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 cultivation in these ecosystems 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 From two cultivated plots and one native grassland plot, 288 soil samples were collected over 7 and 4 months, respectively, at three depths. Samples were analyzed for soil N concentrations, mineral N stocks, and 16 edaphic properties. Using random forest and multiple linear regression (MLR), PTFs were developed incorporating edaphic and remote-sensing covariates. Key results Green onion cultivation altered mineral N distribution, with subsurface nitrate-N reaching 114 mg kg−1, compared to 1.76 mg kg−1 in native grassland. Monthly declines in mineral N stocks reached 75.8 kg N ha−1 in cultivated soils, nearly triple the losses in uncultivated soil. The MLR using edaphic properties performed best for total N and nitrate-N prediction (R2 = 0.80 and 0.64, respectively), while ammonium-N was poorly predicted. Conclusions Shifts in N dynamics were consistent with soil acidity neutralization associated with repeated chicken manure applications, fallow history, and soil texture. These conditions favored subsoil mineral N accumulation, increasing leaching risk. The developed PTFs demonstrate potential to inform site-specific assessments of soil N pools in páramos. Implications Future research should include additional N pools, microbial processes, and isotopic tracers to deepen understanding of N dynamics.