Effects of reduced inorganic fertilization and rice straw recovery on soil enzyme activities and bacterial community in double-rice paddy soils

To decrease nutrient losses in paddy soils, use of reduced inorganic fertilization and crop straw return has gained interest in recent years. However, there is limited understanding about the complex responses of soil microbial communities to chemical fertilizers and straw return, especially in the double-rice cropping system. In this study, we report the responses of soil enzyme activities and the bacterial community structure in a double-rice cropping system in southern China after nine years of fertilizer application. Treatments included conventional high inputs of inorganic fertilization, optimum fertilization with reduced inorganic fertilizers (OPT), the combination of inorganic and organic fertilization with rice straw to partially substitute the inorganic fertilizers and keep the inputs of pure N, P, and K the same as OPT (OPT + S), and a no-fertilization control. The soil bacterial communities were examined using Miseq sequencing. The fertilization after nine years significantly increased the soil N, P, K and organic matter contents, but showed little effect on soil pH. The soil enzyme activities were also largely enhanced by fertilization, which were mostly the highest in the OPT + S treatment. The results of Miseq sequencing indicated that the bacterial diversity and community composition were not significantly changed among different fertilization treatments. Nevertheless, they selectively enriched and inhibited the growth of certain bacterial taxa. The OPT + S treatment enriched the highest number of operational taxonomic units (OTUs), most of which were from the phyla Proteobacteria, Acidobacteria, Chlorobi and Bacteroidetes. Correlation analyses suggested that the available potassium, available phosphorus and soil organic matter emerged as the major determinants of the bacterial community composition. Overall, the OPT + S treatment can be more efficient in improving soil nutrient availability without excessive chemical fertilization.