Residual inorganic soil nitrogen in grass and maize on sandy soil

Nitrogen (N) remaining as inorganic (‘mineral’) soil N at crop harvest (NminH) contributes to nitrate leaching. NminH data from 20 (grass) and 78 (maize) experiments were examined to identify main determinants of NminH. N-rate (A) explained 51% (grass) and 34% (maize) of the variance in NminH. Best models included in addition crop N-offtake (U), offtake in unfertilised plots (U0), and NminH in unfertilised plots (NminH,0) and then explained up to 75% of variance. At low N-rates where apparent N recovery ρ keeps to its initial value ρini, NminH keeps to its base level NminH,0. At N-rates that exceed the value Acrit where ρ drops below ρini, NminH rises above NminH,0 by an amount proportional to (ρini−ρ)A(ρini−ρ)A. About 80% of (ρini−ρ)A(ρini−ρ)A was found as NminH, in grass as well as in maize. The fraction (1−ρini)A(1−ρini)A does not appear to contribute to NminH at low N-rates (A ≤ Acrit) or at high N-rates (A > Acrit).