Abstract:Peat is used as rooting medium in greenhouse horticulture. Biochar is a sustainable alternative for the use of peat, which will reduce peat derived carbon dioxide emissions. Biochar in potting soil mixtures allegedly increases water storage, nutrient supply, microbial life and disease suppression but this depends on feedstock and the production process. The aim of this paper is to find combinations of feedstock and production circumstances which will deliver biochars with value for the horticultural end user. Low-temperature (600 • C-750 • C) gasification was used for combined energy and biochar generation. Biochars produced were screened in laboratory tests and selected biochars were used in plant experiments. Tests included dry bulk density, total pore space, specific surface area, phytotoxicity, pH, EC, moisture characteristics and microbial stability. We conclude that biochars from nutrient-rich feedstocks are too saline and too alkaline to be applied in horticultural rooting media. Biochars from less nutrient-rich feedstocks can be conveniently neutralized by mixing with acid peat. The influence of production parameters on specific surface area, pH, total pore space and toxicity is discussed. Biochar mildly improved the survival of beneficial micro-organisms in a mix with peat. Overall, wood biochar can replace at least 20% v/v of peat in potting soils without affecting plant growth.

(1) Increased atmospheric nitrogen deposition has shifted plant dominance from ericaceous plants to grass species. To elucidate the reduced competitiveness of heather, we tested the hypothesis that additions of nitrogen reduce the concentrations of phenolics and condensed tannins in ericaceous leaves and retard mycorrhizal colonisation in ericaceous plants. We also tested the negative effects of reduced light intensity on carbon-based secondary compounds and mycorrhizal colonisation in ericaceous plants. (2) We performed a field inventory at three heathland sites in the Netherlands varying in nutrient supply and light intensity. Leaves of ericaceous plants and grasses were collected and analysed for concentrations of tannins, phenolics and nutrients. Similarly, we took root samples to record mycorrhizal colonisation and soil samples to measure the soil mineralisation. In addition, we conducted two-factorial experiments with Calluna vulgaris plants, in which we varied fertiliser and shade levels under greenhouse and field conditions. (3) The field inventory revealed that nitrogen addition and shading both negatively affected the concentration of total phenolics. The total phenolics and condensed tannin concentrations were positively correlated (P \ 0.001), but in the field experiment, the condensed tannins were not significantly affected by the treatments. Our results provide the first evidence that the carbon nutrient balance can be used to predict the amount of total phenolics in the dwarf shrub C. vulgaris. (4) In the field experiments, shading of plants resulted in significantly less mycorrhizal colonisation. Only in the greenhouse experiment did addition of nitrogen negatively affect mycorrhizal colonisation. (5) Our results imply that increased atmospheric nitrogen deposition can depress the tannin concentrations in ericaceous plants and the mycorrhizal colonisation in roots, thereby reducing the plants' competitiveness with respect to grasses. Additionally, if ericaceous plants are shaded by grasses that have become dominant due to increased nitrogen supply, these effects will be intensified and competitive replacement will be accelerated.

We hypothesized that the outcome of competition between ericaceous plants and grasses is strongly affected by the concentrations of phenolics in the litter that they produce. To test the effect of phenolic-rich litter on soluble soil nitrogen concentrations, plant nitrogen uptake and inter-specific competition, we conducted a greenhouse experiment with the shrub Calluna vulgaris and the grass Deschampsia flexuosa and their leaf litters. Two litters of C. vulgaris were used, with equal nitrogen concentration but different (high and low) concentrations of total phenolics. The D. flexuosa leaf litter contained lower concentrations of phenolics, but higher concentrations of nitrogen than the C. vulgaris litters. The plants were grown in monocultures and in mixed cultures. Inorganic and dissolved organic nitrogen were measured monthly during the experiment. After four months, we measured above-and belowground biomass and the nutrient concentrations in above-and belowground plant parts. In monocultures, C. vulgaris produced more shoot and root biomass on its own litter than with no litter. Growth of Calluna was reduced on grass litter. D. flexuosa plants produced most biomass on their own litter type, whether in monocultures or in mixed cultures. Addition of Calluna litter stimulated the growth of D. flexuosa both in monoculture and in mixtures. The grass plants outcompeted Calluna both on shrub litter and on grass litter but not when grown without litter. The two C. vulgaris litter types that differed in their concentration of phenolics did not differ in their effects on the competition between the two species or on the production of inorganic and dissolved organic nitrogen. We conclude that the nitrogen content of the litter is more important as a plant feature driving competition between shrubs and grasses than the concentrations of phenolics.

Powdery mildew (PM) is a very serious disease affecting glasshouse-grown roses and tomatoes in the Netherlands. Control is limited because of resistance to existing fungicides. Anhydrous milk fat (AMF) and soybean oil (SBO) emulsions were evaluated for control of PM in roses and tomatoes. Both AMF (14 g/litre) and SBO (14 g/litre) provided powdery mildew control on rose leaves and blooms that was significantly better (P<0.001) than that achieved using formulated dodemorph acetate (2.5 ml/litre), a systemic fungicide commonly used by Dutch rose growers. Powdery mildew control with SBO was as good as that provided by dodemorph acetate on tomato fruit and flowers, while AMF was slightly less effective. Combination of SBO (3.5 g/litre) and AMF (3.5 g/litre) mostly provided PM control rivalling that of SBO (14 g/litre). Spraying roses with whole milk (20% v/v) left white residues. No evidence of phytotoxicity with the use of natural product emulsions was found on floral and fruit tissues.

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