Priming effect in semi-arid soils of northern Ethiopia under different land use types

Input of organic carbon (C) to the soil stimulates soil microbial activity leading to changes in turnover of soil organic matter, a phenomenon referred to as priming effect (PE). However, contribution of various management forms in tropical drylands, the role of land use conversion and the soil depth at which such management induces altered PEs remain largely unclear. In this study, we quantified respiration and PE in semi-arid soils of northern Ethiopia. Soils from three depths (0–30, 30–60 and 60–90 cm) in forest, croplands, exclosure and grazing land use types were sampled. The soil samples were incubated for 23 days and PE and respiration quantified after addition of 14C labeled glucose corresponding to 50% of initial microbial biomass carbon (MBC). Generally, CO2 respired was 30–63% lower in sub than in topsoil with most expressed depth gradients in croplands. The weak negative PEs in subsoil is an indication of highly stabilized C. Contrary, glucose addition induced stronger positive PEs in topsoils collected from forest, exclosure and grazing land. The temporal dynamics of PEs involved a strong positive peak for the first five days after glucose addition and a second smaller peak 10 days after glucose addition in natural ecosystem, corresponding to apparent and real PE, respectively. Lack of positive correlation between PEs and C/N ratio ruled out the N-mining hypothesis, but a positive correlation between PE and MBC suggests co-metabolism as possible mechanism behind the real PE. Higher priming in natural ecosystem compared to cropland is an indication that conversion of natural ecosystem to continuous cropping system leads to depletion of the “primable” C pool in dryland soils. Additionally, this land use conversion negatively affects biogeochemical C cycling by an altered response of soil microbes to C input.