Abiotic and biotic effects of long-term straw retention on reactive nitrogen runoff losses in a rice–wheat cropping system in the Yangtze Delta region

Abstract

Straw retention has been widely recommended as a nutrient-conserving measure that effectively increases soil fertility and soil nitrogen (N) availability. However, despite its efficacy in reducing reactive N (Nr) runoff losses, there is currently little information available regarding the influence of distinct straw return modes with respect to potential Nr runoff loss. In this study, based on a 6-year period of straw incorporation in a field under rice–wheat rotation with a yield of two crops a year, we sought to evaluate the overall responses of Nr runoff losses and N changes in the surface water to soil biotic and abiotic factors. Four different straw return modes (CK: no straw, W: wheat straw, R: rice straw, and S: both wheat and rice straw) were examined. The results revealed that compared with the CK treatment, the three modes of crop residue incorporation significantly reduced N runoff loss by 4.21 %–11.59 % (P < 0.05), of which the S mode of straw returning was found to be the most effective. The reductive effect of straw return was observed during the early period of rice growth (the basal and tiller fertilization stages, P < 0.05). Significant differences in both the soil abiotic and biotic factors were observed not only between the tillering and ripening stages of rice growth (P < 0.01), but also among the four straw return modes (P < 0.05). Structural equation modelling revealed that soil inorganic nitrogen (SIN, r = 0.53, P = 0.007) had the most direct positive effect on Nr runoff loss, but the effect was statistically compromised by the overall negative effects of soil organic carbon (SOC, r = -0.33, P = 0.034), pH (r = -0.15, P> 0.05), and bacterial community composition (r = -0.10, P = 0.041), and to a less extent by α-diversity (r = -0.04, P > 0.05). During the tillering stage of rice growth and in response to the S mode of straw return in particular, there was a significant enrichment of Proteobacteria (P < 0.01), thereby indicating the potentially prominent role of microbial N immobilization in reducing N losses. The links among N contents in paddy standing water, surface runoff, and soil biotic and abiotic factors established in this study will inform straw return strategies to reduce the levels of N-based pollution.