The inherent ‘safety‐net’ of an Acrisol: measuring and modelling retarded leaching of mineral nitrogen

Summary The inherent features of Acrisols with their increasing clay content with depth are conducive to reducing nutrient losses by nutrient adsorption on the matrix soil surfaces. Ammonium (NH 4 + ) and nitrate (NO 3 − ) adsorption by a Plinthic Acrisol from Lampung, Indonesia was studied in column experiments. The peak of the H 2 18 O breakthrough occurred at 1 pore volume, whereas the median pore volumes for NH 4 + and NO 3 − ranged from 6.4 to 6.9 and 1.1 to 1.6, respectively. The adsorption coefficients ( K a in cm 3 g –1 ) measured were 1.81, 1.51, 1.64 and 1.47 for NH 4 + and 0.03, 0.09, 0.10 and 0.17 for NO 3 − , respectively, in the 0–0.2, 0.2–0.4, 0.4–0.6 and 0.6–0.8 m soil depth layers. The NH 4 + and NO 3 − adsorption coefficients derived from this study were put in to the Water, Nutrient and Light Capture in Agroforestry Systems (WaNuLCAS) model to evaluate their effect on leaching in the context of several cropping systems in the humid tropics. The resulting simulations indicate that the inherent ‘safety‐net’ (retardation mechanism) of a shallow (0.8–1 m) Plinthic Acrisol can reduce the leaching of mineral N by between 5 and 33% (or up to 2.1 g m −2 ), mainly due to the NH 4 + retardation factor, and that the effectiveness in reducing N leaching increases with increasing depth. However, the inherent ‘safety‐net’ is useful only if deep‐rooted plants can recover the N subsequently.