Analysis of Soil δ 13 C and δ 15 N Along Precipitation Gradient: Critical Insights into Tree–Grass Interactions and Soil C Sequestration in Savannas

Kebonye Dintwe (), Gregory S. Okin, Frances O’Donnell, William P. Gilhooly, Abinash Bhattachan, Mokganedi Tatlhego, Lixin Wang and Paolo D’Odorico
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Kebonye Dintwe: Department of Environmental Science, University of Botswana, Gaborone 00267, Botswana
Gregory S. Okin: Department of Geography, University of California Los Angeles, Los Angeles, CA 90095, USA
Frances O’Donnell: Department of Civil and Environmental Engineering, Auburn University, Auburn, AL 36849, USA
William P. Gilhooly: Department of Earth and Environmental Sciences, Indiana University, Indianapolis, IN 46202, USA
Abinash Bhattachan: Department of Earth and Environmental Sciences, California State University East Bay, Hayward, CA 94542, USA
Mokganedi Tatlhego: Department of Environmental Science, Botswana International University of Science and Technology, Palapye 00267, Botswana
Lixin Wang: Department of Earth and Environmental Sciences, Indiana University, Indianapolis, IN 46202, USA
Paolo D’Odorico: Department of Environmental Science, Policy, and Management, University of California, Berkeley, CA 94720, USA

Land, 2025, vol. 14, issue 12, 1-19

Abstract: In situ observations of belowground tree–grass interactions are sparse in savanna ecosystems. In this study, we analyzed stable carbon and nitrogen isotopes (δ 13 C and δ 15 N) 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 C 3 and C 4 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 C 3 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 δ 15 N 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.

Keywords: savannas; soil organic carbon; total nitrogen; tree–grass interaction; carbon sequestration; climate change (search for similar items in EconPapers)
JEL-codes: Q15 Q2 Q24 Q28 Q5 R14 R52 (search for similar items in EconPapers)
Date: 2025
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