The Potential of Sitka Spruce Bark as an Alternative to Peat Casing for Mushroom (Agaricus bisporus) Production

Abstract

White button mushrooms are an important crop globally, and due to the role of peat in current cultivation practices, the industry is under increasing pressure to find suitable alternatives. Peat functions as “casing”, a surface layer that, amongst other functions, provides available water to the fungal mycelium and is considered essential for mushroom growth. This research aimed to determine the potential of bark as an alternative casing material and its suitability for commercial mushroom production. Two experiments were conducted, comparing two types of commercially available peat casing with bark-based casings. The bark casing in Experiment A was found to be statistically similar to the peat casing in terms of yield and quality. The two bark casings in Experiment B failed to produce a 1st flush of mushrooms, and total yields were significantly lower compared to the peat casings, highlighting the fact that the consistency of the bark material across both experiments was variable. There were several issues associated with the bark casing, which included water stress and cropping delays, all of which would be unacceptable from a commercial perspective. Further research is required to refine bark-based casing, focusing on a better consistency of the bark feedstock and reduction of contamination risk. This work contributes to ongoing research and development to address the dependency of mushroom production on peat.

1. Introduction

Mushroom production has expanded rapidly in recent decades, with world production increasing from approximately 1 M tonnes in 1978 to 48 M tonnes in 2022 [1]. This trend has only accelerated since the start of the millennium, largely due to the expansion of the Chinese mushroom sector. Between 1997 and 2013, mushroom consumption per capita is estimated to have increased from 1 to over 4 kg per person annually [2]. Button mushroom (Agaricus bisporus) production accounts for around 11% (4.73 M tonnes) of global production with China, Europe and the USA producing 52%, 28% and 7%, respectively [3].

Mushrooms represent Ireland’s most valuable horticultural output. In 2023, exports were valued at €145 M, and this is predicted to increase [4]. Peat plays a central role in current mushroom production practices. It is the main component of mushroom “casing”, which is a pH neutral substrate containing peat and a neutralising agent such as sugar beet lime. It is applied as a 5 cm layer on top of the mushroom compost. The casing layer prevents the evaporative moisture loss from the colonised compost beneath, stimulates the formation of primordia by providing both a suitable microclimate and by hosting beneficial microorganisms, and importantly retains a reservoir of water available to developing mushrooms throughout the duration of the crop. However, peat extraction is under scrutiny due to its impact on peatland habitats, which is linked to loss of biodiversity [5] and greenhouse gas emissions from damaged areas [6]. Consequently, there is increasing impetus to reduce or substitute the use of peat. Many countries have drafted guidance and/or legislation on the use of peat with the goal of transitioning towards more sustainable alternative materials [7,8,9].

Identifying suitable candidate materials to replace the peat in mushroom casing is challenging due to the range of criteria that must be satisfied. Further to being sustainable, any alternative casing material must produce mushroom quality and yield comparable to commercial peat casing. In addition to this, such a material must be economically viable for use by growers [10]. To minimise fluctuations in cost and availability, ideally peat alternatives would be produced locally [9]. Moreover, this would also reduce the environmental impact of long-distance shipping, although casing producers who currently export to international destinations are likely to continue to export peat-reduced casings [11,12]. For this reason, what an ideal alternative casing material looks like may depend on the location of the grower in addition to the cultivation practices employed in that region. Furthermore, any alternative material will be required to have consistent physical, chemical and biological characteristics [13,14,15]. As mushrooms are in direct contact with the casing substrate as they grow, any potential alternatives should be naturally free from microbiological contamination or easily processed to reduce any risk to human health (e.g., composting or pasteurisation [16]). As a result of these requirements, the development of any peat alternative casing material will have to overcome many challenges, most likely requiring considerable refinement before being introduced as a commercial mushroom casing.

In spite of several studies that experimentally assessed the performance of a number of different peat alternative candidates [17,18,19,20,21], there still remains a substantial degree of uncertainty about their efficacy [10]. The coniferous tree Sitka spruce (Picea sitchensis) is the dominant forest tree species in Ireland [22]. Sitka and Norway spruce (Picea abies) are highly important species in plantation forestry across Europe [23,24]. Spruce bark is a by-product of the timber industry, and it is a readily available peat-alternative material. The primary aim of this study was to assess locally sourced Sitka spruce bark as a peat alternative for mushroom casing and to characterise its impact on mushroom cropping performance and quality in comparison with standard commercial peat casing.

2. Materials and Methods

2.1. Casing Treatments

Two commercial casing substrates were obtained from a local peat supplier (Harte Peat, Clones). One was a casing blend delivered in “bulk” bags (2.5 m³); this is referred to as bP in the experiments. The other was a drier “fine” casing blend delivered in smaller 50 litre bags, which is referred to as fP. Both contained blends of deep dug black peat mixed with a small proportion of milled peat and ~10% sugar beet lime (SBL) to neutralise the natural acidity of peat. Two batches of Sitka spruce bark were obtained from a local sawmill (CJ Sheeran, Mountrath). To prepare a casing mix, the bark was mixed with ground limestone and sugar beet lime. In Experiment A, bark from the first batch was mixed with a 50:50 mixture of ground limestone and SBL (+5% by volume). In Experiment B, a second batch of bark was mixed with either a +5% or +10% (by volume) of a 50:50 mixture of ground limestone and SBL. Despite being described as the same bark product by the supplier, the bark used in Experiments A and B was visibly different, with the bark in Experiment B being coarser in size. It was decided to continue with the experiment to see if this variability in the bark material had an impact on crop performance. All casing mixes were prepared 2 days in advance of setting up the crop experiments. Water was added to the mixtures to bring the moisture content up to their maximum water holding capacity while ensuring no water pooled on the material’s surface.

2.2. Mushroom Cropping Details

A commercial strain of Agaricus bisporus (Sylvan A15) was grown in both experiments. Mushrooms were grown in plastic crates, with a cropping area of 0.2 m². Each crate contained 16 kg of commercially sourced fully colonised bulk phase 3 mushroom substrate. The substrate was compressed, and each crate was topped with enough casing to achieve a depth of approximately 5 cm. Then, 600 mL of water was sprayed over the surface of each crate (3 L/m² equivalent). The cased crates were placed in a purpose-built experimental mushroom growing room at the Teagasc Ashtown Mushroom Research facility in Dublin, Ireland. Crops were grown under environmentally controlled conditions following standard commercial growing procedures.

The compost temperature was maintained at 25 °C for the first week to encourage the mycelium to colonise the casing layer. During this time, fungal respiration causes the levels of CO₂ to rise. The temperature was then reduced to 20 °C over a 48–72 h period and filtered, fresh air was introduced (this process is known as airing) in order to lower the levels of CO₂ to 1800 ppm, which is optimal for mushroom production (ambient levels are 400 ppm). The reduction in air temperature coupled with the introduction of fresh air induces the formation of mushroom primordia, known as pins. The relative humidity of the growing room was held at 90–95% initially, dropping to 85–90% after airing. During the first week, plots were watered twice and again at the end of each flush at a rate equivalent to 10 L/m² or approximately 2 L per plot. The application of water was stopped if the casing appeared saturated or showed symptoms of panning, which are not desirable for optimal mushroom cultivation [17,25,26]. Both experimental trials ran for 5–6 weeks, with three flushes of mushrooms being harvested over three to four weeks, predominantly as closed cups (45–60 mm). The fresh mass of these was recorded as the yield. Details of this process are shown in Figure 1.

2.3. Experiments

Two experiments were conducted. In Experiment A, there were three casing treatments—two peat controls and one bark treatment (bP, fP and B5). Each treatment was replicated 8 times to give 24 crates. In the 1st flush of Experiment A, some of the mushrooms were cleared without being yielded, resulting in lower numbers of replicates in this flush (6 replicates of B5 and 5 of bP and fP). In Experiment B, there were four casing treatments—two peat controls and two bark treatments (bP, fP, B5 and B10). Each treatment was replicated 3 times to give a total of 12 crates. The reduced number of replicates was related to the volume of materials that it was possible to procure at that time. Experiment A commenced on 24 February 2022 and concluded on 31 March 2022 (35 days) and Experiment B ran from 13 May 2022 until 21 June 2022 (39 days).

2.4. Casing Water Content

The gravimetric water content (GWC%) of the casing mixes was recorded throughout each experiment. In Experiment A, 5 × 1 g samples were taken from each casing mix approximately once every 7 days. The samples were dried in an oven set to 100 °C for 2 h. In Experiment B, the sample size of the casing material was increased to 5 × 3 g, pooled from each replicate to better account for heterogeneity of the casing mixes. These samples were dried overnight at 105 °C to ensure complete desiccation. The percentage moisture content was calculated in both experiments using the following equation:

GWC % = ((initial weight − dry weight)/dry weight) × 100%

2.5. pH

pH was recorded using a Hanna Instruments (Woonsocket, RI, USA) pH meter. The casing sample was combined in a 1:5 ratio with deionised water; this solution was agitated for 1 h in accordance with EN13037:2012 [27].

In Experiment A, pH was only recorded at the start of the experiment, as a quality check on the casing material. In Experiment B, pH was recorded at the start of the experiment as a quality check on the casing materials and on a weekly basis throughout the experiment. This corresponded with key moments in the progression of the mushroom crop: prior to pinning and at the end of the 1st, 2nd and 3rd flushes for each treatment. At each timepoint, samples were taken from multiple plots of each treatment and pooled to give a single pH reading for each casing treatment at each time point. The pH recorded for each treatment at the start of each experiment is detailed in

Table 1. pH values recorded for each treatment at the start of experiments A and B.

	Experiment A	Experiment B
bP	7.48	7.83
fP	7.01	7.82
B5	7.50	7.76
B10	-	7.94

.

The acceptable pH range for mushroom cultivation is 6.8–9, with the optimum pH being 7.8 [28]. Thus, all initial pH readings fell within the acceptable range.

2.6. Mushroom Colour Assessment

In Experiment A, mushroom colour data were recorded as a metric of mushroom quality [29]. A CR-400 (Konica Minolta, Tokyo, Japan) handheld colorimeter was used to determine the colour of the harvested mushrooms. Readings from a sample of 3 mushrooms per replicate crate were taken at three time points during the trial, on the last day of the 1st, 2nd and 3rd flush. Mushrooms with visually evident indications of discolouration, such as bruising, were discounted. Measurements of lightness (L) were recorded along with chromaticity coordinates (a and b).

2.7. Experimental Design and Statistical Analysis

Both Experiment A and Experiment B were set up according to a randomised block design. Experiment A consisted of 3 casing treatments and 8 blocks with one replicate in each block; Experiment B consisted of 4 casing treatments in 3 blocks, with one replicate in each block.

All statistical analyses were conducted in R and RStudio. ANOVA was used to test for statistically significant treatment effects, with the threshold for significance being 95%. Linear models were used to assess the properties of data and ensure the assumptions of ANOVA were not violated. The Akaike information criterion (AIC) was used to select the most appropriate models in all cases; this is based on the comparative ability of each model to explain the variation in the data. Tukey’s Honestly Significant Difference (HSD) test was applied to identify the significance of the relevant interactions and the effects to be assessed.

3. Results

3.1. Yield

In Experiment A, the three casing treatments were found not to differ significantly from each other with regards to mushroom yield; this was the case within each flush (Figure 2) and for total yield (Figure 3), although the 1st flush yield from the bark casing was slightly lower than for the two peat casings. Mean yield (kg/t) across all flushes was bP = 124, fP = 128 and B5 = 120.

In Experiment B, both bark-based treatments failed to produce a 1st flush at the expected time. For this reason, they are recorded as having no yield at this timepoint. The mean yield (kg/t) across all flushes was bP = 115, fP = 104, B5 = 80.2 and B10 = 78.3. In this experiment the overall yield of peat treatments differed significantly from each of the bark-based casings (p = 0.001 in all cases).

3.2. Moisture

At the start of the trial, in Experiment A there was no significant difference in moisture content among any of the casing treatments (Figure 4). However, by the time point prior to pinning, B5 had a significantly lower moisture content than bP (p < 0.001). By the end of the 1st flush, the same trend was observed, with bP having significantly greater moisture content than B5 (p < 0.001). By the 2nd flush, bP had significantly greater moisture content than either fP or B5 (p < 0.05; p < 0.001, respectively). At the end of the 3rd flush, the difference in moisture content among all casings was found to be insignificant.

Throughout Experiment B, both peat casings had significantly greater moisture content than either of the bark treatments (p < 0.001) (Figure 4). Additionally, bP was found to have significantly greater moisture content than fP (p < 0.001).

3.3. Mushroom Colour (Experiment A Only)

The colour analysis of Experiment A indicates that the whiteness of mushrooms grown on B5 did not differ significantly between the 1st and 2nd flush. However, the 3rd flush had significantly lower whiteness than both subsequent flushes (p < 0.001). There was no significant difference between the whiteness of mushrooms grown on either peat treatment across flushes. Mean L values in each treatment across all flushes: bP = 88.6, fP = 88.9 and B5 = 89.1.

It was found that mushrooms grown on B5 in the 1st flush were significantly lighter than those grown on bP. In the 1st flush, the mean L value of B5 = 90.6 while bP = 87.4 (p < 0.001). In the 2nd flush, whiteness was found not to differ significantly between treatments. However, in the 3rd flush, the mushrooms grown on bP had a mean L value of 89.5 versus a mean L value of 87.6 recorded on mushrooms grown on B5. This difference narrowly qualifies as statistically significant (p < 0.05) (Figure 5).

3.4. Additional Observations

3.4.1. Contamination

All precautions were taken to ensure casing handling and preparation was carried out in a clean, controlled environment. In Experiment A, it was noted that the B5 was contaminated with a basidiomycete fungus, most likely a species of the genera Parasola (Figure 6). Contamination by this wild fungal species became apparent during the 1st flush when fruiting bodies appeared. While this species did not appear to be competing directly with the crop of Agaricus, its spores stained a small number of the crop. Though extremely unlikely, it is possible that this contamination may have occurred by chance or as the result of failure in handling procedures. Moreover, these wild mushrooms disappeared by the 2nd flush.

3.4.2. Consistency of Bark

There were substantial differences in the physical properties of the bark batches used in Experiment A as compared to Experiment B. The bark material used in Experiment A was considerably finer than the material sourced for Experiment B, where the bark was coarser and less finely ground, more closely resembling chipped wood (Figure 7). The latter was also noticeably drier, which was reflected in the results of the moisture content analysis (Figure 4). Despite attempts to water the material to a higher moisture content, the bark material in Experiment B was easily saturated; however, it had poor water retention. Additionally, in Experiment A, B5 was found to have marginal cropping delays in the first flush. However, in Experiment B, both B5 and B10 experienced much more substantial delays, failing to undergo pinning and sporulation at the expected time. This delay in mushroom formation was approximately one week in Experiment B.

4. Discussion

The results of Experiment A revealed that there is some potential for bark-based alternative casing materials. Despite the reduction in yield and cropping delays in the first flush, it is possible that crop management could overcome these challenges. While the performance of B5 in Experiment A was promising, the results of Experiment B were much less encouraging. In Experiment B, there was a delay of approximately one week in the pinning and formation of mushroom fruit bodies in both bark-based casing treatments. Similar changes to crop timing have been reported in previous studies of peat alternative casing [30,31].

The pronounced discrepancy in the performance of the bark-based casing treatments in Experiments A and B highlights the inconsistency of the batches obtained from the same supplier. The physical properties of the bark used in Experiment A were different from those used for Experiment B, despite coming from the same source. Based on visual observations, the material used in Experiment A was finer, whereas the material used in Experiment B was considerably coarser, more closely resembling chipped wood. The bark material used in Experiment A had superior water retention based on the differences in moisture content recorded between the two experiments (Figure 4). In addition, the bark in Experiment B appeared more resistant to absorbing water, reaching saturation more rapidly during initial wetting and subsequent watering. This discrepancy in properties is likely the cause of the pronounced developmental delays observed in Experiment B. Following communication with the supplier, it was determined that the material procured in Experiment A was fine fresh Sitka spruce bark, while the material used in Experiment B was a mixture of medium grain bark and wood. It is also likely that these materials had been stored in heaps for differing periods of time, further influencing the differences in their characteristics. This batch-to-batch inconsistency would not be acceptable for a commercial casing production facility, which is dependent on consistent ingredients to make reliable casing media. It also indicates the necessity for consistency in the characterization of primary materials in future studies.

In addition to improving the consistency of the materials, it is possible that the performance of bark-based casing could be improved by the incorporation of additional materials. Combining multiple alternative materials with disparate physical properties could interact synergistically to produce a casing material with improved characteristics. For example, bark has a lower capacity to retain water than peat, but this may be improved by blending it with alternative materials that have a high capacity to absorb moisture—SBL could be just such a material [32]. In Experiments A and B, SBL and limestone were primarily added to lower the pH of the material, to bring it into line with the pH of the peat casings. Peat casing is typically blended with 10% SBL by volume to neutralize the natural acidity of the material in its natural state, making it slightly alkaline. Additionally, SBL has excellent water holding capacity, further enhancing the casing. The SBL and limestone mixture used in these experiments may have had a divergent effect depending on the particle size of the bark. For example, the liming mixture added to the bark in Experiment B may have only marginally improved its water holding capacity, unable to counteract fully the larger particle size of the bark, but may have had a more pronounced benefit when combined with the finer grade bark used in Experiment A.

The potential of peat alternative casing materials that incorporate multiple materials remains speculative in the context of these experiments. However, there is some evidence that such blended alternative casing materials could have a beneficial impact on yield [14,17,33]. Blended materials with enhanced physical properties will likely be necessary for commercial viability and suitability for commercial scale production. Future experiments could investigate the performance of such materials.

Colouration data was recorded in Experiment A. Colour plays a crucial role in consumer perception of food quality [34]. For consumers, mushroom whiteness is the primary indicator of quality and freshness. The decrease in quality of mushrooms associated with each subsequent flush of mushrooms is largely related to changes in colouration [35,36,37]. Moreover, white button mushrooms are prone to such discolouration [38]. Thus, the economic viability of an alternative mushroom casing may be influenced by the colour of the mushrooms produced.

Given this background, the results of Experiment A are encouraging. The finding that mushrooms grown on bark were significantly lighter (L value) than those grown on the peat control casing at the end of the 1st flush is especially promising as the 1st flush is generally associated with the greatest harvests of mushrooms compared to later flushes during commercial production [39]. It must also be considered that colour measurements were taken for only a small number of mushrooms and at the end of each flush. Therefore, the results may not be representative of the crop as a whole and further testing is required.

While not quantitatively assessed in this study, there was some evidence of a clumped pattern of growth on the bark-based casing in the 1st flush of Experiment A. This would be undesirable from a commercial perspective as this does not allow the fruiting bodies the space required to grow normally. Similar growth patterns have been noted in other alternative casing material [40]. It is possible that clumping could be caused by heterogeneity of microbiota [16], poor dispersion of water within the casing and/or toxic compounds in the casing that are unfavourable to Agaricus growth [15,40].

The appearance of a wild fungus contaminating the bark-based casing treatment in Experiment A indicates that further measures will need to be taken with the processing of bark-based casing materials if they are to be used commercially. The presence of undesirable fungi and moulds was not entirely unexpected, as similar contaminants have been noted in a range of other experimental trials looking at alternative casing materials [19,33,41]. This contamination would be unacceptable in a commercial casing material. However, it is possible that composting of the raw material could help to eliminate these contaminants before the material is utilised as casing. Future studies should include toxicological analysis to understand the potential risks from contamination in greater detail.

In addition to observations on the performance of the bark-based casing in this study, it is important to note the broader context of identifying peat alternative mushroom casings. Coniferous barks have been previously demonstrated to have potential for peat reduction, with mixed casings of peat and bark producing statistically similar mushroom yield to standard peat controls. In one study, where 25% pine bark fines were incorporated by volume, yield of the alternative was reduced by less than 10% compared to the black peat control [32]. This reduction was not reported as statistically significant. Similarly, the incorporation of 70% wattle bark into peat casing was found not to significantly affect mushroom yield as compared to controls. While statistically insignificant, this casing mixture was found to slightly outperform the peat control, producing 17.5 kg/m² compared to 17.0 kg/m² [42]. However, little research has been conducted as to its potential as a peat-free casing material. Evaluation of coniferous bark, and Sitka spruce in particular, as a source of alternative casing material is valuable as coniferous trees are common in plantations globally, meaning that bark by-products should be available in a wide range of locations. This increases the international relevance of this research.

However, there may be other materials that are more locally available in specific regions. For example, coconut coir may be more accessible in tropical and subtropical regions. Coir specifically has been a popular choice of experimental alternative casing material in the scientific literature, and has demonstrated some potential for mushroom production, although its performance varies between studies [20,30,31,43]. The local availability of materials should not be underestimated in the search for sustainable peat-free casing alternatives. Moreover, peat itself varies considerably based on source and geographic origin, which may affect the performance of the casing and the quality of the mushrooms produced in different regions of the world. Moreover, this results in differing demands on growers in different locations regarding the yield and quality of mushrooms produced [10]. Therefore, a casing alternative that would be unsuitable in one region may be perfectly adequate to meet the needs of growers elsewhere. This holistic, international perspective must be kept in mind regarding research in this field.

5. Conclusions

The yield and quality of mushrooms produced on one bark-based casing treatment in this study were promising in terms of identifying a novel peat-free casing material. However, a repeat experiment with a different batch of bark from the same supplier gave different results. This highlights the need to establish the essential characteristics of a prospective bark alternative for use in mushroom casing. Further research is needed to develop acceptable standards and characteristics that such a material should possess to ensure the consistency of the material and its performance in a series of replicated trials. For a bark-based casing alternative to be commercially viable, consistency is of the utmost importance. There is potential to improve this consistency with measures such as a set protocol that could be followed by timber producers in order to obtain consistency. Furthermore, the observations of contamination would present a potential challenge in a commercial setting. Therefore, for this material to be used in commercial settings, it will be necessary to find a way of eliminating any such contamination, perhaps through a standardised process of composting. Finally, while this study identified several challenges that will need to be overcome before a bark-based casing alternative is truly suitable for commercial production, the results are encouraging. This indicates the potential for further development of bark-based casing alternatives and the potential to incorporate additional materials to enhance performance.