Belowground carbon allocation has stronger influence on soil respiration than soil organic carbon content in a dry temperate grassland

As the major carbon sources of soil respiration (R_s) are the soil organic carbon content (SOC) and the belowground carbon allocation, we aimed to reveal their effects on actual CO₂ efflux from soil. For that reason, we measured soil respiration and additional variables in a dry grassland site in Hungary in the same spatial grid (78 points, 0.54 ha) during 23 campaigns in eight years covering a broad range of environmental conditions. The measuring positions at the study site had high spatial variability of topsoil organic carbon content (range was 2-14%, 0-10 cm). The source of belowground carbon allocation is plant photosynthesis, therefore we used gross primary productivity (GPP) as a predictive variable of R_s. GPP was derived from eddy-covariance measurements and downscaled to the measuring positions by using above-ground biomass and vegetation index data. To visualize the multidimensional data, principal component analysis was performed. To describe the partial effects of the measured variables general additive models (GAM) were fitted and the relative importance of predictor variables in GAM models was estimated. According to the results, GPP had similar importance in the models as soil temperature (T_s) and soil water content (SWC), while the importance of SOC was negligible. GPP was the most important predictor variable in the middle of the vegetation period, while SWC was the most important in the first part of the vegetation period and T_s in the late season. The overall relative importance of SWC, GPP and T_s were 35.7%, 31.8% and 29.2%, respectively. The shape of the partial effect of GPP was linear suggesting that the whole range of GPP could be an important factor in soil respiration models.