The effects of clearing and cropping on organic reserves and biomass of tropical forest soils

Twenty Nigerian soil samples, mainly from field experiments at the International Institute of Tropical Agriculture (IITA), were used in a study of the effects of clearing and cropping on the C, N and S reserves of soils under secondary lowland rain forest. Soils that had been cropped for 2yr usually contained less total C and N than the corresponding forest soils. This decline was less rapid when crop residues (maize) were returned to the soil as a mulch than when they were removed.

The soils were incubated in the laboratory either fresh, or after exposure to stress treatments (air-drying or fumigation). The amounts of N mineralised by the fresh untreated soils were small and not clearly related to the cropping history. Both air-drying and fumigation caused a marked increase in the mineralisation of N and the amount released was related to the cropping history. Cropping caused an even greater decrease in these ‘stress-labile’ N reserves than in total soil N, so that cropping causes a decline in both the quantity and “quality’ of soil organic matter. Again, the decline was less where crops were grown under mulches. A term “Per cent Stress-labile N” (PSN), defined as the percentage of the total N mineralised when fumigated soil is incubated under defined conditions, is proposed as a measure of the ‘quality’ of soil organic nitrogen. Under secondary forest the PSN ranged from 3.4–4.2%; on cultivation it declined, in one case to less than 1%.

The amount of C in the soil biomass was calculated from the size of the flush of decomposition caused by fumigation. Biomass C usually decreased on cultivation, the decrease being less under mulches. There was a close linear relationship (r = 0.94) between the flush of N caused by fumigation and the amount of biomass C in a soil and a rough measure of biomass C can be obtained by multiplying the N flush by 8.