Legacy of Land Use: Deep Soil Nitrogen Accumulation Driven by Deforestation and Agriculture in the Southeastern Amazon

ABSTRACT The southeastern Amazon region represents one of the critical hotspots of land‐use change (LUC) in Brazil. Conversion of natural ecosystems to agricultural fields triggers mineralization of soil organic matter and decomposition of residual forest biomass, releasing large amounts of nitrogen (N) that, together with subsequent high N‐fertiliser applications, disrupt N biogeochemical cycles. Therefore, we hypothesize that deforestation disrupts surface soil N‐pools, causing their gradual percolation into deeper layers of these highly weathered tropical soils. To test this, we measured total‐N and key fractions—particulate organic matter N (POM‐N), mineral‐associated organic matter N (MAOM‐N), water‐extractable N (WEN), ammonium (NH 4 + ‐N) and nitrate (NO 3 − ‐N)—to 800 cm depth in forest, recently deforested, single‐crop soybean and double‐crop soybean–corn systems in Mato Grosso, Brazil. Compared to the forest, the deforested site showed a 40% reduction in soil total‐N stocks at 10 cm; however, it was 62% higher at depths of 100–300 cm. In the croplands, the soil total‐N stocks remained similar to those in the forest. The POM‐N was nearly absent in the deforested site, whereas the MAOM‐N was consistently lower in both croplands and deforested soils than in the forest. The deforested site had the highest WEN and inorganic‐N (NH 4 + ‐N and NO 3 − ‐N) stocks to 800 cm, with most concentrated in the top 0–200 cm. Croplands had the greatest NO 3 − ‐N stocks at 300–600 cm. Principal component analysis (PCA) showed that WEN and inorganic‐N in the top 100 cm clustered with the deforested site, whereas POM‐N, MAOM‐N and total‐N at the surface clustered with the forest. Below 100 cm, the soil total‐N stock was clustered with the deforested site. These results provide critical insight into the initial mechanism of deep nitrate formation: the breakdown of the N biogeochemical cycle after deforestation, fueled by decomposition and N mineralization from soil organic matter and remaining forest biomass following slash‐and‐burn events on the Amazon–Cerrado agricultural frontier. However, further studies are needed to confirm the spatial and temporal persistence of these patterns across the region.