower than those in T2 on the 10° and 15° slopes (p < 0.05). The DN concentration in T1 was lowest at the 5° slope (p < 0.05). TN loss in T1 was 14.7–33.9% and 17.9–30.3% lower than those in CK and T2 across all slopes, respectively (p < 0.05). The PN loss in T1 was 56.7% and 53.3% lower than that in T2 on the 10° and 15° slopes, respectively (p < 0.05). DN loss in T1 was 39.3–72.5% lower than that in CK for all slopes (p < 0.05). DN:TN in T2 was lower than that in CK and T1 at the 10° and 15° slopes (p < 0.05). Our results confirm the effectiveness of drainage systems in reducing runoff nutrient losses in a sloped plot and demonstrate that the confluence drainage system is better at reducing N losses in runoff than diverging drainage systems.

Optimal fertilizer rates towards the improvement of nitrogen use efficiency and reduction of nitrogen export in paddy rice-wheat intensive farming

Introduction: Fertilizer application above plant nitrogen (N) requirements, leading to N use inefficiency, has become the primary source of N surplus, posing nonpoint pollution threats. Chao Lake has received N loadings, primarily from the agroecosystem that surrounds it. Based on 10 years of field monitoring and experiment, this study used optimum fertilizer rates (30% N reduction) to evaluate nitrogen use efficiency (NUE) and N export in the Chao Lake watershed of paddy rice-wheat rotational farming.Methods: Five treatments were tested, namely, the Blank Treatment (CK), Conventional Fertilization Treatment (CFT), High Fertilization Treatment (HFT), Reduced Fertilization Treatment (CFT), Reduced Fertilization Treatment plus Straw addition (RFTS). The parameters employed to measure NUE were 1) by calculating NUE indicators; Agronomic efficiency (kg kg-1), recovery efficiency (%), and partial factor productivity (kg kg-1) 2) using the European Union NUE framework as a comparison tool. N export was measured using N export parameters, namely, nitrogen loading (kg ha-1yr-1), nitrogen runoff loss ratio (%), and net nitrogen runoff loss ratio (%).Results and discussion: Results revealed that optimum N fertilizer input levels maintained high crop and biomass yields. The empirical model of y = y0 + a * exp (−0.5 * (x-x0)/b2) explains the relationships between input N fertilizer and corresponding yield returns. Low yield responses to increased N input beyond certain limits were observed, which may lead to N accumulations. N export was much lower from the reduced fertilization treatments than other treatments. N export increased in 10 years for both paddy rice and wheat seasons. Runoff volume influenced N export more than any other environmental factor studied.Conclusion: Overall, optimal fertilizer levels produced high NUE, maintained high economic yield, and produced lesser N exports; NUE was in the order of reduced &amp;gt; high &amp;gt; conventional fertilization treatments, while it decreased in 10 years (2008–2018) across all treatments. Such strategies that further reduce fertilizer levels by accounting for excess supplies are vital for consolidating effective N control measures.

Controlled-Release Blended Fertilizer Combined with Urea Reduces Nitrogen Losses by Runoff and Improves Nitrogen Use Efficiency and Yield of Wet Direct-Seeded Rice in Central China

Controlled-release fertilizer is one of the best fertilizer management strategies for improving the yield and nitrogen use efficiency of transplanted seedling rice. Wet direct-seeded rice has gradually replaced transplanted seedling rice since it saves labor. In addition, it is conducive to mechanization promotion. However, the effects of controlled-release fertilizers on wet direct-seeded rice remain largely unknown. A two-year field experiment aimed to compare the effects of controlled-release blended fertilizer at two rates (basal N to tiller N ratio = 7:3 (CRBF+U), CRBF alone), urea at two rates (basal–tiller ratio of 4:6 (U40), 6:4 (U60)) and a control (no N fertilizer) on the ammonia volatilization (AV) loss, nitrogen runoff loss, accumulation, transport, utilization and yield of rice. The nitrogen runoff loss in wet direct-seeded rice paddy fields was concentrated from sowing to the three-leaf and one-leaflet stage, and the loss rat was lowest after CRBF+U (11.41–12.94%). AV loss rate was lowest after CRBF (3.41%), followed by CRBF+U (3.55–3.89%). CRBF+U increased nitrogen accumulation by extending the duration of rapid nitrogen growth and accelerating maximum nitrogen growth. CRBF+U also increased the nitrogen transport rate of stems, sheaths and leaves from full heading to maturity, and intensified the increase in nitrogen in panicles, increasing the harvest index, agronomy utilization rate and apparent utilization rate of nitrogen. Finally, the grain number per panicle, seed-setting rate and actual yield of rice were significantly improved. In conclusion, CRBF+U can reduce nitrogen runoff loss and AV loss and can improve the yield and nitrogen use efficiency of wet direct-seeded rice.

Estimation of Cropland Nitrogen Runoff Loss Loads in the Yangtze River Basin Based on the Machine Learning Approaches

A quantitative understanding of cropland nitrogen (N) runoff loss is critical for developing efficient N pollution control strategies. Using correlation analysis, a structural equation model, variance decomposition, and machine learning methods, this study identified the primary influencing factors of total N (TN) runoff loss from uplands (n=570) and paddy (n=434) fields in the Yangtze River Basin (YRB) and then developed a machine learning-based prediction model to quantify cropland N runoff loss load. The results indicated that runoff depth, soil N content, and fertilizer addition rate were the major influencing factors of TN runoff loss from uplands, whereas TN runoff loss rate from paddy fields was mainly regulated by runoff depth and fertilizer addition rate. Among the four used machine learning methods, the prediction models based on the random forest algorithm presented the highest accuracy (R2=0.65-0.94) for predicting upland and paddy field TN runoff loss rates. The random forest algorithm based model estimated a total cropland TN loss load in the YRB of 0.47 Tg·a-1 (upland:0.25 Tg·a-1; paddy field:0.22 Tg·a-1) in 2013, with 58% of TN runoff loss load derived from the midstream and downstream regions. The models predicted that TN runoff loss loads from croplands in YRB would decrease by 2.4%-9.3% for five scenarios, with higher TN load reductions occurring from scenarios with decreased runoff amounts. To mitigate cropland N nonpoint source pollution in YRB, it is essential to integrate efficient water, fertilizer, and soil nutrient managements as well as to consider the midstream and downstream regions as the high priority area. The machine learning-based modeling method developed in this study overcame the difficulty of identifying the functional relationships between cropland TN loss rate and multiple influencing factors in developing relevant prediction models, providing a reliable method for estimating regional and watershed cropland TN loss load.

Returned straw reduces nitrogen runoff loss by influencing nitrification process through modulating soil C:N of different paddy systems

Nitrogen (N) runoff loss from paddy fields contributes significantly to non-point source pollution. Straw return to soil may affect N runoff loss by changing soil biochemistry. In situ N runoff monitoring combined with an analysis of soil biochemical indicators was implemented to explore the effect of straw return lasting for five or six years on N runoff loss and its potential mechanism in five rice rotation systems. The results showed that straw return reduced total N (TN) runoff losses by 2.29%− 26.10% through physical and biochemical pathways, and the reduction in inorganic N (IN) was the largest (53.56%–82.42%) through biochemistry. This was mainly achieved by the increase in soil C:N due to straw return, thereby increasing the immobilization of microbial N and reducing the soil IN. At the same time, the abundance of functional genes (AOA, AOB, nxrA) participating in the nitrification process was also conducive to the decrease in TN runoff concentration. More importantly, the reduction in IN and functional genes of the nitrification process did not affect N uptake by rice, and the increase in the atmospheric N fixation gene (nifH) was also beneficial to N supplementation in soil. However, the positive effects of straw return under the five rice rotation systems were different, and the TN loss reduction in double-cropping rice was the lowest. With the lower soil C:N in the single-cropping and paddy-upland systems, the increase in soil C:N after straw was more effective, and had greater potential to reduce TN runoff loss. This study provides a new perspective on N cycling in soil biochemistry by straw return, so as to select optimal measures to protect the water environment in different rice rotation systems.

Effect and Mechanism of Rice-Pasture Rotation Systems on Yield Increase and Runoff Reduction under Different Fertilizer Treatments

This study investigated the effects of crop rotations and different ratios of dairy manure fertilizer on nitrogen loss and rice yield in the Erhai Lake basin. Two kinds of herbages were set in the rotation: Ryegrass (Lolium multiflorum cv.‘Tetragold’) (R) and Villose Vetch (Vicia villosa var. Glabresens) (V). The experiment involved two-year field tests with nine management treatments. The results showed that the rice-Vicia villosa rotation with 70% chemical and 30% dairy cattle manure fertilization increased rice yield significantly, while reducing nitrogen runoff losses and increasing microbial abundance with nitrification and nitrogen fixation. The microbial abundance varied among tested soils, with Alphaproteobacteria, Rhodopseudomonas, Rhizobiales, Bradyrhizobium, and Azotobacter Vinelandii being the highest in 70% chemical fertilizer + 30% manure rice Villose Vetch (R-V) to ameliorate plant growth and strengthen the efficiency of nutrient uptake, whereas that of Planctomycete was comparatively lower to promote long-term N stabilization in soil. The 70% F—30% M R-V treatment also significantly decreased nitrate reductase and ammonia monooxygenase enzyme activity, potentially improving fertilizer use efficiency, and reducing gaseous losses. The LEfSe analysis results indicated that 70% F—30% M R-V fertilizers significantly enhanced the abundances of metabolic genes related to energy and nitrogen. These findings suggested that appropriate agricultural management using rice-Vicia villosa rotation and 70% chemical + 30% dairy cattle manure fertilization can improve the soil quality and sustainability of agroecosystems.

Alleviation of aqueous nitrogen loss from paddy fields by growth and decomposition of duckweed (Lemna minor L.) after fertilization

Runoff loss of nitrogen from paddy fields has received increasing attention in recent years. Duckweed is an aquatic plant frequently found in paddy fields. In this study, the effects of duckweed (Lemna minor L.) in floodwater on aqueous nitrogen losses from paddy fields were systematically investigated. Results demonstrated that the growth of duckweed decreased total nitrogen concentrations in floodwater and nitrogen runoff loss from paddy fields by 16.7%–18.3% and 11.2%–13.6%, respectively. Moreover, compared with NO3−, NH4+ was preferentially removed by duckweed. 15N isotope tracer experiments revealed that the growth and decomposition of duckweed acted as a “buffer” against the nitrogen variation in floodwater after fertilization. During the growth of duckweed, leaves were found to be the principal organ to assimilate NH4+ and release NO3− by using non-invasive micro-test technology. Duckweed degradation increased the content of hydrophobic acids and marine humic-like substances in floodwater, which promoted the migration of nitrogen from floodwater to soil. Redundancy analysis and structural equation models further illustrated that pH and temperature variation in floodwater caused by duckweed played a greater role in aqueous nitrogen loss reduction than the nitrogen accumulation in duckweed. This study suggested that the growth of duckweed in paddy fields was an effective supplementary method for controlling aqueous nitrogen loss during agricultural production.

Study on Nitrogen Loss Rules of Paddy Fields under Different Irrigation and Drainage Modes in Southern China

With the growing problem of agricultural nonpoint source pollution, it is an urgent issue to explore irrigation and drainage modes suitable for rice-growing areas in southern China. Barrel experiments were conducted to study the variation of NH4+-N, NO3−-N and TN concentrations in surface drainage and deep percolation water, as well as the rules of nitrogen runoff losses and leaching losses. Treatments included frequent and shallow irrigation (FSI), drought planting with straw mulching (DPS) and water catching and controlled irrigation (WC-CI). The results showed that the trends of NH4+-N, NO3−-N and TN concentrations in surface drainage were similar, and the N concentration gradually decreased with increasing drainage frequency. The trends of NH4+-N, NO3−-N and TN concentrations in deep percolation water were similar, rising to a peak within 5~7 days after fertilization and then gradually decreasing. NH4+-N loss load was the main form of N loss load in surface drainage and deep percolation water, which accounted for 42.06~89.16%. Compared with FSI, DPS significantly increased surface drainage and N runoff loss loads by 98.67% and 125.86%, respectively, while WC-CI significantly reduced them by 59.21% and 66.38%, respectively. Deep percolation water was reduced by 68.59% and 37.99% for DPS and WC-CI, respectively, and N leaching loss loads were reduced by 74.69% and 43.23%, respectively. Compared with FSI, the total TN loss load was significantly reduced by 76.69% and 43.34% for DPS and WC-CI, respectively, and the pollution control was better for DPS. WC-CI significantly increased rice yield by 7.31%, while DPS decreased by 0.95 % due to long-term water stress. Comprehensively considered, WC-CI has obvious advantages, which can reduce nitrogen loss and maintaining a high yield.

Rice-crayfish pattern in irrigation-drainage unit increased N runoff losses and facilitated N enrichment in ditches

The rice-crayfish (RC) integrated pattern has been developed vigorously in China, but how it affects the nitrogen (N) runoff loss and distribution status during rice production is still poorly studied. Based on this, we selected two types of irrigation and drainage units (IDUs), which adopted the traditional rice-wheat (RW) rotation pattern and burgeoning RC rotation pattern separately, to investigate the effect of the RC pattern on N runoff loss, inorganic N distribution and N balance of the IDU. The results showed that there was a 241 kg ha−1 yr−1 and 135 kg ha−1 yr−1 N surplus achieved under RW and RC, respectively. Among these, the N surplus of RC was 53 % lower than that of RW during the rice growing season and was 37 % lower at other times. The NH4+-N contents of paddy field soils, rice yields and productive traits were not affected by rotation patterns. Nevertheless, the total nitrogen (TN), dissolved organic nitrogen (DON) and NH4+-N concentrations of RC field water were significantly higher (P < 0.01), and the N runoff losses of the RC pattern increased by 103 % to 855 % compared with the RW pattern. In addition, the NH4+-N reserved in RC ditch sediments substantially increased regardless of the dynamic changes during the rice growing season or from the vertical distribution at depths of 0–40 cm. Our results indicated that the RC pattern was beneficial for decreasing the N surplus without impacting the rice yield. However, larger N runoff losses and more available N flowing into crayfish farming ditches still pose great environmental risks. Therefore, more efficient and cleaner measures should be applied for the N management of IDU under the RC pattern.

Effect of Thaw Depth on Nitrogen and Phosphorus Loss in Runoff of Loess Slope

Seasonal freeze-thaw erosion is a form of soil erosion caused by the topographical characteristics and climatic factors of the hilly and gully loess regions. Seasonal freeze-thaw can damage the soil pores and cause its bulk density to change. The effects of thawing depth on runoff and Nitrogen and Phosphorus loss on the rainfall erosion of an artificial slope filled with loess soil were analyzed after a rainfall test that simulated the spring thaw period in China. The results showed that: (1) The maximum runoff yield was 33.35 mm at 4 cm thawing depth, and the minimum was 12.95 mm at 6 cm thawing depth. With the increase in runoff time, the slope infiltration rate had a decreasing trend. The loss rate of available and total Phosphorus increased with the increase in runoff rate. The rate of increase was fastest when the thawing depth was 4 cm. (2) The relationships between runoff rate and Nitrogen loss and Phosphorus loss rate can be explained by linear regression equations, and the loss rate increased as the runoff rate rose for all thawing depths. Within the 0–6 cm thawing depths, the loss of total phosphorus was the largest when the thawing depth was 4 cm, and the loss of available phosphorus was the smallest when the thawing depth was 6 cm. At the shallower thawing depths, the available Nitrogen loss represented a smaller proportion of the total Nitrogen loss compared to nitrate Nitrogen loss. However, there was a gradual rise in the available Nitrogen proportion in the total amount of inorganic Nitrogen as the thawing depth increased. (3) Total Phosphorus was the available Phosphorus with a quadratic function relationship with runoff energy and runoff power. Runoff energy mainly affected the total Nitrogen and available Nitrogen loss in runoff, whereas runoff power mainly affected total Nitrogen loss in runoff. The results of this paper can improve the understanding of runoff and Nitrogen and Phosphorus loss caused by runoff during freeze-thaw conditions.

Apparent variations in nitrogen runoff and its uptake in paddy rice under straw incorporation

Straw incorporation is a widespread practice to promote agricultural sustainability. However, the potential effects of straw incorporation with the prolonged time on nitrogen (N) runoff loss from paddy fields are not well studied. The current study addresses the knowledge gap by assessing the effects of straw incorporation on the processes influencing N runoff patterns and its impacts on crop yield, N uptake, total N (TN), and soil organic matter (SOM). We conducted field experiments with rice (Oryzasativa L.)–wheat (Triticumaestivum L.) rotation, rice–tobacco (Nicotianatabacum L.) rotation, and double-rice cropping in subtropical China from 2008 to 2012. Each rotation had three N treatments: zero N fertilization (CK), chemical N fertilization (CF), and chemical N fertilization combined with straw incorporation (CFS). The treatment effects were assessed on TN runoff loss, crop yield, N uptake, soil TN stock, and SOM. Results showed that TN runoff was reduced by substituting part of the chemical N fertilizer with straw N in the double rice rotation, while crop N uptake was significantly (P<0.05) decreased due to the lower bioavailability of straw N. In contrast, in both rice–wheat and rice–tobacco rotations, TN runoff in CFS was increased by 0.9–20.2% in the short term when straw N was applied in addition to chemical N, compared to CF. However, TN runoff was reduced by 2.3–19.3% after three years of straw incorporation, suggesting the long-term benefits of straw incorporation on TN loss reduction. Meanwhile, crop N uptake was increased by 0.8–37.3% in the CFS of both rotations. This study demonstrates the challenges in reducing N runoff loss while improving soil fertility by straw incorporation over the short term but highlights the potential of long-term straw incorporation to reduce N loss and improve soil productivity.

Long-term straw return with N addition alters reactive nitrogen runoff loss and the bacterial community during rice growth stages

Crop residue return is an effective, eco-friendly tillage method for decreasing reactive nitrogen (Nr) losses via surface runoff. However, the associated variation in Nr characteristics and its prospective mechanisms are not well understood. We systematically evaluated the response of Nr runoff loss and N variation in standing water to the abiotic and biotic parameters of soil in a paddy field after 6 years of straw return. Five experimental treatments of different fertilization strategies in combination with straw return were tested during the rice growth season. The results indicated that under equivalent fertilizer input, long-term straw return significantly reduced Nr runoff loss by 11.5% (P < 0.05), even though the loss increased with N fertilizer addition. We report that variations in abiotic soil properties (P < 0.05) and bacterial communities (P < 0.01) were both responsible for Nr loss differences between the rice growth stages and among the tested fertilizing patterns. Soil inorganic nitrogen (r = 0.18) had a significant positive influence on Nr runoff loss, but this effect was surpassed by the overall negative influence of soil organic carbon (r = −0.43), soil pH, (r = −0.40), and bacterial community composition (r = −0.14), which was especially apparent during the tillering stage. Our results emphasize the importance of jointly considering biotic and abiotic factors in soil and standing water when characterizing the effects of long-term straw return and N addition on Nr runoff loss, which will aid in mitigating N-based agricultural non-point pollution.

Potential of paddy drainage optimization to water and food security in China

Rice production not only consumes large amounts of irrigation water and fertilizer, but also poses a high risk of water pollution by delivering nitrogen (N) through surface runoff. To ensure sustainable rice production, many water-saving irrigation managements have been proposed and implemented, but drainage water managements receive far less attention and need to be further explored. This study aimed to determine the paddy drainage optimization management and assess its potential to water and food security in China via different scale methods (from pot and field experiments to national assessment). The national investigation of water and N fertilizer use in paddy fields implied that diffuse N pollution was expected to continue increasing, especially in the Yangtze river basin. Two-years field experiments at typical sites identified that the tillering and jointing–booting stages were critical risk stages for N runoff loss, and pot experiments on the critical stages were conducted to determine the optimal drainage water level without yield reduction. Then, the applicability of paddy drainage optimization was verified and evaluated by drainage optimization field experiment and precipitation characteristics analysis. Finally, the potential of drainage optimization on mitigating N runoff loss was estimated by scenario analysis at the national scale. After implementing paddy drainage optimization in field experiments, surface runoff and nitrogen runoff loss decreased by 27.97–78.94% and 35.17–67.95%, respectively, without affecting rice yield. By full implementation of the optimal drainage and fertilization management, N runoff loss could be reduced by 0.19 Tg yr−1 at the national scale. These results suggest that paddy drainage optimization is an agro-ecosystems friendly water management for sustainable rice production, and has notable potential to ensure water and food security in China.

Investigation into runoff nitrogen loss variations due to different crop residue retention modes and nitrogen fertilizer rates in rice-wheat cropping systems

Crop residue retention has been established as an effective conservation tillage practice for decreasing nitrogen (N) runoff losses and increasing crop production. However, the impacts on N runoff losses from annual rice-wheat rotation are not well understood, which has prevented identification of optimal straw retention practice. Herein, N dynamics and losses in the runoff, crop yields, and soil properties were assessed for two years in a rice-wheat system after implementing straw return for five consecutive years. The field experiment included six treatments—N1, WN1, RN1: recommended N application with no straw, wheat straw, and rice straw return, respectively; and SN0, SN1, and SN2: both wheat and rice straw return with zero, recommended, and custom N addition, respectively. The results showed that NH4+-N and NO3--N were the major forms of N loads discharged during the rice and wheat season, respectively. The total N loss (TN, P < 0.001) and mean annual crop yield (rice and wheat, P < 0.01) significantly increased as the N fertilizer rate increased. With respect to crop residue retention modes, the cumulative TN loss and mean annual crop yield trends were: N1 > RN1 >WN1 >SN1 (P < 0.05) and SN1 ≥RN1 >WN1 ≈N1 (P < 0.05), respectively. During a one-year rice-wheat rotation cycle, the treatments with simultaneous incorporation of crop residue and N fertilizer reduced N runoff loss by 2.7–20.9% (P < 0.05) and increased crop yield by 8.7–16.9% (P < 0.05), of which the SN1 treatment was found to be the most effective. The straw-induced reducing effect occurred only during the base and tiller fertilization stages of the rice growing period, which was primarily regulated by soil inorganic nitrogen (SIN), soil organic carbon (SOC), and pH during the rice tiller stage. Our findings indicate that straw return decreases N runoff loss by mediating factors in soil and standing water. In order to reduce N runoff loss without jeopardizing crop yield, the practice of double seasons straw retention with recommended N application (SN1) is suggested in the rice-wheat rotation cropping system in southern China.

Estimation of nitrogen runoff loss from croplands in the Yangtze River Basin: A meta-analysis.

Nitrogen (N) runoff loss from croplands due to excessive anthropogenic N additions is a principal cause of non-point source water pollution worldwide. Quantitative knowledge of regional-scale N runoff loss from croplands is essential for developing sustainable agricultural N management and efficient water N pollution control strategies. This meta-analysis quantifies N runoff loss rates and identifies the primary factors regulating N runoff loss from uplands (n=570) and paddy (n=434) fields in the Yangtze River Basin (YRB). Results indicated that total N (TN) runoff loss rates from uplands and paddy fields consistently increased from upstream to downstream regions. Runoff depth, soil N content and fertilizer addition rate (chemical fertilizer+manure) were the major factors regulating variability of TN runoff loss from uplands, while runoff depth and fertilizer addition rate were the main controls for paddy fields. Multiple regression models incorporating these influencing factors effectively predicted TN runoff loss rates from uplands (calibration: R2=0.60, n=242; validation: R2=0.55, n=104) and paddy fields (calibration: R2=0.70, n=189; validation: R2=0.85, n=82). Models estimated total cropland TN runoff loss load in YRB of 0.54 (95% Cl: 0.23-1.33) Tg, with 0.30 (95% Cl: 0.15-0.56) Tg from uplands and 0.24 (95% Cl: 0.08-0.77) Tg from paddy fields in 2017. Guangxi, Jiangxi, Fujian, Hunan and Henan provinces within the YRB were identified as cropland TN runoff loss hotspots. Models predicted that TN runoff loss loads from croplands in YRB would decrease by 0.8-13.7% for five scenarios, with higher TN load reductions occurring from scenarios with decreased runoff amounts. Reducing upland TN runoff loss should focus primarily on soil N utilization and runoff management, while reducing N fertilizer addition and runoff provided the most sensitive strategies for paddy fields. Integrated management of water, soil and fertilizer is required to effectively reduce cropland N runoff loss.

Abiotic and biotic effects of long-term straw retention on reactive nitrogen runoff losses in a rice–wheat cropping system in the Yangtze Delta region

Straw retention has been widely recommended as a nutrient-conserving measure that effectively increases soil fertility and soil nitrogen (N) availability. However, despite its efficacy in reducing reactive N (Nr) runoff losses, there is currently little information available regarding the influence of distinct straw return modes with respect to potential Nr runoff loss. In this study, based on a 6-year period of straw incorporation in a field under rice–wheat rotation with a yield of two crops a year, we sought to evaluate the overall responses of Nr runoff losses and N changes in the surface water to soil biotic and abiotic factors. Four different straw return modes (CK: no straw, W: wheat straw, R: rice straw, and S: both wheat and rice straw) were examined. The results revealed that compared with the CK treatment, the three modes of crop residue incorporation significantly reduced N runoff loss by 4.21 %–11.59 % (P < 0.05), of which the S mode of straw returning was found to be the most effective. The reductive effect of straw return was observed during the early period of rice growth (the basal and tiller fertilization stages, P < 0.05). Significant differences in both the soil abiotic and biotic factors were observed not only between the tillering and ripening stages of rice growth (P < 0.01), but also among the four straw return modes (P < 0.05). Structural equation modelling revealed that soil inorganic nitrogen (SIN, r = 0.53, P = 0.007) had the most direct positive effect on Nr runoff loss, but the effect was statistically compromised by the overall negative effects of soil organic carbon (SOC, r = -0.33, P = 0.034), pH (r = -0.15, P> 0.05), and bacterial community composition (r = -0.10, P = 0.041), and to a less extent by α-diversity (r = -0.04, P > 0.05). During the tillering stage of rice growth and in response to the S mode of straw return in particular, there was a significant enrichment of Proteobacteria (P < 0.01), thereby indicating the potentially prominent role of microbial N immobilization in reducing N losses. The links among N contents in paddy standing water, surface runoff, and soil biotic and abiotic factors established in this study will inform straw return strategies to reduce the levels of N-based pollution.

Ammonia volatilization and nitrogen runoff losses from moso bamboo forests after different fertilization practices

Moso bamboo (Phyllostachys edulis (Carrière) J. Houz.) is a major giant bamboo species. Unreasonable fertilization and nitrogen (N) loss is a serious problem. A field experiment was conducted to determine the effects of application methods (furrow and hole) and depths (0–20 and 20–40 cm) on ammonia (NH3) volatilization and N runoff losses from June to December, 2015. Ammonia volatilization was detected as a single peak curve after fertilization. Three weeks later, fluxes of all fertilization treatments were similar to those of the control and remained stable. After the experiment, the cumulative NH3 volatilization was 17.2–21.4 kg·ha−1. Approximately 90% of NH3 volatilization occurred within the first two weeks. The NH4+-N loss was higher than the NO3−-N loss, and the total runoff losses were relatively low. For the same application method, with the increase of application depth, the NH3 volatilization and N runoff losses reduced; for the same application depth, the NH3 volatilization and N runoff losses after furrow application were lower than those after hole application. Therefore, the deep and furrow application methods were effective in reducing N loss through NH3 volatilization and runoff. In the present study, furrow application at a 20–40 cm depth has been appropriate for field practice.

Grazing Management and Buffer Strip Impact on Nitrogen Runoff from Pastures Fertilized with Poultry Litter.

Nitrogen runoff from pastures fertilized with animal manure, such as poultry litter, can result in accelerated eutrophication. The objective of this study was to evaluate the long-term effects of grazing management and buffer strips on N runoff from pastures fertilized with poultry litter. A 12-yr study was conducted on 15 small watersheds in Booneville, AR, using five management practices: continuous grazing, haying, rotational grazing, rotational grazing with an unfertilized buffer strip, and rotational grazing with a fenced unfertilized riparian buffer. Poultry litter was applied annually at a rate of 5.6 Mg ha. Concentrations and loads of total N, NO-N, NH-N, organic N, and total organic C in runoff varied intra- and interannually and coincided with precipitation trends. Overall, the greatest component of total N in runoff was organic N. Rotational grazing resulted in the highest concentrations and loads of all forms of N in runoff compared with other treatments, including the continuously grazed paddocks, which were grazed almost twice as much. Total organic C concentrations and loads in runoff were also higher from rotationally grazed watersheds than other treatments. Rotational grazing is considered a best management practice that typically reduces soil erosion; hence, the mechanism by which it caused higher N and C runoff is unclear. Nitrogen runoff losses from rotationally grazed pastures were reduced by 44% with unfertilized buffer strips, by 54% with fenced unfertilized riparian buffers, and by 52% by converting pastures to hayfields.

Effect of management intensity on runoff losses of nitrogen and phosphorus in bamboo forests in Zhejiang province

Effect of management intensity on runoff losses of nitrogen and phosphorus in bamboo forests in Zhejiang province