| Popis: |
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. |
