Rapid Root Decomposition Decouples Root Length from Increased Soil C Following Grassland Invasion

Abstract

Plant invasion often increases stand biomass, but higher tissue quality (for example, less lignin and more nutrients) in invasive species might accelerate litter decomposition. This mechanism may minimize increases in soil carbon (C) sequestration despite higher production. Our knowledge about invasion and tissue quality is based on shoots, but roots contribute 50–90% of biomass in vegetation types such as semiarid grasslands. Here we investigate root decomposition rates and tissue quality in the widespread invasive grass Agropyron cristatum, which doubles root mass but not soil C in the Great Plains of North America. Root length was significantly greater beneath Agropyron than native grassland 7 years after minirhizotron installation. However, CO2 evolution from decomposing roots was twice as much for Agropyron roots as for native grass roots (P < 0.05). CO2 evolution from decomposing native grass roots was not significantly different from controls with no root tissue added, suggesting that Agropyron invasion can convert grassland soil to a source of CO2 to the atmosphere. Rapid root decomposition was associated with significantly lower lignin content in Agropyron roots than native grass roots, although root N and lignin:N ratios did not differ. We present the first report of root decomposition rates associated with plant invasion. Increases in root length were accompanied by increased root decomposition rates of low-lignin tissue, such that invasion-driven enhanced productivity did not enhance soil C sequestration. Among-species differences in root tissue quality and decomposition rates could influence soil C dynamics during invasions of systems dominated by belowground production, such as tundra, boreal forests, and semiarid grassland.