Effects of land use on nitrogen stocks and humic fractions in a Plinthosol of the Brazilian Cerrado

The replacement of native vegetation may alter soil organic matter (SOM) quality and nitrogen (N) dynamics, because N is one of the main constituents of SOM. This study evaluated N content and stocks in humic fractions across different soil layers (0.00–0.50 m) of a Plinthosol in Tocantins State, Brazil, under different land uses: native Cerrado forest (control), eucalyptus, agriculture, and pasture. Soil samples were collected from five layers, and N content in the humic fractions, including fulvic acid (FA), humic acid (HA), and humin (Hum), was determined after chemical fractionation and dry combustion; N stocks were then calculated for each soil layer and for the entire profile. Total carbon (TC) content and stocks were also determined to contextualize N dynamics in SOM. Data were analyzed using analysis of variance (ANOVA), Tukey’s test, Cohen’s d effect-size estimates, and principal component analysis (PCA) to assess differences among land uses and identify multivariate patterns. The eucalyptus land use showed the highest TC stocks and the greatest N accumulation in humic fractions, especially in the upper 0–0.40 m. Agricultural land use increased N-FA and N-Hum content and stocks but reduced N-HA relative to the control, whereas pasture reduced N-FA and N-Hum content and stocks while maintaining relatively stable N-HA. Across all land uses, N-Hum was the dominant N reservoir, and humic substances accounted for approximately 59–74% of total soil N stocks. PCA indicated that land use was the main factor associated with N distribution among humic fractions, whereas soil depth had a secondary but consistent effect. These changes indicate that SOM quality was affected by land-use conversion, particularly under more intensively managed or degraded systems, suggesting that improved soil management strategies are needed to preserve N retention and humification processes in converted Cerrado landscapes.