Goal and scope definition
The goal of this study is to characterize a typical commercial mushroom production system in the US using process-based LCA that largely conforms to the International Organization for Standardization (ISO) guidelines for LCA, ISO 14040 and 14,044 (ISO 2006). Mushroom production is typically divided into three distinct phases: composting and composting operations (phase 1), pasteurization and conditioning to finish the compost (phase 2), and growing and harvesting of mushrooms (phase 3). In phase 1, the compost is created by wetting and mixing bulk ingredients, nitrogen supplements, and gypsum, and then composted using processes such as aeration, turning, and watering for 6–14 days. In phase 2, the compost is pasteurized to kill pests and fungi and is conditioned to remove ammonia, all of which can damage the mushroom crop (Beyer 2017).
Unlike most crops where seed is planted, the mushroom is introduced to the compost through spawn. In phase 3, spawn is created by propagating mycelium on a grain material, and then the spawn and additional supplements are mixed into the finished compost. Mushrooms are grown in sealed, insulated rooms where the temperature, humidity, and carbon dioxide (CO2) concentration are carefully controlled. Once the compost, spawn, and supplements are in place, casing (mostly comprised of peat moss) or casing inoculum (casing seeded with mushroom spawn) is applied over the top of the mixture. Mushrooms can be harvested 18–21 days after casing in 7–10-day cycles for 35–60 days (Beyer 2017). After the final harvesting, the remaining material, referred to as spent mushroom substrate (SMS), can be used for other applications, for example, as a soil amendment.
The scope of the LCA is from cradle-to-facility gate with a functional unit of 1 kg (wet basis 92% moisture content) of bulk mushroom product (USDA 2016). Figure 1 illustrates the modeled system, showing each of the three phases, as well as the production of SMS. Material and energy inputs such as pesticides, cleaning products, compost materials, peat, electricity, and fuels; outputs of emissions from fuel combustion and off-gassing; and transport processes are all included in the system boundary. The construction, repair, and decommissioning of facility structures and equipment are excluded.
Life cycle inventory
The foreground system, which is comprised of the processes directly involved in production, is characterized using survey data as described in Sect. 2.2.1, and emission rates and factors for processes such as off-gassing are drawn from literature (Sects. 2.2.3 and 2.2.4). The background system, comprised of the upstream processes (upstream with reference to the supply chains) that produce the inputs or provide the services consumed by the foreground system, is characterized by secondary data, namely reference life cycle inventory (LCI) datasets sourced from the GaBi software tool (Thinkstep 2016) as described in Sect. 2.2.2.
The SMS co-product has no economic value but does have a use as a soil amendment; thus, co-product allocation is handled two ways, first through economic allocation where no production-related impacts are attributed to SMS because of its zero economic value to producers and, second, through a displacement calculation assuming that it is used as a soil amendment, as described in Sect. 2.2.6.
Primary data collection
Two different surveys were administered to US mushroom producers in 2015: one administered by Sureharvest and one by the University of California Davis LCA research team. Both surveys generated data used in this study. Primary data were gathered from 22 mushroom compost and mushroom production facilities nationwide, accounting for approximately one third of total national mushroom production (USDA and National Agricultural Statistics Service 2016). The Sureharvest survey consisted of 285 questions on the on-site use of water, energy, fuel, labor, detailed raw material inputs, composting timelines, the composition of finished compost, recycling and shrink data, growing facility data, yield data, and SMS information. The UC Davis survey was administered as a follow-up survey and contained another 43 questions on spawn, recycling of finished products, co-products, on-site electricity generation, transport of raw materials and interim products, and the uses of insecticides, fungicides, microbicides, and sanitizers. Meetings with facilities were conducted to query certain data points and verify data. The LCIs generated based on the data collected for this study are presented in weighted and unweighted averages/kg of mushroom product (Table S1, Electronic Supplementary Material).
Reference life cycle inventories
The GaBi Professional database (Thinkstep 2016) and the Ecoinvent database (Ecoinvent Center 2016) provided the LCI datasets used in the model. Where available, LCIs based on US conditions were selected. Combining LCIs from two databases was necessary as neither database could provide all the needed LCIs. Table S2 in the Electronic Supplementary Material reports the dataset used for each input. With one exception, the same dataset is used regardless of the location of a mushroom producer. Electricity is the exception, as LCIs differ based on the regional electricity grid of each producer as defined by the North American Electric Reliability Corporation (NERC). Each grid uses different fuels and technologies to produce and deliver electricity, and thus their life cycle impacts are different.
Some inputs to mushroom production do not have existing LCI datasets, such as some pesticide, fungicide, insecticide, and sanitizer inputs. When unavailable in either of the two databases, LCIs were created based on the active ingredients reported in the respective material’s material safety data sheet. Table S2 (Electronic Supplementary Material) also describes the LCIs created for this project.
Transport of materials
The average transport distances for material inputs are based on the material manufacturer and location information reported in facility survey data. For all materials, the transport distances from manufacturer to facility (one-way transport) are averaged separately for each material type (Table S9, Electronic Supplementary Material). The distance for each input is multiplied by the mass of material input and then multiplied by the GaBi reference LCI for US freight truck transport, based on units of freight intensity. Freight intensity units account for average backhaul rates, meaning only one-way transport distances need to be modeled. Transport within facilities is accounted for using reported annual fuel use data.
Compost production
Compost may be produced on site at the mushroom growing facility or may be produced at a compost-only facility and transported to mushroom growers. For facilities that do not produce mushrooms, a calculated ratio is used to equate emission values to a per-mass of mushroom basis (Eqs. (1) and (2)).
$$ \frac{mass\ (tons)\ of\ phase\ 1\ compost}{mass\ (tons)\ of\ phase\ 2\ (finished)\ compost}=1.40 $$
(1)
$$ \frac{mass\ (tons)\ of\ phase\ 2\ (finished)\ compost}{mass\ (tons)\ of\ mushrooms\ produced}=3.63 $$
(2)
These calculated ratios are based on average mass flow at each phase in the production system. The calculated mass of phase 1 compost relative to phase 2 finishing compost equaled 1.40 ± 0.14, and the mass of phase 2 compost to mushroom produced equaled 3.63 ± 0.69.
Compost emissions are released during the composting process, and these emissions include methane (CH4), nitrous oxide (N2O), and ammonia (NH3). Composting emissions can vary based on composting practices (e.g., aeration and turning) and conditions (e.g., temperature, moisture content). Phase 1 may occur either in windrows or tunnels; phase 2 always occurs indoors, in spaces often referred to as tunnels. Emission factors for composting are taken from a peer-reviewed study of a windrow system conducted by Saer et al. (2013): 4.06 × 10−4 kg (NH3)/kg compost, 1.83 × 10−3 kg CH4/kg compost, and 7.50 × 10−5 kg N2O/kg compost.
Tunnel-based composting emits approximately twice the ammonia emissions than windrow-based composting (Cadena et al. 2009). A source for peer-reviewed literature values for tunnel-based compost emissions was not identified. As such, the Saer et al. (2013) windrow-based compost emissions values are used for both windrow and tunnel composting systems. Given the variability in the emissions generated from windrow and tunnel composting systems, this may lead to an underestimation of the tunnel-based composting emissions. It is also important to note that tunnel-based composting emissions can be captured in biofilters which would reduce or eliminate this difference (Cadena et al. 2009; Sánchez et al. 2015); however, the surveys administered for this study did not query biofilter use, and thus filter use and related emissions abatement are not accounted for in the assessment.
Peat emissions
Peat is a carbon-rich soil-like material that is used as casing in mushroom production. Peat forms in bogs and other similar environments where plant matter accumulates, rather than degrading, and accrues and stores carbon for periods of centuries or millennia. Peat begins to degrade upon exposure to oxygen and in doing so releases fossil carbon dioxide (CO2). The peat emission factor for CO2 (0.3726 kg CO2/kg of peat per year) is based on peer-reviewed literature values from a peat study that sourced peat from regions across Canada including Quebec, Ontario, and Manitoba (Moore and Dalva 1997). Moore and Dalva (1997) measured emissions from peat samples held in a controlled environment (i.e., temperatures 15–20 °C (59–68 °F) and aerobic conditions. These environmental conditions are similar to the mushroom growing environment (i.e., ambient temperature ≈ 18 °C (65 °F)) and aerobic conditions. Because, according to the survey data collected for this LCA study, US mushroom producers mainly source peat from Canada, and the environmental conditions (temperature and oxygen levels) in mushroom production are comparable with the environmental conditions in the Moore and Dalva (1997) study, the measured emissions values from that study are assumed adequate for estimation of peat emissions in this study.
SMS co-product and allocation
SMS is the used portion of the growing medium from mushroom production that no longer has all the nutrients needed for growing mushrooms but retains nitrogen (N) and phosphorus (P) content which may have value for other cultivation systems. SMS also contains carbon (C), with a C/N ratio of 9–15:1 (Roy et al. 2015), and could contribute to soil C. The primary use of SMS is as a soil amendment in agricultural systems.
Based on survey data, SMS has no value to the mushroom producer (survey respondents indicated that they received no payment for the SMS), but they also incur no cost of disposal as SMS users pick up the SMS at no cost to the mushroom producers. Two options for considering how to conduct co-product allocation are considered: First, allocation can be applied based on economic value, which leads to no impacts attributed from mushroom production to the SMS because the producer receives no value for SMS generation; second, a displacement calculation can be used to estimate the environmental value of SMS production assuming that it displaces substitutable products. Both approaches are used in this article. In all results, SMS credits are reported separately, along with total results with and without SMS credits. By doing so, both the economic allocation results (which allocate no production-related impacts to SMS) and the results of displacement calculations are reported.
N and P nutrients are typically provided to agricultural soils through synthetic and mineral sources. As such, when SMS is used on agricultural soils, it can displace these nutrient sources and SMS can receive a credit for avoiding their production. While additional C may be beneficial to a soil, SMS is modeled to displace soil amendments that provide nutrients only. It is important to note that the C content in SMS is a labile form that does not contribute to soil aggregate formation nor long-term soil C sequestration (Peregrina et al. 2014); as such, this study applies no C sequestration credits for the C content of SMS. This assumption errs on the side of undervaluing the C content of SMS from a carbon accounting standpoint.
Based on the N and P content of SMS, LCIs for two common fertilizers, an N fertilizer (urea ammonium nitrate; 30% N content) and a P fertilizer (triple superphosphate; 45% P content), are used to estimate avoided impacts due to the production of these fertilizers. The LCIs (sourced from GaBi) are used to calculate the value of SMS as a substitute for the synthetic and mineral fertilizers. The average nutrient content for SMS is 1.12% nitrogen and 0.29% phosphorus on a wet weight basis (Fidanza et al. 2010), and this information is used as described in Eq. (3) to estimate a co-product credit.
$$ Credit\ per\ kg\ SMS=\left(0.0112\ kg\ {N}_{SMS}\right)\times \left({N}_F\ LCI\right)+\left(0.00029\ kg\ {P}_{SMS}\right)\times \left({P}_F\ LCI\right) $$
(3)
where SMS refers to spent mushroom substrate, NSMS refers to nitrogen content of SMS, NF LCI refers to the nitrogen fertilizer LCI (assuming a functional unit of 1 kg N), PSMS refers to the phosphorus content of SMS, and PFLCI refers to the phosphorous fertilizer life cycle inventory (assuming a functional unit of 1 kg P).
Impacts for other material inputs (e.g., wheat straw) that are produced from multi-product systems are calculated using economic allocation and averaged market prices for 2016–2017. All allocation calculations are documented in Table S2 in the Electronic Supplementary Material.
Life cycle impact assessment
LCAs can select from among many life cycle impact methods. These methods provide the means to translate the tracked environmental flows into indicators of impact. Each impact category represents specific environmental issues that can be quantified. These categories define the impacts of an assessment and are evaluated and interpreted to develop conclusions based on the results of the study. This study applies the CML impact analysis method, supplemented with additional impact categories for energy use, freshwater use, and 20- and 100-year global warming potentials (GWPs) with and without carbon-climate feedback. In addition, the Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI) 2.1 methodology is used for a comparison of results and presented in the online resource materials (Tables S7 and S8, Electronic Supplementary Material).
The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report GWPs are used in this study (IPCC 2014). Both the 20- and 100-year GWPs are evaluated with and without climate-carbon feedback mechanisms, in units of carbon dioxide equivalents (CO2e). Primary energy use from renewable and non-renewable sources is calculated and reported in units of megajoule. Non-renewable energy sources include coalbed CH4, crude oil, hard coal, lignite, natural gas, oil sand, peat, pit CH4, shale gas, tight gas, and uranium. Renewable energy sources include geothermic, hydropower, solar, wave, and wind power, as well as resources from primary forests. The total primary energy metric is the sum of the renewable and non-renewable sources. Total freshwater use reported in kilogram of water is the life cycle water use metric used in this assessment. It includes rainwater use, surface water (lakes and rivers), and ground water use. In addition, the CML methodology impact categories and characterization factors developed by Leiden University Institute of Environmental Sciences (2016) are used to assess a suite of environmental impacts including human toxicity potential (HTP), marine aquatic ecotoxicity potential (MAETP), terrestrial ecotoxicity potential (TETP), freshwater aquatic ecotoxicity potential (FAETP), acidification potential (AP), photochemical ozone creation potential (POCP), ozone layer depletion potential (ODP), eutrophication potential (EP), elements abiotic depletion (elements ADP), and fossil abiotic depletion (fossil ADP) per kilogram of mushroom product. Though the CML impact analysis method includes GWP, it is modeled and discussed separately from the other CML impact categories.