Thursday, November 21, 2013, 4:30PM - 5:30PM
RL-1 room 269
Erosion, soil development and nitrogen cycling in a topographically dissected landscape, Osa Peninsula, Costa Rica
Lowland tropical forests are thought to cycle nitrogen in relative excess because old, highly weathered soils are broadly viewed as poor in rock-derived nutrients, such as phosphorus. However, exposure to diverse geomorphologic and hydrologic processes can give rise to differential weathering and erosion across topographically dissected landscapes, especially those in a transient state. Topographic variation has been shown to influence the availability of rock-derived nutrients, such as phosphorus, through effects on soil erosion and residence times, though analogous effects remain unexplored for nitrogen.
In this talk I will demonstrate that spatial patterns in soil N availability are closely associated with landscape position in a highly dissected, wet (~ 3,500 mm annual rainfall) tropical lowland forest on the Osa Peninsula, Costa Rica. I will discuss how correlations between soil biogeochemical data with soil development and weathering indices suggest a strong geomorphic control on the N cycle. Measurements of overland erosional N transport from this forest suggest that slopewash fluxes (~ 64 mg N m-2 yr-1) are relatively small. However, if slopewash is only ~ 7 % of annual erosion rates (which we think it is), then when all erosion is considered the N flux (954 mg N m-2 yr-1) may be high enough to constrain the accumulation of N within the ecosystem, potentially contributing to N limitation in eroding zones. Overall, these results demonstrate how topography and erosion drive within-watershed gradients in soil N availability, which may be relevant to biogeochemical dynamics in watersheds worldwide.