Abstract
Litter decomposition is an ecological process of key importance for forest headwater stream functioning, with repercussions for the global carbon cycle. The process is directly and indirectly mediated by microbial decomposers, mostly aquatic hyphomycetes, and influenced by environmental and biological factors such as water temperature and litter quality. These two factors are forecasted to change globally within the next few decades, in ways that may have contrasting effects on microbial-induced litter decomposition: while warming is expected to enhance microbial performance, the reduction in litter quality due to increased atmospheric carbon dioxide and community composition alteration may have the opposite outcome. We explored this issue through a microcosm experiment focused on early microbial-mediated litter decomposition under stream oligotrophic conditions, by simultaneously manipulating water temperature (10 °C and 15 °C) and litter quality (12 broadleaf plant species classified into 4 categories based on initial concentrations of nitrogen and tannins). We assessed potential changes in microbial-mediated litter decomposition and the performance of fungal decomposers (i.e., microbial respiration, biomass accrual, and sporulation rate) and species richness. We found stronger effects of litter quality, which enhanced the performance of microbial decomposers and decomposition rates, than temperature, which barely influenced any of the studied variables. Our results suggest that poorer litter quality associated with global change will have a major repercussion on stream ecosystem functioning.
Highlights
Litter decomposition is one of the most important ecological processes globally, as most of the organic matter produced annually escapes herbivory [1, 2] and enters detrital food webs, substantially contributing to the global carbon (C) cycle [3, 4]
Differences among treatments in microbial-mediated decomposition rate, microbial respiration rate, fungal biomass, and aquatic hyphomycete sporulation rate and species richness were assessed with mixed ANOVA models, with litter quality category and temperature as fixed factors and species nested within litter quality category, acting as random factor, followed by post-hoc comparisons (Tukey tests)
Litter category had an influence on litter decomposition rates, with mean values increasing with litter quality (Fig. 1A); plant species identity within categories was the Variable and source of variation df F
Summary
Litter decomposition is one of the most important ecological processes globally, as most of the organic matter produced annually escapes herbivory [1, 2] and enters detrital food webs, substantially contributing to the global carbon (C) cycle [3, 4]. Climate warming is expected to increase the relative contribution of microorganisms to litter decomposition in streams, especially at higher latitudes where microbial performance is temperature-limited [12] Another factor that can alter microbial-mediated litter decomposition is litter quality, which is expected to be reduced due to changed riparian community composition [13] and . Afforestation and other human-related impacts such as the substitution of natural forests by plantations [16], biological invasions [17], or emerging diseases [18] can affect the composition of litter and its decomposition rate, altering C flows [19] These alterations render litter inputs characterized by lower nutrient concentrations, as well as higher concentrations of structural compounds and inhibitory secondary metabolites, which often reduce microbial decomposition [20, 21] compared to softer and more nutritious litter [e.g., 22, 23]. It is important to understand how decomposition rates change as a result of such environmental stressors associated to global change
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