Species community shifts in fertilized tundra affect decomposition through changes in litter quantity, quality, and the decomposition microenvironment

As the Arctic warms, soils thaw, decomposition and nutrient mineralization may increase, and deciduous shrubs often grow taller and denser, shifting the competitive environment and affecting soil properties. Similarly, when soil nutrients are experimentally increased, tundra shrubs become more abundant, and plant productivity, biomass, and nutrient content increase. In an Alaskan tundra, we decomposed leaves of three common plant species, using fertilized and ambient tissue, along a chronosequence of fertilization experiments to test the roles of microenvironment, litter quality, and litter quantity in decomposition. Mass loss was lower in fertilized than ambient plots, likely because of cooler summer soil temperatures caused by greater shading and litter inputs. Species differed in mass loss, and all lost more mass if the leaves had been fertilized, indicating an acceleration of decomposition with improved litter quality. These counteracting environmental and litter quality effects on mass loss are overwhelmed by the enormous increase in litter inputs, resulting in greater carbon loss while still accumulating litter. As shrubs increase in abundance, separating the role of litter quality and the environment in determining local carbon cycling will help us to understand how such vegetation changes may affect regional Arctic element cycling.