The effects of clearing and subsequent land-use on abundance and biomass of soil nematodes in tropical forest

The soil nematode fauna at 24 sites, each of about 1 ha, was investigated in semideciduous tropical forest at Mbalmayo, Cameroon over 2 years. Almost all combinations of three factors were investigated: (1) time since clearance - forest undisturbed for at least 70 years, sites cleared less than 2 years ago, forest cleared 3 to 5 years ago or forest cleared 15 to 25 years ago; (2) method of clearance - manually or by bulldozer; (3) use of land after clearance - plantation forestry, small-scale agriculture or natural regeneration. Mean total abundance of free-living and plant-parasitic nematodes in the soil in undisturbed forest was over 2 million m(-2), and biomass 6.5 kg ha(-1) (fresh weight). Both abundance and biomass were affected by the time since clearance, method of clearance and use of land after clearance. Total abundance and biomass were low soon after clearance by bulldozer and at 3 to 5 years after clearance under all land uses. After 15 to 25 years, values tended towards those in undisturbed forest. Total biomass of nematodes feeding on roots showed different relationships between the treatments to total biomass of nematodes not feeding on roots: root feeders were higher than undisturbed forest under natural regeneration at all times after manual clearance, and under forestry less than 2 years after manual clearance, whereas other nematodes were higher in biomass than undisturbed forest under natural regeneration less than 2 years after manual clearance only. Biomass of root feeders was lower than undisturbed forest under forestry 3 to 5 and 15 to 25 years after manual clearance, under agriculture less than 2 years after manual clearance, and under forestry less than 5 years after clearance by bulldozer, whereas biomass of other nematodes was lower than undisturbed forest under forestry less than 2 years after clearance by bulldozer and under all land-uses 3 to 5 years after clearance by either means. These results show that substantial changes occur in soil nematodes after clearance of tropical forest, which may affect the rest of the soil ecosystem.