Dominance, drivers and thresholds of DEN, ANA, and DNRA quantified using a global dataset of cross-ecosystems and machine-learning

Nitrogen retention and loss in the critical zone are regulated by denitrification (DEN), anaerobic ammonium oxidation (ANA) and dissimilatory nitrate reduction to ammonium (DNRA), yet their global partitioning remains uncertain. A global dataset from 72 studies reporting DEN, ANA and DNRA rates plus environmental variables and functional genes was analyzed using random forests, generalized additive models and mixed-effects gene-rate analyses. Across ecosystems, contributions followed DEN (66.3%) > DNRA (21.9%) > ANA (11.8%). Wetlands showed consistently elevated DEN rates (mean 11.35 nmol N g⁻¹ h⁻¹) and the strongest ANA activity (mean 1.39 nmol N g⁻¹ h⁻¹), supporting moisture-rich systems as hotspots for DEN and ANA whereas highlands showed the highest relative DNRA (37.8% contribution; 6.89 nmol N g⁻¹ h⁻¹). Seven principal components explained 63.4% of the variance, highlighting critical zone organic matter pools, anaerobic metabolic intensity, and critical zone moisture and nutrient availability as the main environmental controls. Random forest models indicated that NO₃⁻ is the primary predictor for DEN and ANA (together with moisture and temperature/carbon supply), while DNRA is driven more strongly by NH₄⁺ (together with precipitation and temperature). DEN and ANA initially related to moisture negatively then positively at inflection points of about 15%. All processes were associated with hzsB, but correlations were weaker for DNRA. Together, these findings fill a key gap by providing a global, cross-ecosystem synthesis that jointly quantifies the rates, drivers, thresholds, and functional gene-rate associations of DEN, ANA, and DNRA.