Impacts of Land Use Change on Unsaturated Zone Nitrate Cycling in the Southern High Plains, USA

Nitrate profiles in semiarid unsaturated zones archive land use change (LUC) impacts on nitrogen cycling with implications for agricultural nitrogen management and groundwater quality. This study quantified impacts of LUC on nitrate inventories and fluxes by measuring nitrate beneath natural ecosystems (5 profiles) and beneath nonirrigated (19 profiles) and irrigated (13 profiles) agricultural ecosystems in the southern High Plains (SHP). Low inventories (24-76 kg nitrate-N/ha) are found beneath natural ecosystems, which exhibit bulge-shaped chloride concentration profiles below the root zone resulting from evapoconcentration of precipitation and accumulation of chloride over long times (~10-15 kyr). Rainfed agricultural profiles were either completely (10 profiles) or partially flushed (9 profiles) of chloride due to increased recharge and downward displacement of accumulated chloride resulting from LUC. The partially flushed profiles have 86–1,057 kg nitrate-N /ha (median 265 kg/ha) at depth that corresponds to initial cultivation, dated using soil water chloride. This nitrate most likely originated from cultivation causing mineralization and nitrification of soil organic nitrogen (SON) in old soil water (pre-cultivation), which is microbially mediated. The SON source is supported by N and O isotopes of nitrate. The chloride-flushed zones of all 19 rainfed profiles have 8-874 kg nitrate-N/ha (median 212 kg/ha) in younger water associated with LUC based on soil water chloride dating and resulting from over-application and leaching of fertilizers. Most of the irrigated agricultural profiles (12 of 13) showed the effects of irrigation water throughout the profile and had 77-1841 kg nitrate-N/ha (median 914 kg/ha) attributed to over-application and leaching of fertilizers. Nitrate appears to behave conservatively below the root zone as indicated by high correlation with chloride (median R=0.86, range 0.68-0.96) in irrigated profiles. LUC in the SHP mobilizes nitrate reservoirs by increasing drainage/recharge beneath cropland, which could increase nitrate-N in the underlying Ogallala aquifer by 0.1-29 mg/L.