Litter Traits Regulate Leaf Litter Decomposition In Terrestrial And Aquatic Ecosystems:a Filed Study And A Synthesis

Leaf litter is a major common source of energy and nutrients for heterotrophic communities both in terrestrial and aquatic ecosystems worldwide.Its decomposition constitutes a valuable ecosystem service by recycling nutrients,transferring energy,and sequestrating carbon(C).In general,leaf litter decomposition is controlled by both extrinsic factors(e.g.environmental conditions and decomposer community composition)and intrinsic factors(e.g.litter quality).However,the debate over the relative contribution of extrinsic and intrinsic factors to decomposition is still ongoing and the identification of common drivers of litter decomposition remains elusive.Here,I used a field experiment to explore the role of litter traits in controlling litter decomposition in a terrestrial ecosystem as well as a synthesis study to identify the dominant factors that regulate litter decomposition in aquatic ecosystems worldwide.In the first study(field experiment,Chapter 2),I conducted a “common plot” experiment in a evergreen broad-leaf forest in the Wuyi mountains from May 2017 to August 2018.I studied the effects of experimentally-,stand-age-,and species-induced variation in initial litter quality on leaf litter decomposition,aimed at eliminating environmental factors and ultimately exploring impacts of litter quality on decomposition.Leaf litter were collected from Populus deltoids both in a Nitrogen(N)addition experiment and a stand age experiment at the Dongtai Forest Farm,Liriodendron chinense × tulipifera and Platanus acerifolia on campus Nanjing Forestry University,and Quercus variabils and Pinus taeda at Xiashu Forest Farm.Our results showed that leaf litter of Poplar plantations from N addition sites had much higher decomposition rates.This may result from N addition induced increases in litter quality,indicated by a high N content and a low C:N ratio of leaf litter.Along the age sequence of Poplar plantations,the decomposition rates of leaf litter significantly increased with increasing stand ages.Across the five tree species(with no litter from Poplar plantations under N addition treatment),we found decomposition rates of the coniferous tree(Pinus taeda)was much lower than those of the other four broad-leaf trees on account of its lower litter quality.C:N ratio,Cu,P,K,Ca,Na,N,Mg,N:P ratio and Fe content showed significantly impacts on leaf litter decomposition across all the leaf litter we collected.However,multiple regression analyses indicated that the initial C:N ratio and sodium(Na)content dominated the decomposition.Our results suggest the importance of the initial nutritional controls on leaf litter decomposition in terrestrial ecosystems.In the second study(synthesis study,Chapter 3),I synthesized 1707 records of litter decomposition in streams from 275 studies.I explored variations in decomposition rates among climate zones,tree functional types,and between mesh size groups.Regressions were performed to identify the factors that play dominant roles in litter decomposition globally.The main results are shown below: Litter decomposition rates did not differ among tropical,temperate,and cold climate zones.Decomposition rates of litter from evergreen conifer trees(EC)were much lower than those of deciduous and evergreen broad-leaf trees(DB and EB),attributed to the low quality of litter from EC.No significant differences were found between decomposition rates of DB and EB.Additionally,litter decomposition rates were much higher in coarse-than in fine-mesh bags,which control the entrance of decomposers of different body sizes.Multiple regressions showed that litter traits(including lignin,C:N ratio)and altitude were the most important factors in regulating leaf litter decomposition.These findings suggest that litter traits predominantly control leaf litter decomposition in streams worldwide.While further analyses are necessary to explore whether commonalities of litter traits' predominant role in decomposition exist in both aquatic and terrestrial ecosystems,our findings could contribute to using trait-based approaches in modeling the decomposition of litter in streams globally and exploring mechanisms of land-water-atmosphere C fluxes.Our findings may contribute to using trait-based approaches in modeling the leaf litter decomposition globally and exploring mechanisms of global C fluxes.Further comprehensive analysis is required to uncover whether commonalities of litter traits' predominant role in decomposition exist in aquatic and terrestrial ecosystems,aimed at improving development of the global models.