Spectroscopic Analysis of Canopy Nitrogen and Nitrogen Isotopes in Managed Pastures and Hay Land

Improving watershed nutrient budgets, ecosystem models, and our understanding of the impact of land-use management on ecosystem functioning depends on the development of remote sensing methods that can predict aspects of the nitrogen (N) cycle. This is particularly true for temperate managed grasslands, which constitute a large portion of agricultural land and, at times, export a significant amount of N to aquatic systems and the atmosphere. Although foliar N is often remotely sensed, we explore the use of spectroscopy to predict the foliar isotopic ratio of ¹⁵N to ¹⁴N, i.e., δ¹⁵N. Foliar δ¹⁵N has been shown in global surveys and site-specific studies to reflect N availability and the amount of N lost to the atmosphere. We built a data set of the canopy reflectance of plots in managed pastures and hay lands, which we then harvested for laboratory analysis. For the spectra of dried and ground samples, we calculated the normalized band depth (NBD) of three absorption features most likely to correlate with δ¹⁵N. In these data, foliar N and δ¹⁵N were not correlated, and we found weak, but significant, linear models with δ¹⁵N for the NBD of the 2100-nm feature known to relate to foliar N. The canopy spectra, which inherently reflect the vegetation structure, correlated better with δ¹⁵N than the spectra of dried and ground samples. These results suggest that near-term advances in estimating δ¹⁵N and aspects of pasture management style are likely to be related to, or to include, the quantification of the vegetation structure.