Marigold and Pepper Growth in Container Substrates Made from Biosolids Composted with Carbon-rich Organic Wastes

Composts made from organic wastes have the potential to substitute for peat and bark as components of container growth substrates. Composts for this research were produced in small-scale aerobic bins using biosolids blended with construction debris, storm debris, or horse waste in a 1:3 (v:v ratio). The composts were screened and blended 1:1 (v:v) with douglas fir (Pseudotsuga menziesii) bark to produce substrates. They were compared with a peat–perlite control substrate, a biosolids blend control substrate, and substrates made from a commercial biosolids compost mixed 1:1 with bark and from fiber from an anaerobic digester (dairy manure and food waste) mixed 1:1 with bark. Chemical and physical properties of the substrates were measured before transplanting, and growth, quality, and leaf color of ‘Little Hero Flame’ marigold (Tagetes patula) and ‘Golden California Wonder’ bell pepper (Capsicum annuum) were measured in a replicated greenhouse study comparing the substrates at two rates of nitrogen (N) application. The experimental biosolids composts-bark substrates performed similar to the peat–perlite and biosolids blend controls for growing marigold and pepper. The commercial biosolids compost mixed with bark did not perform as well as the experimental substrates or the controls. Digester fiber-bark was intermediate between commercial biosolids compost-bark and other treatments. Higher N rates improved plant growth and quality across all container substrates in pepper, but had fewer significant effects on marigold. Experimental substrates were coarser texture than the peat–perlite or biosolids blend controls, resulting in higher aeration porosity (AP) and lower water-holding capacity (WHC), but performed well nonetheless under the drip irrigation used in this study. Using locally sourced organic waste materials as container substrates can help capture value from organic wastes and contribute to the sustainability of nursery production practices.

Microbial-Induced Carbon Competition in the Spermosphere Leads to Pathogen and Disease Suppression in a Municipal Biosolids Compost

The aim of this study was to understand whether competition for fatty acids in plant seed exudates by compost-derived seed-colonizing microbial communities could explain the suppression of plant infections initiated by sporangia of Pythium ultimum. The germination behavior of P. ultimum sporangia in response to cucumber seeds was measured to determine the impact of seed-colonizing microbes on pathogen suppression. Seed-colonizing microbial communities from municipal biosolids compost utilized cucumber seed exudates and linoleic acid in vitro, reducing the respective stimulatory activity of these elicitors to P. ultimum sporangial germination. However, when sporangia were observed directly in the spermosphere of seeds sown in the compost medium, levels of germination and sporangial emptying did not differ from the responses in sand. The percentage of aborted germ tubes was greater after incubating sporangia in compost medium for 12-h than the level of germ tube abortion when sporangia were incubated in sand. Abortion did not occur if previously germinated sporangia were supplemented with cucumber seed exudate. Furthermore, removal of cucumber seed exudate after various stages of germ tube emergence resulted in an increase in aborted germ tubes over time. Adding increasing levels of glucose directly to the compost medium alleviated germ tube abortion in the spermosphere and also eliminated disease suppression. These data fail to support a role for linoleic acid competition in Pythium seedling disease suppression but provide evidence for general carbon competition mediated by seed-colonizing microbial communities as a mechanism for the suppression of Pythium seed infections in municipal biosolids compost.

Seed-Colonizing Bacterial Communities Associated with the Suppression of Pythium Seedling Disease in a Municipal Biosolids Compost

This study was designed to characterize seed-colonizing microbial communities that were previously shown to be involved in the suppression of seedling disease caused by Pythium ultimum in a municipal biosolids compost. Selective microbial inhibitors were employed to inactivate portions of the microbial community associated with seed germinated in a compost medium to evaluate their impact on disease suppression. After initial screenings for toxicity to both cucumber and P. ultimum, six selective inhibitors were eventually used to assess the impact of seed treatment on the reduction of bacterial and fungal populations and on disease suppression. Rifampicin was the most effective inhibitor for inactivating disease suppression. Bacterial communities that colonized cucumber seed sown in compost medium for 8 h and seed sown in compost medium for 8 h followed by a 3-h treatment of either rifampicin at 500 ppm or water were dislodged from seed surfaces and subjected to RNA extraction and reverse transcription to cDNA. Differences in the composition of seed-colonizing bacterial communities were assessed using terminal restriction fragment length polymorphisms (T-RFLP) of polymerase chain reaction-amplified 16S rDNA genes. T-RFLP profiles revealed a diversity of distinct bacterial taxa, a number of which dominate seed surfaces within 8 h of sowing. Analysis of similarity (ANOSIM) using terminal restriction fragment (T-RF) presence or absence showed that community profiles of nontreated and water-treated seed were quite similar whereas community profiles from rifampicin-treated seed were distinct. Differences in community profiles based on T-RF abundance (peak height and peak area) indicated that all treatments were unique (ANOSIM, all pairwise comparisons P < 0.05) Peaks heights and areas of relatively few T-RFs were reduced to zero following rifampicin treatment and 34 T-RFs explained 85% of the observed difference between treatments. Tentative taxon assignments for each of the T-RFs that contributed to the treatment differences revealed a preponderance of sequences with affinities to the α-, β-, and γ-Proteobacteria and Firmicutes. Limited sequencing of clones associated with water-treated and rifampicin-treated seed revealed the presence of similar taxa dominated by members of the γ-Proteobacteria. Many species within these taxa (such as Pseudomonas spp., Enterobacter spp., and Bacillus spp.) are known to be suppressive to Pythium diseases. Results of our study have confirmed that Pythium disease suppression in a municipal biosolids compost is mediated by compost-associated bacteria that colonize seed within hours after sowing. By focusing on actively growing microbes in the infection court during important stages of pathogen infection, we believe we can more efficiently determine the mechanisms of disease suppression and the microbes involved. Although specific to this pathosystem and compost, our results have a much broader scope of inference and illustrate the utility of such a targeted approach in identifying a relatively small subset of microbial taxa from complex communities likely to be involved in disease suppression.