The Biological Origins of Soil Organic Matter in Different Land-Uses in the Highlands of Ethiopia

Abstract

In the Ethiopian highlands, clearance of Afromontane dry forest and conversion to crop and grazing land lead to land degradation and loss of soil organic matter (SOM). Eucalyptus is often grown on degraded soils, and this results in the partial recovery of soil carbon stocks. The aim of this work was to assess the biological sources of SOM in this land-use sequence. In top-soils (0–10 cm) of four land-use systems, namely remnant natural forest, eucalyptus plantation, cropland, and grazing land, in the Ethiopian highlands, the origin of SOM was investigated. For this, a sequential extraction method was used, involving a solvent extraction, base hydrolysis, and a subsequent CuO oxidation. In these extracts, biomarkers (molecular proxies) were identified to characterize the SOM of the soil of the four land-uses. Putative lipid monomers of leaf, root, and microbial degradation products suggest that root inputs and microbial inputs dominate in SOM of all the land-uses, except grazing land. The ratios of syringyls, vanillyls, and cinnamyls showed that non-woody angiosperm plants were the predominant source for lignin in eucalyptus, cropland, and grazing land soil. In the soils of the natural forest, lignin originates from both woody angiosperms and woody gymnosperms. Our study shows the importance of root and microbial inputs in the formation of SOM, but also that, in the natural forest, legacies of previous forest cover are present.

Root and branch hydraulic functioning and trait coordination across organs in drought-deciduous and evergreen tree species of a subtropical highland forest

“…The forest covers c. 68 ha at c. 2500 meters above sea level. The forest is considered an old-growth, seasonally dry-evergreen Afromontane forest growing on slopes of 10-40° ( Assefa et al., 2018 ; Belay et al., 2018 ), and is embedded in a mosaic of (degraded) grass- and croplands ( Assefa et al., 2022 ). The climate of the mountainous study area is seasonally dry subtropical, with warm summers and dry winters (Köppen–Geiger classification ‘Cwb’, Peel et al., 2007 ).…”

“…In the Gelawdiwos forest, at least 41 woody species of 29 families have been recorded, dominantly woody angiosperms ( Wassie et al., 2010 ; Assefa et al., 2022 ). The only woody gymnosperm is Afrocarpus falcatus (syn.…”

“…The only woody gymnosperm is Afrocarpus falcatus (syn. Podocarpus falcatus ); at present occurring at relatively low density, it was likely more dominant in the past ( Assefa et al., 2022 ). In 2014, the mean timber volume was estimated as 92.4 m 3 ha -1 with an annual increment of 3.5 m 3 ha -1 ( Sisay et al., 2017 ).…”

The Contribution of Traditional Agroforestry Systems and Challenges of Adoption by Smallholder Farmers in Ethiopia. A review

“…However, 59.65 Mg C ha ⁻1 was discovered in Minjar Shenkora's parkland agroforestry practice, proving the system's necessity due to the higher density of trees (Tsedeke, Dawud, & Tafere, 2021). Compared to grassland and agriculture, the soil of the natural forest had concentrations of C that were roughly 4.4 and 3.7 times higher, respectively (Assefa et al, 2022). When compared to forest soil, grazing area had the greatest reduction in total soil C storage (0-50 cm), followed by cropland (50%) and eucalyptus plantations (47%) (Ahmed, Assefa, & Godbold, 2022).…”

Effect of Eucalyptus globulus Plantations on Soil Physicochemical Properties in the Upper Blue Nile, Ethiopia

“…Further, the results showed that the mean soil pH in the 9-year-old E. globulus plantation was 9.25% lower than in grassland. Eucalyptus trees are known to release certain organic acids into the soil through their root systems and fallen leaves [83]. Tese organic acids can contribute to soil acidifcation, potentially lowering the pH of the soil.…”