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

Aims

As the major carbon sources of soil respiration (R_s) include the soil organic carbon content (SOC) and the belowground carbon allocation, we aimed to reveal their relative effects on actual CO₂ efflux from soil.

Methods

We measured soil respiration and additional variables in a dry grassland site in Hungary in the same spatial grid (78 points, 0.63 ha) during 23 campaigns over nine years. We used gross primary productivity (GPP) as a proxy for belowground carbon allocation, derived from eddy-covariance measurements and downscaled to the corresponding measuring positions. To visualize the multidimensional data, principal component analysis was performed. To describe the partial effects of the measured variables, general additive models (GAMs) were fitted.

Results

GPP was found to be the most important predictor variable in the middle of the vegetation period and during drought periods, while soil water content (SWC) proved to be most crucial factor in the first part of the vegetation period and soil temperature (T_s) dominated in the late season. The overall relative importance of T_s, SWC, GPP and SOC in GAMs were 36.0%, 32.6%, 30.2% and 1.2%, respectively.

Conclusion

GPP i.e., the resulting belowground carbon allocation was found to exert a similar influence on R_s in the models as T_s and SWC, while the significance of SOC was negligible which could be explained by the quality of SOC available to the microbes. Belowground carbon allocation could be the major driver of R_s in some phenological phases, therefore it should be incorporated in R_s models.