Fertility recovery in sandy soils under bush fallow in southern Mozambique

[摘要] English: Bush fallow under shifting cultivation is the most common practised subsistence farmingsystem in southern Mozambique. This system is likely to persist due to the existence oflarge areas sparsely inhabited coupled with financial limitations preventing small scalefarmers from buying fertilizers. The bush fallow is intended to recover naturally theproductive capacities of soils lost during cropping. This study was conducted thereforeto gain a better understanding on the composition and biomass of bush fallowvegetation, climatic factors affecting leaf litter decay of an important tree species andthe dynamics of some soil fertility indicators.Five agroecosystems representing rainfall regions of <400 mm (AE6), 400-600 mm(AE5), 600-800 mm (AE3), 800-1000 mm (AE2), >1000 mm (AE1) and a transitionalagroecosystem of 400-800 mm (AE4) were selected. Within each agroecosystem, fiveland uses (virgin, cultivated, < 5 years fallow, 5-15 years fallow and >15 years fallow)were identified. Descriptions and comparisons of vegetation were performed betweenland uses within agroecosystems and similar land uses across agroecosystems, exceptin cultivated land; effects of soil water content and soil temperature on decomposition ofBrachystegia spiciformis leaf litter were evaluated in recently abandoned agriculturalfields cleared of any vegetation (Bare) and in >15 years fallow fields (15F) at sites in atransect that covered AE2 to AE6; and at every combination of agroecosystem and landuse the dynamics of organic C, total N, CEC, pH, P, Ca, Mg and K were determined inthe 0-50 mm, 50-100 mm and 100-200 mm soil layers.A total of 204 species that including N-fixing species, belonging to 141 genera and 50families divided into tree, shrub and herbaceous layers were identified. The tree layerwas only found in virgin fields and in fields abandoned to bush fallow >15 years,whereas shrub and herbaceous layers occurred in all fields. The tree species in bushfallow fields of coastal and wetter AE1, AE2 and AE3 (dominated by B. spiciformis andJulbernaldia globiflora) outnumber those in inland and drier AE4, AE5 and AE6(dominated by Birchemia discolour and Colophospermum mopane) and have largerdiameter that result in greater biomass. Number of shrubs decreased from coastal an wetter to inland and drier agroecosystems. The herbaceous biomass declined fromyoung to old fallow fields in coastal and wetter agroecosystems, while the converse wasobserved in inland and drier agroecosystems. Nitrogen-fixing species tended to occurmore in bush fallow fields older than 15 years. In inland and drier agroecosystems thetree biomass in 15F fields tended to be higher than in virgin fields due to presence ofsuccession species that differ from the original ones. In the wetter agroecosystems Closs from B. spiciformis leaf litter was faster, whereas in the drier ones it was moresensitive to rainfall pulses. Similarly, C loss was faster in 15F fields than in bare fields.In coastal and wetter AE1, AE2 and AE3 there was a declining trend in organic C andtotal N from virgin to cultivated fields. This trend proceeds to the <5 years fallow fieldsand thereafter the contents of the two indicators increased in older fallow fields. Adifferent pattern was found in the dry AE4 and AE5 where organic C and total N tendedto decline gradually even with longer fallow periods. In the severely dry AE6 no cleartrend was found. The pH in all agroecosystems decreased from cultivated to fallowfields, an effect attributable to a gradual decrease in the basic cations released on thesoil surface by the ash produced during slash and burn. A slight increase in the silt plusclay fraction from AE4 to AE5 was found, which resulted in increased CEC, P, Ca, Mgand K. From the coastal and wetter to inland and drier agroecosystems pH, P and Caincreased, except in AE4 and AE5, which had lower pH and Ca values. The lowervalues of pH resulted in lower contents of P in AE4 and Ca and Mg in bothagroecosystems, which have the same vegetation, suggesting that this should be thedetermining factor.The results from this study showed that a bush fallow period of longer than 15 years isrequired for restoration of soil fertility in abandoned cultivated fields to the same level asin virgin fields. This aspect must be taken into account when strategies are developed toimprove the sustainability of cropping on the sandy soils of southern Mozambique.

[发布机构] University of the Free State