Effects of wood-based compost on soil health and fungal populations in an organic loam

Sabrina Anderson*

Objective

Determine whether wood compost enhances soil microbial life (specifically Trichoderma spp.) and promotes healthy plant growth

Central hypothesis

Wood compost application will increase Trichoderma spp. frequency, active carbon, microbial soil respiration and plant biomass when compared with an organic compost or control

Methods

Experimental Design

• Completely randomized design with three treatment and 10 replicates
• Each experimental unit was a 4" pot of soil from an organic vegetable farm (Friesen Farms, Langley, BC) planted with a single lettuce transplant (cv. 'Hampton')

Randomization scheme (left) and experimental setup

Treatments

1. Organic compost: soil amended with 25% (volume basis) commercial yard waste compost (Boost Class A Organic, Net Zero Waste, Abbotsford, BC)
2. Wood compost: Soil amended with 25% (volume basis) wood-based compost (Equal proportions of 5-10 mm wood shavings and grass clippings composted underground for 45 days)
3. Control: No soil amendment

Study Location and Sampling

• Initial subsample of the bulk soil was analyzed for active carbon (5), soil respiration (5), and CFU counts (7) using Trichoderma specific agar (2) at the KPU Institute for Sustainable Horticulture (ISH) Lab in Langley, B.C. 
• Two week-old lettuce plants were transplanted into the experimental pots on May 30th and grown until July 17th in a passive solar greenhouse (KPU Research & Teaching Farm Dome, Garden City Lands, Richmond, B.C.) 
• Plants were evenly watered, photographed and monitored for signs of stress throughout the growing period. Notes and photographic records were kept weekly.
• Plants were removed from soil after seven weeks, washed, divided into roots and shoots, oven-dried for ~48 h, and weighed to determine dry-weight biomass (KPU Seed and Soil Lab, Richmond, BC)
• Rhizosphere soil samples were air-dried and analyzed for soil respiration (5), active carbon (5), and CFU counts (7) using Trichoderma specific agar (KPU ISH Lab, Langley, BC)

Passive solar dome greenhouse at the KPU Research and Teaching Farm on the Garden City Lands in Richmond, BC

Statistical Analysis

Data were tested for normality using the Shapiro-Wilk test. ANOVA was used to test for treatment effects. Means were separated by Tukey’s Honest Significant Difference Test. All analysis employed the jamovi interface (8) for the R statistical computing environment (6), with alpha = 0.05 maintained throughout. 

Results and Discussion

Active Carbon

More active carbon was found in the two treatments amended with compost than in the unamended soil or the bulk soil sample (Fig. 1). Active carbon served as an indicator of the readily available food and energy sources for the microbial community in the soil. It has been correlated with particulate organic matter, organic matter, aggregate stability, and respiration (3). Because active carbon was not higher in the control treatment than in the bulk soil sample, the increase in active carbon was attributed to the compost amendment rather than the lettuce.

Fig. 1. Active carbon in soil treated with different soil amendments (control, wood compost, and commercial organic compost). Error bars denote standard error of the mean (n=10). Means labeled with the same letter do not differ significantly (alpha=0.05).

Soil Respiration

Soil respiration was higher in the compost treatments than in the untreated control or bulk soil sample, and higher in the wood compost treatment than in the organic compost treatment (Fig. 2). Soil respiration has been positively correlated with active carbon. It indicates soil microbial activity associated with organic matter decomposition, an essential part of soil nutrient cycling that supports plant growth.  Differences in soil respiration could be attributed to the compost inputs rather than the lettuce transplants.

Fig. 2. Respiration in soil treated with different soil amendments (control, wood compost, and commercial organic compost). Error bars denote standard error of the mean (n=10). Means labeled with the same letter do not differ significantly (alpha=0.05).

The wood compost likely had a higher C:N ratio than the organic compost, resulting in slower initial decomposition. Short-term impacts could include nitrogen demobilization, while long-term benefits might include higher water holding capacity and reduced susceptibility to erosion. In the long term, inputs of carbon-rich materials improve soil fertility and quality. The organic compost likely had a more balanced C:N ratio, resulting in an earlier increase in fertility with longer-term benefits to soil structure.

Soil Fungi

The wood-based compost had more soil fungi than the other treatments (Fig. 3). Trichoderma was not detected among the fungi.

Fig. 3. Fungal colony forming unit (CFU) counts from soil treated with different soil amendments (control, wood compost, and commercial organic compost). Error bars denote standard error of the mean (n=10). Means labeled with the same letter do not differ significantly (alpha=0.05).

Lettuce Biomass

Dry weight of lettuce shoots was higher in the commercial compost treatment and lower in the wood compost treatment than in the control (Fig. 4). Despite its potential benefits to the soil microbial populations, the wood-based compost inhibited plant growth in the short term, perhaps due to N-immobilization.

Fig. 4. Dry weight of lettuce shoots grown in soil treated with different soil amendments (control, wood compost, and commercial organic compost). Error bars denote standard error of the mean (n=10). Means labeled with the same letter do not differ significantly (alpha=0.05).

Plants in the wood compost and control treatments exhibited chlorosis on their outer leaves during the first four weeks after transplanting, suggesting N deficiency. In the final three weeks of the trial, plants in wood compost treatment recovered from chlorosis but the larger plants in the commercial compost treatment exhibited stress symptoms (bolting and necrosis) suggesting they were becoming pot-bound.

Conclusions

• Wood-based compost increased active soil carbon, soil respiration, and soil fungal populations. Trichoderma was not detected among the fungal colonies. 
• Although the slow decomposition of the high-lignin wood-based compost results in short-term N immobilization, it could still offer long-term benefits. Additional N inputs may be necessary when using wood-based compost.
• A commercial compost increased active soil carbon and supported faster lettuce growth than the wood-based compost.

*Department of Sustainable Agriculture & Food Systems, Kwantlen Polytechnic University

email sabrina.anderson <at> email.kpu.ca

Acknowledgments:

Special thank you to Dr. Deborah Henderson, Director of the Institute for Sustainable Horticulture, for opening up their lab to complete my research and for suggesting research on Trichoderma. Matilda Tabert, Research Technician, assisted with lab analysis. Net Zero Waste donated organic compost for the trial. Thank you to Andy Smith, KPU Research and Teaching Farm Manager and the rest of the farm team for watering the trial.

References

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3. Launchbaugh, K. 2009. Direct measures of biomass. University of Idaho College of Natural Resources. https://www.webpages.uidaho.edu/veg_measure/index.htm
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6. R Core Team (2018). R: A Language and environment for statistical computing. [Computer software]. Retrieved from https://cran.r-project.org/.
7. Tabert, M. (n.d) Soil dilution plating: protocol 2.21. Institute for Sustainable Horticulture.
8. The jamovi project (2019). jamovi. (Version 1.0) [Computer Software]. Retrieved from https://www.jamovi.org.