Biogeochemical transformations of phosphorus in wetland soils

Phosphorus (P) is an essential element required by all organisms and is a principal nutrient influencing agricultural productivity worldwide. Surface runoff and subsurface leaching of P to surface waters has led to nutrient enrichment of aquatic ecosystems. Nutrient enrichment is the third largest cause of impairment of surface waters in the U.S. Excess nutrients, particularly P, have been linked to changes in flora and fauna, such as encroachment of invasive species and nuisance algal blooms. Eutrophication of historically low-nutrient lakes and wetlands alters trophic structure, favoring species that are more adapted to higher nutrient conditions.; Determining pre-disturbance nutrient loading regimes is a major challenge for aquatic ecosystem researchers. Wetland soils and lake sediments that accrete materials preserve a partial record of ecosystem history that can be used to determine some pre-impact ecosystem characteristics. However, biogeochemical processes continually alter soil properties. Describing the extent of these changes was the central goal of this dissertation. Results from both P and carbon (C) characterization suggest movement of P and C into more recalcitrant organic compounds with time.; I examined changes in soil phosphorus pools over a 5000-year period in two subtropical wetlands using chemical fractionation. Results indicate an increase in the relative proportion of recalcitrant P over time, with minimal change in humic- and fulvic-P fractions. Long-term wetland P accretion was estimated to be approximately 2 mg m2 yr-1, a rate much lower than has been reported for wetland soils. Two thermal methods were developed to better resolve the extent of P recalcitrance. Autoclave-extractable P declined throughout the soil profile from 40% of total P in surface soils to 10% in soils 1 m deep. Soils thermally extracted under N2 atmosphere showed both increasing extractable P with increasing thermal energy, and declining P recovery with depth for any given temperature. Both techniques suggest that susceptibility to thermal degradation may be a useful method to characterize organic P recalcitrance.; Spectrophotometric investigation of organic matter extracts showed little change in average molecular weight with soil depth, though fluorescence and 13C-NMR analyses indicate increasing humic and phenolic character with increasing soil depth. The lignin content of soil fiber separated from peat increased from 35% in surface soil, to 75% at a depth of 1 m. Isotopic characterization of soil fiber seemed to indicate rapid ecological changes in the marsh that occurred at approximately 1000AD.