SUMMER FIELDWORK by Alyssa Johnston

My ARP aimed to investigate the response of common rain garden species to different application rates of biochar in an engineered bioretention soil.  Rain gardens are bioretention systems used for stormwater management in urban centres. Bioretention soil (e.g. mixture of compost, topsoil and sand) within rain gardens effectively filters sediments, heavy metals and nutrients from infiltrated stormwater. Rain gardens also contain a variety of plants which assist in additional uptake of nutrients and metals. Minimal maintenance and other inputs are required from these systems resulting in harsh growing conditions for plants (e.g. intermittent drying and wetting). The use of native plants that are more suited to local conditions and organic soil amendments that produce more hospitable conditions are important to utilize in these systems.

Biochar is an organic matter that is heated at high temperature with little to no oxygen. When added to soil it can provides beneficial properties. These include improved soil fertility and plant growth, increased microbial activity and increased water and nutrient holding capacity which is effective in retaining nutrients and contaminants from runoff. With these known benefits that biochar can provide to soil and plants, my research set out to better understand native plant response to biochar in an engineered bioretention soil.

Near the end of June, I set up a randomized potted plant experiment using two native plants, Carex obnupta and Juncus effusus, 3 different biochar ratios (0.5%, 1.5% and 5% weight for weight) and a control (0% biochar), with 5 replicates for each plant-biochar/control combo. I followed municipal bioretention soil specification to engineer the bioretention soil that was used in the biochar-soil mixtures. Once the biochar-soil mixtures were weighed, mixed and put in pots, soil samples were taken and sent to the lab to assess basic soil fertility (N, P, K), pH, organic matter content, C:N ratio, cation exchange capacity, as well as the sand, silt & clay composition.

Each week over the summer, I measured height, percent cover and took observational notes on colour and any noticeable damage to the shoots to identify if the selected biochar rates were suitable to enhance the survival and growth of native plants in an engineered bioretention soil. At the end of September,  I harvested the plants and took soil samples to be analyzed at the lab. At the BCIT lab, I separated, processed and dried the aboveground biomass (shoots) from belowground biomass (roots). After drying I obtained the final biomass weight of both the shoots and roots. The data I have collected will hopefully be able to contribute to increasing the performance of rain gardens as bioretention filters by using biochar to improve physical, chemical and biological capabilities of the growth media and their ability to support a diversity of native plants.

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