Spatial heterogeneity and sources of soil carbon in southern African savannas Lixin Wang a,c, ⁎, Gregory S. Okin b , Kelly K. Caylor c , Stephen A. Macko a,d a Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22904, United States b Department of Geography, 1255 Bunche Hall, University of California, Los Angeles, CA 90095, United States c Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08544, United States d Program in Geobiology and Low Temperature Geochemistry, US National Science Foundation, Arlington, VA 22230, United States abstract article info Article history: Received 30 June 2008 Received in revised form 2 December 2008 Accepted 20 December 2008 Keywords: Geostatistics Kalahari Savannas Soil δ 13 C Soil organic carbon Stable isotope Knowledge of the southern Africa soil carbon pool, its heterogeneity, sources (from trees or grasses), and potential response to climate is extremely limited. In this study the Kalahari Transect (KT) was used as a representative savanna ecosystem to quantitatively evaluate the spatial heterogeneity of the soil carbon pool and its contributing sources. The KT encompasses a dramatic aridity gradient on relatively homogenous soils. Two sites were chosen along the KT, representing dry and wet conditions. In February–March 2005, soil samples were collected at each site along a 300 m transect. Stable carbon isotope (δ 13 C) and organic carbon content (%C) of the soils were utilized in the assessment in conjunction with geostatistical analysis of the spatial patterns of soil δ 13 C and %C. At the dry savanna site, well-defined patterns in both δ 13 C and %C were observed that were related to the distribution of woody vegetation. At the wet savanna site, the spatial patterns of δ 13 C and %C were somewhat less pronounced, but still were impacted by the distribution of woody vegetation. The relative contributions from C 3 and C 4 vegetation to the soil carbon pool at the wet site were independent of tree locations, but dependent on woody plant locations at the dry site. At the dry site, ~40% of the soil carbon was derived from C 3 vegetation, whereas at the wet site ~ 90% of the soil carbon originated from C 3 vegetation. These results represent an important step in understanding the impact of regional climate change (e.g., rainfall variations) on carbon sequestration in southern Africa by providing quantitative information on soil carbon spatial distributions and sources under different climatic conditions (e.g., different rainfall regimes). © 2008 Elsevier B.V. All rights reserved. 1. Introduction Soil organic matter (SOM) is one of the largest and most dynamic reservoirs of carbon (C) in the global C cycle. The amount of C stored in SOM is about twice that stored in the biosphere and atmosphere combined (Schlesinger,1997). Africa is the second largest continent on Earth (20% of the Earth's land area), but knowledge of the soil C pool in Africa is extremely limited (Williams et al., 2007). The shared dominance of trees and grasses in savannas, the dominant physiog- nomy in southern Africa, and surface soil crust C fixation (e.g., Thomas et al., 2008), add more complexity in soil C pool partitioning and dynamics than is found in landscapes dominated by a single physio- gnomy. Previous work on regional C stack estimates have been treating trees and grasses as one vegetation pool without differentiat- ing their contributions (Williams et al., 2007), which may limit our understand of the African C cycling since tree-grass composition is dynamic both spatially and temporally. By using stable C isotopes and soil organic carbon content in conjunction with geostatistical analysis, this study aims to investigate the spatial variability of the soil C pool, partition the contributions of soil C from trees and grasses, and assess the variations in soil C spatial variability and sources under different climatic conditions. The research was conducted at sites along the Kalahari Transect (KT), one of a set of IGBP (International Geosphere–Biosphere Programme) “megatransects” (Koch et al., 1995; Scholes et al., 2002) identified for global change studies. The soil substrate along the entire Transect is relatively homogenous, being covered by the Kalahari sands. The physical and hydraulic parameters such as soil texture (N 96% of sand) and bulk density (around 1.4–1.5 g cm - 3 along the whole Transect) do not have significant variations along the KT (Wang et al., 2007a). The KT thus provides an ideal setting to investigate changes in ecosystem dynamics, vegetation composition and structure, and C or nutrient cycles along a gradient of precipitation while minimizing confounding effects of soil heterogeneity. As the two main plant functional types, the trees and grasses in African savannas differ in their photosynthetic pathways. Trees in this region utilize the C 3 photosynthetic pathway whereas grasses typically utilize the C 4 pathway (Caylor et al., 2005). Trees provide a browsing habitat for herbivores and provide fuel wood for human harvesting. Grasses are important fuel load for savanna fires therefore play a potential important role in regional climate due to the effect of Geoderma 149 (2009) 402–408 ⁎ Corresponding author. Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ, 08544, United States. Tel.: +609 258 8308; fax: +609 258 1436. E-mail address: lixinw@princeton.edu (L. Wang). 0016-7061/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geoderma.2008.12.014 Contents lists available at ScienceDirect Geoderma journal homepage: www.elsevier.com/locate/geoderma