Changes in Soil Nitrogen-Mineralization and Nitrification Along an Altitudinal Transect in Tropical Rain Forest in Costa Rica

Abstract

(1) Surface soils (0-15 cm) were collected from six sites in mature undisturbed tropical rain forest along an altitudinal transect (100 m-2600 m) on Volcan Barva, Costa Rica. The rates of nitrogen-mineralization and nitrification were measured under (a) field conditions, where the soils were incubated under the litter layer, and (b) laboratory conditions, where the incubations were done at higher temperatures, but at field moisture content. In addition, a measure of nitrogen-mineralization was made under 'improved' conditions, where the incubation temperature was increased, and aeration and moisture content improved. The moisture content, pH and the concentrations of a range of soil nutrients were also measured for each soil. A nutrient amendment experiment was also done, where selected nutrients were added to soils from three altitudes (100 m, 1000 m and 2600 m), and their effect on nitrogen mineralization and nitrification assessed. (2) The moisture content, total nitrogen and carbon, extractable potassium, calcium, magnesium and ammonium-nitrogen increased, and both extractable copper and nitratenitrogen decreased, with altitude. (3) Soil nitrogen-mineralization and nitrification both decreased with increasing altitude under field conditions, and both rates were not increased significantly by laboratory incubation, where the temperature alone was alleviated. However, where both temperature and aeration/moisture content were improved, then nitrogen-mineralization rate increased by between one and three orders of magnitude, and a positive relationship was found with altitude of origin. These combined results suggest that nitrogenmineralization may be limited under field conditions by the high moisture contents found in the montane soils. (4) Partial correlation analysis of these nitrogen turnover rates with other measures of soil chemistry showed that (a) under 'improved' conditions the mineralization rate was positively correlated with resource quality and negatively correlated with pH and perhaps reducing conditions, (b) under field and laboratory conditions the mineralization rate was correlated with the reaction product, and (c) nitrification was correlated with the reaction substrate, implying a resource limitation. (5) In the nutrient-addition experiment, ammonium-nitrogen increased nitrogenmineralization rates at low and high altitudes, but decreased rates at the intermediate altitude. Calcium carbonate addition increased mineralization rates at the low and intermediate altitude, whereas calcium sulphate increased these rates only at the highest altitude. Phosphorus addition had no effect on nitrogen-mineralization at any altitude. (6) Nitrification rates were increased by substrate addition, confirming that the reaction was substrate limited. However, the stimulation was much less than the amount of added substrate, and other limiting factors are clearly important. Calcium sulphate also increased nitrification, but only at the lowest altitude. (7) These findings agree with the general hypothesis that nitrogen-mineralization and nitrification rates are reduced in montane tropical rain forest, and may be a contributory factor controlling growth, species composition and stand structure. The significance of these results for future management of montane tropical rain forest is briefly discussed.