Comparative assessment of temporal changes in soil degradation under four contrasting land-use options along a tropical toposequence

Assessment of land-use types has conventionally been on spatial rather than temporal scales, thus limiting the inference on relative potential of land-use types against soil degradation, needed to validate their allotment to ecosystems on agricultural watersheds. The study assessed the changes in key indices of soil degradation among four land-use systems of West African 'watershed' agroforestry development over about a decade (1999-2011) along a toposequence in Central Ghana. The land-use systems were grass-dominated natural fallow (Fallow), cocoa plantation (CPT), traditional rice farm (TRF), and Sawah ecotechnology for wet inland valley rice farm (Sawah), with sampled land areas covering 2,000, 1,500, 4,500, and 6400 m(2), respectively. In 1999 and 2011, soil samples were taken from 0-20, 20-40, and 40-60-cm depths. Samples were collected from about the same spots in both years. The samples were analyzed for selected physicochemical parameters and, for each land-use option and depth, comparison made between 1999 and 2011. A mean-weighted decadal change in degradation status was used to rank the land-use options. Most of the decadal changes in soil properties (including available micronutrients) were negative, with about two-third of the significant changes in the top (0-20 cm) soils. For instance, soil pH tended to decrease in 2011 compared with 1999; the decreases (14.51%) were significant in the topsoils under Sawah where no other index showed significant decline. Topsoil total carbon decreased under TRF (49.08%) and CPT (32.80%). Available phosphorus marginally increased only under Sawah. The overall aggregate scores for the mean-weighted decadal change in degradation status showed that the land-use options differed in their potential to resist degradation: Sawah (slight), Fallow (mild), and TRF/CPT (moderate). Our findings highlight the extent of temporal changes in soil degradation under different topo-positions in tropical African ecosystems while pointing to the superiority of Sawah ecotechnology. Increased use of this soil-and-water management system in lowland rice farming could, therefore, have some soil conservation benefits.