Estoques de carbono e nitrogênio nas frações da matéria orgânica em argissolo sob eucalipto e pastagem

Abstract

The cultivation of eucalyptus in areas of pasture has resulted in changes of C and N storage in soil, mainly due to the different characteristics of each plant and their biomass management; the largest stock of C and N can be found under eucalyptus, originating from the biomass at the soil surface layer and increasing the C/N in decomposition of plant residues. These differences may cause a decrease in the storage of C and N fractions in the chemical, physical and biological soil, both in the more labile, an the more recalcitrant fractions. This study aimed to evaluate the stocks of total organic carbon (TOC) and total N (TN), the stock of C and N in humic substances (fulvic acids, humic and humin), the light fraction (C-MOL and N-MOL) and the microbial biomass (C-BM and N-BM) in samples of collected from the layers 0-0.10, 0.10-0.20, 0.20-0.40, 0.40-0.60 and 0.60-1.00 m in depth, line (EL) and interline (EEL) from the soil under eucalyptus in the fourth rotation with six years of age, and from an area of cultivated pasture with Brachiaria (P). The total organic carbon stock and carbon in the light fraction in the first layers of soil (0.60 m) was higher in the interline of eucalyptus compared with that from the cultivated pasture and the line of eucalyptus, probably favored by harvest residue from the previous cycle of eucalyptus buried in the current interline. By determining the natural abundance of 13C in the 0-0.20m soil, it was found that 29% and 37% of carbon present in the soil organic matter (SOM) after 28 years of cultivation in the line and interline, respectively, was derived from eucalyptus (C 3 plants), which corresponded to an average rate of cycling of SOM in this soil of 1.04% (line) and 1.32% (interline) per year. In the soil interline it was observed that the C-C 3 from the eucalyptus increased the stock of soil organic C, even with the replacement of C-C 4 from the pasture, after 28 years of eucalyptus cultivation. For nitrogen in humic substances, a similar stock was obtained for the soil of the interline of eucalyptus and that of the pasture, which was higher than that of the soil line of eucalyptus, up to 1 m deep. The soil of eucalyptus (EL and EEL) showed a stock of C and N in microbial biomass similar to that of cultivated pasture.