Carrier potential of spent mushroom substrate-peat formulations for legume nodulating bacteria in biofertilizer production

Abstract

Globally, legume inoculants are an invaluable agricultural technology especially in legume crop production. In Uganda, Makerere University is promoting the technology as MAK_BIO_N_FIXER product. Presently, using peat for LNB inoculants production increases production costs as landlord (Owner) is paid for each trip of peat collected. Additionally, peat mining from wetlands degrades the environment hence renders peat excavation undesirable in future. The study objective was to identify alternative carrier materials for inoculants production in Uganda. Specifically, the aim was to explore the potential of Spent Mushroom Substrate (SMS), a mushroom cultivation by-product, as a carrier material. A social economic feasibility analysis for the use of SMS in agriculture was conducted among mushroom farmers from Kampala District Farmers Association (KADIFA). Evidently, SMS offered a viable economic alternative as a carrier material for inoculants production. Many farmers were not maximally utilizing this material for agricultural purposes hence were willing to sell the SMS to a prospective buyer at a modest cost. Presently, some farmers dispose SMS through burning and dumping on garbage skips of a few others use the by product for backyard gardening. Further, laboratory and greenhouse studies were undertaken to assess the efficacy of SMS as LNB inoculants carrier material. The SMS was then collected from KADIFA farmers, composted for eight weeks and dried afterwards. The dried and processed SMS mixed with peat in different ratios (0-100% SMS-Peat) were pre-tested to obtain the suitable carrier formulations for experimentation. The selected 10 and 20% SMS-peat formulations were either gamma irradiated, pasteurized, autoclaved or unsterilized. The carriers were then used to produce bean (Phaseolus Vulgaris) nodulating bacteria (BNB) strain CIAT 899 inoculants. Surviaval of BNB was assessed using cultural and MPN techniques under laboratory and growth room conditions. In addition, the symbiotic potential of the test inoculants was assessed under screen house. Autoclaved and pasteurized 10 or 20% SMS mixed with respective 90 and 80% peat formulations significantly improved the suitability of SMS as inoculants carrier material. Furthermore, the pasteurized and autoclaved carrier materials improved the microbial quality/shelf life of inoculants. Gamma irradiation mainly improved the quality of peat inoculants but with a negative effect on SMS inoculants resulting into a much lower survival rate of BNB compared to unsterile SMS. The study indicated validity of using SMS as inoculants carrier when smaller amounts of 10 and 20% SMS are formulated with peat (90 and 80% peat). Furthermore, the pasteurized / autoclaved carriers improved the quality /shelf life of inoculants. The yield benefits from these inoculants were realized during legume production in the greenhouse. More studies recommended on the performance of the formulated carriers on the different legume bacterial species, additives to improve SMS as a BNB carrier material, cost of sterilizing and duration/number of cycles of SMS, investigating the possibility of metabolic by products in SMS before and after sterilization, effect on LNB growth of the different substrates where SMS is generated form and the long term effect on storage of such inoculants.