Analysis of Soil δ13C and δ15N Along Precipitation Gradient: Critical Insights into Tree–Grass Interactions and Soil C Sequestration in Savannas

In situ observations of belowground tree–grass interactions are sparse in savanna ecosystems. In this study, we analyzed stable carbon and nitrogen isotopes (δ13C and δ15N) in soils and plants from four study sites in an African savanna ecosystem along the Kalahari moisture gradient. The homogeneous soil texture, primarily sandy soils, is well-drained and nutrient-poor, influencing vegetation and water retention uniformly across the region. At each site, soil samples were collected from a 120 cm deep soil profile. We used a 2-endmember mass balance approach to calculate the relative contributions of C3 and C4 plants to SOC in the 120 m soil profile. The wettest site was dominated by trees, whereas the driest site was dominated by shrubs. The intermediates had the highest amount of grass biomass. Our results revealed that tree- and shrub-derived SOC was highest in the wettest and driest sites, respectively. The contribution of C3 plants was 63.8% and 55.8%, in the wettest and driest sites, respectively, when integrating the 120 cm depth. Grass-derived SOC was highest (69.4%) in the middle of the precipitation gradient when integrating the 120 cm depth. The δ15N values were highest in the middle of the precipitation gradient (10.7‰) and lowest in the wettest site (5.2‰). Our findings indicate that belowground tree–grass interactions and nitrogen cycling in savanna ecosystems are more complex than previously thought and do not conform to the traditional concept of the two-layer roots hypothesis. This lack of conformity could be attributed to several factors, including overlap in rooting depth and ecological drivers, such as wildfires and herbivory, which could stimulate production of belowground biomass. We used space-for-time substitution to leverage the region’s steep north–south precipitation gradient and homogeneous soil texture. Our results indicated that trees and shrubs would become an important SOC source in the two extreme sites of the transect, while grass would become an important SOC source in the middle of the precipitation gradient.