Assessing the sustainability of land use management of northern Ethiopian drylands by various indicators for soil health

Land use change and agricultural intensification in developing countries affect terrestrial carbon (C) stocks, CO2 efflux, microbial communities and overall soil health. This study assesses the effects of four land use types typical for northern Ethiopia (forests, exclosures, grazing lands and intensively cultivated croplands) on various soil health indicators. We quantified and compared microbial biomass carbon (MBC), water extractable organic carbon (WOC), metabolic quotient (qCO2), substrate use efficiency (SUE) and dynamics of 14C-labelled glucose added to soil. Irrespective of the land use, MBC but not SUE decreased 2- to 8-fold with increasing depth, demonstrating the C limitation of subsoil microbial communities under all land use forms. Sandy soils, however, which permit seepage and leaching of WOC into lower layers and promote subsoil microbial communities, adapted to frequent input of easily accessible C substrates. Significantly higher qCO2 were recorded in subsoils compared to topsoils, especially in croplands with low MBC. In croplands, high glucose-14C incorporation (≈20%) into their low microbial biomass indicates a high SUE and reflects a better nutrient supply of these microbial communities. Mineralization of up to 95% of 14C-labeled glucose in topsoils of forest and grazing lands was higher than in croplands, and exclosures never reached the level of natural ecosystems. This demonstrates that 6–10 years of exclosure establishment does not result in soil microbial communities and soil C dynamics resembling those of natural forests. Our study demonstrates that land use can negatively affect the ecological performance of microbial communities and that these impacts are more severe in sandy than in clayey soils. Mitigation strategies such as minimum tillage or residue retention in intensively cultivated croplands can increase microbial abundance and activity and help ensure environmental sustainability and mitigation of climate change. Nonetheless, such measures need to be carefully accompanied by monitoring indicators of soil health to confirm the sustainability of the chosen mitigation strategies.