Nitrogen mineralization potential depletion in pampas (Argentina) croplands following conversion from native grasslands

Abstract

In the Argentinean Pampas, where Phaeozems soils are predominant, the conversion of native grasslands into croplands has negatively affected soil fertility and health. Nitrogen (N) mineralized during an anaerobic incubation (N_an), an estimator of N potential mineralization, is frequently used to evaluate soil health. The objectives of our study were to determine the magnitude of the N_an depletion caused by agriculture (dN_an), and to assess the relative contribution of land use, climate, and soil properties on dN_an in the Argentinean Pampas. We conducted a soil survey at 409 sites distributed throughout the Argentinean Pampas region. At each site, pairwise samples (0–20 cm) were collected from uncultivated and agricultural soils. At each site, different edaphic, climatic, and productive variables were evaluated. The direct and indirect effect of different variables over dN_an was evaluated through a path analysis. The conversion of native grasslands from the Argentinean Pampas into croplands generally decreased N_an values, with N_an reductions ranging from 0 to 216 mg kg⁻¹ (representing 0 to 79 % of the N_an value at uncultivated soils). The magnitude of the N_an depletion was influenced by edaphic variables [i.e. original N_an value (N_an at uncropped soils), soil organic matter depletion, clay content] and climatic variables (i.e. precipitation). These four variables explained 85 % of the dN_an variability. From the path analysis, it was determined that soils with greater original N_an values (r = 0.89) and greater soil organic matter depletion (r = 0.65) presented larger dN_an, while increases in clay content (r = 0.30) or mean annual precipitation (r = 0.25) resulted in lower dN_an values. The identification of the drivers of N_an depletion can aid in the classification of areas with greater vulnerability to soil degradation and in the development of models to predict N_an depletion in soils. Such models would hold significant potential as tools for policymakers and program developers engaged in the formulation and implementation of targeted interventions aimed at halting or reversing this degradation process.