Soil carbon stocks in planted woodlots and Ngitili systems in Shinyanga, Tanzania

Abstract

Our understanding of the processes influencing the storage and dynamics of carbon (C) in soils under semi-arid agroforestry systems in Sub-Saharan Africa (SSA) is limited. This study evaluated soil C pools in woodlot species of Albizia lebbeck (L.) Benth., Leucaena leucocephala (Lam.) de Wit, Melia azedarach (L.), and Gmelina arborea Roxb.; and in farmland and Ngitili, a traditional silvopastoral system in northwestern Tanzania. Soil organic carbon (SOC) was analyzed in the whole soil to 1 m depth and to 0.4 m in macroaggregates (2000–250 μm), microaggregates (250–53 μm), and silt and clay-sized aggregates (<53 μm) to provide information of C dynamics and stabilization in various land uses. Synchrotron-based C K-edge x-ray absorption near-edge structure (XANES) spectroscopy was also used to study the influence of these land use systems on the soil organic matter (SOM) chemistry to understand the mechanisms of soil C changes. Whole soil C stocks in woodlots (43–67 Mg C ha−1) were similar to those in the reserved Ngitili systems (50–59 Mg C ha−1), indicating the ability of the planted woodlots on degraded lands to restore SOC levels similar to the natural woodlands. SOC in the woodlots were found to be associated more with the micro and silt-and clay-sized aggregates than with macroaggregates, reflecting higher stability of SOC in the woodlot systems. The continuous addition of litter in the woodlots preserved recalcitrant aromatic C compounds in the silt and clay-sized aggregates as revealed by the XANES C K-edge spectra. Therefore establishment of woodlots in semi-arid regions in Tanzania appear to make significant contributions to the long-term SOC stabilization in soil fractions.