Soil organic matter (SOM) is crucial to the sustainability of smallholder agriculture that relies heavily on nutrient mineralized from organic matter due to small amounts of fertilizers used. Long-term changes in soil organic carbon and nitrogen were measured on fields cultivated for different ages after woodland clearance for smallholder farming on three different soils: Kalahari sand (5% clay + silt), granitic sand (12% clay + silt) and a red clay soil (50% clay + silt).  A commercial farming site characterized by high external inputs was also studied on the red clay soil. Soil organic carbon (SOC) contents under reference woodlands were largest (53.3 t C ha^-1) in the red clay soil, followed by the granitic sand (22.8 t C ha^-1) and least (19.5 t C ha^-1) in the Kalahari sand. More than 50% of the SOC was lost in the initial 5 years of cultivation when woodland soil was cleared for maize cultivation under smallholder management characterized by use of small amount of inputs and removal of stover to feed livestock. New equilibrium was attained within 10 years on all soils. Greatest losses occurred in soils that initially contained most carbon and nitrogen in the order: red clay (22.4 t C ha^-1) > granitic sand (13.2 t C ha^-1) > Kalahari sand (10.6 t C ha^-1). On the clay soil, commercial farming with intensive use of mineral fertilizers and incorporation of maize stover led to more gradual decline: at equilibrium, contents of SOC in the soil with inputs of mineral fertilizers and crop residues were 15 t C ha^-1 greater than the contents in the soils on smallholder farms, which were cultivated continuously without mineral fertilizers or crop residues.

In the Kalahari sand, the d¹³C value of organic C remained constant after woodland clearance, and maize contributed less than 10% of the total C even after 55 years. The ¹³C signature increased slightly with increasing duration of cultivation by smallholders in the granitic sands and red clay soil where maize contributed 29% and 35% of the C at equilibrium. Under more productive commercial farming, the carbon derived from maize accounted for 50% of the total after 10 years of cultivation and 67% at equilibrium. The persistence of woodland carbon in the sandy soil is attributed to chemical stabilization resulting from large concentrations of lignin and polyphenols in the tree litter, or as charcoal.

Keywords: carbon 13, soil organic carbon, cultivation, crop residue, carbon dynamics.