Altered Microbial Community Development Explains Litter Mixture Non-additive Effects on Litter Decomposition

Abstract:

Litter decomposition is a fundamental process of biogeochemical cycles. It is very important to know the microbial processes and mechanisms of litter decomposition, and to fully understand the carbon cycle of the terrestrial ecosystems. Mixing litter of different species can induce non-additive effects (NAEs) on decomposition processes. Microbial decomposer communities will change during litter decomposition due to biotic interactions and changed nutrients availability. Though the abundance of microbial decomposers can change due to the mixing of different litters, linking these shifts to the non-additive effects (NAEs) of litter mixture is very scarce, which hinders the understanding mechanisms of litter decomposition. So we carried out a leaf litter decomposition experiment including four species in monocultures and all possible litter combinations (11 litter mixture treatment, i.e. 2-species, 3-species, 4-species) in a secondly mixed broad-leaved monsoon forest. We extracted phospholipid fatty acids (PLFAs) from leaf litterbags after 1.5-, 3- and 11 months of litter decomposition. The results showed that: (1) Whether based on litter mass loss or total microbial PLFA concentrations of litter mixtures, Litter mixing exhibited more frequent antagonistic effects than synergistic effects in the 16-month-decompostion incubation experiment; however, the NAEs changed from antagonistic to synergistic at the latter stage of experiment. The magnitude of NAEs increased as decomposition progressed. The four-species mixture resulted in more antagonistic effects than two- and three-species mixture, while the magnitude of NAEs showed no significant relationship with litter species richness. (2) At the first two sampling time (1.5- and 3-month), total PLFA concentrations were higher on single litter types than litter mixtures, but were lower after 11 months. Similarly, fungal-to-bacterial ratios showed the same pattern as total PLFA concentrations. Microbial community composition, as indicated by principal components analyses (PCA), differed due to both litter mixing and stage of litter decomposition. PLFA biomarkers i15:0 and 18:1w7c, which indicate gram-positive and gram-negative bacteria respectively, were considered the main reason for driving these shifts. (3) Total PLFA correlated significantly with single litter mass loss at latter stages of the decomposition, but not at earlier stages. But the correlation was opposite for mixture litter mass loss with total PLFA concentrations. Our study for the first time evidenced that that litter mixing alters microbial community development, which can contribute to the antagonisms and synergisms in litter mixture decomposition. These findings further advance our understanding of how changing forest biodiversity can alter microbial communities and the ecosystem processes they mediate.