Joint control of precipitation and CO2 on global long-term patterns of plant nitrogen availability.

Plant nitrogen availability critically limits plant growth and thus constrains terrestrial carbon sequestration. Growing evidence points to a recent decline in plant nitrogen availability, yet the global consistency of this trend and its underlying drivers remain unclear. Here, we reconstruct the spatiotemporal trajectory of plant nitrogen availability (1980-2020) using four machine-learning algorithms applied to a global database of 37,268 foliar stable nitrogen isotopes measurements and ancillary climate data. We first find high spatial heterogeneity in foliar stable nitrogen isotopes, primarily linked to mean annual temperature. Then, we discover that foliar stable nitrogen isotopes declined primarily during the 1980s, and subsequently remained stable across 44% of land areas. Moreover, the dominant driver in temporal variations of foliar stable nitrogen isotopes shifts from CO2 (1980-1988) to precipitation (1989-2020). These patterns collectively reveal divergent plant nitrogen availability trajectories and highlight the increasing role of precipitation in shaping terrestrial nitrogen cycles over the past decades.