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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. [ABSTRACT FROM AUTHOR]

Wood ash has a variety of uses including land application, geotechnical construction and daily and final covers for landfills. The objective of this study was to determine the relevant chemical and physical properties of selected mixtures of aspen burner-ash, log-yard waste scrapings (sandy soil plus wood waste) and a sandy soil as an intermediate landfill cover. The treatment mixtures, per weight basis, included: (1) 100% sand; (2) 80/20 log-yard waste/ash; (3) 50/50 sand/ash; (4) 40/30/30 sand/log-yard waste/ash; (5) 60/40 ash/log-yard waste; (6) 100% ash; (7) 100% log-yard waste. All treatments containing ash had a very high pH (12.8–13.3) and electrical conductivity (39–105 dS m-1). The ash contained significant amounts of beneficial nutrients, but soil solution dominance by sodium carbonate suppressed concentrations of calcium and magnesium. Treatments containing ash showed lower bulk densities but relatively higher soil strength values (2.5–3.5 kg cm-2) than that for the log-yard waste scrapings or the sandy soil, due to its self-cementing properties. Mixtures of soil material and ash had lower hydraulic conductivity rates (∼1.0 × 10-4 cm s-1) and lower infiltration rates compared to the sandy soil (∼5 × 10-3 cm s-1) or log-yard waste scrapings or pure ash. Water holding capacity increased with treatments containing ash. A soil–water balance model used to determine percolation rates through a 60 cm layer of treatment material showed that treatments containing a mixture of ash and soil material decreased the downward flow of water throughout the year. [ABSTRACT FROM AUTHOR]