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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 m2, 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. [ABSTRACT FROM AUTHOR]

Inland valleys (IVs) of Ghana offer a very high potential for sustainable agricultural production due to their relatively high soil fertility and water availability. Within the IVs, different land use systems can be identified along the toposequence. The land uses selected for this study were: traditional rice farming in the valley bottom (TR): mixed cropping (MC), young fallow (YF), and cocoa plantation (CP) in the lower and upper slopes; and primary forest (PF) at the crest or summit. Soil reaction, cation exchange capacity (eCEC), and levels of major soil nutrients were determined for each. Results showed that the pH of the soils was moderately to slightly acid and that MC had the lowest pH. The observed topsoil mean values were TR, 5.5; MC,1.9; YF, 6.4; CP, 6.1; and PF, 6.2. With the exception of TR, pH generally decreased with depth. Exchangeable cations, especially potassium, calcium, and magnesium, and effective cation exchange capacity were higher in the upland systems than in the valley bottom, showing the degradation caused by traditional slash-and-burn rice cultivation. Recorded mean topsoil eCEC (cmnol (+)/kg) was TR, 7.2; MC, 6.1; YF, 11.1: CP, 14.1; and PF, 14.7. Total nitrogen (TN) levels ranged from 0.2% in TR to 0.4% in PF. Vegetation cover, litter fall, and organic matter accumulation seemed to influence the distribution of total carbon (TC) and TN. Available phosphorus (Bray 1), on the other hand, was higher in TR than in the upland land use systems, possibly due to the hydromorphic and relatively coarse nature of the soils. Sustainable lowland farming systems, such as the Asian type sawah system, need to be established. At the same time, because effective management of the upland is reflected in the valley bottom soils, proper management of the upland soils could help increase food production and conserve the environment. [ABSTRACT FROM AUTHOR]