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

Abstract 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 (δ 15 N) serves as a valuable metric for assessing changes in nitrogen cycling and potentially in plant nitrogen availability. However, existing observations of δ 15 N 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 δ 15 N spanning from 1984 to 2022 globally from Landsat spectra. We found that the Landsat‐derived δ 15 N effectively captured the observations, with an R 2 of 0.77 and a normalized root mean square error of 0.15. Globally, we found widespread temporal changes in δ 15 N with significant decreases for 44% and increases for 16% of vegetated ecosystems. Foliar δ 15 N mostly declined in forest ecosystems but increased in non‐forest land cover types. Gross primary productivity and its trend consistently explained spatiotemporal variation of δ 15 N globally, indicating increasing plant demand could lead to decreasing δ 15 N. 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.