PS 68-146 - Utilization of watershed scale soil moisture and C/N dynamics: Possibilities for quantifying nitrogen cycling “hotspots”

Thursday, August 11, 2011
Exhibit Hall 3, Austin Convention Center
Peter Baas1, Jacqueline Mohan1, Daniel Markewitz2 and Jennifer D. Knoepp3, (1)Odum School of Ecology, University of Georgia, Athens, GA, (2)Warnell School of Forestry and Natural Resources, The University of Georgia, Athens, GA, (3)Coweeta Hydrologic Laboratory, USDA Forest Service Southern Research Station, Otto, NC

The high level of heterogeneity in nitrogen cycling hinders our ability to develop an ecosystem- wide understanding of these processes. In southern Appalachia, processes determining nitrogen cycling (redox, C/N dynamics) are highly dependent on elevation. In this study we focused on unraveling spatial and temporal patterns of soil moisture and C/N dynamics needed for ecosystem-level insight. Specifically, we focused on the following questions: 1) How do spatial and temporal patterns in soil moisture and C/N dynamics change over an elevational gradient in southern Appalachia? 2) How well do soil moisture and C/N dynamics explain nitrogen cycling hotspots? Sites (80 m x 80 m) distributed over an elevation gradient (788 m-1389 m) were sampled every other month from April 2010 through March 2011. Belowground soil moisture patterns were estimated at shallow (~0.5 m) and deep (~3.5m) depths using a non-destructive technique based on electromagnetic induction (EMI). Near infrared reflectance spectroscopy (NIRS) was used to determine soil organic carbon and nitrogen content over a depth gradient. In November 2010, cores (20 cm deep) were collected based on soil moisture and C/N dynamics for specific estimates of potential denitrification rates at 0-5, 5-10, and 10-20 cm using the acetylene block method.


Repeated measurements for conductivity using EMI, which are used to estimate soil moisture, had significant temporal and spatial variation. Differences in soil moisture estimates were mainly controlled by elevation and proximity to stream (p<0.05). Measurements taken before and after a storm event in May 2010 in the cove site reveal specific spatial patterns of subsurface flow into the stream. During the February sampling, higher elevation sites showed significantly greater (p<0.05) average soil moisture, most likely due to snow melt. The site at the highest elevation had significantly greater %C (p<0.05) and %N (p<0.05) than all other sites except the cove site. Overall, %N was highest at 0-5 cm depth (p<0.0001) and %C was highest (p<0.0001) at 0-5 compared to 10-20 cm.  While C/N ratios did not vary significantly between sites, they explained 17% of the potential denitrification rates (linear regression, p<0.05). Overall, EMI is a promising technique in tracing subsurface water flow paths and soil moisture patterns. However, coupling of soil moisture patterns with potential denitrification was weak suggesting  additional attributes (C/N dynamics, clay content, vegetation) must be incorporated into this model to explain observed variability in potential denitrification.

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