Human activities shape global patterns of decomposition rates in rivers

Rivers and streams contribute to global carbon cycling by decomposing immense quantities of terrestrial plant matter. However, decomposition rates are highly variable and large-scale patterns and drivers of this process remain poorly understood. Using a cellulose-based assay to reflect the primary constituent of plant detritus, we generated a predictive model (81% variance explained) for cellulose decomposition rates across 514 globally distributed streams. A large number of variables were important for predicting decomposition, highlighting the complexity of this process at the global scale. Predicted cellulose decomposition rates, when combined with genus-level litter quality attributes, explain published leaf litter decomposition rates with high accuracy (70% variance explained). Our global map provides estimates of rates across vast understudied areas of Earth and reveals rapid decomposition across continental-scale areas dominated by human activities. Plant detritus is continually added to rivers and streams, where it is stored and decomposed, forming the base for aquatic food webs. Despite the major role that rivers play in carbon cycling and greenhouse gas production, the factors that influence organic matter decomposition in rivers are poorly understood. Tiegs et al. addressed this knowledge gap with a distributed experiment of cellulose decomposition (CELLDEX) replicated in more than 500 streams across six continents. Environmental variables explained cellulose decomposition rates and, when combined with data on litter quality, also explained published rates of leaf litter decomposition. This work could help to improve prediction of global change effects on the carbon cycle. ?Bianca Lopez