Temperature‐Dependent Root Responses to Water Deficit Modulate Biological Nitrogen Fixation and Rhizosphere Dynamics in Soybeans

Abstract

Although soil warming and water scarcity are frequently associated with reductions in soybean productivity and biological nitrogen fixation (BNF), their combined effects remain poorly understood. This study evaluated how soil temperature and water regime influence nodulation, BNF efficiency, plant physiology and metabolism, soil enzymatic activity, and rhizosphere microbial communities in soybean plants grown at two soil temperatures (24°C and 36°C) under two water regimes: well‐watered (WW) and water deficit (WD). The WD treatment was the main limiting factor, reducing plant growth, nodule number, and biomass, ureide accumulation, and integrated BNF indices. Surprisingly, nodular efficiency and photosynthetic rate were higher under WD. Root‐zone warming under adequate water availability promoted greater plant and nodule biomass, higher ureide accumulation, and increased integrated BNF efficiency despite a reduction in nodule number. In addition, soil warming increased malondialdehyde and citrate concentrations in shoots as well as nodular concentrations of N, P, K, S, and B. Soil enzymatic activities and rhizosphere bacterial community structure varied among treatments, whereas fungal communities remained relatively stable. Overall, water deficit structured BNF limitation, while soil warming modulated metabolic responses. These results indicate that water availability and soil temperature jointly regulate biological nitrogen fixation and soil–plant–microbe interactions in soybeans, highlighting the importance of considering these factors together when developing management strategies under climate change scenarios.