Internal Nitrogen Cycling in Tropical Forest Soils

Growing human population and activities such as the continued increase in fossil fuel consumption, fertilizer use, and biomass burning have considerably increased the amount of atmospheric nitrogen (N) deposition. Until recently, elevated N deposition has been mainly a concern in temperate, highly industrialized regions but deposition of N increasingly occurs also in fast-developing and industrializing tropical regions like Latin America and South-East Asia. The most harmful impacts of elevated N deposition are increases in soil N2O (a long-lived greenhouse gas also contributing to the depletion of stratospheric ozone) and NO emissions (important for the formation of tropospheric ozone and acid rain) as well as N-leaching to ground and surface waters. The few studies that have been conducted in tropical forests showed that the N status (i.e. high or low N availability) of ecosystems is the key to estimate reactions to elevated N input. For this reason, the present thesis assessed the internal N cycling of tropical soils under old-growth forests in Ecuador, Costa Rica and Panama. Beforehand, two ways of measuring gross N cycling rates in soils were contrasted, one based on in-situ processing and incubation of soil samples and the other one based on laboratory processing and incubation. Both treatments were followed by the 15N pool dilution technique. This first investigation revealed that reliable data of gross N cycling rates in tropical forest soils can only be obtained from samples that were processed (i.e. injected, incubated and extracted) in-situ. In view of decreasing gross N mineralization rates and increasing gross nitrification rates occurring from soil storage and laboratory incubation before analysis, it followed that the processing of soils has to be undertaken in-situ.Subsequently, we followed this methodology to examine the gross rates of soil N cycling in forest sites across two Andosol toposequences in Ecuador and Costa Rica and across one Ferralsol toposequence in Costa Rica. The two Andosol sequences showed opposite trends in soil N cycling changes with elevation. While gross N mineralization rates decreased across the altitudinal gradient in Ecuador, an increasing pattern was observed in Costa Rica. For the Costa Rican Ferralsol toposequence, we obtained a decrease in gross mineral N production with elevation. Comparing young Andosols with old, strongly weathered Ferralsols, our data revealed similar gross N production rates. Nevertheless, net N mineralization rates (used as an indicator for plant available N) in Andosols exceeded those of Ferralsols, a finding that was attributed to a lower microbial NH4+ immobilization in these Andosols. This was in contrast to the general assumption that old, highly weathered soils do not limit ! plant growth by the insufficient supply of N but instead by the limitation of rock derived nutrients. On the other hand, young soils were reported to have relatively low available N.Furthermore, we assessed the factors that determined the changes in gross N mineralization across these three altitudinal gradients. In Ecuador, changes were paralleled by a declining degree of soil development, while this was not observed along the Costa Rican toposequences. In all toposequences, mean annual air temperature controlled gross N mineralization rates in soils either directly or indirectly by influencing the organic layers. In Costa Rica the mean annual precipitation appeared to be an additional factor behind the mineral N production rates, again, by controlling the organic layers across both altitudinal gradients. In general, organic layers of sites had a great influence on mineral N production in soils. This was manifested in the Ferralsol toposequence, where both quantity and quality of the organic material in topsoil controlled the gross N mineralization rates by regulating the microbial biomass. In the Costa Rican Andosols, it was only the quality of organic substrate that was responsible for changes in mineral N production by controlling the microbial biomass. Across all gradients, we also consider the abundance of N fixing legumes to be responsible for the changes in N cycling rates we observed. No doubt that these factors controlling the soil N cycling are interdependent to some extent. But the fact that these factors varied among the three toposequences suggests that the effect of one factor may counteract and possibly outweigh another, dependent on the soil characteristics and locations.Finally, we measured changes in soil N cycling rates after experimental chronic N-addition in a tropical lowland forest in Panama. Elevated N input resulted in increasing rates of gross N mineralization, induced by the improving quality of incoming organic substrate. Chronic N-addition decreased pH and tended to reduce the microbial biomass in the top soil. This was reflected in lower microbial NH4+ immobilization rates in the N-addition plots. Due to these changes, more NH4+ was available for nitrification which was manifested in higher soil extractable NO3- concentrations. Based on these results, we expect gross N mineralization rates to increase with elevated atmospheric N deposition in the tested sites in Ecuador and Costa Rica. The extent may vary according to the factors that controlled the gross N mineralization rates at each of the three gradients. We expect this increase in gross N mineralization rates to be paralleled by increasing N losses through soil gaseous N-oxide emissions and/or leaching. In general, we assume that possible losses may follow the patterns of mineral N availability (gross N mineralization rates) across the tested toposequences since patterns of nitrification rates resembled the trends of N availability and N losses are strongly connected with nitrification activities.