Perennial cover crop biomass contributes to regulating soil P availability more than rhizosphere P-mobilizing capacity in rubber-based agroforestry systems

Cover crops used in agroforestry systems (AFS) are generally perceived to improve soil quality, but mechanisms underlying phosphorus (P) cycling in complex rhizosphere system are less clear, because the impacts of incorporated crops are often determined by inclusion time, residue management and species. We quantified 7-year cumulative effects of perennial legume cover crop–Flemingia macrophylla on rhizosphere P cycling and availability during the dry and wet seasons in young and mature rubber-based AFS (i.e., YAFS and MAFS) in tropical China by analyzing cover crop biomass and P content, as well as net rhizosphere changes in P-mobilizing capacity (pH, organic acids and phosphatase activity) and P fractions. Compared to their monoculture counterparts, YAFS enhanced plant biomass and P stock in both seasons, whereas MAFS exhibited an opposite trend, except for increased biomass and P stock of floor litter in the dry season. Moreover, YAFS had a greater effect on net rhizosphere properties by reducing pH and increasing organic acids concentrations and acid phosphatase activity, but MAFS hardly alter them. However, these changes had little effect on net rhizosphere P fractions in both seasons, except the contribution of calcium–P to increasing P availability to a certain extent. Net rhizosphere organic P and microbial biomass P in YAFS were enhanced in both seasons by abundant residue returns from cover crop. Net rhizosphere available P surplus occurred in YAFS in the dry season and maintained balance in the wet season, but it almost reached a deficit in MAFS in both seasons. These results suggest that rhizosphere P cycling was more improved by cover crop in YAFS than MAFS, and biomass of cover crop residue contributed to regulating rhizosphere P cycling and availability more than rhizosphere P-mobilizing capacity. Our study highlights the importance of residue management of permanent cover crops on rhizosphere P cycling and availability in AFS with different ages.