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

Litter decomposition is a fundamental ecosystem service that can be affected by the nonadditive effect of litter mixing. We hypothesize that drought will slow the decomposition of litter mixtures due to a decline in complementary resource use. The presence of soil fauna could favor litter decomposition and mitigate the drought effect, but this depends on soil nitrogen (N) availability to soil fauna. To test these hypotheses, we compared the decomposition of single and all possible litter mixtures (11) of four leaf litter types in litterbags with three mesh sizes (i.e., 0.05-mm, 1-mm, and 5-mm) in control and treatment plots (drought, N addition or both) in an N-enriched bamboo forest ecosystem. Increasing the litter species richness moderately accelerated litter carbon (C) and N loss. The positive effect of litter diversity on decomposition was due to more complementary resource use. Both drought and N addition slowed the C and N loss from diverse litter mixtures, and N addition exacerbated the negative drought effect in this N-enriched forest. These negative effects of abiotic manipulation on litter decomposition were partly attributed to less complementary resource use. Specifically, the complementarity effects of litter mixing on C loss declined significantly, by 50%, with drought, and was even more pronounced in the drought and N addition treantment. The three-way interactions among drought, N addition, and decomposer communities demonstrated that the presence of soil fauna supports complementary resource use and alleviates thedrought constraint, but not the restriction in decomposition caused by N addition in this N-enriched forest. These findings have implications for projected decomposition rates under future global change scenarios, since they suggest that environmental changes affect decomposition through direct effects on soil biota and indirect impacts on within- and among-trophic interactions in forest ecosystems. •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.