Enzymatic activity and organic carbon contents under different land uses in Neosols (Leptosols and Regosols) of the Sul-Riograndense Shield, Southern Brazil

Soil organic matter (SOM) stability is primarily controlled by mineral associations and physical protection within aggregates. In sandy loam Neosols, these mechanisms are limited, making biological attributes essential for understanding SOM dynamics and stability. Indicators such as permanganate-oxidizable carbon (POXC), fluorescein diacetate hydrolysis (FDA), and the enzymes arylsulfatase and β-glucosidase are sensitive to land-use changes. This study evaluated the effects of land use on SOM and microbial activity in Neosols. Surface soil samples from 17 sites were grouped into native grassland, cropland, orchards, cropland/orchard systems, and native forest. Mean contents of total organic carbon (TOC), FDA, POXC, arylsulfatase, and β-glucosidase were determined, and Pearson correlation analyses were performed among variables. TOC and FDA decreased with increasing land-use intensity, with the highest values observed under native forest (37.86 g kg⁻¹,13.70 µg fluorescein mL⁻¹) and the lowest under cropland (14.00 g kg⁻¹, 7.58 µg fluorescein mL⁻¹), indicating a progressive loss of functional complexity and SOM stability. Arylsulfatase was the most sensitive indicator of land-use change, showing maximum activity under native forest (2,331.29 µg p-nitrophenol g⁻¹ h⁻¹), intermediate values in grasslands and orchards (661.90–882.29 µg p-nitrophenol g⁻¹ h⁻¹), and marked reductions under cropland (14.00 g kg⁻¹, 7.58 µg fluorescein mL⁻¹). The strong correlation between FDA and TOC (r=0.9275, p=0.0232) indicates that SOM content controls the intensity of biological activity in these soils. POXC showed significant (p=0.0208) and strong correlations with arylsulfatase (r=0.9324) and β-glucosidase (r=0.9326), demonstrating that the labile carbon fraction represents the main metabolic substrate for soil microorganisms.