Influence of gross primary productivity and soil carbon content on soil CO2 efflux in dry grasslands

Soil respiration of grasslands is highly variable both in time and space reflecting the topographic characteristics, the changing environmental constrains and biological activity of the vegetation. The aim of this study was to describe the effect of gross primary productivity (GPP) and soil organic carbon content (SOC) on soil respiration under varying environmental conditions in a dry grassland site in Hungary. We made spatially explicit measurements of variables including soil respiration, aboveground biomass, green vegetation index, soil water content, and soil temperature during an 8-year study in the vegetation periods. Sampling was conducted 23 times in 80 x 60 m grids of 10 m resolution with 78 sampling points. Altitude, slope, and soil organic carbon were used as background factors at each sampling position. Site-level GPP data were derived from eddy-covariance measurements and used for the estimation of GPP at every sampling position as a weighted metric on the basis of the biomass and green vegetation index of the positions. Data were analyzed using generalized additive models (GAM). Spatially, soil respiration had negative correlation with soil temperature, altitude and slope, while it was positively correlated with soil water content, aboveground biomass, green vegetation index and SOC. Soil respiration was significantly different between SOC groups (low-medium-high carbon content), mean soil respiration increased with soil carbon content. According to the GAM analysis, the shape of the GPP response was almost linear in each SOC groups and GPP had a strong influence on soil respiration in all of the groups besides temperature and soil water content. The results suggest that GPP and the resulting belowground carbon allocation affecting mainly the autotrophic components of soil respiration have similar influence on soil respiration as the main environmental variables.