Long-term changes in organic matter of woodland soils cleared for arable cropping in Zimbabwe

Subsistence farmers in Africa depend largely on the soil organic matter to sustain crop productivity. Long-term changes in soil organic carbon and nitrogen were measured after woodland clearance for smallholder subsistence farming or for commercial farming. The contents of organic carbon and nitrogen in soil under reference woodlands were largest (53.3 t C ha-1, 4.88 t N ha-1) in a red clay soil (~ 50% clay + silt), followed by a granitic sand (~ 12% clay + silt; 22.8 t C ha-1, 1.47 t N ha -1) and least (19.5 t C ha-1, 0.88 t N ha-1) in a Kalahari sand (~ 5% clay + silt). Organic carbon declined rapidly under cultivation to attain new equilibria within 10 years on all smallholdings. Greatest losses occurred in soils that initially contained most carbon and nitrogen in the order: red clay (22.4 t C ha-1 and 1.0 t N ha -1) > granitic sand (13.2 t C ha-1 and 0.8 t N ha -1) > Kalahari sand (10.6 t C ha-1 and 0.5 t N 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 carbon and nitrogen were 15 t C ha -1 and 1.7 t N ha-1 greater than on smallholdings with similar soil and climate. In the Kalahari sand the ¿13C 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.