The Effect of Fertilizer Application on Greenhouse Gas Emissions from Willow Short Rotation Coppice Systems in Southern Ontario, Canada
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Willow (Salix spp.) short-rotation coppice (SRC) systems on marginal lands are effective at providing carbon (C) neutral bioenergy. However, nitrogen (N) fertilizer application, used to enhance aboveground biomass productivity, can result in greater soil-derived carbon dioxide (CO2) and nitrous oxide (N2O) emissions and negate the C neutrality of the willow biofuels. This study presents the effect of fertilizer application on GHG emissions, soil characteristics, winter freeze-thaw emissions and total annual emissions in willow SRC systems. Mean CO2-C emissions were 95.1 and 111.0 mg CO2-C m-2 h-1 in 2014, and 69.4 and 92.7 mg CO2-C m-2 h-1 in 2015, in fertilized and unfertilized treatments, respectively. Soil CO2-C emissions exhibited seasonality, with the greatest emissions occurring in the summer and decreasing in autumn. Elevated summer emissions were due to preferable soil temperature and moisture regimes stimulating microbial respiration, and elevated air temperatures and sunlight availability increasing root respiration. Soils under willow clone SX67 (Salix miyabeana) consistently emitted more CO2-C emissions than clone SV1 (S. dasyclados), as SX67 was more efficient at SOC accrual, which is an energy substrate for microbes. Total annual CO2 emissions were 19.73 Mg CO2 ha-1 yr-1 from fertilized treatments, and 26.30 Mg CO2 ha-1 yr-1 from unfertilized treatments. Of this, only 7.2 and 13.4% were derived annually from winter emissions, respectively, and freeze-thaw cycles did not create a pulse of CO2 emissions. Mean N2O-N emissions were 26.5 and 17.2 μg N2O-N m-2 h-1 in 2014, and 22.9 and 18.2 μg N2O-N m-2 h-1 in 2015, from fertilized and unfertilized treatments, respectively. In both years, fertilizer application increased NH4+ and NO3- availability in the soil, resulting in a pulse of N2O-N emissions. Thus, elevated N2O-N emissions were due to inorganic N availability, which stimulated microbial nitrification following fertilizer application. The fertilizer amendment did not result in a substantial increase in willow biomass yields, which was 10.24 ± 1.86 odt ha-1 in fertilized treatments and 8.33 ± 0.97 odt ha-1 in unfertilized treatments; thus, willow SRC systems exhibited very low N use efficiency. There was no pulse of N2O-N following spring thaw events. The willow SRC systems had total annual emissions (expressed as CO2 equivalents) of 20.43 Mg CO2-eq ha-1 yr-1 from fertilized treatments, and 26.90 Mg CO2-eq ha-1 yr-1 from unfertilized treatments. N2O-N emissions only accounted for 2.2 to 3.4% of total emissions, whereas CO2-C emissions accounted for 97.8 and 96.6%. When C sequestration in above and belowground biomass, and litter fall contribution to SOC were quantified, willow SRC systems acted as a C sink in fertilized treatments, with a C sequestration potential of 10.79 Mg CO2-eq ha-1 yr-1. Unfertilized treatments acted as a slight C source, with net emissions of 1.19 Mg CO2-eq ha-1 yr-1, but may become a C sink as willow SRC systems mature and accrue more C. This thesis proposes that fertilizer application be limited in willow SRC systems, as willows exhibited very low N uptake, to eliminate the annual pulse of N2O-N following fertilizer application, but to maintain willow yields at ~10 odt ha-1 and ensure that willow SRC systems are a net C sink. Willow SRC systems are potential C sinks, and can ameliorate atmospheric GHG emissions. This research contributes to a comprehensive understanding of N fertilizer dynamics in willow SRC systems.