Tropical forests are generally regarded as naturally nitrogen (N)-rich ecosystems where N availability is in excess of biological demands. These forests are usually characterized by increased soil N cycling rates such as mineralization and nitrification causing loss of N through leaching and denitrification from the ecosystem. Loss of N, in turn, has many negative consequences, including soil and surface water acidification, plant nutrient imbalances and related adverse effects on biological diversities. Increased atmospheric N deposition that is anticipated for tropical regions may further aggravate these negative consequences. Thus, an improved understanding of how increased atmospheric N deposition impacts N retention efficiency of tropical forests is needed. However, the fate of deposited N in tropical forest ecosystems and its retention mechanisms remains elusive. This PhD thesis used the stable nitrogen (N) isotope 15N to uncover two aspects of N cycling in tropical forests: i) the patterns of ecosystem natural 15N abundance (δ15N) in relation to the 15N signature of deposition N, and its response to increased N deposition; ii) the fate of ambient and increased N deposition in the same forests. The study took place in the Dinghushan Biosphere Reserve in southern China, including an old-growth (N rich) and a pine dominated forest, which is relatively N depleted. This study is the first to use continuous 15N-labeling and to quantify the fate of atmospheric N in the major plant and soil pools as well as N export in soil water in tropical forests.
Total annual atmospheric deposition of N to the forest in the study period was 51 kg N ha-1yr-1. Nitrogen deposition was dominated by NH4-N due to intensive agricultural NH3 emissions in nearby areas. Nitrate dominated leaching loss from the soil, and it was similar to the input amount. As a consequence of the 15N-depleted atmospheric N deposition plants and soils were generally also 15N-depleted, indicating a tight coupling between input and retention of N. Plants in the old-growth forests were more enriched with 15N than those in the pine forest. Addition of fertilizer N with 15N abundance close to the atmospheric signature showed that addition slightly increased N content in the pine forest, but tended to decrease it in the more N-saturated old-growth forest. Plant species variation in their N and 15N contents was also investigated in the old-growth forest. The plants displayed consistent difference in their N and 15N content.
Fates of N were investigated by tracing the 15N labeled N under ambient and increased deposition at 50 kg N ha-1yr-1 in the old-growth forest. The tracing was based on how much 15N could be recovered in plant and soil pools four months after the last addition and by monitoring leaching of 15N in soil water on a monthly basis. The result showed that deposited N is effectively retained in plant and soil pools resembling and exceeding that observed for temperate forests. Furthermore, increased N input decreased the N retention efficiency from 75% to 55% and increased N leaching from 14% to 23%. Comparison of short-term fate of deposited N in the old-growth and pine forests also showed no apparent significant difference, although the old-growth forest retained more N in both plants and soils. In general, the results from these tracer studies indicate that short-term retention mechanisms of deposited N (plant N uptake and soil N retention) could be similar in N-rich and N-limited forests, but N cycling rates (decomposition, mineralization, and nitrification) may differ considerably. Conclusions of this study are, however, based on short-term observations of retention traits in the forests, and longer-term studies are needed to evaluate the role of plants and soils as more permanent sinks for the N deposition these tropical forests.