Net mineralization, net nitrification and potentially available nitrogen in the subsoil beneath a cultivated crop and a permanent pasture

The brigalow clay soils of central Queensland in eastern Australia contain large quantities of nitrate‐N in the subsoil beneath shallow rooting cultivated crops. A laboratory incubation study was conducted to determine whether nitrate accumulation at depth beneath these crops was due to in situ nitrogen mineralization. Intact soil cores, 5 cm long and 5 cm diameter, were obtained at four depths to 120 cm beneath cultivated black gram (Vigna mungo) and green panic (Panicum maximum var trichoglume) permanent pasture and incubated for 12 weeks at 60% water‐filled pore space and 25°C. Net mineralization of organic N occurred in all soil cores obtained from under black gram with values ranging from 4.3 to 9 mg N kg soil at 12 weeks. Beneath the pasture, net mineralization had not commenced by the end of 12 weeks. Potentially available nitrogen (N) ranged from 1.2 to 62.7 kg N ha under black gram, and from 10.2 to 136.9 kg N ha under pasture. A significant relationship was observed between N and total N beneath both crops, and between N and total C under the pasture. Leaching of N mineralized in the surface layers of soil appears to be the main avenue of nitrate build‐up in the subsoil beneath black gram, with subsoil mineralization making only a partial contribution to the accumulated nitrate pool. Copyrigh