Does Including Alfalfa in a Crop Rotation Reduce Net Greenhouse Gas Emissions?
Environment
Biota
Siobhan Stewart, Aaron Glenn, Mario Tenuta, and Brian Amiro, Department of Soil Science, University of Manitoba, Winnipeg, MB R3T 2N2
Objective
• To investigate the short‐term impacts of including a perennial forage (alfalfa) in an annual cereal/oilseed crop rotation on: (1) C and N cycling in agricultural soils; (2) net GHG emissions (CO2 and N2O); and
(3) soil health
Methodology
TGA
Site Description and Design
• TGAS MAN is located 20 km south of Winnipeg at the University of Manitoba’s National Centre for Livestock and the Environment (NCLE)
• The study site consists of four, 4‐hectare plots on Red River and Osborne clay with two crop treatments imposed: cereal/oilseed and alfalfa (Figure 1)
• After decades in annual rotation, plots 1 and 4 were sown to alfalfa in May 2008, while plots 2 and 3 were planted to spring wheat • High erucic acid rapeseed was sown onto plots 2 and 3 in June 2009
Photo 1. Trace Gas Analyzer (TGA) in trailer with datalogger and reference gas cylinders
SAT
Photo 2. Wheat plot with gas sampling station and sonic anemometer‐thermometer (SAT)
• Spring 2008 (Figure 2) showed a loss of carbon from both cropping systems, with rapid C uptake by the spring wheat, drawing in CO2 with plant growth. Due to the slow establishment of alfalfa in the first year, the alfalfa plots took longer to become a net CO2 sink. With grain maturation and harvest, the spring wheat plots became a C source while the alfalfa continued to photosynthesize longer into the fall
• In 2009, the alfalfa began taking up C as early as May; while the rapeseed was not seeded until early June, becoming a net C sink in early July
GI
First cut
Photo 3. Setting up SAT in over‐wintering alfalfa plot Photo 4. Gas sampling station in alfalfa plot showing the two gas intake (GI) levels
Figure 2. Cumulative carbon dioxide (CO2) fluxes from both cropping systems.
• In June of both years, the annual treatment (Figure 3) emitted N2O following fertilizer application and precipitation, whereas absence of N fertilizer addition to alfalfa resulted in reduced emissions. A small N2O release was detected from the alfalfa in 2008, while in 2009, it was negligible
Photo 5. Weather station at TGAS site
Photo 6. Rapeseed plots in mid‐July 2009.
Fertilizer and precipitation
• Micrometeorological instrumentation at the site measures surface GHG emission fluxes continuously throughout the year from the four experimental plots
• A highly sensitive trace gas analyzer (TGA100A Campbell Scientific Inc., Photo 1) measures CO2 and N2O concentrations (Photo 2 and 3)
• Fluxes of each gas are determined from the gas concentration difference between two heights (Photo 4) and the turbulent transfer of energy by the atmosphere
• A weather station (Photo 5) provides supporting environmental measurements, such as soil temperature, soil moisture content, precipitation and solar radiation inputs, that influence GHG emissions
• Soil samples are taken monthly during the growing season to distinguish inorganic nitrogen as a driver of N2O emissions and indicator of soil health.
Acknowledgements
A big thank you to Jenna Rapai and Brad Sparling for technical support. Funding provided by CFI, CRC in Applied Soil Ecology, NSERC Discovery and the Province of Manitoba, MSAPP.
Figure 1. Diagram of site layout and instrumentation location (A) and cropping treatments (B) at the TGAS MAN site. Each of the four plots are numbered.
Soil Ecology
Laboratory
at The University of
Manitoba
Findings
Background
• Agriculture is responsible for 10% of Canada’s greenhouse gas (GHG) emissions. In Manitoba, agriculture is responsible for >30% of emissions, the largest contributing sector to the provincial GHG inventory
• Nitrous oxide (N2O) has a GHG warming potential approximately 300 times greater than carbon dioxide (CO2), and is predominantly lost from manure and fertilizer application to agricultural fields
• Agricultural soils have the potential to be a net carbon sink, through C sequestration, when farmers adopt perennials into crop rotations
• Prairie farmers have converted to annual cropping systems from perennial crops due to changing market economics, alternative crop demand, and use of biofuels as an alternate energy source
• Trace GAS Manitoba (TGAS MAN) is the only site in Western Canada that measures surface CO2 and N2O exchange continuously throughout the year
• TGAS MAN is ideal for determining the short‐term effects on net GHG emissions by introducing perennials (ie. alfalfa) into annual crop rotations.
Management
contact info: [email protected]
Harvest
Flood
Figure 3. Nitrous oxide (N2O) fluxes from both cropping treatments throughout the 2008 and 2009 growing seasons [100 g ha‐1 N2O‐N is equivalent to 1/10 lbs N ac‐1].
Conclusions
• Alfalfa accumulated 1250 kg ha‐1 of C in 2008, whereas the wheat was a net C sink of approximately 480 kg ha‐1, after accounting for straw and grain removal • By September 2009, the perennial had fixed more atmospheric CO2 than the annual treatment. Further work will account for the biomass harvested in the net 2009 C budget • Inclusion of N2O fluxes provides much needed realistic estimates of direct GHG emissions during perennial phase establishment
• Findings from this project will be useful for future policy changes in Canada, the value of perennials to a crop rotation, and the possible modeling of GHG emission levels