Effects of river type and microtopography and seasonality on soil phosphorus pools in forested floodplains of the southeastern United States

Forested floodplains play a significant role in the regulation of nutrient transport and retention by rivers and thus water quality. During flood events, dissolved nutrients including phosphorus (P) can be retained through sedimentation, uptake and complexation mechanisms. As part of a larger study of nutrient retention in forested floodplains, we examined the effects of river type, seasonality and within site microtopography, on soil P pools to better understand the mechanisms regulating P dynamics in these systems. We selected four sites from a set of established research plots in floodplain forests across the southeastern United States: two on alluvial rivers and two on blackwater rivers. At each site we collected soil cores at four ridge and four swale locations in December 2005 (post growing season), April 2006 (early growing season) and August 2006 (peak growing season). Soils were analyzed using a sequential fractionation scheme allowing us to measure P, Fe and Al in six components of the total soil pools. In addition, we measured microbial P, total P and crystalline and amorphous Fe and Al pools. Total soil P pools varied almost three-fold across sites (from 357.2 ± 16.4 to 895.6 ± 29.4 kg ha^-1). Soil P also varied between ridges and swales within all but one site (P ≤ 0.001, paired t-tests) and across the growing season. Amorphous Fe was the best predictor of soil P (R = 0.64 in the ridges and 0.84 in swales, P ≤ 0.001) suggesting metal binding plays a key role in soil P retention.