Effect of water, temperature and fertilizers on soil nitrogen net transformations and tree growth in an elfin cloud forest of Colombia

In the 'elfin' cloud forest of Serrania de Macuira, exchangeable ammonium and nitrate, and the rates of soil nitrogen mineralization and nitrification were measured in soil samples under different water, temperature and mineral nutrient additions. The effects of nitrogen, phosphorus and nitrogen plus phosphorus fertilization on radial trunk growth were measured in three tree species. In the cloud forest soils, concentrations of ammonium were much higher than those of nitrate. Nitrate was higher in samples collected during the afternoon than during the morning, probably as a result of leaching during the night or nitrification during the day. When samples were incubated under different water and temperature treatments, rates of nitrogen mineralization and nitrification increased more with changes in soil water content than with changes in temperature. Nitrification was significantly increased in soils amended with ammonium or with ammonium plus phosphorus, suggesting that nitrification is substrate-limited. Fertilization with nitrogen and phosphorus resulted in significantly increased girth increments in Guapira fragrans (Dum. -Cours.) Little and Rapanea guianensis Aublet. Myrcianthes fragrans (Sw.) D.C. did not respond to the fertilization. The results of this study support the hypothesis that the characteristics of montane rain forest in small and large tropical mountains (the 'Massenerhebung' effect) are greatly controlled by soil water conditions and related soil nitrogen availability.