Mineral nitrogen losses in two contrasting tropical soils under intensive water input: roles of ammonium retention and native nitrogen pools

Abstract Purpose Mineral nitrogen (N) losses from tropical soils are often inferred from texture. We evaluated how contrasting soil types and N application rates affect N losses via ammonium (NH 4 + ) and nitrate (NO 3 − ) leaching and nitrous oxide (N 2 O) emissions. Methods One-meter columns packed with a sandy (28 g kg − 1 clay) or clayey (626 g kg − 1 clay) soil received ammonium sulfate at 0 (control), 600, 1200, or 1800 mg N column − 1 (0–240 kg N ha − 1 ) and 1200 mm simulated rainfall over 112 d. We recovered every leachate, monitored N 2 O flux and water-filled pore space, and sectioned profiles at termination. Results Cumulative NH 4 + -N leaching was greater from the sandy soil at all fertilized rates but showed no consistent rate response. Cumulative NO 3 − -N leaching increased linearly with N rate only in the sandy soil, yet was greater from the clayey soil at each fertilized rate. The sandy soil drained 6.7% more water, whereas flow-weighted NO 3 − -N concentration was greater in the clayey soil, showing that its larger NO 3 − export under fertilization was concentration-driven. Nitrogen-induced acidification was strongest near the surface in the clayey soil but extended through 10–70 cm in the sandy soil. N 2 O-N emission was approximately sixfold greater from the clayey soil (emission factor 0.17% versus 0.03%). Conclusion NO 3 − -N dominated mineral-N leaching from both soils. Weak NH 4 + retention favored its mobility in the sandy soil, whereas greater background N availability and wetter conditions in the clayey soil favored NO 3 − export and N 2 O production. These contrasts cannot be attributed to clay content alone.