Nitrogen and Organic Loss Following Land Use Change and Overgrazing in Soolegan Catchment

Grassland soils in Zagros Mountains are being seriously degraded under cultivation and overgrazing. This study investigated the impacts of land use shifting and overgrazing on soil erosion, soil fertility, and Organic C and N losses via surface runoff produced by rainfall simulator. Land uses were two levels of pasture degradation including moderately (MDP) and heavily degraded pasture (HDP), classified based on vegetation cover (the former had 20-25 % and the latter had 5-10% plant cover), and cultivated field. Significantly (p<0.05) lower amounts of organic carbon and total nitrogen were found following overgrazing (38 and 27%) and cultivation (54.5 and 41%), respectively. Much higher runoff, sediment and nitrogen (dissolved and particulate) losses observed following land use changes. Total nitrogen lost under cultivated (46.3 g/m2) and HDP (22.5 g/m2) were 5 and 2 times greater than that under MDP (8.7 g/m2), respectively. Much higher amounts of dissolved ammonium and nitrate were lost under cultivated (25.5 and 96.8 mg/m2) and HDP (26.6 and 88.6 mg/m2) than that under MDP (10.7 and 42.1 mg/m2), respectively. This study indicated that overgrazing and land use change caused not only sever soil erosion, but also fertility decline and nitrogen removal via catchment flows.