Forms and Amounts of Soil Nitrogen and Phosphorus Across a Longleaf Pine–Depressional Wetland Landscape

Abstract

Forms and amounts of soil N and P were measured across transects from freshwater depressional wetlands into longleaf pine–wiregrass forests of southwestern Georgia to evaluate changes in labile vs. recalcitrant N and P and C:N:P ratios across drainage gradients. Plant‐available NO−3–N (3.7 μg cm−3) and organic N (2000 μg cm−3) were significantly greater in wetland than upland soils (NO3–N = 0.03 μg cm−3, organic N = 890 μg cm−3) and C:N increased from wetlands (17:1) into uplands (43:1). Forms and amounts of P were not as strongly related to landscape position as N. Labile organic P (Po, 2.6 μg cm−3) was significantly greater in wetland than upland soils (0.88 μg cm−3). Recalcitrant organic compounds accounted for 95 to 97% of the N and 50 to 82% of the P stored in wetland and upland soils. Wetland soils stored a disproportionately large share of N as compared with upland soils even though soil organic matter (C) content was uniform across the landscape. Landscape position (drainage, degree of wetness) is an important determinant of nutrient retention in sandy soils of the southeastern coastal plain. Periodic waterlogging favors sequestration of biological (organic) forms of N and P with proportionally greater storage of N relative to P. Soil waterlogging by promoting accumulation of N more than P favors a shift from N limitation in upland soils towards P limitation in wetland soils of the southeastern coastal plain.