The assessment of physicochemical factors influencing phosphorus mineralization and immobilization in an Illinois floodplain soil
Persistent losses of phosphorus (P) from soil to aquatic environments, provoked by human activity, degrade downstream water quality. Thus, it is critical to gain a deeper understanding of the biogeochemical processes that govern P bioavailability/mobility in the soil, including P mineralization and immobilization. Through microbially mediated immobilization, inorganic P is transformed into an organic form while during mineralization, organic P is converted into a labile, inorganic form, which is more susceptible to loss. In this study, the occurrence of mineralization/immobilization in Illinois floodplain soils was thoroughly examined. First, a comprehensive examination of mineralization and immobilization parameters in a floodplain and its adjacent upland was carried out. Due to higher total organic P and microbial P relative to the upland soil, it was concluded that the floodplain soil buffers P in part through microbial immobilization (Chapter 2). However, the degree of P immobilization varied throughout the floodplain, which motivated further investigation of physicochemical parameters that may be influencing the process, namely organic C and inorganic N. The following objectives were subsequently developed: 1) to evaluate the influence of native leaf residue on P reaction dynamics as a function of the C composition of the residue and the soil organic C:P ratio and 2) to investigate if the P mineralization rates vary as a function of inorganic nitrogen (N) species. Separately, floodplain soils were amended with 1) three species of native leaf residue with varying C compositions and 2) nitrate (NO3-N) and ammonium (NH4-N). Phosphorus mineralization/immobilization dynamics (i.e., wet chemistry analysis and NMR spectroscopy) were examined during long-term laboratory incubations. In the C study, residues with low aromaticity promoted P mineralization, and residues with high aromaticity and hydrophobicity led to P immobilization (Chapter 3). In the N study, NH4-N was more effective at boosting phosphatase activity and P mineralization rates than NO3-N (Chapter 4). It was concluded that both C and N have a large influence on P mineralization and immobilization in floodplain soil. On one hand, the incorporation of vegetation that specifically produces litter with more recalcitrant C compounds could optimize soil P sequestration in constructed floodplain buffers and improve downstream water quality. On the other hand, excess inorganic N, especially NH4-N, could promote mineralization and reduce P sequestration. These conclusions should be considered in the management of floodplains, wetlands, and P rich agricultural soils to minimize the release of P to aquatic environments.
Soil Chemistry, Nutrient Cycling, Biogeochemistry, Phosphorus, Mineralization
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign