Planting Density Effect on Soil Functions With Specific Response to Rhizobacteria in Drought‐Prone Maize Field in the Yunnan Plateau

Soil multifunctionality (SMF) governs terrestrial biogeochemical cycles and ecosystem services, yet whether belowground SMF exhibits saturating density‐dependent responses analogous to the aboveground constant final yield law remains unclear. In a two‐year field experiment across five planting densities (D1–D5) in drought‐prone maize farmland on the Yunnan Plateau, we found that SMF displayed a distinct nonlinear saturating response. It increased sharply at low‐to‐moderate densities (D1–D3) before reaching an intrinsic saturation plateau at higher densities (D3–D5). This pattern extends the constant final yield law from plant productivity to soil biogeochemical functioning. Notably, plant growth‐promoting rhizobacteria (PGPR) inoculation did not alter this saturating trajectory but instead elevated the saturation plateau. PGPR resulted in negligible effects at low‐to‐moderate densities yet increased SMF by 472% and 957% at high densities (D4 and D5), respectively. Mechanistically, PGPR restructured bacterial communities under the high‐density conditions by enhancing α‐diversity, which optimized taxonomic composition and increased co‐occurrence network complexity. Concurrently, it boosted extracellular enzyme activities and the functional potential of carbon‐ and phosphorus‐cycling processes while maintaining functional redundancy. We conclude that planting density sets the baseline saturation threshold of SMF, whereas PGPR acts as a modulator that raises the saturation ceiling without overriding the underlying density‐driven pattern.