Plant Nitrogen Preferences Mirror Underground Nitrogen Cycling in Natural Ecosystems

ABSTRACT 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 15 N 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.