Different mechanisms underlying the divergent responses of soil respiration components to an introduction of N2-fixer tree species into Eucalyptus plantations

Soil respiration (Rs), primarily rhizospheric (Rr) and heterotrophic respiration (Rh) is an important CO2 efflux from soil to the atmosphere. Previous studies demonstrated that introducing N-2- fixing species into plantations could increase soil carbon stock and modify carbon chemical composition by altering microbial community and associated enzymes activities. However, their responses of Rs and its two components (Rr and Rh) remain poorly understood. This study investigated how Rs and its components responded to the introduction of N-2-fixer tree species in a subtropical eucalyptus plantation and to further ascertain its underlying mechanisms. We measured Rs and its components (Rr and Rh) and used phospholipid fatty acids (PLFAs) as bioindicators to study soil microbial communities and activities of four extracellular enzymes as indicators of soil microbial functioning in an 8-year-old pure Eucalyptus urophylla plantation (PP) and an 8-year-old mixed E. urophylla and Acacia mangium (N-2-fixer tree species) plantation (MP) in subtropical China. We found that Rs was significantly reduced by approximately 12.5%, with divergent responses of Rr and Rh in the MP. Compared with PP, the Rr decreased by 41.7% and Rh increased by 36.6% in plantation with N-2-fixer species. The decreased Rr in the MP was largely associated with a reduction in fine root biomass. In contrast, the increased Rh in the MP was induced by an increase in total microbial biomass, changing the microbial community structure and enhancing the activities of beta-1,4-glucosidase. The enzymatic changes were associated with an increase in N availability. Our results indicate that shifts in both biotic and abiotic factors resulting from the introduction of N-2-fixer species shaped changes in Rs and its components.