The presence of soybean, but not soybean cropping frequency has influence on SOM priming in crop rotation systems

Purpose: The legume crops are advocated to integrate in crop rotation systems, and legume included in cereal-based rotations have a substantial effect on improving soil fertility and health. It is not yet clear whether the frequency of legumes included in crop rotation system influences soil biochemical properties and soil organic matter (SOM) mineralization. Methods: : An incubation experiment was conducted with 13 C-glucose addition to evaluate the influences of the soybean ( Glycine max (L.) cropping frequency on SOM mineralization under long-term wheat ( Triticum aestivum L.)- and maize ( Zea mays L.)-based rotation systems. PLFAs and 13 C-PLFAs were measured to explore microbial biomass, community structure and microbial utilization of glucose in wheat and maize systems. Results: : Glucose addition increased native SOM mineralization, i.e. positive priming effect. Compared with less soybean cropping frequency under long-term wheat- and maize-based rotation systems, Wheat-soybean-soybean-soybean rotation and maize-soybean-soybean rotation increased the total biomass (PLFAs), fungal biomass and decreased the ratio of bacteria to fungi. Furthermore, the ratio of bacteria to fungi was negatively correlated with SOM priming, indicating that greater fungal biomass played a key role in stimulating the priming effect. That the proportion of 13 C-glucose in G- and fungi had a positive relationship with PE also proved the point. The presence of soybean in crop rotation increased SOM priming, while the soybean cropping frequency had no significant influence on SOM priming. However, in contrast to maize-based rotation system, the same frequency of soybean in wheat-based rotation system had fewer total PLFAs and higher B/F ratio. Conclusions: : Our findings indicated that the presence of soybean increased SOM priming because of the increasing fungal biomass and the ratio of fungi to bacteria, while the soybean cropping frequency had no significant influence on SOM priming. Furthermore, the presence of soybean in maize system promoted more SOM priming than that in wheat system with glucose addition.