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

Nitrate (NO3-) and nitrite (NO2-) leaching threatens groundwater quality. Soil C:N ratio, i.e., the ratio of soil organic carbon to total nitrogen, affects mineralization, nitrification, and denitrification; however, its mechanism for driving soil NO3- and NO2- accumulation and leaching remains unclear. Here, a field investigation in a fluvo-aquic soil and a soil column experiment were performed to explore the relationships between soil C:N ratio and soil NO3- and NO2- leaching in three soil layers (0-20, 20-40, and 40-60 cm) under heavy rainfall (rainfall rate > 25 mm d(-1)). The field investigation results showed that both soil NO3--N and NO2--N contents decreased exponentially (P < 0.001) with increasing soil C:N ratio in each soil layer. Furthermore, negative exponential relationships (P < 0.001) were found between soil C:N ratio and both NO3--N and NO2--N concentrations in soil solution in each soil layer under heavy rainfall. The soil column divided into three layers was leached with simulated heavy rainfall; the results confirmed negative exponential relationships (P < 0.05) between soil C:N ratio and both NO3--N and NO2--N concentrations in the leachate from each soil layer. A total of 18 soil samples obtained from three depths at six field sites during the rainy season were used to elucidate the microbial mechanisms induced by soil C:N ratio using high-throughput sequencing and real-time polymerase chain reaction. High abundances of ammonifying bacteria (Flavobacterium, Bacillu, and Pseudomonas), ammonia-oxidizing bacteria (Nitrosospira), and nirS/K gene were observed when soil C:N was low, concomitant with low abundances of NO2--oxidizing bacteria (Nitrospira) and narG gene. Partial least squares path modeling showed that the high NO3- and NO2- levels at low soil C:N ratio might be attributed to the inhibition of NO3- reduction (i.e., low narG gene) and NO2- oxidation (i.e., low Nitrospira) and thus the accumulation of soil NO3- and NO2-, respectively. Therefore, the leaching of NO2- and NO3- in low C:N soils requires more attention during the rainy season.