Greenhouse gas emissions are of ever-increasing concern as levels of these gases are linked to the greenhouse effect and climate change. Agriculture may have a significant role to play in reducing greenhouse gas emissions, and for this reason, the La Broquerie Research Project includes consideration of the greenhouse gas dynamics that occur in a forage-based manure management system.
The most important greenhouse gases are nitrous oxide, methane, and carbon dioxide. These gases have many sources, including agricultural components such as animals (methane), manure (methane and nitrous oxide) and soil (methane, nitrous oxide and carbon dioxide). The rate of release of greenhouse gases from these components depends, in part, on agricultural management practices. It is also possible to offset greenhouse gas emissions through carbon sequestration, where carbon is captured in the form of plant roots and organic matter, thus reducing carbon dioxide levels in the atmosphere.
Why Reduce Greenhouse Gases?
Reducing greenhouse gas emissions has long-term environmental benefits, but that's not all! Greenhouse gases lost from agricultural systems represent losses in productivity (for example, milk or meat production) and profit as well.
- When nitrous oxide is released from manure or the soil, valuable nitrogen is lost
- When carbon dioxide is released from the soil, soil organic matter is lost
- When more methane is released from animals, less forage energy is available for meat or milk production
Therefore, reducing greenhouse gas emissions is good for the bottom line as well as the environment!
Emissions of greenhouse gases were measured in a variety of ways:
- methane emissions from cattle were measured by collecting samples of the expired air from grazing animals in stainless steel canisters for 24 hours, once every 28 days. Gases collected were later analyzed for their methane content.
- methane and nitrous oxide emissions from soil, dung, and urine were measured using static-vented chambers at numerous points across the research site and in all treatments throughout the growing season.
- root mass was measured to see how much carbon was being captured. This was done by taking 24 soil samples per plot (2 inch depth and 3 inch diameter), and then washing, drying and weighing the roots contained in each sample.
Methane Emissions from Cattle
Methane emissions from cattle were unaffected by manure application on forage land, even though forage quality was higher in paddocks that received manure. This is likely because grazing animals were able to select the higher quality forages (i.e. legumes) which were more abundant in the unmanured plots (see Forage and Cattle Productivity for the botanical composition of the various treatments). Other studies have shown that including legumes such as alfalfa in pastures (25%) can reduce methane emissions by 10%.
Methane and Nitrous Oxide Emissions from Soil
Total greenhouse gas emissions were higher in manured plots than in unmanured ones. When nitrous oxide and methane emissions were converted to carbon dioxide equivalents and averaged over two years (2004 and 2005), emissions were several times higher where manure had been applied compared to unmanured plots (Figure 1).
Figure 1. Average (2004-2006) nitrous oxide and methane emissions from control, split and full application manure treatments, expressed as CO2 equivalents.
Greenhouse gas emissions from soil were also affected by soil moisture. Soil moisture at the research site gradually increases across the site, with the west side being wetter and the east side being drier. On the wettest edge of the site, methane was the major greenhouse gas emitted, while on the dry side of the site, nitrous oxide was the major greenhouse gas emitted. This was true both in control plots and manured plots.
Carbon Capture in Root Growth
In a forage system, carbon is captured and held in plant roots. Measuring plant root density (mg root material per cubic cm of soil) in the various manure management systems allowed us to estimate the amount of carbon sequestration in each system.
Root density measurements were used to calculate total carbon per hectare. Total carbon in the spring manure treatment was 3588 kg C/ha, almost 900 kg higher than in the control (2695 kg C/ha). This 900 kg/ha of captured carbon is equal to about 3300 kg CO2 equivalent/ha.
Looking at Figure 1 above, average greenhouse gas emissions from the split application treatment and the spring application treatment were 132.4 and 187.0 kg CO2 equivalent/ha higher than the control, respectively. Therefore, the carbon captured in plant roots appears to more than offset the greenhouse gas emissions associated with manure application.
These are preliminary results and the long-term stability of the carbon captured in forage plant roots and the impact on soil organic matter is being examined in Phase 2 of the La Broquerie Research Project.
- Methane emissions produced from feed fermentation in the rumen were not affected by manure application on pasture
- Manure application increased nitrous oxide and methane emissions
- Nitrous oxide and methane emissions were lower with spring-fall application than spring application of manure
- Variability is soil moisture caused great variability in greenhouse gas production throughout the site
- Root weight increased with manure application
- Higher root weight in manured plots may signify increased carbon sequestration, although the stability of this sequestered carbon is not known
- Carbon capture due to increased root weight appeared to overshadow greenhouse gas emissions associated with manure application