Changes in soil C and N stocks and nutrient dynamics 13 years after recovery of degraded land using leguminous nitrogen-fixing trees

In tropical forest areas with highly weathered soils, organic matter plays an important role in soil functioning and forest sustainability. When forests are clear-cut. the soil begins almost immediately to lose organic matter, triggering a series of soil degradation processes, the extent and intensity of which depends on soil management. Depending on the level of soil degradation, the rate at which the system can re-establish itself can be slow and may require the use of degraded land restoration techniques. This study aimed at evaluating the potential of pioneer leguminous nitrogen-fixing trees to recuperate degraded land. The area studied - located in the coastal town of Angra dos Reis in the State of Rio de Janeiro, Brazil - was planted with seven species of fast-growing leguminous nitrogen-fixing trees in 1991. The nutrient concentrations (Ca, Mg, P and K) and N and C stocks in the soil and litter were determined, in addition to the free- and occluded-light fractions of soil organic matter. Soil samples were also collected from two reference areas: (1) an area of undisturbed native forest; and (2) a deforested area spontaneously colonised by Guinea grass (Panicum maximum). The nutrient stocks in the litter of the restored area were similar to those found in native forest. The recuperation technique used was able to re-establish the soil C and N stocks after 13 years. C and N increased by 1.73 and 0.13 Mg ha(-1) year(-1), respectively. The free-light fraction was highest in the recuperated area and lowest in the deforested area. The occluded-light fraction of the recuperated area was higher than that of the native forest only in the 0-5 cm layer. Both the free-light and occluded fractions were higher in the native forest and recuperated areas than in the deforested area. Since the free-light and the occluded-light fractions are the result of litterfall and decomposition, these results - combined with the data of litter stocks and soil C and N stocks - indicate that the use of legume trees was efficient in re-establishing the nutrient cycling processes of the systems. These results also show that recovering degraded land with this technique is effective in sequestering carbon dioxide from the atmosphere at high rates. (C) 2007 Elsevier B.V. All rights reserved.