The effect of fire and overgrazing disturbes on soil carbon balance in the Dry Chaco forest

It is usually assumed that much of CO2 released into the atmosphere results from soil degradation in tropical and subtropical land, particularly from deforestation and conversion of forest into cropland and cultivated pastures. Accordingly, a considerable research effort has been devoted to the understanding of soil carbon balance in these ecosystems. By contrast, little attention has been given to disruption of carbon soil balance in arid regions, which is mostly produced by overgrazing and occasionally fire. We compared soil carbon balance (CO2 production in relation to soil and litter organic carbon) in areas under varying degrees of disturbance by overgrazing and fire in the Dry Chaco woodland. Litter and soil were sampled in both dry and wet seasons. Soil carbon under undisturbed conditions was relatively constant throughout the year (range 23-24 g kg−1), whereas at sites both burned and grazed carbon values fluctuated markedly (range 31-21 g kg−1). In non-grazed, burned areas soil carbon content increased (16%), whereas at burned and overgrazed sites it decreased (38%). Our results suggest that overgrazing has a more significant, adverse effect on soil carbon balance than fire when both factors act separately. Burned but ungrazed areas appear to show a tendency to recover the initial carbon balance, whereas in chronically overgrazed sites there is a permanent tendency to carbon loss.