Plant Nitrogen Preferences Mirror Underground Nitrogen Cycling in Natural Ecosystems

Reliable prediction of plant nitrogen (N) acquisition strategies is critical for interpreting ecosystem productivity. We propose a process-based framework that connects plant N preferences to soil microbial N cycling and environmental conditions. We compiled data from 66 ¹⁵N labeling studies, yielding 336 triplet observations, each consisting of plant organic-N, ammonium-N, and nitrate-N uptake measurements (Dataset 1). Additionally, 2030 observations of gross soil N transformations from 270 studies were compiled to predict the spatial variation of these rates globally, with the aim of populating Dataset 1. We found that ammonium-N was the primary contributor to N uptake in forests (49% ± 1.84%) and wetlands (55% ± 3.29%), whereas nitrate-N was the dominant source in grasslands (41% ± 1.52%). Plant ammonium-N and nitrate-N preferences were lowest in temperate and tropical regions, respectively. Nitrification capacity-autotrophic nitrification (the process where ammonium is oxidized to nitrate) to gross N mineralization (GNM; the conversion of organic N to ammonium) ratio-was the main regulator of plant ammonium-N and nitrate-N preferences. Terrestrial environments with high nitrification capacity (e.g., temperate or grassland soils) resulting from high soil pH and low carbon-to-N ratio exhibited higher plant nitrate-N preference, while adverse conditions (e.g., tropical, forest, or wetland soils) exhibited higher ammonium-N preference. Interestingly, dissimilatory nitrate reduction to ammonium (DNRA) process redirected plant preference toward ammonium-based nutrition in organic carbon-rich, low-oxygen soils. Climate-driven shifts in plant N preference are mediated by gross soil N transformations, as increased precipitation and/or temperature accelerated GNM and/or DNRA while inhibiting nitrification capacity, promoting plant ammonium preference. Soil N cycling and environmental conditions explained little variation in plant organic-N preference, suggesting that other variables (e.g., mycorrhizal associations and plant functional traits) may be at play. We highlight that plant N acquisition is not purely plant-driven, but it mirrors underground N transformations, with environmental conditions acting as pivotal modulators of this relationship.