Straw return counteracted constraints caused by microplastics and enhanced soil multifunctionality by regulating...

Microplastic pollution threatens agricultural soil multifunctionality (SMF) by disturbing microbial communities and nutrient cycling. Straw return is widely used to improve soil quality, but whether it can offset the decline in SMF caused by microplastics (MPs) and which microbial pathways are involved remain unclear. We conducted a soybean pot experiment for 90 days with four treatments: control (CK), 2% polyethylene microplastics (PE), 1% corn straw (CS), and PE combined with corn straw (PC). Soil physicochemical properties, enzyme activities, microbial biomass, soybean growth and SMF were measured at the seedling, flowering, filling and harvesting stages, while microbial diversity, keystone taxa and predicted functions were analyzed at the flowering stage. PE significantly reduced enzyme activities, microbial biomass, soybean growth and SMF, and intensified microbial phosphorus limitation. Across the four stages, PE decreased SMF by 30.65% to 51.35% compared with CK (P < 0.05), whereas CS and PC increased SMF by 65.91% to 97.22% and 36.36% to 75.00%, respectively (P < 0.05). MPs constrained SMF by altering fungal diversity, keystone taxa and functions, and decreasing microbial biomass and extracellular enzyme production. These changes aggravated microbial phosphorus limitation, thereby impairing nutrient transformation and plant growth. In contrast, straw return supplied labile carbon and nutrients, stimulated microbial proliferation and enzyme synthesis, restored bacterial and fungal functional complementarity and relieved microbial resource limitation. Overall, straw return mitigated the reduction in SMF caused by MPs by rebuilding microbial diversity, function and resource acquisition efficiency, providing practical guidance and theoretical support for managing microplastic pollution in agricultural soils.