Organic matter and isotopic composition of soils under different management systems in western Paraná State, Brazil

No-till (NT) contributes to improving soil quality. This study aimed to quantify the organic carbon (C) content of light SOM fractions at sites under pasture, native forest, or NT for different periods and assess the influence of management systems on C and nitrogen (N) stocks and C/N ratio in a Ferralsol in Paraná State, Brazil. Soil samples were collected from the 0–5, 5–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm layers at sites managed under NT for 6, 14, or 22 years (NT6, NT14, and NT22, respectively); a site under NT management for 12 years, followed by 4 years of Brachiaria intercropping (NT12 + B); a permanent pasture (PP); and a native forest (NF) site. A density fractionation was carried in 0–5 and 5–10 cm layers and then were obtained the organic C contents in fLF (fLF-C) and oLF (oLF-C). The stocks of fLF-C, oLF-C and total C stocks in the light fraction were then calculated. It was also determined the natural abundance of 13C and 15N and the contents of C and N were used to calculate the C/N ratio of each layer. Management systems promoted changes in C dynamics in light SOM fractions, δ13C, δ15N, and C/N ratio. Differences in 13C isotopic composition were observed only at depths greater than 30 cm, with the NF site differing from the others (− 25‰ δ13C). For NT6, NT14, NT22, NT12 + B, and NF, δ15N values indicated isotopic enrichment with depth. At the PP site, there was a decrease in isotopic enrichment with depth, ranging from just over 12‰ δ15N in topsoil to 10‰ δ15N in the 60 cm layer. Multivariate analyses were used to group sites according to the analyzed variables. In the 0–5 cm layer, PP soil was associated with 15N isotopic composition, whereas NT6, NT14, NT22, and NT12 + B had a strong correlation with C/N ratio. NF showed a strong association with C contents and stocks in light fractions. In the 5–10 cm layer, PP had a strong correlation with δ13C and C/N ratio. For NF soil, there was a correlation with light fraction C stock, free light fraction C content and stock, and δ15N isotopic composition. In contrast to the other management systems, NT12 + B was strongly correlated with occluded light fraction C content and stock in the 5–10 cm layer. These results underscore the importance of evaluating labile SOM fractions when assessing the quality of anthropized environments and how management practices adopted over time can affect the SOM light fraction dynamics. The introduction of other crops, such as Brachiaria, during the production cycle becomes a solution for increasing C stocks in cultivated soils.