Fruit and vegetable waste can be upcycled using different technologies to produce building blocks for bio-based fertilisers blends: upscaling from lab to field

Introduction
Production of novel bio-based fertilisers from fruit and vegetable waste can reduce the dependence on mineral fertiliser production and mining, and decrease the nutrient losses in the form of agricultural residues. Moreover, applying such bio-based organic fertilisers into the field can increase soil health. In the Rustica project, 5 divergent technologies producing fertiliser building blocks from organic residues are considered: carboxylic acid platform (CAP), microbial biomass production, electrodialysis, insect breeding yielding biomass and frass, and biochar production. Starting from lab production over small-scale testing towards pilot production, blending and field trials, the project aims to develop environmentally sustainable and marketable end-products.
Methodology
First of all, biochar, insect biomass and frass, and microbial biomass and NPK solution from electrodialysis of CAP solution, all produced at lab/small scale from various fruit and vegetable waste, were analysed for chemical and biological parameters (nutrients, stability, carbon contents, etcetera). Based on this information, blends composed of these building blocks (with compost as a potential additional one) were designed to match fertiliser and soil quality requirements for several soils, crops and regions (BE, FR, IT, ES and Colombia). Effects of addition of different blends and their building blocks were assessed in incubation trials. During 30 days, changes in respiration, nutrients, microbial biomass and enzymes after blend addition to soil were monitored in the lab.
Results and discussion
Microbial and insect biomass showed to be very rich in nitrogen (6-12% N), with somewhat higher water extractable NO3- and NH4+ contents for the microbial biomass. Both had a high and easily degradable organic matter (OM)content, interesting to boost the soil microbial life. Compared to the biomasses, insect frass samples showed smaller total N (TN) contents but higher water extractable NO3- and NH4+ contents (up to > 1 mg N/g frass) and contents of other nutrients (especially K). Electrical conductivity (EC) was also highest for this building block, potentially harmful for salt sensitive plants. High pH (up to 10.5), low EC, low TN (0.8-1.7%) contents and high contents of stable OM were observed for the biochars. The NPK concentrate showed a rather high salt, high K and low N and P content. Feedstocks (particular fruit and vegetable waste) influenced partly the building block characteristics, especially for insect frass.
Results of the incubation trials of soil amended with building blocks showed that microbial and insect biomass caused a significant increment in N availability and in the content and activity of microbial biomass, while biochar did not affect these parameters. Insect frass showed an intermediate behaviour between biomasses and biochar.
Adding blends of the building blocks to soil increased microbial biomass, CO2 respiration, enzyme activity and P availability, in most cases. Extractable N and NO3- increased for soil amended with blends that included microbial biomass, whereas high biochar contents in blends reduced the values obtained for these parameters. More incubation trials will be performed in 2023 and presented at the RAMIRAN conference.
Conclusion
The widely varying characteristics of the different building blocks produced from fruit and vegetable waste allow fot the design of diverging fertiliser blends that are adapted to soil and crop requirements. Compositions of the blends will be finetuned according to the outcomes of the incubation trials and further tested in field trials in 4 European regions (BE, FR, IT, ES) and Colombia. In parallel, a multi-actor approach supported by market, lifecycle and legal analyses will foster the implementation potential of these technologies.