METHODS
Study Area &
Experimental Design
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We collaborated with Edmonton City to select 36 sites as experimental plots, each featuring one Burr Oak tree (Figure 1). Based on the natural conditions of these sites, the 36 trees were divided into 12 blocks, with each block containing three different biochar application rates (0, 20, 40 tons/hectare; Table 1).
Figure 1. Study area
Table 1. Experimental design
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As shown in Figure 2, the size of the tree pit is approximately 2.25 cubic meters. The tree was planted with its root ball intact. After planting, top-soil was applied and then biochar was mixed into the soil at a depth of 5 cm from the surface. A collar was installed around the tree to facilitate subsequent measurements of greenhouse gas emissions.
Figure 2. Design and field diagram of biochar application
Sample Collection
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GHG flux measurements were conducted at each site over a one-year period, initially with a sampling frequency of three times per week for the first two weeks. This was followed by twice weekly measurements for the next two weeks, then once a week, and eventually reduced to once every week. The rationale for this frequency is that at the beginning of the experiment, right after biochar was applied, the substantial disturbance could cause significant fluctuations in greenhouse gas emissions. Therefore, a higher frequency was necessary initially, which was gradually decreased as the treatment stabilized. We used an integrated GHG analysis system based on off-axis integrated cavity output spectroscopy (LI-7810SC, Licor Environmental, Lincoln, NE, USA), along with measurements of substrate water content and temperature (Figure 3).
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Measurements of monthly tree growth and performance include diameter at breast height (DBH), tree height, crown diameter, crown height, base to crown height, and internode extension.
Figure 3. Greenhouse gas measurements
Sample Analysis
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The methods employed to measure the soil parameters were as follows: Soil pH was determined using a pH meter; Electrical Conductivity (EC) was measured using a conductivity meter; Available Nitrogen and Available Ammonium concentrations were quantified using colorimetric analysis; Total Organic Carbon (TOC) and Total Carbon (TC) were analyzed with a TOC analyser, as depicted in Figure 4; Total Nitrogen (TN) was assessed using the combustion method (Dumas method); Microbial Biomass Carbon (MBC) and Nitrogen (MBN) were estimated using the fumigation-extraction method. These methods collectively facilitated a comprehensive evaluation of the soil properties.
Figure 4. Total Organic Carbon Analyser (left) and Element Analyser (right)
Data Analysis
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In our data analysis, we incorporated blocks as random factors to handle inherent variability and applied a linear mixed-effects model for differential analysis. This approach allows me to effectively control for confounding influences and improve the accuracy of our findings.
