Marandu palisadegrass pasture management intensity and relation to soil organic carbon fractions

Human interference in natural ecosystems can increase or decrease the soil organic carbon (SOC). This occurs because the humic substances can be produced or destroyed in soil depending on the biomass production (quality and quantity) and litter return to soil, which are directly affected by grazing intensity and environmental conditions. The present study evaluates the organic carbon (OC) in soil and in the humic fractions, obtained by chemical separation, to study the effect of grazing intensities in Marandu palisadegrass pastures. The experimental area is located in UNESP, Jaboticabal, SP. Grazing intensities corresponded to the sward heights of 15, 25 and 35 cm maintained by beef steers under continuous stocking for seven years. The fractions fulvic acid (FA), humic acid (HA), and humin (HU) were separated considering the solubility in alkali and, or, in acid, using 0.1 mol L^-1 NaOH and HCl, respectively. The OC in both soil and fractions were determined by wet oxidation method. Data were submitted to variance analysis considering the effects of pasture height, soil depth and interaction using proc mixed from SAS, and the comparisons were made using orthogonal polynomial contrasts (P < 0.05). As expected, OC in the soil decreased with depth. The effect of pasture height was significant in 0-5 cm and 5-10 cm, being observed a quadratic effect, and no effect in the 10-20 cm. Higher values for the 35 cm pasture height in the 0-5 cm represent an increase of 46% in relation to other heights. This same tendency observed for OC in the soil was observed for HU, since it represents the major part of total soil organic matter. A quadratic effect for pasture height was also observed for FA, HA only in the 0-5 cm, and for sum of FA and HA in 0-5 and 5-10 cm. FA represent 86% of the sum of FA and HA in the 15 and 25 cm pasture heights, and 68% in the 35 cm pasture height. The results obtained herein show an effect of grazing intensities in relation to distribution of chemical fractions of soil organic matter. It is important to notice that, besides the higher value for 35 cm in total SOC and humin fraction, higher values for the sum of FA and HA (less humidified fractions that present higher cation exchange capacity) were registered for the intensities of 15 and 35 cm.