Isotopic evidence for shifts in N cycling across rain/snow transitions in the Sierra Nevada

Background/Question/Methods

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 (δ¹⁵N_F) 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 δ¹⁵N_F of trees to understand how a rain-snow transition across an elevation gradient affects forest N cycles. Models of climatic controls on δ¹⁵N_F 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 δ¹⁵N_F.

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.

Results/Conclusions

The two watersheds have significantly different δ¹⁵N_F (p = 0.0002740) and δ¹³C_F (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 δ¹⁵N_F. 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 δ¹⁵N_F as a function of temperature and precipitation derived from global climate data would predict the opposite trend: more negative δ¹⁵N_F 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.