Topographic controls over nitrogen cycling in a lowland tropical rainforest

Background/Question/Methods

A common paradigm for the lowland tropics holds that in-tact forests are nitrogen (N) rich, with N cycling in excess of biological demand. This pattern has been attributed to relative geo-climatic stability of the lowland tropics over long periods of time. However, substantial geomorphologic variation within this biome means that many forests contain diverse topography. Topographic variation has been shown to influence the availability of rock-derived nutrients, such as phosphorus (P). Here we explore the possibility that gradients in topography may also drive meaningful shifts in soil N cycling in denuding landscapes, influencing the march toward and maintenance of an N-rich steady state. To investigate geomorphic drivers of N dynamics, we measured multiple indices of soil development and soil N cycling across a topographically dissected, wet (~ 4,000 mm MAP) lowland forest on the Osa Peninsula, Costa Rica. We measured total and extractable N pools, including δ¹⁵N enrichment of total soil N, as well as potential and net rates of N cycling across topographic gradients.

Results/Conclusions

Within this wet tropical forest, we observed a negative linear relationship between slope angle and soil ¹⁵N enrichment, indicating a decline in N richness as slope angle increases. Focusing on one catena within this landscape, we observed distinct patters in soil profile development and soil organic matter contents and stoichiometry from the ridge-top down to the fluvial-riparian zone. These changes in edaphic traits correlated with clear shifts in N cycling. Soils on flat, stable ridge-tops showed more evidence of in-situ weathering, suggesting a longer residence time. These soils were relatively N-rich, exhibiting high nitrification potential, large pools of soil nitrate, elevated net N transformation rates, low soil C: N ratios and enriched δ¹⁵N values. By contrast, these indices of N-richness all exhibited significant declines on slopes where soils displayed weaker profile development, indicative of shorter times for in-situ weathering. Our results highlight the potential role of topographic variation in driving significant local-scale gradients in N availability linked to differences in soil residence times across tropical forests.