Patterns and drivers of nitrogen availability in floodplain soils

Background/Question/Methods

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.

Results/Conclusions

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.