The floating system is a successful strategy for producing baby leaf vegetables. Moreover, compost from agricultural and agri-food industry wastes is an alternative to peat that can be used as a component of growing media in this cultivation system. In this study, we experimented with three composts containing tomato (Solanum lycopersicum L.), leek (Allium porrum L.), grape (Vitis vinifera L.), and/or olive (Olea europaea L.) mill cake residues, which were used as the main component (75/25 volume/volume) of three growing media (GM1, GM2 and GM3) to evaluate their effect on the growth and quality of red baby leaf lettuce (Lactuca sativa L.). We used a commercial peat substrate as a control treatment (100% volume) and in mixtures (25% volume) with the composts. The plants were cultivated over two growing cycles, in spring and summer, and harvested twice in each cycle when the plants had four to five leaves. We found that the percentage of seed germination was significantly higher in plants grown in peat than in those grown in compost growing media. The yield was affected by the growing media in the summer cycle, and we obtained the highest value with GM1. Furthermore, the second cut was more productive than the first one for all the growing media in both cycles. The lettuce quality was also affected by the growing media. In general, the total phenolic content and antioxidant capacity in the leaves was higher in plants grown in the compost growing media, particularly in the second cut, but the nitrate content in the leaves was greater in some of the compost treatments compared with the peat treatment. In addition, an in vitro suppressive activity study demonstrated that the interaction between different fungi and bacteria observed through metagenomics analysis could contribute to the effectiveness of the compost in controlling Pythium irregulare. The use of compost as a component of the growing media in the production of baby leaf vegetables in a floating system does not only favor the crop yield and product quality, but also shows suppressive effects against P. irregulare.

Composts of spent mushrooms substrates can be an alternative for the partial replacement of peat as growing media in horticulture. Three mature composts from Agaricus bisporus (Ag), Pleurotus ostreatus (Pl), and 70% Ag:30% Pl (AgPl) production were used as partial components of peat growing media, used at a 1:4 compost:peat ratio for growing red baby leaf lettuce. They showed higher yields, between 3 and 7 times more than that for peat itself, even under the pressure of the plant pathogen Pythium irregulare. AgPl showed the higher suppressiveness (50%) against Pythium irregulare than Ag- (38%) or Pl- (15%) supplemented media. The combination of these raw materials and a suitable composting process is important for obtaining mature compost for use as a partial component of peat-based growing media.

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The presence of elevated concentrations of heavy metals (Cu and Zn) in pig slurry and, particularly, in the solid fraction obtained after solid-liquid separation is a limiting factor for their use in agricultural soils. These metals are further concentrated if compost is produced from solid pig slurry. This paper studies the influence of the composting of the separated solid phase of pig slurry on the concentrations and solubility of Cu and Zn, and their distribution in the different particle size fractions, as well as evaluating their potential toxic effects on seed germination and seedling growth. Two composts were prepared with the solid fraction of pig slurry in a piglets and sows farm, using two different bulking agents (cereal straw and cotton gin waste). The concentrations of Cu and Zn in the mature compost were higher due to organic matter degradation; however, their solubility decreased from 0.72 and 1.76% in the solid fraction of pig slurry to 0.18 and 0.30% of total Cu and Zn, respectively, in the compost prepared with cotton gin waste. Zinc was concentrated in the smallest particle size fraction, while the Cu concentration was highest in the largest particles, and associated to the organic matter/humic fraction. The elimination of the smallest particle size fraction would not reduce significantly the total heavy metal concentration of the composts. Nevertheless, the low solubility of both metals in the composts avoided any significant toxic effect on seed germination and also in the growth test when compost was present at low rates.

Anaerobic digestion of agro-industrial and livestock waste generates considerable digestate volumes that are important sources of nitrogen (N). However, on some occasions, the high concentrations of N present in the digestates may represent an obstacle to its use locally as fertilizer, since it can cause an environmental impact (European Community, 2000). This study analyzes the efficiency of gas-permeable membranes (GPM) in the recovery of the ammoniacal nitrogen (NH + 4 ) present in the swine manure (SM, Control) and three digestates generated from the anaerobic co-digestion of mixtures of SM, fruit and vegetable sludge (FVS) from the vegetable industry (peppers and artichokes), and by-products of the tomato processing industry (TW) (skins and seeds), which were mixed at three different proportions [TW + FVS]: [SM]. Their NHx + content was 2,240 mg L −1 for the SM and 4,670-5,370 for the digestate mixtures. Throughout the duration of the Necovery experiment (96 h), the percentages of NH + 4 removal and recovery achieved were consistent among treatments, approximatelly 78 and 96%, respectively. The recovery of NH 4 + for the digestate mixtures seems to be dependent on the treatment time, since the potential of N recovery was higher when the treatment time increased. However, for the control experiment (SM), with lower N concentration, the maximum N recovery was obtained at the experimental time established, without increasing the treatment time.