Organic matter recovery in sandy soils under bush fallow in southern Mozambique

Bush fallow under shifting cultivation is commonly practised by subsistence farmers in southern Mozambique. This farming system is likely to persist due to the existence of large sparsely inhabited areas, coupled with financial limitations preventing smallholders from buying fertilisers. The bush fallow is intended to naturally restore soil production capacity lost during the cropping period. This study quantified soil organic matter (OM) content and investigated factors determining its dynamics in sandy soils under varying duration of bush fallow in southern Mozambique. Agroecosystems representing rainfall regions of <400, 400–600, 600–800, 800–1,000, >1,000 mm and a transitional agroecosystem of 400–800 mm were selected. Within each agroecosystem, five land uses (virgin, cultivated, <5 years fallow, 5–15 years fallow and >15 years fallow) were identified and compared for three shallow soil layers (0–50, 50–100 and 100–200 mm). Organic C and total N were used as indicators of soil OM and the study examined whether OM content increased with higher rainfall or longer bush fallow. Higher rainfall did not consistently increase soil OM content, which was highest in the wettest agroecosystem but did not decline gradually towards the driest. For example, the lowest OM content was found in the second wettest agroecosystem (800–1,000 mm rainfall). Duration of bush fallow was also inconsistent, as OM content continued to decrease for some time after cultivated land was abandoned to bush fallow and an increase in OM content occurred sooner in wetter agroecosystems than drier at a 600 mm rainfall threshold. After 15 years of bush fallow, OM content was still lower than in the virgin land of the agroecosystems. Relative to virgin land, both the loss and gain of OM were smaller in the drier agroecosystems. Further studies are needed to establish optimum bush fallow periods for full recovery of soil OM in southern Mozambique.