Carbon and nitrogen in degraded Brazilian semi-arid soils undergoing desertification

Although the effects of grazing exclusions on the recovery of soil properties have been extensively reported, few studies have addressed the different soil organic matter (SOM) fractions in desertification threatened areas from tropical regions. The objective of this work was to evaluate the effect of overgrazing (Og) and grazing exclusion (Exc) on SOM content and its pools in a desertification nucleus of the Brazilian semi-arid region. Six experimental plots with two treatments (Exc and Og) were studied. Soils from Exc plots showed higher total organic carbon (TOC: 4–37%) and total N (TN: 1–29%) contents when compared to soils from Og plots. At the Exc plots, the OC (organic carbon) contents in the light, labile and humin fractions were 38, 29 and 36% greater, respectively. The N contents at the Exc areas were greater in the light and humin fractions. The Carbon Management Index (CMI) was also greater in all Exc areas, ranging from 16 to 49%. Despite the low CMI values (<100) found for both treatments, indicative of highly degraded soils, Exc increased CMI in ∼20%. Although the period of seven years seems to be insufficient for large TOC and TN accumulations, the results indicate that exclusion may be an important management strategy for the recovery of desertified lands in Brazilian semi-arid regions.