Optimization of bio-conversion and nutrient release from coal fly ash amended cow dung – waste paper composts using earthworms (eisenia fetida) and effective micro-organisms

Abstract

Improving the fertilizer value of fly ash can create an abundant nutrient source in agriculture, whilst reducing the landfill deposition of this coal waste. This study was undertaken to address this challenge and was guided by the following specific objectives, (i) to characterize fly ash samples from selected South African coal powered electricity stations (ii) to optimize the fly ash incorporation into cow dung – waste paper mixtures for enhanced biodegradation and nutrient mineralization (iii) to determine the ideal Eisenia fetida (E. fetida) stocking density for the vermicomposting of fly ash - cow dung-waste paper mixtures (FCP) (iv) to determine the effectiveness of Effective Micro-organisms (EM) during vermicomposting in enhancing the biodegradation of FCP (v) to determine the effectiveness of FCP vermicompost substituted into pine bark compost on media physico-chemical properties and marigold growth. The eight South African fly ashes evaluated had a total phosphorus content ranging from 553.3 to 1514 mg/kg and the Olsen extractable P from 130 to 345.5 mg/kg. Across all three soils, fly ash incorporation increased extractable P content from a P deficient level to about 20 mg P/kg. Except for Cu, all metal species investigated (Cr, Pb, Ni and Fe) showed significantly (P ≤ 0.05) low extractability under fly ash treated soils compared to the soil alone control. To enhance the nutrient mineralization in fly ash, it was mixed with optimized cow dung – waste paper (CP) mixtures at ratios of (F: CP) 1:1, 1:2, 1:3, 2:1, 3:1 and CP alone and composted for 14 weeks with or without E. fetida earthworms . Based on C: N ratio, the extent of vermi-degradation of the waste mixtures followed the decreasing order (F: CP) of 1:3 > 1:2 > 1:1 > CP alone > 2:1 > 3:1. Olsen P was significantly higher (P < 0.05) where earthworms were added. The mean percentage increase in extractable P was in the order CP alone > 1:2 > 1:3 > 1:1 > 2:1 > 3:1, with earthworm addition almost doubling P release across the 1:1; 1:2 and CP alone treatments. Fly ash incorporation at the 1:2 ratio proved to be the most appropriate, as it enhanced biodegradation and nutrient release to a greater extent than all other treatments with fly ash incorporation. To optimize stocking density of fly ash vermicompost, the optimum fly ash incorporation level of 1: 2 was treated with four stocking densities of 0; 12.5; 25 and 37.5 g –worm/kg; including and control with cow dung- waste paper only (CP alone). Though the treatments 12.5; 25 and 37.5 g – worm/kg all resulted in mature vermicompost, a stocking density of 25 g-worm/kg and above resulted in faster maturity; higher humification parameters and a low C: N ratio range (11.1 – 10.4). The activity of ß-glucosidase and fluorescein di-acetate hydrolysis enzymes showed faster stabilization at stocking densities of 25 g-worm/kg and above, indicating vermicompost stability and maturity. These enzymes also showed significant (P < 0.05) correlation with changes in C: N ratio in the vermicompost. Similarly, a stocking density of 25 g-worm/kg resulted in the highest potential to increase nitrate/nitrite and Olsen extractable P. The treatments 0; 12.5; 25 and 37.5 g-worm resulted in a net Olsen P increase of 16.3%; 38.9%; 61.0% and 53.0%, respectively, after 10 weeks. Though vermicompost maturity can be attained at a stocking density of 12.5 g-worm/kg, for faster more humified and nutrient rich fly ash vermicompost, a stocking density of 25 g-worm/kg seemed most appropriate. Inoculation of fly ash vermicompost with EM alone did not result in significantly (P < 0.05) different changes in C: N ratio and dissolved organic matter (DOC) compared to the control. Also, though inoculation with EM together with E. fetida resulted in greater changes in C: N ratio and DOC compared to the E. fetida alone treatment, this difference was not statistically significant. Inoculation with EM coupled with E. fetida, however, resulted in significantly (P< 0.05) higher rates of Olsen phosphorus release compared to the E. fetida alone treatment. On average, the EM + E. fetida treatment resulted in a rate of weekly Olsen P release of 54.32 mg/kg, with the E. fetida alone, EM alone and control releasing 48.39; 28.71 and 16.56 mg-P/kg/week, respectively. It was observed that inoculation of fly ash based composts with EM alone is not beneficial, whilst combining EM with E. fetida may result in faster compost maturity and greater Olsen P release. By substituting pine bark growing media with fly ash vermicompost up to 50% significant improvements in water holding capacity, total porosity and air filled porosity was observed, with germination over 90%. The 25% FA treatment also resulted in significantly higher number of flowers and buds compared to the 50% and 75%, despite the higher concentration of essential nutrients in the 50% FA treatment. For effective marigold seedling germination and growth, the 50% FA: 50% PB growing medium is recommended whilst for maturity and flower production, the 25% FA: 75% PB combination is to be preferred as it performed better than all treatments regardless of the nutrient composition of the media. The results of this study have shown that fly ash can be an effective phosphorus source when vermicomposted at an incorporation ration of 1:2 (fly ash: cow dung – waste paper) and a stocking density of 25 g-worm/kg. The fly ash vermicompost can be an important part of planting media which can be used for land reclamation and horticultural crop production. However, optimization of EM incorporation may be required for significant effects of on biodegradation and mineralization of fly ash vermicomposts following EM inoculation.