Partitioning soil surface CO 2 efflux into autotrophic and heterotrophic components, using natural gradients in soil δ13C in an undisturbed savannah soil

Abstract

We used natural gradients in soil and vegetation δ 13C signatures in a savannah ecosystem in Texas to partition soil respiration into the autotrophic (Ra) and heterotrophic (Rh) components. We measured soil respiration along short transects from under clusters of C 3 trees into the C 4 dominated grassland. The site chosen for the study was experiencing a prolonged drought, so an irrigation treatment was applied at two positions of each transect. Soil surface CO 2 efflux was measured along transects and CO 2 collected for analysis of the δ 13C signature in order to: (i) determine how soil respiration rates varied along transects and were affected by localised change in soil moisture and (ii) partition the soil surface CO 2 efflux into Ra and Rh, which required measurement of the δ 13C signature of root- and soil-derived CO 2 for use in a mass balance model. The soil at the site was unusually dry, with mean volumetric soil water content of 8.2%. Soil respiration rates were fastest in the centre of the tree cluster (1.5 ± 0.18 μmol m −2 s −1; mean ± SE) and slowest at the cluster–grassland transition (0.6 ± 0.12 μmol m −2 s −1). Irrigation produced a 7–11 fold increase in the soil respiration rate. There were no significant differences ( p > 0.5) between the δ 13C signature of root biomass and respired CO 2, but differences ( p < 0.01) were observed between the respired CO 2 and soil when sampled at the edge of the clusters and in the grassland. Therefore, end member values were measured by root and soil incubations, with times kept constant at 30 min for roots and 2 h for soils. The δ 13C signature of the soil surface CO 2 efflux and the two end member values were used to calculate that, in the irrigated soils, Rh comprised 51 ± 13.5% of the soil surface CO 2 efflux at the mid canopy position and 57 ± 7.4% at the drip line. In non-irrigated soil it was not possible to partition soil respiration, because the δ 13C signature of the soil surface CO 2 efflux was enriched compared to both the end member values. This was probably due to a combination of the very dry porous soils at our study site (which may have been particularly susceptible to ingress of atmospheric CO 2) and the very slow respiration rates of the non-irrigated soils.