Short rotation culture of willow clones across Canada : growth requirements and implications for soil nutrients and greenhouse gas balances

Abstract

The cultivation of willow (Salix spp.) is being investigated as a potential feedstock for biomass energy in the Canadian prairies. For this purpose, and despite willow’s high nutrient and water demand, high rates of productivity can ideally be achieved and maintained while minimizing inputs and greenhouse gas (GHG) emissions. Therefore, plantations of Salix purpurea ‘Hotel’ grown at seven sites across Canada—encompassing a large climate gradient and a variety of soils—provided an opportunity to assess the impacts of site quality on biomass production. Soil and climate variables as well as internal measures of nutrition and water stress were correlated with harvested biomass via linear, non-linear and multiple linear regression. Soil at nine sites were compared to reference sites in order to examine the effects of willow plantation establishment on soil nutrient pools and GHG efflux. The effects of initial nutrient concentration and growth rates on change were also examined. Greenhouse gas efflux (CO2, CH4, N2O) was directly measured using the vented chamber method at two of the sites. Lastly, a new method for measuring standing biomass via ground based digital imaging was developed and validated. Total elemental composition of soil, in particular the presence of Ca, was found to have the greatest correlation with willow productivity (r = 0.89, P < 0.01). Surprisingly, precipitation was not the dominant control for the majority of the sites. Nutritionally, N (r = 0.44, P < 0.01 total soil N) and K (r = 0.52, P < 0.01 foliar K) were dominant for increasing willow productivity although both N (P < 0.05) and K (P < 0.10) were depleted from the previous land use with willow plantation establishment. This indicates increasing nutrient deficiencies of N and K may be become problematic in future rotations for maintaining productivity. Carbon was consistently diminished in the upper 20 cm of soil (P < 0.05) regardless of previous conditions or productivity although previous land use is a contributing factor with less depletion on cultivated soils. However, soil CO2 emissions were greatest under larger trees indicating increased C cycling with increasing productivity. Seasonal differences in N2O emissions and CH4 consumption from traditional agriculture and mature tree stands were not detected. The calcareous soils common in the Canadian prairies provide opportunity for willow production, though fertilization with N and K may be required to sustain adequate growth.