Long-Term Changes In Organic Matter Of Woodland Soils Cleared For Arable Cropping In Zimbabwe

Summary

Subsistence farmers in Africa depend largely on the soil organic matter to sustain crop productivity. Long￢ﾀﾐ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￢ﾀﾃt C ha￢ﾈﾒ1, 4.88￢ﾀﾃt N ha￢ﾈﾒ1) in a red clay soil (￢ﾈﾼ￢ﾀﾃ50% clay + silt), followed by a granitic sand (￢ﾈﾼ￢ﾀﾃ12% clay + silt; 22.8￢ﾀﾃt C ha￢ﾈﾒ1, 1.47￢ﾀﾃt N ha￢ﾈﾒ1) and least (19.5￢ﾀﾃt C ha￢ﾈﾒ1, 0.88￢ﾀﾃt N ha￢ﾈﾒ1) in a Kalahari sand (￢ﾈﾼ￢ﾀﾃ5% clay + silt). Organic carbon declined rapidly under cultivation to attain new equilibria within 10￢ﾀﾃyears on all smallholdings. Greatest losses occurred in soils that initially contained most carbon and nitrogen in the order: red clay (22.4￢ﾀﾃt C ha￢ﾈﾒ1 and 1.0￢ﾀﾃt N ha￢ﾈﾒ1) > granitic sand (13.2￢ﾀﾃt C ha￢ﾈﾒ1 and 0.8￢ﾀﾃt N ha￢ﾈﾒ1) > Kalahari sand (10.6￢ﾀﾃt C ha￢ﾈﾒ1 and 0.5￢ﾀﾃt N ha￢ﾈﾒ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￢ﾀﾃt C ha￢ﾈﾒ1 and 1.7￢ﾀﾃt N ha￢ﾈﾒ1 greater than on smallholdings with similar soil and climate.

In the Kalahari sand the ￎﾴ13C of organic C remained constant after woodland clearance, and maize contributed less than 10% of the total C even after 55￢ﾀﾃyears. The ￎﾴ13C 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￢ﾀﾃ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.