Biodiversity, Nitrogen Deposition, and CO2 Affect Grassland Soil Carbon Cycling but not Storage

Grasslands are globally widespread and capable of storing large amounts of carbon (C) in soils, and are generally experiencing increasing atmospheric CO2, nitrogen (N) deposition, and biodiversity los-ses. To better understand whether grasslands will act as C sources or sinks in the future we measured microbial respiration in long-term laboratory incubations of soils collected from a grassland field experiment after 9 years of factorial treatment of atmospheric CO2, N deposition, and plant species richness on a deep and uniformly sandy soil. We fit microbial soil respiration rates to three-pool models of soil C cycling to separate treatment effects on decomposition and pool sizes of fast, slow, and resistant C pools. Elevated CO2 decreased the mean residence time (MRT) of slow C pools without affecting their pool size. Decreasing diversity reduced the size and MRT of fast C pools (com-paring monocultures to plots planted with 16 spe-cies), but increased the slow pool MRT. N additions increased the size of the resistant pool. These effects of CO2, N, and species-richness treatments were largely due to plant biomass differences between the treatments. We found no significant interac-tions among treatments. These results suggest that C sequestration in sandy grassland soils may not be strongly influenced by elevated CO2 or species los-ses. However, high N deposition may increase the amount of resistant C in these grasslands, which could contribute to increased C sequestration. Key words: C sequestration; elevated CO2; FACE experiment; soil C cycling; biodiversity; nitrogen deposition