Soil fertility, humic fractions and natural abundance of 13C and 15N in soil under different land use in Paraná State, Southern Brazil

Humic fractions of soil organic matter (SOM) and measurements of 13C and 15N isotope can be used to highlight differences between management systems with different intensities of land use. This study characterized soil fertility, quantified carbon levels in the humic fractions and evaluated the natural abundance of 13C and 15N in systems cultivated under no-tillage system (NTS) and conventional tillage system (CTS) or used with secondary forest or perennial pasture in Marmeleiro, Parana State, Southern Brazil. NTS was more efficient than the conventional tillage system (CTS) in increasing pH (0.0-0.10 m layer), Ca (0.0-0.05 m layer), P (except 0.05-0.10 m layer) and N (0.0-0.10 m) levels, total organic carbon (TOC) stocks (0.0-0.20 and 0.0-0.40 m layers); carbon of the humin fraction (C-HUM) in 0.0-0.40 m; the fulvic acid fraction (C-FAF) and humic acid (C-HAF) in 0.0-0.05 m. The use of grasses, in NTS and pasture, increased TOC stocks compared to the other soil use or management systems evaluated in the 0.0-0.40 m layer. In the topsoil layer, the anthropogenic influence of plowing and harrowing in CTS promoted greater loss of carbon in C-HUM, C-FAF and C-HAF than NTS, forest and pasture. In CTS, growing corn for 42 years after the removal of forest cover did not alter the 13C at 0.0-0.40 m. In pasture, the absence of legumes, constant deposition of cattle manure and a more stable organic matter favored high 15N levels (except at 0.0-0.05 m in CTS). The decrease in 15N values from the 0.0-0.10 to 0.10-0.20 m layer in CTS indicates that soil turnover (by plowing and harrowing) has the potential to disturb the depth-related variation in soil 15N, accelerating decomposition and compromising N transformations. Among the variables analyzed, the determination of carbon in humic fractions and 15N values were efficient in identifying soil changes produced by land use or management systems.