Decomposition of diverse litter mixtures affected by drought depends on nitrogen and soil fauna in a bamboo forest

Litter decomposition is fundamental to ecosystem functioning and can be affected by the nonadditive effect of litter mixing. We hypothesize that drought decelerates decomposition of litter mixtures, partly being attributed to decreased complementary resource use. The presence of soil fauna could facilitate litter decomposition and mitigate the drought effect, but this process depends on soil nitrogen (N) availability. To testify these hypotheses, we exposed single and all possible litter mixtures (11) of four leaf litter types to litterbags with three mesh sizes (i.e., 0.05-mm, 1-mm, and 5-mm) among control and treatment plots (drought, N addition or both) in an N-enriched bamboo forest ecosystem. Our results show that increased litter species richness moderately accelerated litter carbon (C) and N loss, as indicated by the positive net diversity effects, which were clearly driven by complementary resource use. Both drought and N addition decelerated the C and N dynamics of diverse litter mixtures, and N addition further exacerbated the negative drought effect in this N-enriched forest. We further found that these negative effects of abiotic manipulations on decomposition were partly attributed to decreased complementary resource use. The complementarity effects of litter mixing on C loss were significantly reduced by drought (by 49.7%), and this negative drought effect on complementarity was intensified after adding N. Furthermore, three-way interactions among drought, N addition, and decomposer communities demonstrated that the presence of soil fauna could mitigate the negative drought effects on decomposition by facilitating complementary resource use, but this mitigation effect vanished after soil N was elevated in this N-enriched forest. Our results revealed that projected global environmental changes not only directly change biotic activities but also indirectly affect within- and among-trophic interactions to alter ecosystem processes and function. This study provides new insights for a better prediction of ecosystem processes and functioning under projected species loss and global environmental change conditions. • Synergistic effect of litter-mixing on decomposition increase with species richness. • Drought and N addition reduced litter C and N loss of diverse mixtures. • N addition exacerbated negative drought effect on decomposition. • Reduced complementary resource use contributes to drought effect on decomposition. • Soil fauna facilitate complementary resource use under drought without adding N.