New sources of S-based fertilizers from industrial wastes to improve crop performances and agroecosystems sustainability

The continuous growth of global population is accompanied by an increasing demand for agricultural products, both for food and non-food uses. Meeting the challenges of food security, social equity and environmental sustainability requires substantial improvements in the efficiency and productive performance of agroecosystems. Enhancing yield potential of currently cultivated land is consistent with the objective of ensuring universal access to adequate and safe food, while avoiding the conversion of natural ecosystems into cropland. Within this context, the transition toward more sustainable production systems is closely linked to the principles of the circular economy, which promote the recovery and valorisation of waste streams. The industrial sector generates large quantities of by-products that are often discarded, despite their potential to be recycled into valuable resources for agriculture. Therefore, recovering nutrients from these waste streams represents an important strategy to reduce the environmental impacts of agricultural systems and support the development of new green markets. In addition to nitrogen, phosphorus and potassium, sulfur is an essential secondary macronutrient involved in a wide range of biological processes. It is a structural component of aminoacids and proteins, contributes to the synthesis of defence compounds, and the modulation of plant stress responses. Therefore, sulfur availability is a major determinant of crop growth, quality and overall productivity, especially in contexts where environmental conditions make this element a potentially limiting factor. Among the various sulfur sources, gypsum (CaSO4 x 2H2O) is commonly used in agriculture for the remediation of acidic or saline soils. Gypsum can be obtained from quarries (as the one used withing organic agriculture framework) or can be sourced as waste product from several chemical processes. However, this industrial waste stream is frequently discarded in landfill, without being valorised for proper nutrient recovery. Therefore, the present study explores the use of industrial chemical calcium sulfate as a recycled sulfur fertilizer for herbaceous crops, focusing on its role as a nutrient source rather than a soil conditioner. The general aim of the project was to develop a positive synergy between agricultural and industrial sectors, identifying technical solutions for the recycling and valorisation of industrial wastes into fertilizers, thereby fostering the promotion of circular and sustainable production chains. To achieve these results, sulfur granular fertilizers were produced at FerroDuo s.r.l. facilities using waste gypsum derived from a local chemical plant. Agronomic performances were primarily studied through multi-year trials, both at mesocosm level and at field scale, on Camelina sativa (L.) Crantz, Cicer arietinum L. and Triticum aestivum L. These species were selected to represent different botanical families and end-use purposes, including food and non-food applications, in order to provide a comprehensive assessment of sulfur fertilization across diverse crops and production chains. Crop performance was assessed through the evaluation of dry matter production and allocation, seed yield and yield components, with particular attention to fertilization use efficiency and yield formation processes. In addition to agronomic performance, the environmental implications of sulfur fertilization were also investigated though a consequential Life Cycle Assessment carried out using camelina and chickpea trials as case studies. Regarding the structure of the present dissertation, Chapter 1 begins with a general overview of waste streams valorisation within circular economy context. Then was presented the state-of-art of sulfur dynamics within agroecosystems, with particular attention to its biogeochemical cycle, its role in plant metabolism and its agronomic management. The last part of the introduction is dedicated to the description of Life Cycle Assessment methodology and its modelling framework. The introduction is followed by Chapter 2 “Aim of the project” outlining the specific objectives of the research and the schematic workflow (fertilizer formulation, agronomic trials, environmental assessment) underlying this thesis. The subsequent Chapter 3 “Sulfur fertilizer production” presents the results of the experimental activities related to the development of a granular fertilizer, starting from waste chemical gypsum and the scaling-up of the production process from pilot plant to industrial level. Chapter 4 “Agronomic and environmental benefits of recovered gypsum for sulfur fertilization in camelina for biofuel production” evaluates the responses of camelina (var. ‘Calena’) to different N-S fertilization strategies (same amount of nitrogen, and three rates of sulfur), compared to an unfertilized control and the common fertilization management, which implies just the distribution of nitrogen. Results shows that combined N-S application at medium rate (60 kg N ha-1 + 40 kg S ha-1) significantly increased camelina productivity compared to the business-as-usual strategy, improving fertilization efficiency, and without negatively affecting seed quality. LCA results (comprehensive of indirect Land Use Change computation) indicated a reduction in environmental impacts compared to conventional fertilization, highlighting the potential of moderate sulfur inputs to enhance both productivity and sustainability of camelina cropping systems. Chapter 5 “Sulfur supply from recycled source: an effective strategy to improve chickpea productivity and sustainability” investigates the agronomic and environmental performances of two commercial chickpea varieties (‘Pascià’ and ‘Reale’) under Mediterranean conditions in response to different sulfur rates (30 or 60 kg S ha-1). Moderate sulfur application significantly increased seed and biomass yields compared to the unfertilized control, mainly through enhanced pod formation, with no additions benefit at higher rates. LCA showed that improved productivity reduced environmental impacts across all categories, emphaticizing the role of optimized sulfur inputs in improving the efficiency of chickpea cropping systems. Chapter 6 “Sulfur supplementation in wheat: agronomic performance and implications for spike traits”, presents the results of a mesocosm experiment on common wheat (var. ‘Bologna’), testing two nitrogen levels (150 and 112 kg N ha-1) with and without sulfur application (30 kg S ha-1), compared to an unfertilized control. Despite the relatively low sulfur requirements of wheat, sulfur distribution improved both kernel yield and fertilization use efficiency. Analysis of yield components indicated that sulfur positively influenced spike fertility traits, primarily through an increase in kernel number. The improved fertility of distal florets within spikelet, suggests that sulfur may exhibit physiological effects comparable to those reported for nitrogen nutrition. Overall, this research adopts a systemic approach, examining a real-life case study in which sulfur was recovered from industrial waste streams and formulated into a granular fertilizer already available on the market. By employing fully operational and scalable technologies, the study demonstrates that the proposed approach can be already implemented at commercial level, to enhance the overall sustainability of cropping systems. The author hopes that this contribution will advance the current understanding of nutrient-related constraints on crop performances and encourage a broader perspective on the interconnections among the different components of agricultural production systems.