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Soilless substrates are widely used for plant cultivation. However, little is known about how soilless substrate components, plant growth, or their interactions impact microbial communities in soilless media. The objectives of this study were to analyze microbial communities in typical pine bark substrates used for nursery crop production and determine the impacts of substituting peat with a compost substrate, and planting, on microbial community dynamics over a production cycle. Three soilless substrate mixtures were compared. The substrate mixes consisted of 80:20:0, 80:10:10, and 80:0:20 (volume:volume:volume) ratios of pine bark:peatmoss:leaf compost, respectively. One set of each treatment was planted with a single birch (Betula nigra ‘Cully’) liner and another set was not planted. The treatments (n 5 3) were maintained in a nursery production setting, and samples were taken after 0, 1, 2, 3, and 4 months. Bacterial and fungal communities were characterized by sequencing polymerase chain reaction-amplified 16s rRNA genes and internal transcribed spacer regions. Initially, the two substrate mixtures that contained compost had more phyla than the substrate mixture that only contained peat and bark. After 1 month, microbial communities in all treatments contained similar phyla, but at different relative abundances based on the amount of compost they contained. Over time, Nitrosomonadaceae and Acetobacteraceae were the most abundant bacterial families in substrate mixes containing 10% and 20% compost, but they were absent from treatments without compost. The communities were dynamic and changed the most over the first 2 months. Microbial communities and their dynamics were similar between planted and unplanted treatments. Planting had less of an effect on microbial communities than compost amendment. Among the fungal communities, differences were observed based on both compost amendment and plant presence. Ascomycota and Basidiomycota were the most abundant fungal phyla and resembled those originally in the peat and compost, respectively. These findings could be used to understand the importance and dynamics of specific microbial communities present in substrate components and how they develop during greenhouse production. [ABSTRACT FROM AUTHOR]

• Nutrient solution management controls yield and quality in floating system lettuce. • Diluting the nutrient solution slows lettuce growth but maintains plant nutrition. • High nitrogen concentration can toxically accumulate in lettuce. Lettuce is a leading greenhouse-grown vegetable highly appreciated by consumers for its nutritional properties texture and flavor. However, leafy vegetables are known to accumulate an excessive amount of nitrate and this constitutes a strong health concern in Europe. Among the factors that most affect the yield and quality of hydroponic crops, the composition and management of the nutrient solution stand out. In this work, different strategies have been studied in the management of the nutrient solution for hydroponic lettuce cultivation. Production nutritional status, and quality parameters were evaluated. The concentrations of nutrients in the nutrient solution significantly influence the harvested product, 100 to 50 % H treatment resulted in highest yields with significant increase of 14 % compared to 50 % H at 34 DAT, from the point of view of both biomass production and quality, regarding the contents of anthocyanins, chlorophylls, and calcium, constituents that add value to the product due to their importance in the human diet. A significant reduction of nitrates in lettucewas observed one (14 %) and two (22 %) weeks after modifying the nutrient solution (100 to 50 % H). After one week there was no reduction in lettuce growth. In addition, no significant reduction in the concentration of most macro- and micronutrients was observed. [ABSTRACT FROM AUTHOR]

Lettuce (Lactuca sativa L.) is a leading greenhouse-grown vegetable. However, nitrate (NO3−) accumulation in leaves remains a major issue. The aims of this research were: (i) to test the modified intermittent Nutrient Film Technique (NFT) in the cultivation of soilless lettuce in which plants are grown on peat blocks in trays and supplied with an intermittent flow of nutrient solution, and (ii) to calibrate the fertilization scheme to increase yield performance, while keeping NO3− concentration under control. Two greenhouse trials were performed between autumn 2013 and spring 2014. Results showed that a 30-day cycle is the optimum duration in terms of fresh biomass yield, both for autumn and spring cultivation. Reducing N fertilization in the last cropping days never affected NO3− concentration in leaves during autumn trial, due to unfavourable growing conditions. Conversely, suspension of fertilization 2 days before harvest had a consistent effect during the spring trial, when NO3− concentration in leaves was highly reduced (from 20 to 36%) without yield penalties. Thus, suspending fertilization 2–4 days before harvesting in intermittent NFT may reduce, on average, NO3− accumulation by 29–58% and the fertilization rate by 7–16%, respectively. Yet, growing conditions are crucial to make this system effective.