Optimising nitrogen transformations in mushroom production (MU17004)

The project team has been focused on adding nitrogen-containing supplements to compost during cropping, and on adding microbial inocula during spawn run to promote mycelial growth. 

Targeted supplementation of cropping compost with nitrogen immediately before each flush was shown in the previous project period to increase mushroom yield by about seven per cent. To realise this on a commercial scale, a drip irrigation method was designed by which liquid nitrogen supplements could be dispersed specifically into the compost during cropping, without affecting the casing. Use of the drip irrigation method alone gave an immediate increase in yield, and work is continuing to test the effect of adding nitrogen supplements using this system. The system is designed for addition of required nutrients at specific points during growth when demand for these nutrients is at its highest (e.g. at pinning or at cap expansion), and has the potential also to be used to supplement other compounds (e.g. fatty acids, selenium, etc).

Addition of microbial inocula during spawn run stimulates mycelial growth, but the effect on yield is not known. The compost bacteria Pseudoxanthomonas, Bacillus and Chelatococcus were co-inoculated into spawn run together with the dominant fungus Mycothermus (formerly called Scytalidium). An increase in mycelial growth rate was observed even at low levels of inoculation, suggesting that the effect is caused by a symbiotic effect on the mycelium, rather than just by adding additional microbial biomass as nutrition. Efforts to measure the effect of inoculation on mushroom yield in a full scale cropping experiment were frustrated in the current project period by pest problems, and are continuing.

This project explores the fate of nitrogen in mushroom compost and mushroom production, allowing growers to optimize the rate and timing of nitrogen additions to achieve maximum yield and crop nutritional value. Work in the current project period has focused on the timing of supplements added to compost during cropping, and on the efficacy of microbial inocula added during spawn run in promoting mycelial growth.

Supplementing crops with nitrogen immediately before each flush provided an increased yield of about 7 per cent, with much of this increase occurring in second flush. The nitrogen content of mushroom caps and stipes was found to be significantly enhanced in mushrooms from second and third flush, which contained 7-8 per cent N (dry wt) compared with 4-5 per cent N (dry wt) in first flush mushrooms. Similar results were found on a commercial farm, suggesting that late-flush mushrooms may be described as “high nitrogen” versions of the crop.

Test of mycelial growth promotion with combinations of abundant compost bacteria including Pseudoxanthomonas, Bacillus and Chelatococcus revealed that stimulation of mushroom growth was dependent on co-inoculation with the dominant fungus Mycothermus (formerly Scytalidium). Work is continuing to establish the effect of these treatments on crop yield.

Since the last project update, work has focused on completing an overall nitrogen balance for mushroom composting and cropping, reviewing alternative nitrogen sources for Australian composters, and designing and testing microbial consortia for compost inoculation.

Detailed measurements taken during Phase 2 of composting showed nitrous oxide production much lower than expected, and ammonia released by the compost largely reabsorbed. Major loss of nitrogen during composting therefore occurs primarily during Phase 1, partly as leachate and partly as ammonia release.

Calculating an industry average for these losses is difficult because of variation in the proportion of recycled leachate used by different composters for straw wetting. Further limitations to nitrogen balance calculations were identified in the overall mass losses experienced by all composters.

Possible alternative nitrogen sources for the Australian mushroom industry were reviewed, focused on agricultural by-products already tested overseas. The need for an up-to-date inventory of the types, quantities, and localities of agricultural and food production by-products was highlighted. Local availability is particularly important in establishing test experiments for these feedstocks.

Ten bacterial taxa were identified as potential consortium for compost inoculation. The dominant genera are Pseudoxanthomonas, Chelatococcus, Chelatovorans, Thermus and Thermobacillus. These will be combined with Mycothermus, the dominant fungus in Phase 2 compost, for stability and functionality testing.

While some impact due to COVID-19 restrictions disrupted research progress, the team continues to work to mitigate any delays.

Since the last project update, a survey of 10 Australian mushroom composting facilities across four States has been completed. It included a comparison of composting management processes and compost bacterial activity. The results delivered initial insights to inform the selection of compost yards for further nitrogen management analysis.

The survey revealed that an average of 10 per cent of input nitrogen is incorporated into the mushroom crop, and about 20 per cent of the total is lost as leachate or nitrogen-containing gases. Nitrogen losses also occur from the compost and casing during cropping. To better understand this process, two cropping trials have been established in the Marsh-Lawson Mushroom Unit. Analysis has unfortunately been delayed due to Covid-19 impacts.

The team have isolated specific microorganisms from high temperature composts, with the dominant strains identified by DNA sequencing and characterised. Interactions between the two main compost fungi (Mycothermus thermophilus and Agaricus bisporus) and the dominant Phase 2 bacterial taxa (Pseudoxanthomonas spp) have been examined in more detail. Due to the high relevance of these bacterial taxa, their entire genetic sequence has been determined, with analysis of their functional capabilities ongoing.

The compost isolate collection contains 175 isolates of 58 different species. This is now sufficient for the design of potential compost inoculation treatments aimed at optimising the composting process.

The reporting period coincided with Covid-19 related work and travel restrictions, resulting in considerable disruption to research progress. The team will continue to share results with industry as opportunities become available.

The project team commenced work in January 2019 and have already isolated and characterised a substantial collection of bacterial strains taken from a range of Australian compost yards in New South Wales, Victoria, South Australia and Tasmania. These samples represent the dominant cultivable bacteria in composts and will underpin the rest of the project.

Many of the most prevalent strains identified were very similar, despite coming from different geographical areas, confirming the conserved biological nature of mushroom composting across Australia.

Analysis of the composts revealed that many of the dominant species have not yet been captured in the strain collection. These are now being targeted using specifically designed growth media and selective conditions.

A detailed survey regarding current nitrogen management by Australian composters was started, with field trips to facilities in Victoria, New South Wales and South Australia, and visits planned to Queensland. The results of this confidential survey will be used to identify composting yards that represent the diversity of processes in the Australian industry, informing the project’s detailed nitrogen balance studies.

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Read an overview of the project in this article, Optimisation of nitrogen use in mushroom production, published on pages 34-35 in the spring 2019 edition of the Australian Mushrooms Journal.