Nutrition of Hybrid Willows for Bio-fuel Production
Seven different clones of willow grown as a potential bio-fuel crop were evaluated in their response to irrigation and N, P, K and S fertilization. After three growing seasons, prior to harvesting the plots for biomass measurements, seven stems per plot were be sampled and analyzed for N, P, Ca, Mg, Al, Fe, Mn, Cu, Zn, B, and Na concentrations. As well residual nutrient concentrations in the soil down to 60 cm (24 in) were assessed.
IPNI-2010-CAN-SK42
07 Mar 2011
Methodology
The study is being carried out in the existing Canadian Forest Service two-year-old hybrid willow plantation, located on 14th Street and Circle Drive in Saskatoon. The plantation is a clonal trial with seven different clones of willow arranged in a randomized complete block design, replicated three times, using a 60 x 60 cm spacing within each triple-row bed and 200 cm spacing between beds, resulting in a density of approximately 15,625 stems/ha. Due to insufficient willow material at time of planting, there are unequal numbers of beds among clones (Figure 1). Consequently, only a single bed will be used for the imposed treatments in this study and only with those clones having more than one bed, namely: Charlie (S. alba x S. glatfelteri) and SV1 (S. dasyclados). In the spring of 2008, one bed from each clone were coppiced and will be allowed to grow for the standard three-year period before being harvested. A 3 x 3 factorial design of three different rates of both irrigation and fertilizer treatments were imposed on each bed, resulting in a split-split-plot experimental design (i.e., clone as the whole plot factor, irrigation rate as the subplot factor, and fertilizer rate as the sub-subplot factor; Figures 1 and 2). The three irrigation treatments consist of either no additional water added above rainfall or drip irrigation used to maintain soil moisture at 75 (half water) or 100 % (full water) field capacity, measured using soil moisture probes installed within each plot (Spaans and Baker, 1992). A Campbell Scientific CR10x is being used to monitor soil moisture and control irrigation timing. The three fertilization treatments include no fertilizer or fertilizer applied once annually over the three-year rotation either at the recommended rate (Fert Treatment #1) or 2x the recommended rate (Fert Treatment #2). The recommended rate consists of a balanced fertilizer blend of 100:30:80:50 (N:P:K:S), which is intended to not only match hybrid willow growth requirements, but also replenish nutrients exported when harvesting willow with annual biomass production of 15-22 Mg/ha (Perttu, 1993; Danfors, 1998; Adegbidi et al., 2001). The 2x recommended rate is intended to test the upper limit of willow growth response to added fertilizer, when grown on Class 2-3 agricultural soil under optimal moisture conditions, along with quantifying its influence on chemical and physical wood properties at time of harvest. Previous studies report negligible nitrate leaching from heavily fertilized (i.e., up to 240 kg N/ha applied annually) willow plantations after the first growing season with established root systems (Dimitriou and Aronsson, 2004). Consequently, leaching is not expected to be a problem in this study with the 2x recommended fertilizer rate treatment of 200 kg N/ha applied annually, because of its current two-year-old root system and the heavy clay soils at this site. Each year the irrigation and fertilizer treatments will be initiated in early June to avoid exacerbating potential frost damage in late May and also ensure the willow are vigorously growing, in order to increase the fertilizer use efficiency (Abrahamson et al., 2002). Likewise, irrigation will cease at the beginning of September, in order to prepare the willow for a possible early frost episode.
After three growing seasons, prior to harvesting the plots for biomass measurements, seven stems per plot (representing the diameter range) will be sampled and then a 10 cm long section extracted from the centroid (i.e., balancing point) of each stem (Mosseler et al. 1988). All of the stem samples will be debarked using a vegetable peeler (Tharakan et al. 2003) and the volume of the fresh bark and wood samples determined by water displacement and specific gravity calculated based on fresh volume and oven-dry weight (Sennerby-Forsse, 1985; TAPPI, 1999). The fresh weights of bark and wood will be assessed before dried to a constant weight at 65 oC and re-weighed for calculating the ratio of bark to wood and moisture content. Bark concentration (g/kg) will be determined using the bark and wood fresh weights (TAPPI, 1999). The seven bark and wood samples from each plot will be bulked separately and analyzed for nutrient contents. All samples will be dried to a constant weight at 65 oC, weighed, and then ground to a fine powder in a ball mill for nutrient analyses. Foliar levels of N, P, Ca, Mg, Al, Fe, Mn, Cu, Zn, B, and Na will be determined using a H2SO4 digest (Thomas et al., 1967) and the extract measured using either inductively-coupled plasma spectrometry (Perkin Elmer Optima 3000-DV; Perkin Elmer, Shelton, CT) or colourimetrically (Technicon AutoAnalyzer; Technicon Industrial Systems, Tarrytown, NY). Stem nutrient-use efficiencies, which are based on the ratio of biomass production per unit of stem nutrient, will also be determined (Laclau et al., 2000). Ash content will be determined by heating samples to 600 oC (Conn and Tillman, 2000). The logistics for analyzing calorific energy value and the relative amounts and composition of extractives, cellulose, hemicelluloses, and lignin have yet to be determined, but will involve sending samples away to either Dr. Shawn Mansfield (Department of Wood Science, UBC), Forintek (Vancouver, BC), or Dr. Tim Volk (SUNY-ESF) for the analyses.
After harvesting the willow, the impact of imposed intensive irrigation and fertilization treatments on nutrient distribution throughout the soil profile will be assessed by collecting a 60 cm deep soil core (2.5 cm diameter) within each plot using a hydraulic punch (Assefa et al., 2004; Stumborg et al., 2007). Soil cores will be sectioned into 0-10, 10-20, 20-30, and 30-60 cm increments prior to sample preparation. When processed, all soil samples will be air-dried to constant weight, ground with a rolling pin, mixed, and sieved (< 2 mm fraction retained) and then analyzed for the aforementioned nutrients.