Sustainable peat alternatives, such as composts and management residues, are considered to have beneficial microbiological characteristics compared to peat-based substrates. Studies comparing microbiological characteristics of these three types of biomass are, however, lacking. This study examined if and how microbiological characteristics of subtypes of composts and management residues differ from peat-based substrates, and how feedstock and (bio)chemical characteristics drive these characteristics. In addition, microbiome characteristics were evaluated that may contribute to plant growth and health. These characteristics include: genera associated with known beneficial or harmful microorganisms, microbial diversity, functional diversity/activity, microbial biomass, fungal to bacterial ratio and inoculation efficiency with the biocontrol fungus Trichoderma harzianum. Bacterial and fungal communities were studied using 16S rRNA and ITS2 gene metabarcoding, community-level physiological profiling (Biolog EcoPlates) and PLFA analysis. Inoculation with T. harzianum was assessed using qPCR. Samples of feedstock-based subtypes of composts and peat-based substrates showed similar microbial community compositions, while subtypes based on management residues were more variable in their microbial community composition. For management residues, a classification based on pH and hemicellulose content may be relevant for bacterial and fungal communities, respectively. Green composts, vegetable, fruit and garden composts and woody composts show the most potential to enhance plant growth or to suppress pathogens for non-acidophilic plants, while grass clippings, chopped heath and woody fractions of compost show the most potential for blends for calcifuge plants. Fungal biomass was a suitable predictor for inoculation efficiency of composts and management residues.

The cultivation of cut flowers such as roses at higher latitudes is highly energy demanding, due to the need for artificial light to supplement sun radiation in the winter. Energy and labour are the two main cost components in commercial rose cultivation in the Netherlands. Saving energy while maintaining the high production and quality standard is an important driver for innovation. The replacement of the commonly used high pressure sodium (HPS) lamps by the more energy-efficient light emitting diodes (LED) lamps, can make possible to reduce the electricity demand for artificial light. An added advantage of LED's is that they allow to tailor the light spectrum in the greenhouse. However, the replacement of HPS lamps by LEDs in greenhouse increases the heat demand in winter to compensate for the loss of radiative heat. This is why "hybrid" lighting systems consisting of a mixture of both types of lamps are an interesting alternative. This paper describes an experiment that was conducted in a commercial rose nursery with the cut rose Avalanche+, where a "hybrid" light system was installed. The light installation consisted of 103 mu mol m(-2) s(-1) PAR from HPS lamps combined with Valoya LED lamp G1 with a light spectrum specially engineered for a hybrid installation to be mixed with HPS. Two different LED light intensities: 57 or 103 mu mol m(-2) s(-1) PAR were combined with the HPS light. Compared to the conventional installation of the company (191 mu mol m(-2) s(-1) HPS), the spectrum of the hybrid installation in both intensities increased the light use efficiency (LUE) by the crop, expressed as grams of produce (roses) per mol light (natural + artificial) received. This offers possibilities for energy saving, provided the energy efficiency of the lamps used is at least 1.7 mu mol PAR per watt electric.

Nature management residues (i.e., biomass generated from the management of nature reserves) are promising peat alternatives for horticultural substrates and may have a positive effect on disease suppression because of their microbiological characteristics. Moreover, addition of fertilizer may also affect the rhizosphere microbiome and, accordingly, disease suppression. In this study, we determined the effect of two management residues in horticultural substrates (i.e., chopped heath and acidified soft rush) and two fertilization regimes (i.e., pure nitrogen fertilizer and compound fertilizer) on the suppression of Phytophthora spp. on Chamaecyparis lawsoniana. The bacterial and fungal rhizosphere community was characterized using 16S ribosomal RNA and internal transcribed spacer 2 gene metabarcoding. Soft rush with a compound fertilizer (R2) and chopped heath with a pure nitrogen fertilizer (H1) showed a disease-suppressive effect and showed the largest shifts in microbial community composition compared with peat-based substrates. The disease-suppressive treatments showed differences in their microbial communities. Different genera associated with described biocontrol agents for Phytophthora spp. were found in higher amounts in those treatments. Aspergillus and Trichoderma spp. were highly abundant in H1, while Actinomadura and Bacillus spp. had a high abundance in R2. In addition, the relative abundances of 24 bacterial and 9 fungal genera were negatively correlated with disease severity. Several of those genera, including Bacillus, Chaetomium, and Actinomadura, were significantly more abundant in one of the disease-suppressive treatments. This study shows that disease suppressiveness in sustainable horticultural substrates is dependent on fertilization and can be linked to changes in the microbial rhizosphere communities.

Previous research has demonstrated that composts (COM) and woody residues from nature management (MR) are potential peat replacers for growing media, but their compositions are highly variable. Our goal is to make growing media more sustainable by optimizing the selection of local and sustainable alternatives for peat. Different batches of COM and MR were incubated to assess the microbial activity based on (1) the N drawdown risk, (2) the C mineralization and (3) the inoculation efficiency of a commercially available biocontrol fungus. The various batches were characterized based on biochemical, chemical (pH, available and total nutrients) and microbiological biomass analysis. COM and MR were scored based on chemical or stability characteristics to assess their suitability to replace peat, lime and fertilizers in growing media. This score allowed for a clear differentiation between the materials; MR received higher scores on average than COM. Five composts were further tested for the effect of storage after blending with an acidic MR, acidification with elemental S, or removal of the finer fraction. One batch of chopped soft rush was acidified with elemental S. Blending and acidification were the most effective treatments as they resulted in a clear increase of the suitability score.

To optimize the pH of management residues for optimal use in growing media, liming or acidification can be used.Previous research has shown the effectiveness of these treatments to increase or decrease the pH of management residues.However, the effect of these treatments on the microbiome of management residues remains unclear.In this study, two types of management residues, forest sods and heath chopper, were limed, while a third type, soft rush, was acidified.We assessed the effect of these two optimization treatments on the microbial community of the management residues using 16S rRNA and ITS2 gene metabarcoding, PLFA analysis and Biolog EcoPlates.Results indicate that the type of management residue is important to determine the effect of liming.In forest sods, liming seemed to have a negative effect for the use in growing media, with a decrease in bacterial and fungal diversity, and microbial biomass.In heath chopper, liming seemed to have a positive effect for the use in growing media, with an increase in bacterial diversity, microbial biomass and activity, and functional diversity.Acidification of soft rush increased fungal diversity, but decreased microbial biomass and activity and functional diversity, which makes it unclear whether it is an appropriate strategy for the use in growing media.

Three characteristics are considered key for optimal use of composts in growing media: maturity, pH and organic matter content. Maturation is a critical step in the processing of composts contributing to compost quality. Blending of composts with chopped heath biomass, sieving out the larger fraction of composts and acidification of composts by adding elemental sulfur may be used either to increase organic matter content or to reduce pH for a better fit in growing media. While several studies have shown the effectiveness of these treatments to improve the use of composts in growing media, the effect of these treatments on the compost microbiome has merely been assessed before. In the present study, five immature composts were allowed to mature, and were subsequently acidified, blended or sieved. Bacterial and fungal communities of the composts were characterized and quantified using 16S rRNA and ITS2 gene metabarcoding and phospholipid fatty acid analysis. Metabolic biodiversity and activity were analyzed using Biolog EcoPlates. Compost batch was shown to be more important than maturation or optimization treatments to determine the compost microbiome. Compost maturation increased microbial diversity and favored beneficial microorganisms, which may be positive for the use of composts in growing media. Blending of composts increased microbial diversity, metabolic diversity, and metabolic activity, which may have a positive effect in growing media. Blending may be used to modify the microbiome to a certain degree in order to optimize microbiological characteristics. Acidification caused a decrease in bacterial diversity and microbial activity, which may be negative for the use in growing media, although the changes are limited. Sieving had limited effect on the microbiome of composts. Because of the limited effect on the microbiome, sieving of composts may be used flexible to improve (bio)chemical characteristics. This is the first study to assess the effects of maturation and optimization treatments to either increase organic matter content or lower pH in composts on the compost microbiome.

ISHS II International Symposium on Growing Media, Soilless Cultivation, and Compost Utilization in Horticulture Nature conservation management residues as alternative for farm yard manure: effects on nutrients, carbon and disease suppression