Low soil C:N ratio resulted in the accumulation and leaching of nitrite and nitrate in agricultural soil under heavy rainfall

Nitrate (NO3-) and nitrite (NO2-) leaching threaten groundwater quality. Soil C:N ratio affects mineralization, nitrification and denitrification, while its mechanism for driving soil nitrate and nitrite accumulation and leaching remains unclear. Here, a field investigation in a fluvo-aquic soil region combined with soil column experiment was performed to explore the relationships between soil C:N ratio and soil nitrate and nitrite leaching in three depth profiles (0-20, 20-40 and 40-60 cm) under heavy rainfall (rain rate >25 mm d-1). The field investigation results showed that both soil nitrate-nitrogen (NO3--N) and soil nitrite-nitrogen (NO2--N) decreased exponentially with the increasing soil C:N ratio at each soil layer (p<0.001). A negatively exponential relationship was determined between soil C:N ratio of each soil layer and both NO3--N and NO2--N concentration in field soil solution in corresponding soil layer under heavy rainfall (p<0.001). The column with three-depths soil was leached with simulated heavy rainfall, and the results confirmed the negatively exponential relationship between soil C:N ratio and both NO3--N and NO2--N concentration of leachate from each soil layer (p<0.05). A total of 18 soil samples from three depths of 6 field sites in rainy season were used to elucidate microbial mechanisms induced by soil C:N ratio based on high-throughput sequencing and real-time PCR. Higher ammonifying bacteria (Flavobacterium, Bacillu and Pseudomonas), ammonia-oxidizing bacteria (Nitrosospira) and nirS/K gene abundance were observed in the low C:N soil, while lower nitrite-oxidizing bacteria (Nitrospira) and narG gene abundance were detected. Partial least squares path modeling showed that high nitrate and nitrite level in low C:N soil might be attributed to the inhibition of nitrate reduction (i.e. low narG gene) and nitrite oxidation (i.e. low Nitrospira), respectively. These results imply the inhibition of nitrate reduction and nitrite oxidation caused by low soil C:N ratio under heavy rainfall resulted in soil nitrate and nitrite accumulation. Therefore, the nitrite and nitrate leaching in low C:N soils deserve more attentions during rainy season.