**4. Effect of cropland conversion to tropical pastures on soil carbon**

Cropland conversion to pastures has recently become a common practice and is believed to have considerable potential to store significant quantities of additional SOC [19, 38, 80]. For example, Conant et al. [33] and Conant [81], reviewed studies worldwide and concluded that cropland conversion to grasslands can create a significant carbon sink, with a mean 5% annual increase in SOC. In the mid–western United States, agricultural land conversion to perennial grassland showed a constant rate of 62 g C m−2 year−1 SOC accumulation over 40 years in the top 10 cm [82]. Similarly, Abberton et al. [83] reported that, in temperate regions, most grasslands can be considered soil carbon sinks of up to 40 g C m−2 year−1 following cropland conversion. Post and Kwon [48], further estimated that land use change from cropping to grassland could result in an increase of 33.2 g soil C m−2 year−1 in the USA. While a meta–analysis in temperate grasslands showed that at the 0–30 cm soil depth over 20 years SOC sequestration reached 44 g C m−2 year−1 which is half of the rate (95 g C m−2 year−1) at which SOC is lost over a 20 year period following permanent grassland conversion to an annual crop [84]. These estimates suggest that SOC recovery is possible but is usually slower than initial loss. Research in the south eastern United States also suggested up to 100 g C m−2 year−1 could be sequestered in soil following conversion of cropland into optimally grazed pastures (where the available pasture matches the animal needs). These increases have been attributed to the fast growth habit of pastures, negligible erosion and the minimal disturbance to soil compared to cropping [28]. Although focused principally on temperate grass pasture species, these studies demonstrate the potential increase of SOC as a result of cropland conversion to grasslands.
