Replenishment of C, N, and P in a degraded alfisol under humid tropical conditions: Effect of fallow species and litter polyphenols

The capacity of vegetation fallow to replenish carbon and nutrients in degr aded soil is related largely to the nature of the fallow vegetation, partic ularly the chemical composition. Therefore, a study was conducted at Ibadan , southwestern Nigeria (humid tropics), to look into these relationships us ing, fallow species with varying chemical compositions. The treatments incl ude three woody species, Senna siamea, Acacia leptocarpa and Leucaena leuco cephala, planted in 1989, and the natural shrub Chromolaena odorata. A cont inuous cropping of maize/cassava was maintained as a control (no fallow). C omposite surface soil (0-15 cm) at three distances from a tree hedgerow (0. 5, 2.0, and 3.5 m) was sampled in 1996 for the determination of soil C and nutrient stocks. Maize and cassava were planted as a test crop of soil prod uctivity after fallowing. Litterbags were placed to determine the N immobil ization during the decomposition of fallow leaves. Soil. organic C (SOC) wi thin 0-15 cm ranged from 19,100 (continuous cropping) to 26,400 kg ha(-1) ( Leucaena fallow), whereas total N ranged from 1820 (continuous cropping) to 3110 kg ha(-1) (Leucaena fallow). High polyphenols in fallow leaves favore d SOC and N accumulation. The amount of potentially mineralizable N was inc reased from 41 kg ha(-1) in continuous cropping to 159 under Leucaena fallo w and 176 under der Acacia fallow. The percentage increase in mineralizable N stock under fallow was, on average, 5.5 times greater than that in total N, implying that fallow causes change in soil organic matter quality.. The available P (Olsen) was 8.6 kg ha(-1) in the continuous cropping and range d from 14.1, kg ha(-1). (natural fallow) to 29.2 kg ha(-1) (Leucaena fallow ). Greater maize and cassava yields were obtained in the fallow plots than in the control during, the subsequent cropping. The maize grain yield after fallow could be predicted by the potentially mineralizable N in surface so il (0-15 cm). Leucaena and Acacia are promising species for planted fallows for soil regeneration in the humid tropics. The binding of protein by poly phenols during leaf decomposition, as confirmed by higher N immobilization with the increase in leaf polyphenols, could be the main mechanism in the c ontribution of polyphenols to SOC and N replenish, ment in the degraded soi l. The study suggests the possibility of stabilizing C and N in tropical ec osystems by manipulating polyphenols in vegetation.