Straw-Increased C/N Ratio Mitigates Nitrate Leaching in Fluvial Soil by Enhancing Microbial N Pool and Reducing N Mineralization

Excessive application of nitrogen (N) fertilizer increases the risk of soil NO3--N leaching in fluvial soil, threatening soil and groundwater quality and safety. Enhancing soil carbon (C) by returning straw to the field can efficiently improve soil quality. The process of increasing C/N by straw returning to regulate soil nitrogen transformation and mitigate NO3−-N leaching, and the ecological threshold of straw application rate in fluvial soil need to be further explored. This study aims to research a series of soil C/N ratio treatments (including no straw, CK; C/N of 15, 20, 25, 30, 35 and 40), which were set up by adding straw at different application rates, and to investigate the underlying process of increasing C/N ratio by incorporating straw to mitigate NO3−-N leaching. As the soil C/N ratio increased, the total soil nitrogen showed a fluctuating increase with the highest value in S40 treatment (increased by 358 mg kg−1), while the NO3−-N leaching amount reached the lowest value at the C/N ratio of 20, with an average reduction of 45% (decreased by 29.3 mg kg−1) . Increasing soil C/N ratio significantly increased soil microbial biomass, cellulase, urease and N-acetyl-β-D-glucosaminidase activities while it decreased the net N mineralization rate, ammonification rate and nitrification rate. Principal component analysis showed that the NO3--N leaching was positively correlated with the ammonification rate, nitrification rate and net N mineralization rate, and negatively correlated with the abundances of bacteria, fungi and nitrogen-fixing genes (nifH) (p < 0.01). Structural equation model analysis showed that straw-regulated C/N, dissolved organic N and soil fungi were the most important factors affecting NO3−-N leaching, followed by the ammonification rate. Overall, increasing soil C/N by adding straw could enhance soil microbial biomass (especially fungi) and enzyme activities to promote soil N storage and reduce net N mineralization, ammonification and nitrification to decrease NO3−-N leaching.