Eutrophication triggers contrasting multilevel feedbacks on litter accumulation and decomposition in fens

Abstract

Eutrophication is a major threat for the persistence of nutrient-poor fens, as multilevel feedbacks on decomposition rates could trigger carbon loss and increase nutrient cycling. Here, we experimentally investigate the effects of macronutrient (NPK) enrichment on litter quality of six species of sedge (Carex sp.), which we relate to litter decomposition rates in a nutrient-poor and nutrient-rich environment. Our research focused on four levels: we examined how eutrophication alters (1) fresh litter production ("productivity shift"), (2) litter stoichiometry within the same species ("intraspecific shift"), (3) overall litter stoichiometry of the vegetation under the prediction that low-competitive species are outcompeted by fast-growing competitors ("interspecific shift"), and (4) litter decomposition rates due to an altered external environment (e.g., shifts in microbial activity; "exogenous shift"). Eutrophication triggered a strong increase in fresh litter production. Moreover, individuals of the same species produced litter with lower C:N and C:P ratios, higher K contents, and lower lignin, Ca and Mg contents (intraspecific shift), which increased litter decomposability. In addition, species typical for eutrophic conditions produced more easily degradable litter than did species typical for nutrient-poor conditions (interspecific shift). However, the effects of nutrient loading of the external environment (exogenous shift) were contradictory. Here, interactions between litter type and ambient nutrient level indicate that the (exogenous) effects of eutrophication on litter decomposition rates are strongly dependent of litter quality. Moreover, parameters of litter quality only correlated with decomposition rates for litter incubated in nutrient-poor environments, but not in eutrophic environments. This suggests that rates of litter decomposition can be uncoupled from litter stoichiometry under eutrophic conditions. In conclusion, our results show that eutrophication affects litter accumulation and -decomposition at multiple levels, in which stimulatory and inhibitory effects interact. The cumulative effect of these interactions ultimately determine whether peatlands remain sinks or become sources of carbon under eutrophic conditions.

Decomposition of Standing Litter Biomass in Newly Constructed Wetlands Associated with Direct Effects of Sediment and Water Characteristics and the Composition and Activity of the Decomposer Community Using Phragmites australis as a Single Standard Substrate

“…Variables related to the presence of macroinvertebrates were present in most of our models, and are thus also recognized as a driving factor (partly) explaining decomposition rates. These observations corroborate with earlier findings on, for example, the effects of nutrients (Rejmánková and Houdková 2006;Sarneel et al 2010;Emsens et al 2016a) and invertebrates (König et al 2014;Raposeiro et al 2014) on decomposition rates in established wetland systems. However, sediment and water variables could only explain a small part of variation in the fraction of detritivores.…”

“…A high availability of nutrients from surface water or eutrophic sediments potentially leads to high biomass production and higher availability of nutrients in the litter (Tanner 1996;Hoagland et al 2001; Lee and Bukaveckas 2002;Fennessy et al 2008;Trinder et al 2009;Emsens et al 2016a, b). However, decomposition rates will also be high (Rejmánková and Houdková 2006;Sarneel et al 2010;Emsens et al 2016a), since high plant tissue nutrient levels will increase decomposition (Serna et al 2013) of the most easily degradable water-soluble compounds and nonlignified carbohydrates (Berg and Laskowski 2005). In contrast, high concentrations of N can also decrease decomposition rates of lignified carbohydrates and lignin, which are decomposed in later stages, due to their inhibiting effect on lignin degrading enzymes (Berg and Laskowski 2005).…”

Impact of Temperature and Nutrients on Carbon: Nutrient Tissue Stoichiometry of Submerged Aquatic Plants: An Experiment and Meta-Analysis

“…This may counteract positive effects of fertilization on plant growth rates and carbon sequestration. Furthermore, eutrophic conditions can enhance plant litter quality (Emsens et al, 2016) and plants with lower carbon:nutrient ratios decompose faster than those with higher ratios (Wang et al, 2017), indicating an accelerated release of sequestered carbon and nutrients. We therefore hypothesize that eutrophication can affect carbon stocks in submerged aquatic vegetation, through changes in their nutritional quality (e.g., reduced carbon:nutrient stoichiometry) and subsequent effects on grazing and decomposition.…”

Potentially peat‐forming biomass of fen sedges increases with increasing nutrient levels

“…On the one side, increased nutrient availability in a system apparently either inhibits or does not impact litter decomposition (Agethen & Knorr, 2018;Bridgham & Richardson, 2003;Emsens, Aggenbach, Grootjans, et al, 2016). On the other side, increased nutrient availability during plant growth can increase decomposability of the resulting litter (Aerts & de Caluwe, 1997;Emsens, Aggenbach, Grootjans, et al, 2016). At the same time, Carex tissue decomposition is highly dependent on tissue type, with roots decomposing slowest (Graf & Rochefort, 2009;Scheffer & Aerts, 2000;Thormann et al, 2001).…”