Subsoil retention of organic and inorganic nitrogen in a Brazilian savanna Oxisol

Nitrogen (N) loss by leaching poses great challenges for N availability to crops as well as nitrate pollution of groundwater. Few studies address this issue with respect to the role of the subsoil in the deep and highly weathered savanna soils of the tropics, which exhibit different adsorption and drainage patterns to soils in temperate environments. In an Anionic Acrustox of the Brazilian savanna, the Cerrado, dynamics and budgets of applied N were studied in organic and inorganic soil pools of two maize (Zea mays L.) - soybean (Glycine max (L.) Merr.) rotations using 15N tracing. Labelled ammonium sulphate was applied at 10 kg N ha-1 (with 10 atom% 15N excess) to both maize and soybean at the beginning of the cropping season. Amounts and isotopic composition of N were determined in above-ground biomass, soil, adsorbed mineral N, and in soil solution at 0.15, 0.3, 0.8, 1.2 and 2 m depths using suction lysimeters throughout one cropping season. The applied ammonium was rapidly nitrified or immobilized in soil organic matter, and recovery of applied ammonium in soil 2 weeks after application was negligible. Large amounts of nitrate were adsorbed in the subsoil (150-300 kg NO3--N ha-1 per 2 m) matching total N uptake by the crops (130-400 kg N ha-1). Throughout one cropping season, more applied N (49-77%; determined by 15N tracers) was immobilized in soil organic matter than was present as adsorbed nitrate (2-3%). Most of the applied N (71-96% of 15N recovery) was found in the subsoil at 0.15-2 m depth. This coincided with an increase with depth of dissolved organic N as a proportion of total dissolved N (39-63%). Hydrophilic organic N was the dominant fraction of dissolved organic N and was, together with nitrate, the most important carrier for applied N. Most of this N (>80%) was leached from the topsoil (0-0.15 m) during the first 30 days after application. Subsoil N retention as both adsorbed inorganic N, and especially soil organic N, was found to be of great importance in determining N losses, soil N depletion and the potential of nitrate contamination of groundwater.