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Effect of Forage Stand Termination Method and Fertilization History on Greenhouse Gas Emissions, Nutrient Supply Rates, and Soil Carbon Dynamics.

Holmes, M.. 2018.


While the majority of land used for growing forages in Saskatchewan is not fertilized on an annual basis, nitrogen (N) fertilization is often used to revitalize declining stands or for grass seed production. Once a stand is ready to be terminated, typically either a combination of tillage and herbicide or herbicide alone is used to kill the vegetation. Termination method is anticipated to have a significant effect on the rates and amounts of greenhouse gas (GHG) production, as well as affect carbon (C) and nutrient cycling in the soil. The objective of this thesis work was to examine the influence of grass forage stand termination method on GHG production, nutrient cycling, and dynamics of various soil C pools. Additionally, the influence of two previous years of N fertilizer addition versus no N fertilizer addition was examined. In a laboratory incubation of intact soil cores collected from two forage grass seed production fields in northeastern Saskatchewan (Arborfield brome grass in August 2013 and Carrot River timothy sites 1 and 2 in May 2014), termination by a combination of tillage and glyphosate caused a reduction of up to 16% in carbon dioxide (CO2) emissions compared to glyphosate alone. The tillage/glyphosate termination also tended to decrease nitrous oxide (N2O) emissions when compared to glyphosate alone. Prior N fertilization for two years resulted in increased emissions of both CO2 and N2O, as well as slightly lower phosphate (PO43-) supply rates in the surface soil. Nitrogen supply rates were generally increased by N past fertilization, especially the ammonium (NH4) supply rate, which was as much as 18% higher than in unfertilized plots. The field experiment conducted on the two Carrot River sites (CR1 and CR2) from August, 2013 to October, 2014 examined changes in soil organic C (SOC) pools. Prior N fertilization increased the amount of light fraction, water extractable, and microbial biomass C (LFOC, WEOC, and MBC, respectively) compared to the unfertilized plots. Termination with tillage significantly increased the LFOC concentrations in the following year but this difference disappeared by the end of the 2014 season. Tillage also tended to reduce the concentrations of WEOC and MBC over the course of the study. There were no significant differences between treatments in any of the C pools at the end of the study. Therefore, the conclusion of this thesis work is that the current practice of grass forage stand termination through a combination of tillage and glyphosate is a beneficial management practice in the soils studied through reduced greenhouse gas emissions.