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
¹⁵
N to
¹⁴
N, i.e., δ
¹⁵
N. Foliar δ
¹⁵
N 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 δ
¹⁵
N. In these data, foliar N and δ
¹⁵
N were not correlated, and we found weak, but significant, linear models with δ
¹⁵
N 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 δ
¹⁵
N than the spectra of dried and ground samples. These results suggest that near-term advances in estimating δ
¹⁵
N and aspects of pasture management style are likely to be related to, or to include, the quantification of the vegetation structure.