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BACKGROUND: Nitrogen (N) is the most limiting nutrient in rice production. N loss via denitrification and ammonia (NH3) volatilization decreases N utilization efficiency. The effect of periphyton (a widespread soil surface microbial aggregate in paddy soil) on N‐cycling processes and rice growth in paddy soils remain unclear. The purpose of this study was to reveal the interactions of periphyton with the overlying water and sediment in paddy soils on denitrification/NH3 emissions and rice yield by combining pot experiments and path analysis modeling. RESULTS: The sediment exerted significant direct and positive effects on denitrification. The periphyton both directly and indirectly enhanced denitrification, mainly by regulating the ammonium (NH4+)‐N content in the sediment. The total contribution of periphyton to denitrification was stronger than that of the overlying water but smaller than that of the sediment. The pH in the overlying water and the NH4+‐N content in the sediment had a strong positive effect on NH3 volatilization. Although the periphyton biomass and chlorophyll a directly prohibited NH3 emissions, this was counterbalanced by the indirect stimulation effects of the periphyton due to its positive alteration of the pH. Moreover, periphyton facilitated rice yield by 10.2% by releasing N. CONCLUSION: Although the periphyton may have driven N loss by regulating the NH4+‐N content in the sediment and the pH in the overlying water, our study also found that the periphyton was considered a temporary N sink and provided a sustained release of N for rice, thus increasing the rice yield. © 2022 Society of Chemical Industry. [ABSTRACT FROM AUTHOR]

Excessive nitrogen (N) fertilization in intensive agricultural areas in the Taihu Lake region of East China has resulted in low N utilization efficiency and serious environmental problems, giving rise to the need for an urgent reduction in the N fertilization rate. However, no holistic evaluations of rice ( Oryza sativa L.) yield effect and environmental effects of N fertilization have been conducted when recommending an optimal N rate. The current study provides an economic indicator and an evaluation model to account for the environmental effects of different N losses after N fertilizer application in the ecological and economic N rate for one rice season in the Taihu Lake region. Based on the assembled data and economic index, a general economic evaluation model to measure efficiently the cascading costs of the chemical N cycle at the regional scale was developed. Thereafter, fertilizer-stimulated benefit curves and fertilizer-induced cost curves were generated to determine an economically and ecologically optimal N application rate. The results revealed that the maximum net benefits were 3,123 yuan ha at 202 kg N ha for one rice season in the Taihu Lake region. Additional N application up to a rate of 263 kg N ha would increase rice production, but the increase in the total marginal costs would be slightly greater than the increase in marginal benefits. Among the marginal costs, the fertilizer and acidification costs were the greatest expenses, amounting to 1,716 yuan at 263 kg N ha, followed by eutrophication and global warming costs. When compared with the conventional N fertilization rate, this recommended rate could decrease the amount of N applied to rice from 10 to 40%, thereby, enabling optimum economic and ecological results. [ABSTRACT FROM AUTHOR]

The nitrogen (N) budget calculation approach is a useful means of evaluating the impact of human activity on the N cycle. Field scale N budget calculations may ignore the interactions between landscapes, and regional scale calculations rely on statistical data and indirect parameters. Watershed scale budget calculations allow for a more direct quantification of N inputs and outputs. We conducted N budget calculations for a rice paddy-dominated agricultural watershed in eastern China for 2007-2009, based on intensive monitoring of stream N dynamics, atmospheric deposition, ammonia (NH) volatilization and household interviews about N-related agricultural activities. The results showed that although total N input to the watershed was up to 280 kg N ha year, riverine discharge was only 4.2 kg N ha year, accounting for 1.5% of the total N input, and was further reduced to 2.0 kg N ha year after reservoir storage and/or denitrification removal. The low riverine N output was because of the characteristics of the rice paddy-dominated landscape, which intercepts run-off and enhances soil denitrification. The watershed actually purified the N in rainwater, as N concentrations in river discharge were much lower than those in rain water. Major N outputs included food/feed export, NH volatilization from chemical fertilizer and manure, and emissions from crop residue burning. Net reactive gaseous emissions (emissions minus deposition) accounted for 5.5% of the total N input, much higher than riverine discharge. Therefore, the agricultural N cycle in such paddy-dominated watersheds impacts the environment mainly through gas exchange rather than water discharge. [ABSTRACT FROM AUTHOR]

Simple and rapid chemical indices of soil nitrogen (N)-supplying capacity are necessary for fertilizer recommendations. In this study, pot experiment involving rice, anaerobic incubation, and chemical analysis were conducted for paddy soils collected from nine locations in the Taihu Lake region of China. The paddy soils showed large variability in N-supplying capacity as indicated by the total N uptake (TNU) by rice plants in a pot experiment, which ranged from 639.7 to 1,046.2 mg N pot at maturity stage, representing 5.8% of the total soil N on average. Anaerobic incubation for 3, 14, 28, and 112 days all resulted in a significant ( P < 0.01) correlation between cumulative mineral NH-N and TNU, but generally better correlations were obtained with increasing incubation time. Soil organic C, total soil N, microbial C, and ultraviolet absorbance of NaHCO extract at 205 and 260 nm revealed no clear relationship with TNU or cumulative mineral NH-N. Soil C/N ratio, acid KMnO-NH-N, alkaline KMnO-NH-N, phosphate-borate buffer extractable NH-N (PB-NH-N), phosphate-borate buffer hydrolyzable NH-N (PB-NH-N) and hot KCl extractable NH-N (H−NH-N) were all significantly ( P < 0.05) related to TNU and cumulative mineral NH-N of long-term incubation (>28 days). However, the best chemical index of soil N-supplying capacity was the soil C/N ratio, which showed the highest correlation with TNU at maturity stage ( R = −0.929, P < 0.001) and cumulative mineral NH-N ( R = −0.971, P < 0.001). Acid KMnO-NH-N plus native soil NH-N produced similar, but slightly worse predictions of soil N-supplying capacity than the soil C/N ratio. [ABSTRACT FROM AUTHOR]

Due to increased economic and environmental concerns, developing statistical models of crop yield has become one of the most important steps in determination of the cost effective rates (CERs) of nitrogen (N) fertilization. Although quadratic models are commonly used to describe wheat and paddy rice yield response to fertilizer rates in the Taihu Lake region of China, few studies have investigated why this model is selected over others. This study evaluated quadratic, exponential and square root models describing the wheat (Triticum aestivum L.) and rice (Oryza sativa L.) yield response to N fertilizer when determining the CERs, while also considering the environmental costs of N losses. All models fit the data almost equally well when evaluated using the variability and standard error statistics. However, there were marked discrepancies among models when calculating the CER of fertilization and the economic returns form Z-test. The quadratic model had a greater CER value (194 kg N ha–1 for rice and 185 kg N ha–1 for wheat) averaged over all sites than the exponential and square root models. The residuals obtained from the quadratic models were closer to a normal distribution than those of the other two models, indicating a less systematic bias. The mean economic uncertainties resulting from the quadratic model were more dependable than the other two models evaluated. These results show that the quadratic model best describes the rice and wheat yield responses and tends to indicate the optimal rates of fertilization while considering the environmental and economic effects of over fertilization for rice and wheat in the Taihu Lake region. [ABSTRACT FROM PUBLISHER]