| In the humid tropics, large quantities of nutrients can be rapidly leached under disturbance regimes that bare the soil frequently, such as short-rotation cropping. To determine the impact of disturbance frequency on nutrient loss from a fertile soil in the humid lowlands of Costa Rica, leaching was measured from plantations of three tree species, each of which was subjected to three cutting cycles: annual, every four years, and uncut. Leaching was determined by combining measurements of mineral nutrients in soil water with estimates of water percolation rates. The latter were estimated using a water balance model; inputs to the model were obtained from (1) a study of evapotranspiration from the stands, (2) soil hydraulic conductivity and water retention measurements, and (3) micrometeorological measurements. Soil water for nutrient analysis was drawn from porous ceramic cup samplers located 1 m below the soil surface. Uptake of nutrients into plant tissues, nitrogen mineralization, and nitrification were measured to assess their effects on leaching rates.;Estimated annual evapotranspiration differed by as much as 10 per cent among the three tree species, indicating that broad-leaved tree species can affect leaching through their effects on stand hydrology. Nitrate leaching was much greater under the annual cutting cycle than in uncut stands; there also were increases in potassium and other cations, but they were not as significant. Once trees in the 4-yr rotation plots were cut, nitrate leaching increased substantially, but the duration of the increase was brief The magnitude of nitrate leaching immediately after cutting was similar in 1-yr and 4-yr rotations (range 29-41 kg/ha). The increased nitrate leaching was due to a combination of increased nitrogen mineralization and nitrification in concert with decreased uptake. After several rotations, trees in the 1-yr rotation stands began to show diminished growth; decreased leaf nitrogen concentrations indicate that they were probably suffering from nitrogen limitation. In these soils, short-rotation cropping has a deleterious impact on nitrogen retention leading to nutrient limitations of productivity. |
