Browsing by Author "Kristensen, J.A."
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item The combined effect of termite bioturbation and water erosion on soil nutrient stocks along a tropical forest catena in Ghana(Catena, 2019-03) Kristensen, J.A.; Boëtius, S.H.; Abekoe, M.; Awadzi, T.W.; Breuning-Madsen, H.In the tropical moist semi-deciduous forests of West Africa, soil catenas with extremely gravel-rich soil horizons at the summits and upper slopes and largely gravel-free profiles at the lower slope are common. Previous investigations have suggested that these gravel layers are the result of macro-invertebrates mining of fine-grained soil material from the subsoil leaving behind the gravel, to build galleries at the surface subsequently exposing it to water erosion transport downslope. We examined the indirect effect of this process on the distribution along a soil catena of crucial base cations (Ca2+, Mg2+, K+) and plant available phosphorous (P), which is often growth-limiting in these tropical ecosystems. We found that the export of fine-grained soil material at the top of the catena reduces the soil stocks (to 1 m) of these elements by up to 60%, while the soil fertility downslope did not change significantly. This important long-term (100–1000 yr scale) reduction in soil fertility at the top of slopes resulting from bioturbation and water erosion is overlooked in contemporary literature, which primarily focus on the beneficial impact termites and ants have on ecosystem functioning in more level savannah landscapes. As the type of catena studied is widespread across tropical environments, this effect is likely ecologically substantial. Future research should aim at understanding such long-term consequences of bioturbation on landscape ecology as well as soil heterogeneity and fertility, so we do not overlook potential negative ecosystem effects.Item Sustainable long-term intensive application of manure to sandy soils without phosphorus leaching: a case study from Ghana(Acta Agriculturae Scandinavica Section B: Soil and Plant Science, 2015-08) Asomaning, S.K.; Abekoe, M.K.; Dowuona, G.N.N.; Borggaard, O.K.; Kristensen, J.A.; Breuning-Madsen, H.Long-term application of manure to sandy soils to ensure high crop productivity may lead to phosphorus (P) leaching, which, in turn, may deteriorate the quality of recipient waters because of eutrophication. The risk of P leaching depends on contents of aluminum (Al) and iron (Fe) oxides that are strong P sorbents and of calcium (Ca) that can form sparingly soluble Ca–P compounds. While P sorption by Al and Fe oxides is limited, the formation of sparingly soluble Ca–P may theoretically continue as long as enough Ca is available, i.e. under such conditions long-term application of manure may be environmentally sustainable. This hypothesis has been tested on soils formed on beach sand at Keta, Ghana, which have been used since the 1930s for intensive cultivation based on manure application. Soil samples were collected from different depths down to 80 cm in profile pits on cultivated soils that received manure over 70 years and on uncultivated plots (controls). The samples were analyzed for texture, pH, total C and P and poorly ordered Al and Fe oxides as well as different P forms as assessed by the Hedley fractionation method. The results showed a fourfold P increase in the top 40 cm of the cultivated soils, whereas in the deepest soil layers the P contents in the cultivated and uncultivated soils were almost the same indicating very limited downward P transport despite long-term manure application. This was supported by comparable P concentrations in groundwater taken under cultivated and uncultivated sites. P fractionation showed that the cultivated top soils were dominated by sparingly soluble Ca–P compounds that accounted for about 70% of the P gain. Thus, the results suggest that intensive crop production under tropical semi-arid conditions may be environmentally sustainable when based on manure fertilization ensuring enough Ca supply and neutral to alkaline pH. © 2015 Taylor & Francis.