In-situ local litter decomposition was mainly microbial and differed between pastures and mature forests but not along early succession in a tropical dry forest ecosystem

Organic matter decomposition in tropical dry forest ecosystems is poorly understood, especially under the diversity of land covers resulting from human activities and increasing predominance of secondary vegetation over primary vegetation. We hypothesized that 1) litter quality decreases progressively from pastures to successional forests and to mature forests as a result of increasing recalcitrant materials with increasing tree cover, 2) as a consequence of those litter quality differences, decomposition rates also decrease progressively from pastures to successional forests and to mature forests, and 3) bacteria dominate the microbial community in the first part of the process, consuming the most labile materials; fungi dominate afterwards, consuming the recalcitrant material; mesoand small macrofauna access to bags increases decomposition. Bags made from mesh of two opening sizes and containing bulk litter from the site were placed in each site when the rainy season started and removed after 1, 2, 4 and 9 months. Mass loss curves were remarkably similar in all sites despite being composed of different litter materials and exposed to different local environmental conditions in each site. Local litter in situ decomposition showed a rapid decomposition phase during the first two months, in which close to half of the initial mass was lost, and virtually no further decomposition in the subsequent seven months. Half-life of the litter was approximately 3 months and the shape of the mass loss curves indicated the local bulk litters were rich in labile materials. Differences in litter quality were minor, were weakly related to decomposition and had no relation to the successional age of the sites. Decomposition rates were significantly higher as hypothesized, in pastures than in mature forests and intermediate in successional sites although the differences observed were of low magnitude. During the rapid decomposition period the fungi:bacteria abundance ratio decreased from four in initial bulk litters to one after the first two months. As predicted, bacteria proliferated in the initial stages whereas fungi decreased but we were not able to measure if fungi dominated the microbial community in the second, slow period. Allowing access of mesoand small macro-fauna to litter bags did not increase decomposition. These results suggest that decomposition is a resilient function in this ecosystem, as it is only slightly reduced in pastures and recovers rapidly through early succession.

Automated summary

This study investigated organic matter decomposition in tropical dry forest ecosystems. It was found that litter quality and decomposition rates decrease as tree cover increases, but differences in litter quality had minor impact on decomposition rates. The microbial community dynamics during decomposition were also examined, showing that bacteria dominate in the initial stages while fungi dominate later on.