Inhibition of nitrate accumulation in tropical grassland soils: effect of nitrogen fertilization and soil disturbance

SUMMARY In acid soils in the Eastern Plains of Colombia, forage grasses planted on land prepared before the previous dry season produced 40–50% more dry matter than when land was prepared immediately before planting. Virtually no NO 3 − accumulated in surface (0–10 cm) soil from three native undisturbed savanna sites. Where land was ploughed before the dry season, NO 3 − levels increased gradually after a 2–3 month lag, and dropped at the beginning of the rains. In samples incubated for 4 weeks, more NO 3 − accumulated in the wet than the dry season. A similar 2–3‐month lag occurred when land was ploughed after the dry season. NH 4 + levels were higher in ploughed than savanna soils, and rose in all soils at the beginning of the rains. More NO 3 − and NH 4 + accumulated on incubation in pots than in soil cores. Forage grasses inhibited NO 3 − accumulation in the soil, relative to plant‐free plots, and legumes stimulated it. N fertilization overcame this inhibition except in the case of Brachiaria humidicola .