Exploring the variability of soil nutrient outflows as influenced by land use and management practices in contrasting...

Improving our understanding of how different land uses and management practices affect soil nutrient outflows in sub-Saharan Africa could aid in developing sustainable practices. The objective of this study was to analyse the variation in outflows of soil total nitrogen (TN) and available phosphorus (P_av) as influenced by land use types (cropland, grazing land, and bushland) and land management practices (soil bunds for cropland and exclosures for non-croplands) in the three contrasting agro-ecological zones of the Upper Blue Nile basin, Ethiopia. Field data were collected for TN and P_av outflows by water erosion (Eo), leaching (Lo), product harvest (Ho), and gaseous emissions (Go) from 18 runoff plots (30 m × 6 m) for two years (2018-2019). TN and P_av losses significantly varied (p < 0.05, p < 0.01) among agro-ecological zones, land use types, and management practices. Losses of TN ranged from 39.6 to 55.5 kg ha^-1 yr^-1 and those of P_av from 4.1 to 5.9 kg ha^-1 yr^-1, with a total replacement cost of US$26-38 ha^-1 yr^-1. Nutrient losses and cost generally increased from lowland (Dibatie) to midland (Aba Gerima) to highland (Guder), although the highland and midland sites did not differ significantly (p > 0.05) in nutrient losses. Cropland showed significantly higher TN and P_av losses than other land use types, but TN loss did not differ significantly between grazing and bushland. In all land use types at all sites, the magnitude of nutrient losses was Ho >Eo >Lo >Go. In plots with land management practices, TN and P_av losses associated with Eo, Lo, and Go were reduced on average by 44-76%, 9-47%, and 1%-36%, respectively. These practices were most effective to reduce Eo nutrient losses. Further study is required to analyzing the soil nutrient balance and identify priority areas for implementing mitigation measures.