Globally Mapping the Nitrogen Stable Isotope Ratios of Terrestrial Vegetation From 1984 to 2022

Nitrogen cycles control the structure, function, and composition of ecosystems globally. Despite their importance, our understanding of long-term changes in global nitrogen cycles remains limited. The foliar nitrogen stable isotope ratio (δ¹⁵N) serves as a valuable metric for assessing changes in nitrogen cycling and potentially in plant nitrogen availability. However, existing observations of δ¹⁵N suffer from spatial bias and temporal discontinuity with contradictory findings across biomes, hindering our ability to detect and attribute drivers of change. Leveraging ground-based observations as our calibration source, we derived annual maps of foliar δ¹⁵N spanning from 1984 to 2022 globally from Landsat spectra. We found that the Landsat-derived δ¹⁵N effectively captured the observations, with an R² of 0.77 and a normalized root mean square error of 0.15. Globally, we found widespread temporal changes in δ¹⁵N with significant decreases for 44% and increases for 16% of vegetated ecosystems. Foliar δ¹⁵N mostly declined in forest ecosystems but increased in non-forest land cover types. Gross primary productivity and its trend consistently explained spatiotemporal variation of δ¹⁵N globally, indicating increasing plant demand could lead to decreasing δ¹⁵N. Our study presents an innovative approach to effectively monitor and track potential changes in global nitrogen cycles over the past four decades, setting the stage for more impactful management and conservation strategies.