Soil functions are amplified along increasing planting densities via nutrient trade-off driven by rhizobacteria in drought-prone environment

It is unclear how plant growth-promoting rhizobacteria (PGPR) affect soil multifunctionality (SMF) and production function (SPF) along planting densities. To address this issue, Bacillus licheniformis (PGPR) was inoculated in maize fields with five planting densities (D1–D5 from low to high) in the drought-prone region of the Yunnan Plateau, southwest China, from 2022 to 2023. Data indicated that under non-inoculation (CK), SMF tended to increase from D1 to D4 (p<0.05) and then stabilize in D5 across two growing seasons. Yet, SPF was observed to elevate steadily with increasing densities. PGPR inoculation significantly improved SMF (0.129) and SPF (0.508) under increasing planting densities. Meanwhile, soil carbon and phosphorus cycling indices significantly improved by 0.272 and 0.069 (p<0.05), respectively, whereas no significant change was observed in the nitrogen cycling index (p>0.05), relative to CK. Increased carbon cycling index was significantly associated with improved soil soluble organic carbon (2.26%) concentration, and enhanced carbon-related extracellular enzyme activities (9.57%). Similarly, phosphorus-related extracellular enzyme activity significantly increased by 10.51% (p<0.05). Interestingly, no significant changes were observed in the levels of soil total nitrogen, ammonium nitrate,n and nitrate nitrogen, and the activities of nitrogen-related enzymes across planting densities. The above-mentioned phenomenon can be mechanistically explained by the variations in rhizosphere microbiomes, and the accelerated carbon exchanges with nitrogen/phosphorus, which amplified SMF for higher SPF by reshaping the nutrient trade-off in the plant-soil-microbe system through PGPR-enhanced microbial activity.