Phosphorus (P) is despite its omnipresence in soils often unavailable for plants. Rhizobacteria able to solubilize P are therefore crucial to avoid P deficiency. Selection for phosphate-solubilizing bacteria (PSB) is frequently done in vitro; however, rhizosphere competence is herein overlooked. Therefore, we developed an in planta enrichment concept enabling simultaneous microbial selection for Psolubilization and rhizosphere competence. We used an ecologically relevant combination of iron-and aluminium phosphate to select for PSB in maize (Zea mays L.). In each consecutive enrichment, plant roots were inoculated with rhizobacterial suspensions from plants that had grown in substrate with insoluble P. To assess the plants' P statuses, nondestructive multispectral imaging was used for quantifying anthocyanins, a proxy for maize's P status. After the third consecutive enrichment, plants supplied with insoluble P and inoculated with rhizobacterial suspensions showed a P status similar to plants supplied with soluble P. A parallel metabarcoding approach uncovered that the improved P status in the third enrichment coincided with a shift in the rhizobiome towards bacteria with plant growthpromoting and P-solubilizing capacities. Finally, further consecutive enrichment led to a functional relapse hallmarked by plants with a low P status and a second shift in the rhizobiome at the level of Azospirillaceae and Rhizobiaceae.

Strawberry cultivation is associated with high mineral fertilizer doses and extensive use of chemical plant protection products. Based on previous research, we expected that chitin application to peat substrate would increase the nutrient availability and activate the plant’s systemic defense response, resulting in higher strawberry yields and less disease symptoms. We set up two experiments in which the temporal variability and differences in initial nutrient concentrations of the growing media were taken into account. Chitin treatment resulted in the attraction of plant-growth promoting fungal genera towards the plant root, such as Mortierella and Umbelopsis. In addition, by the end of the experiments 87 mg mineral N/L substrate was mineralized, which can be related to the observed increase in plant shoot biomass. This however led to nutrient imbalances in plant shoots and fruits: N concentration in the leaves increased over 30%, exceeding the optimal range, while P and K deficiencies occurred, with concentrations lower than 50% of the optimal range. This may explain the decreased fruit yield and disease resistance of the fruits towards Botrytis cinerea. In contrast, chitin caused a clear defense priming effect in the strawberry leaves, with a strong induction of the jasmonic acid response, resulting in less foliar disease symptoms. Chitin causes positive effects on shoot growth and foliar disease resistance, but caution needs to be taken for nutrient imbalances leading to negative influences on root growth, fruit production and disease susceptibility towards B. cinerea.

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.

Substantially extending the life span of peat-and perlite-based growing media is a measure to increase the sustainability of soilless cultivation. The extraction of peat from pristine peatlands threatens these sensitive ecosystems and carbon sinks, meanwhile resulting in increased emissions of greenhouse gasses. Each batch of peat that is reused, results in a clear reduction in CO 2 emissions and a lower impact on the climate. After using growing media for one cultivation, we aim at reusing the spent material as growing medium for another crop. Spent peat and perlite based growing media from strawberry and cucumber cultivation were upcycled after steam treatment. We tested the effectiveness of steaming to reduce phytosanitary risks. The hygienisation efficiency of the steam treatment was confirmed: plant pathogenic fungi, larvae of vine weevils and weed seeds added or already present before the process were killed by the steam treatment. As the upcycled spent growing medium already contained high nutrient levels, the fertilizer application in the reused growing medium should be reduced, especially for P and K. Five indicators for assessing stability of the materials were used: CO 2 flux measurements, oxygen uptake rate (OUR), biodegradation potential, mineral N content and risk for N immobilization. The spent growing media had a low decomposition rate and the release of nutrients in a leaching experiment was lower than for a fertilized peat-based growing medium, being a reference blend for open field cultivated Chrysanthemum. N mineralisation and P uptake were tested in an incubation and pot trial, respectively, and the upcycled spent growing medium was found to be an important source of plant-available K and P. Steam treatment did not severely affect the microbial biomass and diversity of the spent growing media. Blending the steam-treated spent media with other materials or inoculating by a commercially available biocontrol fungus also had a limited effect, indicating that newly introduced microorganisms do not easily establish in steamed-treated spent growing media (SSGM). Acidification of the SSGM was achieved by a low dose of elemental S. The steam-treated growing medium was tested for growing Chrysanthemum cuttings and plantlets. Spent growing media were not able to supply sufficient mineral N, but the stored amounts of P and K in the media were sufficiently plant available for optimal crop growth.

Biochar has been reported to play a positive role in disease suppression against airborne pathogens in plants. The mechanisms behind this positive trait are not well-understood. In this study, we hypothesized that the attraction of plant growth-promoting rhizobacteria (PGPR) or fungi (PGPF) underlies the mechanism of biochar in plant protection. The attraction of PGPR and PGPF may either activate the innate immune system of plants or help the plants with nutrient uptake. We studied the effect of biochar in peat substrate (PS) on the susceptibility of strawberry, both on leaves and fruits, against the airborne fungal pathogen Botrytis cinerea. Biochar had a positive impact on the resistance of strawberry fruits but not the plant leaves. On leaves, the infection was more severe compared with plants without biochar in the PS. The different effects on fruits and plant leaves may indicate a trade-off between plant parts. Future studies should focus on monitoring gene expression and metabolites of strawberry fruits to investigate this potential trade-off effect. A change in the microbial community in the rhizosphere was also observed, with increased fungal diversity and higher abundances of amplicon sequence variants classified into Granulicella, Mucilaginibacter, and Byssochlamys surrounding the plant root, where the latter two were reported as biocontrol agents. The change in the microbial community was not correlated with a change in nutrient uptake by the plant in either the leaves or the fruits. A decrease in the defense gene expression in the leaves was observed. In conclusion, the decreased infection of B. cinerea in strawberry fruits mediated by the addition of biochar in the PS is most likely regulated by the changes in the microbial community.