Pathways, Mechanisms, and Consequences of Nutrient-Stimulated Plant Litter Decomposition in Streams

Excess nitrogen (N) and phosphorus (P) inputs to streams occur globally, and affect not only stream autotrophs, but also heterotrophic microbes and detrital carbon processing. Detrital carbon, such as leaf litter, supports stream food webs and their connectivity via downstream detritus fluxes. Nutrient enrichment increases litter decomposition rates across multiple scales and trophic levels by stimulating activity of microbial decomposers and enhancing interactions among microbial decomposers, detritivores, and physical abrasion. Nutrient effects on microbial and detritivore-mediated decomposition are typically greater for recalcitrant vs. labile litter, especially when coupled to low initial nutrient concentrations. Recent studies and syntheses show that (1) dissolved N and P affect litter by stimulating fungal activity and nutrient immobilization, thus, increasing detrital nutrient content, (2) nutrient effects are greatest with N and P together (vs. individually) and when detritivores are present, and (3) ecosystem-level effects of nutrient enrichment can be predicted from small-scale measurements. Despite extensive studies of leaf litter decomposition, its application as a tool to manage nutrient enrichment issues trails comparable tools for autotrophic (i.e., algal) pathways. Thus, better understanding of the consequences of nutrient enrichment on leaf litter and other detrital carbon is important to predict how nutrients will affect stream ecosystem functioning.