Soil C/N and pH together as a comprehensive indicator for evaluating the effects of organic substitution management in subtropical paddy fields after application of high-quality amendments

Organic substitution management (OSM) is a key technology employed to reduce the amount of chemical fertilizer used in agricultural operations with the goal of reducing environmental pollution and ensuring green and sustainable agricultural development in China. However, there is still limited information regarding the underlying interactions between soil nutrients, enzyme activities and microbial community structures after long-term partial substitution of inorganic N with organic amendments, and no suitable evaluation indicators of organic substitution effects have been identified. Here, distance-based redundancy analysis (dbRDA), principal component analysis (PCA), the partial least squares method (PLS) and the partial least squares path model (PLS-PM) were used to better understand the impact of substitution effects on soil biochemical indexes in a 34-year field experiment. We found that soil C/N significantly directly affected rice yield, and that a soil C/N ranging from 10.12 to 10.19 could sustain a rice yield between 7000 and 6800 kg ha−1. Moreover, the soil hydrolase activities of the carbon and fungi communities were significantly influenced by both C/N and pH, and the carbon-cycling enzyme activities were found to be more susceptible to C/N than nitrogen-cycling enzyme activities. The low soil C/N and high pH after OSM decreased the ratio of G+ to G− and fungi to bacteria, indicating OSM increased the nutrient availability and benefited growth of the bacterial community. Hence, we believe that soil C/N and pH together can be used as a comprehensive index for suitable evaluation of the effects of OSM.