The low value of nitrogen use efficiency (NUE) (around 30%) of crop production in China highlights the necessity to adopt reasonable N managements in national scale. After the implementation of ‘National Soil Testing and Formulated Fertilization’ program in 2005, many field experiments have reported an increase of NUE for crop productions in China. This has prompted discussion regarding the extent to which NUE in crop production has been improved. Here, we analyzed the temporal and spatial changes in NUE (crop N uptake/total N input) and cumulative synthetic and non-synthetic N fertilizer recovery efficiency of crop production in China during 1980–2014, and evaluated the relationship between NUE and economic growth (purchasing power parity, PPP) at national and provincial scale. The results showed that the overall NUE of crop production in China clearly increased from 35 to 42% during 2003–2014, and an increase in NUE was further evidenced by increases in cumulative recovery efficiency of both synthetic and non-synthetic N fertilizer. The relationship between NUE and PPP can be described by an environmental Kuznets curve at the national scale, with NUE first decreasing then increasing with PPP. However, this relationship exhibited large spatial variation: 1) In economically developed (e.g., Guangdong and Zhejiang) and undeveloped provinces (e.g., Yunnan and Guizhou), NUE generally decreased and then remained at low levels (20–35%) as PPP increased. 2) In major agricultural provinces with high (e.g., Shandong and Jiangsu) or intermediate levels (e.g., Hunan and Hebei) of economic development, a pronounced increasing trend in NUE with PPP was observed. These results highlight the necessity of developing region-oriented N management strategies to further increase the NUE of crop production in China, particularly in the economically developed and undeveloped provinces.

Almost 81% of nitrogen fertilizers are applied in form of urea but most of it is lost due to volatilization and leaching leading to environmental pollution. In this regard, slow-release nano fertilizers can be an effective solution. Here, we have synthesized different Fe₃O₄-urea nanocomposites with Fe₃O₄ NPs: urea ratio (1:1, 1:2, 1:3) ie. NC-1, 2, and 3 respectively, and checked their efficacy for growth and yield enhancement. Oryza sativa L. cv. Swarna seedlings were treated with different NCs for 14 days in hydroponic conditions and significant up-regulation of photosynthetic efficiency and nitrogen metabolism were observed due to increased availability of nitrogen and iron. The discriminant functional analysis confirmed that the NC3 treatment yielded the best results so further gene expression studies were performed for NC-3 treated seedlings. Significant changes in expression profiles of ammonia and nitrate transporters indicated that NC-3 treatment enhanced nitrogen utilization efficiency (NUE) due to sustained slow release of urea. From pot experiments, we found significant enhancement of growth, grain nutrient content, and NUE in NC supplemented sets. 1.45 fold increase in crop yield was achieved when 50% N was supplemented in form of NC-3 and the rest in form of ammonium nitrate. NC supplementation can also play a vital role in minimizing the use of bulk N fertilizers because, when 75% of the recommended N dose was supplied in form of NC-3, 1.18 fold yield enhancement was found. Thus our results highlight that, slow-release NC-3 can play a major role in increasing the NUE of rice.

Straw and biochar amendments have been shown to increase soil organic carbon (SOC) stocks in arable land; however, their effects on hydrological fluxes of dissolved organic carbon (DOC), which may offset the benefits of C sequestration amounts remain uncertain. Therefore, we conducted a three-year field study that included four treatments (CK, control with no fertilizer; NPK, synthetic N fertilizer; RSDNPK, synthetic N fertilizer plus crop residues; BCNPK, synthetic N fertilizer plus biochar of crop straw) to investigate the effects of straw and biochar amendment on DOC losses through hydrological pathways of overland flow and interflow from a wheat-maize rotation system in the subtropical montane agricultural landscape. We detected substantial intra- and inter-annual variations in runoff discharge, DOC concentration, and DOC fluxes for both overland flow and interflow pathways, which were primarily attributed to variations in rainfall amount and intensity. On average, the DOC concentrations for interflow (2.98 mg C L⁻¹) were comparable with those for overland flow (2.71 mg C L⁻¹) throughout the three-year experiment. However, average annual DOC fluxes for interflow were approximately 2.60 times greater than those for overland flow, which probably related to higher runoff discharges of interflow than overland flow. Compared to the control, on average, the N fertilization treatments significantly decreased the annual DOC fluxes of overland flow and significantly increased annual DOC fluxes of interflow. Relative to the application of synthetic N fertilizer only, on average, crop straw amendment practice significantly increased annual DOC fluxes of interflow by 28.7%, while decreasing annual DOC fluxes of overland flow by 12.0%; in contrast, biochar amendment practice decreased annual DOC fluxes of interflow by 25.3% while increasing annual DOC fluxes of overland flow by 44.6%. Overall, considering both overland flow and interflow, crop straw amendment significantly increased hydrological DOC fluxes, whereas biochar had no significant effects on hydrological DOC fluxes throughout the three-year experiment. We conclude that crop straw incorporation strategies that aim to increase SOC stocks may enhance hydrological losses of DOC, thereby in turn offsetting its benefits in the subtropical montane agricultural landscapes.

Decision-making related to nitrogen (N) fertilization is a crucial step in agronomic practices because of its direct interactions with agronomic productivity and environmental risk. Here, we hypothesized that soil apparent N balance could be used as an indicator to determine the thresholds of N input through analyzing the responses of the yield and N loss to N balance. Based on the observations from 951 field experiments conducted in rice (Oryza sativa L.) cropping systems of China, we established the relationships between N balance and ammonia (NH₃) volatilization, yield increase ratio, and N application rate, respectively. Dramatical increase of NH₃ volatilizations and stagnant increase of the rice yields were observed when the N surplus exceeded certain levels. Using a piecewise regression method, the seasonal upper limits of N surplus were determined as 44.3 and 90.9 kg N ha⁻¹ under straw-return and straw-removal scenarios, respectively, derived from the responses of NH₃ volatilization, and were determined as 53.0–74.9 and 97.9–112.0 kg N ha⁻¹ under straw-return and straw-removal scenarios, respectively, derived from the maximum-yield consideration. Based on the upper limits of N surplus, the thresholds of N application rate suggested to be applied in single, middle-MLYR, middle-SW, early, and late rice types ranged 179.0–214.9 kg N ha⁻¹ in order to restrict the NH₃ volatilization, and ranged 193.3–249.8 kg N ha⁻¹ in order to achieve the maximum yields. If rice straw was returned to fields, on average, the thresholds of N application rate could be theoretically decreased by 17.5 kg N ha⁻¹. This study provides a robust reference for restricting the N surplus and the synthetic fertilizer N input in rice fields, which will guide yield goals and environmental protection.

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.

The crop-livestock system is responsible for a large proportion of global reactive nitrogen (Nr) losses, especially from China. There are diverse livestock systems with contrasting differences in feed, livestock and manure management. However, it is not yet well understood which factors greatly impact on the nitrogen (N) budgets and losses of each system. In this study, we systematically evaluated the N budgets of the crop-livestock production system from 1980 to 2050 in China by identifying the differences of 20 distinct livestock systems. During 1980 to 2010, the total N flow through the crop-livestock system increased from 21.4 to 49.7 Tg, with large variations in different input/output pathways, due to the strong livestock transitions of production towards to a monogastric and landless industrial system. Different systems contributed differently to the total N budgets in 2010. For example, the landless industrial system contributed 67% of livestock product N output, but accounted for 80% of total mineral N fertilizer use and feed N imports by the whole crop-livestock system. The mixed system had the highest rate of N use efficiency at system level due to high dependence on recycled N. N losses were diversely distributed by different systems, with the mixed ruminant system responsible for the majority of NH₃–N emission in livestock production, and the grazing ruminant system dominant in NO₃–N losses in feed production. The total N entering the crop-livestock system is estimated to be 53.9 Tg with total N losses of 41.3 Tg in 2050 under a business-as-usual scenario. However, this amount could be significantly decreased through combined measures that indicate a considerable potential for future improvements. Overall, our results provide new insights into N use and the management of livestock production.

Hydrochar (HC), an environment-friendly material, enhances soil carbon sequestration and mitigate greenhouse gases (GHGs) emissions in croplands. In this study, the water-washed HC (WW-HC) was applied to paddy soil to investigate effects on nitrous oxide (N₂O) and methane (CH₄) emissions during rice growth period. Four treatments, namely control (without N fertilizer and WW-HC), N fertilizer (WW-HC00), N fertilizer with 0.5 wt% WW-HC (WW-HC05) and N fertilizer with 1.5 wt% WW-HC (WW-HC15), were established. Results showed the WW-HC addition reduced N₂O and CH₄ emissions, global warming potential (GWP) and greenhouse gas intensity (GHGI) during the growing season. Moreover, the WW-HC application reduced N₂O cumulative emission (P < 0.05) (by 28.6% and 23.8% for WW-HC05 and WW-HC15, respectively). It was mainly due to the reduced ratio of (nirK + nirS) to nosZ under WW-HC15 (P < 0.05). Compared with WW-HC00, the WW-HC05 reduced CH₄ cumulative emissions by 14.8%, while the WW-HC15 increased by 9.7%. This might be ascribed to the significantly reduced expression of the methanogenic mcrA gene and ratio of mcrA to pmoA by WW-HC (P < 0.05). The WW-HC05 amendment decreased GWP and GHGI by 18.6% and 32.5%, respectively. Furthermore, the WW-HC application greatly improved nitrogen use efficiency by 116–145% compared with the control. Our study indicates the WW-HC application is a promising GHGs mitigation practice in paddy fields.

Nitrate leaching caused by overusing or misusing nitrogen (N) fertilizers in field vegetable cropping systems in China is a leading contributor to nitrate contamination of groundwater. Identification of the critical fertilizer N input rate could support management decisions that maintain yields while reducing the impact of nitrate leaching on groundwater. A four-season field experiment involving six N treatments (0, 60, 120, 180, 240, and 300 kg N ha⁻¹) was undertaken to investigate the impacts of various N rates on N use efficiency (NUE), seasonal nitrate leaching loss (SNLL), nitrate residue (NR), and radish yield, and to identify the critical N fertilizer rate for both optimum yield and minimum nitrate leaching loss in a field vegetable (radish, Raphanus sativus L.) cropping system in northern China. The results showed that radish yield enhanced quadratically and NUE reduced linearly with increasing N addition, while the NR and SNLL increased exponentially. The yield did not increase markedly when N fertilization exceeded 180 kg N ha⁻¹. SNLL and nitrate concentrations in the leachate averaged 11.5–71.5 kg N ha⁻¹ and 5.1–35.6 mg N L⁻¹, respectively, under N rates of 60–300 kg N ha⁻¹. The results showed that N fertilizer rate ranging from 180 to 196 kg N ha⁻¹ resulted in high yields and low nitrate leaching losses. Compared with those in response to the N fertilizer amount applied by local farmers, the NUE, NR, and SNLL in response to the N fertilizer amount identified in this study increased, decreased by 30.9%–35.0%, and decreased by 49.9%–55.7%, respectively, without any yield loss. Thus, a critical N fertilizer rate ranging from 180 to 196 kg N ha⁻¹ is recommended to obtain optimum yields with minimal environmental risks in radish fields in northern China.