The depolymerization of soil organic matter to low molecular weight organic sources of nitrogen (N) is critical to N cycling and plant N nutrition. The physical environment exerts major control over the rate of depolymerization by affecting the kinetics of enzymatic reactions that affect the form and quantity of available N. Theoretically, organic N availability and uptake should be greatest in cold ecosystems where decomposition rates are slow, and lowest in warm ecosystems where rates of mineralization and nitrification are high. In New England, mountainous landscapes enable us to study the effects of temperature on the availability and form of N taken up from the soil. In this study we used natural abundance δ¹⁵N signatures of foliar tissue and bulk soil to understand organic N uptake along an elevation gradient in the White Mountains, NH, USA. We complemented stable isotope analyses with measurements of rates of organic and inorganic N cycling. We hypothesized (1) that rates of N cycling would decline with elevation, (2) that soil amino acid availability would increase with elevation, and (3) that plant dependence on organic forms of N would increase with increasing elevation, and be reflected in decreasing foliar δ¹⁵N relative to bulk soil δ¹⁵N.  

Results/Conclusions

We found that N availability (organic and inorganic forms) peaked at mid elevation. Foliar δ¹⁵N relative to bulk soil δ¹⁵N increased significantly with increasing elevation (r²=0.948), suggesting decreasing dependence on organic sources of N along the elevation gradient. Our results contradict several other studies showing increasing depletion of foliar δ¹⁵N across broad latitudinal gradients from temperate to arctic ecosystems. N deposition and precipitation increase with elevation in the northeast and both may be driving the observed δ¹⁵N pattern. N deposition increases nitrification rates which fractionates against ¹⁵N and precipitation increases leaching losses of available pools dominated by ¹⁴N effectively enriching the plant available pool of N in ¹⁵N. Measurements of sporocarp natural abundance δ¹⁵N and the results of a double label 13C/15N uptake experiment suggest that plant dependence on organic N sources cannot be explained by temperature and soil N availability in the White Mountains, NH.