PS 68-150 - Isotopic evidence for shifts in N cycling across rain/snow transitions in the Sierra Nevada

Thursday, August 11, 2011
Exhibit Hall 3, Austin Convention Center
Sara K. Enders and Benjamin Z. Houlton, Land, Air and Water Resources, University of California, Davis, Davis, CA

Understanding how climate-driven changes in the form of precipitation (rain vs. snow) affect nitrogen (N) cycling processes is important to predicting future changes in carbon (C) and nutrient cycles of forest ecosystems. Natural abundance N stable isotopes of leaves (δ15NF) have been used as integrators of ecosystem N cycles, with higher values reflecting greater N availability and isotopic expression of key N cycle processes. Here we use δ15NF of trees to understand how a rain-snow transition across an elevation gradient affects forest N cycles. Models of climatic controls on δ15NF predict a linear function of mean annual precipitation (MAP) and temperature (MAT) within the biosphere, but have not considered precipitation type. We hypothesize that the rain:snow ratio will affect dominant processes in the N cycle at ecosystem to regional scales, manifesting coherent changes in δ15NF.

This study is located at the Southern Sierra Critical Zone Observatory, within the Kings River Experimental Watershed of the U.S. Forest Service. Two watersheds were chosen between which parent material, aspect, and precipitation amounts and timing are similar, but in which the lower site (1730-1990 m, MAT of 11.3±0.8°C) receives both rain and snow (35 to 60% of precipitation as snow), while the upper site (2185-2490 m, MAT of 7.8±1.4°C) is snow-dominated (75 to 90% as snow). Leaves were collected from 22 individuals in each watershed. δ15N, δ13C, N contents and other measures of soil and stream N concentrations were analyzed.


The two watersheds have significantly different δ15NF (p = 0.0002740) and δ13CF (p =2.428e-05), with the upper watershed enriched in 15N by 1.2‰ and in 13C by 1.7‰ (n=44). Leaves from the upper watershed show 0.23% higher average N content (p= 0.01921). The C isotope data indicate decreased stomatal conductance at the upper site, likely due to a greater leaf-to-air humidity gradient. The higher efficiency of N uptake per mole of water lost indicated at the upper site may affect δ15NF. The N isotope data suggest that isotope-fractionating processes are more important in the snow-dominated forest, likely owing to accelerated rates of N cycling. Models of δ15NF as a function of temperature and precipitation derived from global climate data would predict the opposite trend: more negative δ15NF values in the colder watersheds. This implies that such models, while useful in identifying large-scale gradients in δ15N, may miss regional changes in δ15N and the dynamics of N cycling associated with climate changes.

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