PARENT SESSION
Monday, August 7, 5:00-6:30 pm
Poster Session 6 - Biogeochemistry and nutrient cycling
Exhibit Hall, Ballroom Level, Cook Convention Center

Nutrient cycling interactions of rhizodeposition and water flow along roots in a temperate forest.

Thorsos, Eileen^*,1, Jackson, Robert¹, ¹ Duke University, Durham, NC

ABSTRACT- Water percolates heterogeneously through soil, following stable preferential flow paths. Plant roots create macropores that guide many such flow paths. Flow paths also have higher carbon (C), nitrogen (N), and microbial biomass than bulk soil. Plants also release organic C to the rhizosphere, the soil zone that live roots physically and chemically modify. Rhizosphere and flow path soil do not always overlap. Rhizodeposition can prime rhizosphere microbes to degrade organic matter, and water speeds decomposition. Phosphorus (P) is less mobile than N, and the P depletion zone typically extends only a few millimeters from roots. Soil regions with higher P concentrations, solubilization rates, or mineralization rates may be attractive rooting zones for plants. In this study, we examine water flow heterogeneity and phosphorus distributions in a temperate coniferous forest on sandy loam soil and clay loam soil (Duke Forest, NC) by applying 1.5 mm of Brilliant Blue FCF food dye to distinguish flow path and non-flow path soil. We identified rhizosphere soils in and out of flow paths. Initial data indicate that the dye does not significantly alter extracted phosphate or microbial C or N (p>0.05). In the sandy loam soil, E horizon (5 to 30 cm) extractable PO4- levels are 3 to 10 times higher than A horizon (0 to 5 cm) PO4- levels. In the A horizon, flow path PO4- levels are higher than non-flow path PO4-. Conversely, in the E horizon, non-flow path PO4- levels are higher than flow path PO4-. Non-rhizosphere soil has marginally higher PO4- than rhizosphere soil, potentially from plant uptake, but these data as a whole indicate that flow paths contribute more substantially to soil phosphate heterogeneity than do rhizospheres. Further, flow path depth is greater in clay loam soil (mean = 17 cm) than sandy loam soil (mean = 6.3 cm). Due to flow path effects on nutrient distributions, we may observe more nutrient heterogeneity in structured clay soil than unstructured sandy soil. In the top 30 cm of the clay loam soil, 70% of roots grow in flow paths, while in the sandy loam soil only 25% of roots are in flow paths. We may observe a positive interaction between flow paths and rhizospheres on C, N, and P mineralization rates, but soil type may influence plant access to flow path soil and interactions between flow path and rhizosphere chemistry.

Key words: phosphorus cycling, temperate forest, biogeochemistry