Impacts of agricultural land use histories on soil organic matter dynamics and related properties of savannah soils in North Cameroon

UBL - phd migration 201

Impacts of agricultural land use histories on the properties of the main soils of the Diamare plain in Nort Cameroon

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Land-use related organic matter dynamics in North Cameroon soils assessed by 13C analysis of soil organic matter fractions

Topsoil samples from cultivated and adjacent non-cultivated fields on three major agricultural soils in North Cameroon were fractionated into particle-size fractions that were analysed subsequently for their C and 13C contents. The aim was to obtain further insight into the dynamics of soil organic matter (SOM) in relation to land use in Cameroon. Since organic carbon contents of the fractions were often very small, samples and analyses were extensively replicated to obtain robust statistical estimates of observed differences. For each soil type, differences in δ13C values between fields could be related to changes in the input and decomposition of organic matter arising from soil type, land management and, for example, the nature and abundance of weeds. Turnover of organic matter appeared to be fastest in the sand fraction, which is in line with results from earlier studies. In the finer fractions, clear differences in reaction to changes in input and decomposition were observed, that seem to be linked to differences in clay mineralogy. The results illustrate that SOM in the various fractions is much less stable and more strongly affected by changes in land use than might be assumed on the basis of changes in total SOM contents alone. At the same time, they demonstrate the relevance of 13C isotope analyses of SOM for studies on the impact ofland use on these savannah soils with little SOM that are highly susceptible to degradation.

Myths about slash and burn in physical degradation of savanna soils: impacts on Vertisols in north Cameroon

A study was conducted to assess the impact of agricultural land use history on macro-aggregate (4.0–4.8 mm) stability in the 0–5 cm layers of Chromic and ‘Hydromorphic’ Vertisols in north Cameroon. Macroaggregate stability to water drop impact was determined and the ASI50 index calculated. Macroaggregates from fallow and zero-tilled cropped soils disaggregated in a stepwise manner. Macroaggregates from ploughed cropped soils collapsed in one step into semi-liquefied microaggregates and primary particles. On both soil types, the ASI50 index of samples from ploughed land was 10.0 mJ, compared to 16.4– 21.9 mJ from zero-tilled slash and burn land use. The stepwise disintegration of macroaggregates indicated the existence of a hierarchy of aggregation within the size range 2–5 mm. Slash and burn land use on zero-tilled Vertisols significantly increased sand-sized organic carbon content and the stability of macroaggregates to water impact