Tropical soil microorganisms are major recyclers of biosphere organic carbon. However, the link of tropical microorganisms to the two primary carbon inputs (roots and litter) remains unclear. Here, we studied the effects of excluding living roots and litter on microorganisms in leaf litter and topsoil in rainforests and plantations in Sumatra, Indonesia. Unexpectedly, excluding living roots strongly decreased microbial biomass and basal respiration in litter but not in soil, indicating that tropical trees prioritize mining for nutrients in litter layer rather than mineral soil. Contrary to litter, soil microbial communities were predominantly influenced by long-term factors related to land-use history. Litter removal neither significantly affected microbial biomass nor community structure in soil, suggesting compartmentalized carbon processing between litter and soil. Our study sheds new light on fundamental-ecosystem processes in the tropics and calls for greater consideration of the litter layer and for including root-derived resources in global carbon cycling models.

Belowground life is traditionally considered to rely on leaf litter as the main basal resource, whereas the importance of roots remains little understood, especially in the tropics. Here, we analysed the response of 30 soil animal groups to root trenching and litter removal in rainforest and plantations in Sumatra and found that roots are similarly important to soil fauna as litter. Trenching effects were stronger in soil than in litter with animal abundance being overall decreased by 42% in rainforest and by 30% in plantations. Litter removal little affected animals in soil, but decreased the total abundance by 60% both in rainforest and rubber plantations but not in oil palm plantations. Litter and root effects were explained either by the body size or vertical distribution of specific animal groups. Our findings highlight the importance of root-derived resources for soil animals and quantify principle carbon pathways in tropical soil food webs.