Soil age is considered a primary driver of soil fertility, particularly because the loss of rock-derived nutrients over time has been demonstrated on several chronosequences. However quantifying soil age in eroding landscapes is difficult because the residence time of soil is not equivalent to the age of the underlying parent material. Recently, cosmogenic radionuclide analyses (CRN) have been used to constrain landscape scale erosion rates, which, when coupled with soil thickness, can be used to elucidate soil ages. We used these tools to ask how soil age differs between two intact tropical forests in eroding landscapes, and whether the relationship between age and fertility observed on chronosequences could be invoked to explain these differences. We quantified CRN-derived erosion rates, phosphorus fractions, effective cation exchange capacity, and soil depletion relative to parent material in the lowland forests of Imataca, Venezuela, and the premontane to montane forests of Las Alturas, Costa Rica. We expected that pronounced differences in geologic setting would result in much lower erosion rates, and consequently older soil ages, in Imataca. Our prediction was that this would be reflected in dominance of more recalcitrant forms of P, lower base saturation and effective cation exchange capacity, and more profound depletion of parent-material derived elements.
CRN-based erosion rates vary by almost two orders of magnitude between our sites, suggesting large differences in soil age. Although the Oxisols of Imataca receive half the rainfall (2m vs. 4m yr -1), total phosphorus concentrations are five times lower there than in the Ultisols of Las Alturas. The fraction of P in the most recalcitrant pools is also higher at Imataca. Interestingly, in Imataca only a small fraction of organic P is in easily extractable (NaHCO3, NaOH) organic forms, whereas in Las Alturas roughly 50% of the P is in this pool. P losses from parent material, calculated by reference to an immobile element, exceeds 90% in Imataca and are negligible in Las Alturas. Exchangeable base cations are depleted in both sites (<10 cmol(+)/kg). However, base saturation was lower in Las Alturas than in Imataca, suggesting that time may have different effects on P and cation availability. These results suggest that soil age plays an important role in driving differences in fertility in eroding tropical landscapes, a result that was expected based on chronosequence data, but had not yet been demonstrated in more dynamic geomorphic settings.
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