Application of 13C natural abundance to elucidate compositional changes of soil organic matter following woodland clearance for maize cultivation in Zimbabwe

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. 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 δ13C values ranging between -9 to -17 %. 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 δ13C 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. δ13C of soil organic carbon increased slightly with increasing length of smallholder cultivation in the Masvingo granitic sands and Chikwaka clay soil where maize contributed 29% and 35% of the soil organic carbon at equilibrium. Under more productive commercial farming the soil organic carbon derived from maize accounted for 50% of soil organic carbon after 10 years of cultivation and 67% at equilibrium. The small contribution of maize to soil organic matter at the smallholder sites can be attributed to the unproductive management leading to small inputs of organic matter. In addition, as maize organic matter is chemically labile, the situation at Mafungautsi could have been worsened by the lack of physical protection of SOC due to the little clay content. The persistence of a fraction of woodland carbon in the sandy soil is attributed to chemical stabilization resulting from high concentrations of lignin and polyphenols in the tree litter, or as charcoal. There is low potential to sequester C in sandy soil under smallholder management and opportunities to improve the SOC stocks of these soils must hinge on use of resistant C sources, such as cattle manure, which may persist in these soils. Commercial management led to maize contributing a higher proportion of the SOC in the long-term. This highlights the essential role of mineral fertilizers in promoting sequestration of C through improving productivity, thereby making available large amounts of biomass to replenish SOC