Despite its importance to plant growth and susceptibility to be leached out of most systems, large, stable pools of nitrate have been observed in tropical forests, savannas, and croplands. Anion exchange capacity is a mechanism proposed to explain how nitrate can accumulate without being leached out of the soil, but is rarely quantified or tied to nitrate sorption capacity. In this study, we measure anion exchange capacity, soil ammonium and nitrate concentrations, soil pH, and soil nitrate sorption capacity from 8m deep soil cores from the southeast Amazon on a gradient from native, intact forest to intensive single-cropped soy, and intensive double-cropped soy-maize sites. We combine these measurements to answer the following questions: how does soil inorganic nitrogen vary with soil depth and land use? Do these soils exhibit high anion exchange capacity, and if so, does it vary with depth or among land uses? What is the maximum nitrate sorption capacity of these soil profiles, and does it vary among land uses?
Results/Conclusions
We have found that soil nitrate varies with depth and increases in the cropping systems compared to the forest, there is net anion exchange capacity in soils under all land uses, and that the soils across all land uses have high nitrate sorption capacity. Conversion of forests to intensive agricultural systems is changing the regional N budget of the Amazon and Cerrado biomes, globally-important hotspot of biodiversity. We aim to determine for how much longer the soil’s physical properties will serve as a buffer to prevent reactive nitrogen losses from the large input of nitrogen to the region via nitrogen-fixing crops and nitrogen fertilizer.