Response of soil microbes to a reduction in phosphorus fertilizer in rice-wheat rotation paddy soils with varying soil P levels

Reducing chemical fertilization is one of the best management practices in agro-ecosystems where soil phosphorus (P) has accumulated to excessive levels. However, few studies have focused on the response of soil microbes to decreases in P input in a rice-wheat rotation. To determine the feasibility of a P reduction regime and its impacts on soil microbes, we modified P fertilizer inputs in a four-year pot experiment. Our treatments contained P fertilization during both rice and wheat seasons (PR + W), P fertilization during wheat-growing season only (PW), P fertilization during the rice season only (PR), and no P fertilization during either rice or wheat season (Pzero) as the control. We used ten paddy soils with varied P levels, including P-rich, P-moderate, and P-low. After four years of rice-wheat cropping, the PW treatment maintained crop yields but reduced soil Olsen-P accumulation by 20.8-88.7% (p < 0.05) compared to current farming practices (PR + W); however, the PR and Pzero treatments significantly decreased wheat yield (p < 0.05). The critical levels of soil Olsen-P were 4.19-5.24 and 7.00-16.8 mg kg−1 for optimal rice and wheat yield, respectively. Analysis of 454 pyrosequencing data from fresh samples revealed that the fertilizer regime was the main factor impacting the soil microbiome, followed by soil properties and available P. Subsequent stepwise regression analysis further indicated that soil pH was a major driver of bacterial community composition. Furthermore, the relative abundances of Alphaproteobacteria and Gammaproteobacteria were higher in P-fertilized soils. There were no significant changes in the taxonomic composition of the nine dominant bacteria phyla between the PW and PR + W treatments in soils form Yixing city, suggesting that this P fertilization reduction regime did not alter the bacterial community composition in this region. The results could help to understand the effects of P fertilizer reduction on the soil microbial community in rice-wheat rotation paddy soils.