Dold, Christian: Soil attribute changes along chronosequences of land use in the littoral wetlands of Lake Naivasha, Kenya. - Bonn, 2014. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Christian Dold}},
title = {Soil attribute changes along chronosequences of land use in the littoral wetlands of Lake Naivasha, Kenya},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2014,
month = nov,

note = {Lake Naivasha is a freshwater lake in the East African Rift Valley, which was affected by a continuously declining water level between 1980 and 2011. The newly exposed littoral area has been gradually put under agricultural land use by pastoralists and small-scale farmers, forming chronosequences of land use with distance to the lake shore (space-for-time approach). Transects representing land use durations of 1 to 30 years (as well as reference sites) were established, comprising soils of alluvial and lacustrine sediment origin in the pasture land and of lacustrine origin in the cropland. We assessed changes in soil moisture, carbon and nutrient content between November 2010 and December 2011. An additional greenhouse experiment studied the responses of kikuyu grass (proxy for pasture vegetation) and maize (proxy for crops) in potted topsoil. With increasing distance from the lake shore and duration of land use, we observed a exponential decline (p < 0.05) in soil organic carbon, potassium permanganate oxidized and non-oxidized carbon as well as N contents under both pasture and cropland uses. Additionally, carbon in particulate organic matter decreased in all size fractions, revealing that both the labile sand-bound and the stable silt- and clay-bound carbon were affected by the time of use. In the case of soil organic carbon, the rate constants of decline were -0.021 under pasture (15 years time span) and -0.016 per year under crops (30 year time span). In the case of soil N, the rate constants were -0.019 and -0.012 per year for pastures and cropland, respectively. Thus, carbon and nitrogen losses were similar on both soil types and land management systems. The soil water content decreased significantly (p < 0.05) with the duration of land use. Consequently, the associated change in soil aeration status is probably the key driver of the observed soil fertility decline, with soil type and land management having little influence. On chronosequence positions ≥ 20 years the upper soil layers (0 – 60 cm) dried up temporarily, owing to a drop in groundwater depth and insufficient rainfall. In croplands, this water deficit in the topsoil could only be partially compensated by supplementary irrigation. Observed changes in the plant-available Olsen-P fraction were not related to the duration of land use. Only the ion exchange resin-adsorbed P fraction decreased significantly with land use duration under pasture use (lacustrine soils), and was mainly associated with soil organic carbon and amount of rainfall and irrigation. The dry matter accumulation in potted soil of both kikuyu grass and maize declined with the duration of land use. As soil moisture was kept constant, this reduction with time of land use was primarily related to changes in soil nitrogen content. The reduction in plant available water and soil nutrients with continuous agricultural production is likely to entail the observed declining production potential on both, pastures and cropland. The chronosequence model provides a suitable tool to study edaphic and hydrological change processes and their impact on production and land productivity.},
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