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

Abstract

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 À1 , 4.88 t N ha À1 ) in a red clay soil ($ 50% clay þ silt), followed by a granitic sand ($ 12% clay þ silt; 22.8 t C ha À1 , 1.47 t N ha À1 ) and least (19.5 t C ha À1 , 0.88 t N ha À1 ) in a Kalahari sand ($ 5% clay þ 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 À1 and 1.0 t N ha À1 ) > granitic sand (13.2 t C ha À1 and 0.8 t N ha À1 ) > Kalahari sand (10.6 t C ha À1 and 0.5 t N ha À1 ). 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 À1 and 1.7 t N ha À1 greater than on smallholdings with similar soil and climate.In the Kalahari sand the 13 C of organic C remained constant after woodland clearance, and maize contributed less than 10% of the total C even after 55 years. The 13 C 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.

Land Cover, Extent, and Properties of Arenosols in Southern Africa

“…Research efforts in Southern Africa have been less intense although there is a fair body of literature from Zimbabwe (e.g., Jeranyama, Hesterman, Waddington, and Harwood, 2000;Nyamangara, Gotosa, and Mpofu, 2001;Zingore, Manyame, Nyamugafata, and Giller, 2005) and South Africa (Mills and Fey, 2004;Moodley, Johnston, Hughes, and Titshall, 2004;Verdoodt, Van Ranst, and Van Averbeke, 2003). Exchange of experiences and research results on the properties and management may be beneficial for land use planning and farmers in both African regions.…”

Effect of farmer management strategies on spatial variability of soil fertility and crop nutrient uptake in contrasting agro-ecological zones in Zimbabwe

“…The observed decrease in SOC, total N and available soil P in soils belonging to poorer farmers in sub-humid conditions has been reported elsewhere (Mtambanengwe and Mapfumo 2005;Zingore et al 2007a) and attributed to differences in the nutrient resources available to the different classes of farmers. As the poorer farmers add little or no fertility amendments to their soils, fertility is likely to decline very rapidly within a few years of continuous cultivation on granitic and Kalahari sands (Zingore et al 2005). Resource-endowed farmers often have access to livestock manure and financial resources to purchase mineral fertiliser.…”

Nutrient constraints to tropical agroecosystem productivity in long‐term degrading soils

“…ISF can decline upon forest clearance and subsequent transition to agriculture because SOM, a main source of plant nutrients, is actively mineralized as a consequence of continuous disturbance of the soil aggregates with cultivation (Collins et al, 2000;Solomon et al, 2007) and a high proportion of plant nutrients lost or removed from the system with annual harvest (Zingore et al, 2005) and through runoff and leaching (Bertol et al, 2007). Studies indicate that the rate of loss of SOM slows with time as a new SOM equili- Table 6 Net benefit (per ha) and benefit to cost ratio of different nitrogen (N) and phosphorus (P) fertilizer rates LR 0 N 120 N 120 N 120 N 0 N 120 N 120 N 120 N 25 P 0 P 25 P 100 P 25 P 0 P 25 P 100 P Net benefit is calculated as returns due to increase in yield above control minus the cost of fertilizer on a per hectare basis; BCR is the benefit-cost ratio calculated as returns due to increase in yield above control as a ratio of the cost of fertilizer; 76 Kenya shillings (Ksh) 5 1 USD.…”

“…After clearing and tillage, the soil plant-available nutrient pool increases as a consequence of mineralization of soil organic matter (SOM). However, high fertility is short-lived (Solomon et al, 2007) if the export of nutrients through harvest and leaching is not balanced by fertilizer input (Zingore et al, 2005;Lal, 2007). The depletion rates of specific nutrients depend on a number of factors including management, soil type and climate (Davidson & Ackerman, 1993;Wopereis et al, 2006;Ngoze, 2008;Tittonell et al, 2008).…”