Measuring Environmental Sustainability of Soilless Systems: Life Cycle Assessment and Sustainable Development Goals for innovative agricultural productions

With the projected increase in world population (expected to exceed 9 billion people), agriculture faces critical challenges to keep up with food production. To increase productivity, farmers must rely on increased use of potentially harmful inputs, such as pesticides and fertilizers, while expanding land and water usage. This drive for increased productivity comes at the expense of biodiversity, soil degradation, groundwater contamination and depletion, not to mention the potential damage to human health. Nonetheless, a potential solution might be to move part of agriculture into artificially controlled environments. In detail, Controlled-Environment Agriculture (CEA) enables optimal control and monitoring of vital plant parameters such as temperature, light, nutrients, and humidity. The most common form of CEA is the soilless system. Soilless systems represent a soil-independent form of cultivation where plant growth is achieved through the use of a substrate in place of natural soil. The substrate can be of organic origin (e.g. coconut fiber) or inert (e.g. perlite) and could be a water-based nutrient-rich solution or a solid one. Soilless systems promise several benefits in terms of water and fertilizer saving, potentially reducing the environmental pressure on agriculture. Additionally, soilless systems may help achieve the Sustainable Development Goals (SDGs). For instance, the higher yield and productivity of soilless systems may help to reduce hunger (SDG-2) potentially leading to improvement in health and well-being (SDG-3). Moreover, the reduction in agrochemical and water consumption as well as optimal use of fertilizers may contribute to the achievement of other multiple SDGs (SDGs 6, 12,14 and 15). However, in the pursuit of higher yield and optimization, these systems may become complex, requiring higher levels of technology, materials and energy, potentially offsetting their initial benefits and rising concerns about their environmental sustainability as well as their ability to achieve SDGs. Therefore, various aspects should be considered and properly assessed to understand not only the environmental sustainability of soilless systems, but also if their implementation can really help in achieving SDGs. In this context, holistic assessment methods, like Life Cycle Assessment (LCA), are necessary to comprehensively evaluate the environmental impacts of these systems. Furthermore, understanding the potential relationships between soilless systems and SDGs is also needed to ensure their effective implementation in achieving sustainable development targets. Hence the purpose of this thesis is three-fold: i) to review the state-of-the-art of LCA studies applied to soilless systems and analyze the methodological choices adopted; (ii) to assess and compare the environmental impacts of a soilless system against conventional in-soil cultivation applying LCA to a case study; and (iii) to analyze the relationship between soilless systems and SDGs through textual, sentiment and manual in-depth analysis. To address these objectives, this thesis is structured in 5 chapters. Chapter 1 introduces the research theme and outlines the thesis structure. Chapter 2 addresses the first objective through a three-step approach. First, an overview of soilless techniques was conducted to understand the nomenclature of different types of soilless systems. Second, a review of literature reviews on environmental impact assessment methods was performed to identify the most commonly adopted method. This review confirmed LCA as the predominant methodology. Third, the identified soilless techniques were combined with LCA to systematically collect relevant case studies. The selected case studies were then analyzed through bibliometric analysis, technical analysis to characterize the systems under study, and systematic analysis to examine the methodological choices adopted in the LCA studies. Drip irrigation systems with solid substrates, particularly perlite, represented the most common soilless cultivation method, primarily cultivating tomatoes and lettuce. The geographical distribution of studies showed a concentration in Spain, with commercial systems predominantly in rural areas and experimental ones in urban settings. Regarding the LCA method, attributional LCA models were most common, typically employing mass-related functional units and cradle-to-farm gate system boundaries. Electricity consumption, fertilizer use, and structural materials emerged as the primary impact sources in soilless systems. The review also revealed limited application of consequential approaches, and most studies made assumptions for transportation and materials lifespan, due to data unavailability. Additionally, only half of the analyzed studies conducted sensitivity analyses. These findings highlighted the need for expanding research considering alternative crop species, innovative techniques, and substrates. Building on these results, Chapter 3 presents a comparative LCA study between soilless and conventional soil-based strawberry cultivation systems in Maletto. The choice of strawberry cultivation addresses a significant gap in the literature, as LCA studies on soilless strawberry production systems were found, in Chapter 2, to be scarce. Following the most common methodological choices previously identified, the assessment adopted a cradle-to-gate approach with 1 kg of fresh harvested strawberries as functional unit. System boundaries encompass greenhouse structure, irrigation system, field operations (only traditional system), crop production, waste management, transport and emissions. Environmental impacts are assessed using the Environmental Footprint 3.0 method. A distinctive feature of this study is the evaluation of suitability of emission estimation methods, which were originally developed for soil-based systems, through sensitivity analysis. This evaluation is particularly relevant as soilless substrates exhibit different physical and chemical properties compared to soil, potentially affecting emission dynamics. The soilless system demonstrated a better environmental performance in most impact categories, primarily due to higher productivity, reduced water consumption, and lower pesticide requirements. However, the in-soil system performed better in Human Toxicity, Eutrophication, Freshwater Ecotoxicity, and Resource use categories. The sensitivity analysis revealed significant variations across emission estimation methods. These results not only highlight the potential environmental benefits of soilless systems but also raise the need to develop substrate-specific methods and models for emission estimation. Having assessed both methodological aspects and environmental impacts of soilless systems in Chapters 2 and 3, Chapter 4 explores their broader implications for sustainable development through an analysis of their relationship with SDGs. The analysis followed a systematic four-step methodology: systematic literature search, text analysis, sentiment analysis, and in-depth manual analysis. Text analysis mapped the connections between soilless systems and SDG targets, and sentiment analysis provided the presence of positive or negative sentiment indicating potential positive or negative relationships. An in-depth manual analysis was then necessary to examine and validate the actual mechanisms behind these connections. Overall, soilless systems were found to be strongly connected to SDGs 6, 12 and 2, with a predominant positive association between soilless systems and SDGs. However, the in-depth analysis revealed a complex but promising relationship between soilless systems and SDGs. While opportunities range from resource use efficiency and environmental protection to technological innovation and socio-economic benefits, these systems also face implementation challenges. As identified in Chapter 2, soilless systems can achieve better environmental performance through reduced water consumption and pesticide use, yet they may increase other environmental pressures through intensive energy use and resource-demanding infrastructure. The analysis highlights potential synergies between different SDGs, where progress in resource efficiency can drive improvements in food security and environmental protection. While trade-offs exist, such as high-tech solutions potentially conflicting with economic inclusivity, these challenges can be effectively addressed through targeted public and private interventions, including policy support, financial incentives, and technological innovation. These interventions need to be carefully tailored to specific geographical and socio-economic contexts to maximize the potential of soilless systems in achieving sustainable development objectives. The findings of this thesis offer insights for different stakeholders. Chapter 2 results provide LCA-practitioners with methodological guidance for LCA studies. The potential environmental advantages of soilless systems are further strengthened by Chapter 3, which also highlights a crucial methodological gap in emission estimation methods. Chapter 4 extends beyond environmental aspects, revealing to farmers, consumers, and policymakers the opportunities, challenges, and potential interventions for soilless systems implementation, while uncovering the dynamics between these systems and SDGs. However, this research has several limitations: it focused on plant-only cultivation systems, excluding integrated systems such as aquaponics, also, it focused primarily on environmental sustainability, and the qualitative nature of SDG relationship analysis. Despite the significant potential of soilless systems, considerable work remains to be done. Future research should quantitatively assess the causal relationships between soilless systems and SDGs, expand the sustainability assessment of soilless systems through other life cycle thinking methods such as LCC and S-LCA, and develop substrate-specific emission estimation methods. While the path toward comprehensive understanding of soilless systems' sustainability is long, it appears both promising and achievable.