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The 13C natural abundant technique was used to trace the fate woodland soil organic carbon (SOC) following clearance of woodland for maize cultivation, and to estimate the subsequent contribution by the maize to the SOC pool along cultivation chronosequences on three different soil types in Zimbabwe [1]. Differentiation of SOC according to its source is made possible by differences in depletion of the 13C isotope between woodland vegetation (mostly C3) and the maize crop (C4). Plants with a C3 photosynthetic pathway (most herbs and trees) discriminate more strongly against 13CO2 and their δ13C values range from-22 to-32‰, whilst C4 plants (tropical grasses and cereals) are less depleted and have δ 13C values ranging between-9 to-17‰[2]. Three sites under subsistence maize production were selected at Mafungautsi under Kalahari sands (clay~ 3%), Masvingo under granitic sands (clay~ 10%) and Chikwaka under dolerite derived clay soil (clay~ 35%). Parallels were drawn with measurements taken at a commercial maize production site in Chikwaka where management involved use of large amount of mineral fertilizers and stover return, to assess the effect of contrasting management on dynamics of SOM derived from different sources.

In the Kalahari sand, the δ 13C value of SOC remained remarkably constant after woodland clearance, and maize contributed less than 10% of the total SOC even after 55 years. The bulk of the SOC present in the cultivated soils was that derived from the native woodland vegetation. δ 13C of soil organic carbon increased slightly with increasing length of smallholder cultivation in the Masvingo …