Metagenomics of the Effect of Long-Term Straw Return on the Phosphorus Cycle in Meadow Black Soil

Returning crop straw to the soil is an effective measure to increase soil fertility and maintain crop yield in agroecosystems. In this study, we conducted a 34-year field experiment (1984–2017) to investigate the relationships among the impacts of straw return on soil nutrients, phosphorus (P) fractions, phosphorus transformation-related functional microbial communities, and key genes in black soil. The results of the soil chemical properties showed that straw return could slow the decline in soil pH and increase soil nutrients, i.e., soil organic carbon (SOC), available K (AK), and available P (AP) concentrations, with the straw addition (PKS) treatment resulting in the highest soil AP and AK contents. Compared with conventional fertilizer (NPK) and conventional fertilizer with straw (NPKS), straw return alone (S) significantly increased the soil AP and AK contents by 1.1–42.8 and 38.3–114.3 mg/kg (p < 0.05), respectively. The results of the phosphorus fraction experiment indicated that long-term straw return promoted labile and moderately stable phosphorus content by altering the composition and increasing the relative abundances of functional genes regulating soil P activation (ugpQ, ppk, phoD, and gcd). The compositional changes in the phosphorus-cycling-related functional genes of the soil were affected mostly by soil total P (TP), AK, and NO3−-N, whereas pH, SOC, and available N (AN) were the main environmental factors influencing the phosphorus-cycling-related microbial community composition. These direct measurements provide insight into the soil phosphorus fractions, phosphorus-cycling-related functional genes, and microbial communities associated with straw returning, which enhances our understanding of the chemical and biogeochemical behavior of soil phosphorus upon straw incorporation.