COS 87-10 - Patterns and drivers of nitrogen availability in floodplain soils

Thursday, August 11, 2011: 11:10 AM
Ballroom F, Austin Convention Center
Alison Appling, Nicholas School of the Environment, Duke University, Durham, NC, Emily S. Bernhardt, Department of Biology, Duke University, Durham, NC and Jack A. Stanford, Flathead Lake Biological Station, Division of Biological Sciences, The University of Montana, Polson, MT

Forest stands of contrasting ages are commonly juxtaposed within floodplains, a consequence of the spatial heterogeneity of flood disturbances. Flooding and ecological succession also drive wide variation in soil nitrogen (N) cycling across floodplain landscapes. We hypothesized that N pools and fluxes could be explained primarily by years since scouring (stand age) and secondarily by the quantity and N content of deposited soil and the composition of the plant community. For each of 55 plots within a 15 km2 alluvial floodplain in northwest Montana, we estimated the stand age using tree rings from the largest trees in that 400 m2 plot. We then investigated the dependence of soil N pools (NH4+, NO3- + NO2-, and total N) and fluxes (N fixation, denitrification, and N mineralization) on stand age and other plot-specific characteristics.


During succession, dominant vegetation transitioned from cottonwood seedlings to mature cottonwoods to mixed cottonwoods and spruces to mature spruces. From recently disturbed gravel bars to 150-year-old spruce stands, total soil N at 0-15 cm ranged from <25 to >400 g N m-2 (<150 to >2100 µg N [g sieved soil]-1), a mean accumulation rate of 9.3 µg N [g soil]-1 yr-1 (r2=0.38; p<0.0001). Thickness of cobble-free soil increased at ~2 cm yr-1 with stand age (r2=0.25; p<0.001). Despite the variable N concentrations of newly deposited sediments (70-990 µg N [g soil]-1), soil thickness explained an additional 7% of variation in soil nitrogen after stand age (p<0.01). Midsummer soil N fixation rates (0-100 µg N m-2 hr-1) exceeded denitrification rates (rarely >4 µg N m-2 hr-1) and peaked in the mature cottonwood stage of succession. While alders grew in only 16 of 55 plots, herbaceous legumes were found at most early-to-mid-successional plots and may be more consistent N sources in this floodplain than in others described in the literature. Our results suggest that patch-scale differences in hydrology and vegetation may indeed contribute to spatial patterns in N accumulation rates across the floodplain.

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