IMPACT OF CREEPING VEGETABLE COVER CROPS ON MINERAL N AND MICROBIAL GROUP POPULATION OF A SANDY LOAM ULTISOL UNDER IMMATURE RUBBER PLANTATION IN SOUTHERN NIGERIA

The use of forage legume species as cover crops in the management of immature rubber plantation soils is not attractive to smallholder rubber farmers (owners of about 75% of the global acreage under rubber production). We sampled immature rubber plantation soils under the following respective creeping vegetable cover crops: Vegetable cowpea (Vigna unguiculata L., Walp. Ssp. Sessquipedallis.), Egusi melon (Cucumeropsis manni Naaudi) and Broadleaf pumpkin (Cucurbita pepo) and a forage legume species (Centrosema pubescens) for four consecutive years at the Rubber Research Institute of Nigeria, Benin. The samples were analyzed for mineral N (NO3- and NH4+) content, and microbial group populations at the onset and end of rains, as well as selected physical and chemical properties. Generally, the result showed that the creeping vegetable cover crop species had a comparative positive effect on the soil properties assessed relative to the forage legume species. The two sampled period mean values of NO3- and NH4+ (10.62 and 8.18 mgkg-1, respectively) obtained from the plantation that had vegetable cowpea were significantly (P≤ 0.05) higher than those of the Egusi melon and Broadleaf pumpkin, but slightly (not significant) lower than that of the Centrosema pubescens. The soil microbial groups (Fungi, Bacteria and Actinomycetes) populations were significantly affected by the cover crop species. At the onset and end of rains, plantations that had vegetable cowpea and Centrosema pubescense cover crops had the maximum number of bacteria (29.58 and 29.50 C.fug-1drywt.soil ×10-6, in that order), while the maximum number of fungi was found in the broadleaf pumpkin (23.66 C.fug-1drywt.soil ×10-6) and Egusi melon (22.92 C.fug-1drywt.soil ×10-6) cover crop plantations. The broadleaf pumpkin and Egusi melon had significant positive effect on the soil surface layer (0-15cm) pH, Org. C, base cations, bulk density and percentage water filled pore space. These findings suggest that the creeping vegetable cover crops could be considered as alternative to the forage legumes in the management of immature rubber plantation soils.