Nitrogen pools in páramo soils under green onion cropping: vertical distribution, prediction, and management implications

Context. High-mountain páramos provide essential hydrological services across the Andes.However, green onion (Allium fistulosum) cultivation in this region has led to nitrogen (N) losses, compromising water quality. Aims. This study aimed to improve understanding of soil N dynamics in páramo agroecosystems by analyzing the vertical and temporal distribution of total and mineral N in páramo soils, and by developing pedotransfer functions (PTFs) to predict these pools. Method. 288 soil samples were collected from two cultivated plots and one native grassland plot over seven and four months, respectively, at three depths. We measured soil N concentrations, mineral N stocks, and 16 edaphic properties. PTFs were developed using Random Forest and multiple linear regression (MLR), with edaphic and remote sensing covariates. Key results. Green onion cropping altered mineral N distribution, with subsurface N-NO 3 -reaching 114 mg kg⁻¹, compared to 1.76 mg kg⁻¹ in native grassland. Monthly declines in mineral N stocks reached 75.8 kgN ha⁻¹ in cultivated soils, nearly triple the losses in uncultivated soil. MLR using edaphic properties performed best for TN and N-NO 3 -predictions (R 2 = 0.80 and 0.64, respectively), while N-NH 4 + was poorly predicted. Conclusions. N dynamics was shaped by the neutralization of soil acidity from repeated chicken manure applications, with additional influence from fallow use and soil texture. This management favors subsoil mineral N accumulation, limiting plant uptake and increasing leaching risk. PTFs can guide improved nutrient management. Implications. Future research should include additional N pools, microbial processes, and isotopic tracers to deepen understanding of N dynamics.