Examining soil nutrient balances and stocks under different land use and management practices in contrasting agroecological environments

Agricultural sustainability and land degradation can be monitored through studying soil nutrient dynamics. This study was conducted to investigate the balance and stocks of nitrogen (N) and phosphorus (P) under major land use types and land management (LM) practices in three contrasting agroecological environments in Ethiopia. We employed a nutrient monitoring approach and assessed soil N and P inputs and outputs at watershed and plot scales. Field data were collected from 72 sites representative of the major land uses of the three watersheds for the watershed scale analysis. In addition to the watershed data, 16 experimental runoff plots were established and the response of nutrient balances to improved LM practices for the major land uses, including cropland with three treatments (control, farmyard manure [FYM] and soil bund), grazing land with three treatments (control, exclosure and FYM + exclosure) and bushland with two treatments (control and exclosure). Nutrient balances, flows and stocks varied significantly among agroecological settings, land-use types and LM practices. Severe and negative N balances were found in all agroecological environments, with midland (-70.8) > highland (-61.7) > lowland (-50.3 kg ha(-1) yr(-1)), whereas slightly positive P balances were found in all agro-ecosystems. As a result, the midland (4.2% yr(-1)) and highland (2.1% yr(-1)) sites had greater N depletion rates than the lowland site (1.0% yr(-1)). Our results also demonstrated that applied LM practices improved N balances: by 32%-40% in cropland through FYM and mineral fertilizer, by 7%-18% in grazing land via FYM and exclosure, and by 6%-36% in degraded bushland via exclosure, implying that, implementing FYM and mineral fertilizer for cropland and implementing FYM with exclosure on non-croplands can reverse nutrient depletion in the study sites and in other areas having similar agroecological conditions.