Plant roots are more strongly linked to microorganisms in leaf litter rather than in soil across tropical land-use systems

Tropical soil microorganisms are major recyclers of the biosphere organic carbon. However, the link of tropical microorganisms to the two main pathways of how carbon enters the belowground system, i.e., plant roots and leaf litter, is poorly understood. To investigate the relative importance of these pathways, we studied the response of microorganisms in leaf litter and underlying topsoil to root trenching (exclusion of living roots) and litter removal in tropical rainforests, and how this is altered by rainforest conversion to rubber and oil palm plantations. Unexpectedly, the linkage between living roots and microbial biomass was stronger in litter than in soil across land-use types, indicating that tropical trees prioritize mining for nutrients in leaf litter rather than in soil. After trenching, microbial biomass and basal respiration in leaf litter decreased by 32% and 38%, respectively, whereas microbial biomass and basal respiration remained unchanged in soil. However, fatty acid profiles indicated that root trenching decreased the proportion of total and mycorrhizal fungi in soil, with the effect on mycorrhizal fungi being especially strong when litter was present. Litter removal neither significantly affected microbial biomass nor community structure in soil, suggesting compartmentalization of carbon processing between litter and soil. Overall, the results suggest that across tropical land-use types root-derived resources shape microorganisms in soil and litter via different pathways, with litter microbial communities relying heavily on root-derived carbon, whereas microbial communities in soil being structured predominantly by long-term factors related to land-use history.