Microbial Dynamics and Nutrient Mineralization in Soil Amended with Cacao Pod and Water Hyacinth Composts: Implication for Nitrogen Fixed by Soybean

Abstract

Composting organic wastes can be relatively cheap and environmentally-friendly sources of mineralizable nutrients for legumes. We amended 5 kg topsoil (0–15 cm) samples of an Ultisol (Lixisol) with 0, 2.5 and 5 t ha−1 each of cacao pod (CC) and water hyacinth (WC) composts with and without starter fertilizer (F) (25 kg N ha−1 as urea and 26 kg P ha−1 as single super phosphate). The amended soil samples were used for 16-week laboratory incubation experiment and two-consecutive 8-week plantings of soybean (Glycine max. L. Merril) in the screenhouse. Measurements were carried out on some indices of microbial activity, namely, microbial respiration, N and P mineralization as well as on nodulation and N2 fixed by soybean. The microbial respiration, and N and P mineralization increased with application rates of the composts, but generally declined with incubation duration. Applications of 5 t ha−1 each of CC and WC with or without F increased (p < .05) the microbial respiration (24–93%), N mineralized (21–35%), and P mineralized (32–180%) during the incubation period. The highest of these increments was, however, obtained with WC. Increases of 1162 and 981% (CC2F and WC2F) and 28% (CC2F) were also recorded in nodulation and the amount of N2-fixed, respectively. It was concluded that 5 t ha−1 water hyacinth or cacao pod composts with starter fertilizer is capable of enhancing soybean N2 fixation by serving as mineralizable nutrient sources prior to the onset of nodulation and N2 fixation.