Interactive effects of elevated CO2 and nitrogen deposition accelerate litter decomposition cycles of invasive tree (Triadica sebifera)

Abstract

Litter decomposition is an important component of forest nutrient recycling potentially impacted by elevated CO2, nitrogen (N) deposition, and biological invasions. Each can affect decomposition indirectly via changes in litter quantity, litter quality, and soils, which may increase or offset direct effects of elevated CO2 on decomposition. To date, no study has examined how elevated CO2 and N deposition interact to determine exotic plant litter cycling or the importance of variation among native and invasive populations of exotic plants for these effects.In this study, we examine elevated CO2 and N deposition effects on litter production and decomposition of native (China) and invasive (USA) populations of tallow tree (Triadica sebifera). First, we investigated litter production in greenhouse chambers. Then, we conducted three additional experiments using experimentally created litter and soils to examine CO2 and N effects on litter decomposition via changes in litter quality, soils, and atmospheric environment.Elevated CO2 and N deposition increased litter production with their combined effects more than additive. Nitrogen deposition increased litter production of native populations more than that of invasive populations. Litter produced in elevated CO2 had higher C:N and decomposed more slowly than litter produced in ambient CO2 but N deposition weakened these effects. Decomposition was slower in soils with histories of N deposition or plant association. Elevated CO2 had no direct effects on litter decomposition.The net effect of elevated CO2 alone on litter decomposition throughputs was negative for invasive populations due to strong reductions in litter decomposability. The net effect of elevated CO2 and N deposition together on litter throughputs was positive, especially for native populations due to strong litter production increases.Results suggest that elevated CO2 and N deposition impact litter cycling primarily through changes in litter quantity and quality with smaller effects via soils. Elevated CO2 and N deposition had strong positive effects on litter throughputs together due to synergistic positive effects on litter production and offsetting effects on decomposability. Although elevated CO2 and N deposition had consistent effects on native and invasive populations, the relative magnitudes of their effects on litter quantity and quality impacted litter cycling, suggesting the importance of considering plant trait variation in an overall estimation of global change effects on nutrient dynamics in forests and other ecosystems.