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Addition of labile carbon (C) inputs to soil can accelerate or slow down the decomposition of soil organic matter (SOM), a phenomenon known as priming effect (PE). However, the magnitude and direction of PE is often difficult to predict, consequently making its relationship with labile C inputs and nutrient availability elusive. To assess this relationship, we added 13C labelled glucose (corresponding to 50% of initial soil microbial biomass C) to two soil types (Vertisol and Acrisol) with different concentrations of available N and from four land use systems (agricultural, pasture, grassland and shrubland). Parallel laboratory incubations i.e. short-term (6 days) and long-term (6 months), were set up to determine the effect of land use and soil type (N availability) on PE. Addition of labelled glucose in solution led to the retardation of SOM mineralization (negative PE) in both soil types and across all land use systems. This is attributed to preferential substrate utilization characterized by the higher mineralization of added glucose. Land use systems and soil types with higher N-availability displayed weaker negative PE, which is in line with the stoichiometric decomposition theory. In conclusion, our study demonstrate that N-availability plays a major role in determining mineralization of labile C inputs, magnitude and direction of PE in the studied dryland soils and land use systems. The fact that 15–27% of the added 13C remained in the soil at the end of the 6 months incubation and PE was negative, indicates that continuous labile C inputs could contribute to C immobilization and stabilization in these semiarid soils. Moreover, 13C glucose remaining in soils after 6 months in semi-natural pastures was comparable to those under natural grassland and shrubland systems especially in Acrisols. This demonstrates that incorporation and maintaining a perennial cover of native pastures has the potential to increase C sequestration in African semi-arid agricultural soils and landscapes. [ABSTRACT FROM AUTHOR]

Soil microbial indicators help monitor soil quality. Limited studies have determined how land use in drylands affects soil microbial indices. Top soil (0–10 cm) from four land use systems in African drylands: (1) shrubland (natural), (2) grassland (natural), (3) pasture (agricultural) and (4) cropland (agricultural) occurring on two soil types: (1) Vertisol and (2) Acrisol, was used in laboratory incubations (6 days) to assess the effects of land use changes on organic carbon (C org) mineralization, microbial biomass C (C mic), mineralization quotient (q M), metabolic quotient (q CO 2), C mic :C org ratio and sensitivity indices of these microbial indicators. Experimental plots were organized into a completely randomized design (n = 3) for every combination of land use and soil type. Cumulative CO 2 produced from native C org mineralization was the highest in Acrisol (108 ± 2.7 μg CO 2 –C g−1 soil) and the lowest in Vertisol (53 ± 2.5 μg CO 2 –C g−1 soil) croplands. Vertisol shrubland (1.34 ± 0.09 mg C g−1 soil) and Acrisol cropland (0.28 ± 0.07 mg C g−1 soil) had the highest and the lowest C mic , respectively. Acrisol cropland (1.29 μg CO 2 –C g−1 h−1) had the highest q M, approximately five times higher than the lowest q M (0.26 μg CO 2 –C g−1 h−1) in a Vertisol cropland. Highest q CO 2 was observed in an Acrisol pasture (12.04 μg CO 2 –C g−1 C mic h−1), which was approximately 30 times higher compared to the lowest q CO 2 observed in a Vertisol shrubland (0.41 μg CO 2 –C g−1 C mic h−1). The C mic :C org ratio was the highest in a Vertisol shrubland (0.097), approximately five times higher than the lowest observed in an Acrisol pastureland (0.019). Our study demonstrated that the measured soil quality indicators' magnitude, direction, and sensitivity varied depending on land use and soil type. Higher N availability in Vertisols increased the biological stability of soil organic carbon (SOC) resulting to decreased SOC mineralization than Acrisols. In conclusion, the measured microbial soil quality indicators showed that Acrisols are prone to accelerated SOC mineralization after disturbance than Vertisols in the studied semi-arid dryland ecosystems. Thus, there is a need to manage natural ecosystem conversions to support sustainable crop and pasture production in African drylands. [Display omitted] [ABSTRACT FROM AUTHOR]