The rate at which landscapes are eroding can influence the nutrient status of the ecosystems they host, but even in a single watershed erosion rates can vary widely at a small spatial scale. In particular, landscapes in geomorphic disequilibrium can host remnant broad flat ridges, which erode slowly and host old, highly weathered soils that contrast with narrower ridges that have already equilibrated to the erosion rate on adjacent slopes, and thus experience a rejuvenated supply of rock-derived nutrients. We explored the nutrient status of twelve ridges in two adjacent watersheds in the Luquillo Experimental Forest of Puerto Rico. The ridges ranged in width from 5 – 54 m, and we interpreted the broad ridges as remnants that were eroding more slowly. We hypothesized that broader ridges would thus host more highly weathered soils. To test this hypothesis, we determined the erosion rate on each ridge by measuring cosmogenic Berillyium-10 (10Be) in soil quartz grains. We also measured soil carbon, nitrogen, d15N, pH, phosphorus fractions, and depletion of elements relative to parent material using niobium as a putatively immobile index element.
Contrary to our expectations, 10Be concentrations did not indicate differences in erosion rates among ridges (rates ranged from 0.026 – 0.044 mm/yr). Nevertheless, some soil properties did vary with ridge width. The broadest ridges had lower pH, and greater depletion of Si, Fe, and Al relative to parent material. Soil C:N was positively, and soil d15N was negatively, correlated with ridge width (p=0.0009 and 0.09, respectively). Both bicarbonate extractable P and sodium hydroxide extractable P were positively correlated with ridge width (R2= .74, P<.001; R2= 0.46, P= 0.012, respectively). This heightened P availability may result from more reducing conditions on broad ridges. Our results indicate that, despite similar erosion rates, ridge width does influence soil weathering and perhaps nutrient cycling. Changes in hydrology, or the balance of physical versus chemical erosion could be responsible for the observed trends, but further research is warranted to determine the true underlying cause.
Tamayo C.D., Brocard G., Porder S., (2014): Geomorphologic influences on tropical soil weathering and nutrient status. 99th ESA Annual Meeting (August 10 -- 15, 2014).
This Paper/Book acknowledges NSF CZO grant support.