Composting and vermicomposting of cattle manure and green wastes under tropical conditions: carbon and nutrient balances and end-product quality

Composting and vermicomposting are interesting options for utilisation in restoration and improvement of weathered and infertile tropical soils. The aim of this study was to assess quality of composts and vermicomposts produced from cattle manure (CM) and green wastes (GW) blended at different ratios under tropical conditions. Chemical, biochemical, and biological indicators were used to identify the role of earthworms and the factors affecting organic matter (OM) stability and nutrient balance in the thermophilic (days 0–65) and stabilisation (days 66–183) phases. Total carbon (C) losses averaged 55% and were greater during vermicomposting and for products with a high GW content. One-third of C losses occurred during the stabilisation phase. This phase presented a high level of C mineralisation, which would be linked to high ambient temperatures (~30°C). Although OM content was similar for all of the final products, respiration measurements indicated that OM stability was greater for vermicomposts. These results indicated that humification and OM decomposition occurred simultaneously during vermicomposting. The material source ratio did not affect the OM biodegradability of the final products. No losses were observed for phosphorus (P), calcium (Ca), and magnesium (Mg). However, high potassium (K) (45%) and nitrogen (25%) losses were detected and were higher during vermicomposting and for the products with a high GW content. Final vermicomposts were enriched in P, Ca, and Mg, and slightly depleted in K compared with normal composts. Nitrogen content was similar for both composting methods. Plant response to amendment addition was higher for intermediate rates of GW and CM (e.g. 40–60% for CM), with no differences between composts and vermicomposts. The results of this study indicated that vermicomposting was a process that favoured OM stabilisation, which is a key factor for the adoption of this practice in the tropics.