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Horticultural biomass has a high potential for circularity. This paper focuses on biomass and biomass by-product streams as well as potential valorisation strategies in a horticultural context. Valorisation can be achieved by both high-tech and low-tech solutions. Depending on the location of production, these solutions can be realised by growers and farmers on-site, but the need for high financial investments can lead to off-site solutions that force off-site partnerships and thorough planning of logistics. In case that further use as material (e.g. as building material, chemical feedstock, fertilizer, soil improver) or as food or feed can be ruled out, it can be repurposed as alternative fuel source. This mini-paper highlights the state-of-art use of horticultural biomass streams, explores local solutions and identifies potential markets that can benefit from horticultural by-products.

Changes in the hydrology of sediments in tidal marshes or landfills may affect the uptake of metals in the vegetation. Leaf and stem samples of Salix cinerea (grey sallow) were collected during four consecutive growing seasons at six contaminated plots on a polluted dredged sediment landfill and one plot on an uncontaminated reference site. The first three contaminated plots were already emerged in the first half of the first growing season, while the other three were submerged in the first year, but became increasingly dry over the study period. Foliar and stem cutting concentrations for Cd, Zn and Mn increased on the latter three plots over the four years. Willow bark contained high concentrations of Cd, Zn and Mn. In two consecutive greenhouse experiments with willow cuttings from different origins (uncontaminated and contaminated sites) and grown under different soil conditions (uncontaminated and contaminated), we observed an important translocation of Mn from bark to shoots. In a third experiment with willow cuttings collected on soils with a range of heavy metal concentrations and, thus, with a broad range of Cd (4–67 mg/kg dry matter), Zn (247–660 mg/kg dry matter) and Mn (38–524 mg/kg dry matter) concentrations in the bark, high Mn concentrations in the bark were found to affect the budburst of willow cuttings, while no association of delayed budburst with Cd and Zn concentrations in the bark was found. We conclude that wood and, especially, bark are not a sink for metals in living willows. The high Mn concentrations in the bark directly or indirectly caused delayed or restricted budburst of the willow cuttings.

Crop residues of field vegetables are often characterized by large amounts of biomass with a high N-content. Despite low soil temperatures during autumn, high rates of N mineralization and nitrification still occur causing important N-losses through leaching1. Crop residues pose a possible threat to maintaining water quality objectives, while at the same time being a vital link in closing the nutrient and organic matter cycle of soils. Appropriate and sustainable management is needed to obtain the full potential of crop residues. In this research some novel strategies for vegetable crop residue management are being investigated. The outcome of this work will help in defining the best practices in vegetable production in order to better meet the requirements set by the nitrate directive, whilst making best use of a valuable organic material. Vegetable crop residues constitute an important link in the soil nutrient and organic carbon cycle and aid in maintaining soil quality and fertility. However during autumn large amounts of N are released by vegetable crop residues causing important N-losses through leaching. Appropriate and sustainable management is needed to obtain the full potential of crop residues while meeting water quality objectives. In this research the potential of removal and valorisation of vegetable crop residues by means of ensilage or composting was evaluated.

Organic farmers in Flanders use manure from extensive conventional livestock systems due to a lack of animal manure from organic producers. The research question was if on-farm prepared compost mainly consisting of vegetal residues can be a good alternative. A long-term fertilization trial with a 4-year crop rotation of maize - potatoes - spring barley - red clover is carried out on two fields with a time difference of one year. The fertilization treatments are on-farm prepared compost, applied as a single dose and a double dose, farmyard manure, slurry and slurry combined with composted municipal waste. This paper concerns the experimental results of the potato crop in 2006 and 2007. The nitrate content of the plant juice was monitored and the potato yield and nitrate content in the soil profile at the end of the growing season were determined. In 2006 the potato yields were significantly higher for both farm compost treatments while the nitrate residues in soil were significantly lower. The early, high and constant leaf blight disease pressure in 2007 resulted in lower yields and less marked treatment effects. This investigation demonstrated that application of mature compost can result in a faster development of the potato crop in the first weeks of the growing season, which is important for sufficient yields in organic potato growing.