Nitrification and denitrification in humid tropical ecosystems: potential controls on nitrogen retention

1 Nitrification is an important microbial process in humid tropical ecosystems because of its effects on potentials for nitrogen loss via nitrate leaching and denitrification and on the hydrogen ion cycle. Nitrification rates in humid tropical soils vary markedly among different forest types but in general appear to increase following disturbance. Proximal controls on nitrifiers include ammonium and oxygen; other major controls (water, temperature, net nitrogen mineralization, plant uptake, cation exchange capacity (CEC), respiration rates and aggregate structure) operate chiefly through their effects on the availability of ammonium and oxygen. There are a number of more distal controls, particularly plant community structure (the composition and physical stature of the plant community), which can be greatly influenced by human activity. 2 Denitrification has been less extensively studied in the humid tropics. A priori predictions suggest that denitrification rates ought to be high in the humid tropics as long as nitrate is available: soils tend to be highly aggregated and are often moist, resulting in low 0 2 diffusion potentials, and carbon inputs from primary producers tend to be high. Recent results from in situ studies suggest that denitrification trends among rain forest sites may be similar to those for nitrification: high where nitrogen mineralization is high and low where nitrogen mineralization is low. Thus denitrification tends to be lowest in mid-successional sites where nitrate availability limits denitrifier activity. 3 Nitrification and denitrification can both lead to high nitrogen losses after vegetation clearing: denitrification directly and nitrification via its effects on nitrate leaching and denitrification potentials. Additionally, both processes may play an important role in the maintenance of soil exchange properties following disturbance. Nitrification, by producing hydrogen ions, will tend to force the soil exchange system to a net positive charge (to a net anion exchange capacity) and denitrification, by consuming some of the H + ions produced by nitrifiers, will tend to restore negative charges. Feedbacks in the system following disturbance may tend to make soil pH equilibrate near the soil's point of zero charge, a pH at which exchangeable ions are held only loosely against leaching from the rooting zone.