Nutrient improvement and soil acidification inducing contrary effects on bacterial community structure following application of hairy vetch ( Vicia villosa Roth L.) in Ultisol

Empirical studies have intensively clarified the effects of livestock manure on soil fertility, crop yield and mi-crobial community, with little information about their response to application of peanut straw and green manure in dryland of Ultisols. In this study, quantitative polymerase chain reaction (qPCR) and next-generation sequencing were used to compare bacterial abundance, community structure and predicted metabolic function under four agricultural practices in Ultisol: without fertilization (control), mineral fertilization (NPK, conven-tional regime), mineral fertilization with returning composting peanut straw (NPKS) and with planting hairy vetch (NPKG). Compared to the control, the NPKS and NPKG significantly decreased soil pH. The NPKG was associated with the highest soil nutrient contents and peanut yield. Compared to control, the NPKS and NPKG increased bacterial abundance and decreased bacterial alpha-diversity. Fertility improvement and soil acidifi-cation were responsible for increasing bacterial abundance and decreasing alpha-diversity, respectively. Phylogenetic clustering was found in soil bacterial community, with the stronger clustering in NPKS and NPKG, suggesting that soil acidification might induce stronger niche-based filtering to decrease bacterial alpha-diversity. Compared to control, the NPKS and NPKG altered soil bacterial community composition, but only NPKG changed the predicted metabolic functions. The NPKG triggered larger changes in bacterial community composition than NPKS, implying that dramatic shifts in community composition might lead to modification of bacterial functions. Soil bacterial community composition and predicted function co-varied with soil pH and nutrient contents, but nutrient improvement had stronger (i.e., 23.3% vs. 11.5% in composition; 21.8% vs. 13.6% in function) and contrary effects (i.e., the opposite arrows) than soil acidification. In addition, the NPKG was associated with higher relative abundance of plant-beneficial bacterial taxa (Mycobacterium and Pseudo-monas). Overall, mineral fertilization with planting hairy vetch might be an effective practice for improving soil fertility, optimizing bacterial community structure and increasing peanut yield in dryland of Ultisol. However, the further soil acidification should be taken into account following planting hairy vetch longer.