Methane Emissions From Domestic Animals
Methane produced during digestion is a significant
part of the global methane budget. As
food is digested, microbes break down the organic matter creating methane by
enteric fermentation. Ruminant
animals, such as cows, emit an especially large amount of methane through
their digestive process. In Rhode
Island, the most significant methane from animal sources originates from
livestock on farms. The
calculated emissions from this source were 3,566 MTCE in 1990, and 3,465 MTCE
in 1996. These emissions account
for only approximately 0.03% of the total greenhouse gas emissions in Rhode
Island.
The methodology used comes from the Workbook.
The data that are needed to estimate these emissions are the populations of
domestic animals.
The population data are multiplied by an emission factor that is
provided by the workbook for each type of animal.
The data used for these calculations are shown in the Appendix
- Agriculture - Table 1-1.
There was an overall decrease in the amount of emissions that resulted
from the digestive process of domesticated animals from 1990 to 1996, as shown
in the Domesticated Animal Figure This
was caused by an overall decrease in the number of domestic animals in the
State over this time.
The animal population data came from the 1996 National Agricultural Statistics Service. Data were not available
for all animals, for every year. Some
of the values used where interpolated between years that were accounted for.
This adds to the uncertainty of the calculations.
The census of animals occurred on January 1st of each of the
census years. Fluctuations in
animal populations throughout the year are not taken into account in the
numbers used, which has some influence on the emissions.
Also, other factors not directly taken into account in the calculations
include: the feeding situation of the animals, work performed by the animals,
and the feed digestibility. There
was insufficient data to incorporate these factors into the calculations, so
the average values provide in the Workbook were used.
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Methane
Emissions From Manure Management
Methane is produced by the anaerobic decomposition of the organic matter
in manure. The amount of methane
produced by manure varies depending on the storage system used to manage it.
Methane emissions from manure management appears to make an insignificant
contribution to the total greenhouse gas emissions in Rhode Island, amounting
to only 0.005% of all emissions in 1996.
The total emissions from manure in 1990 were 762 MTCE, in 1996 they
where 635 MTCE.
The methodology used comes from the Workbook:
Chapter 7. The emissions are
estimated as a function of the domestic animal population, and the types of
waste management systems used. The
calculations and data that were used to estimate the emissions are located in
the Appendix
- Agriculture Tables 2-1 through 2-13.
There was an overall decrease in the amount of methane emitted from
manure management from 1900 to 1996, as shown in the graph below. This
decrease is due largely to a decrease in the amount of mature dairy cattle in
Rhode Island over the inventory years. The
emissions estimated for each animal group are shown in the Appendix
-Agriculture: Table 2-14.
The same population values were used in this section as in the
section for methane emissions from Domestic Animals.
The same difference between the animal populations that were used in
these calculations from the average applies in this section.
Another source of uncertainty was the different manure management
systems used in the state. Statewide
averages were used to estimate the amount of manure that went into different
systems. Since, state data were not available for the management systems used
for sheep manure, the national averages were used.
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Emissions from Agricultural Soils
Agriculture soil management practices are a net source of nitrous
oxide. Other agricultural sources of nitrous oxide emissions include, manure
management practices, and lime applied to soils.
The total nitrous oxide emissions from soils were 2,796 MTCE in 1990
and 2,189 MTCE in 1996. Emissions
from agricultural soils represent approximately 0.03% of all of Rhode Islands
greenhouse gas emissions.
The methodology used to estimate the emissions from agricultural
soils comes from the Workbook, Chapter 9.
The emissions from synthetic fertilizers was estimated by multiplying
the total amount of fertilizer nitrogen used by an emissions factor.
This emissions factor is the amount of N2O, in kilograms,
emitted in each year, per kilogram of nitrogen applied to the soil in that
year. The N2O
emissions from manure was estimated as a function of domestic animal
population in the state in the inventory years. Details of these calculations
are located in Appendix
- Agriculture, Table
3-2.
The change in emissions from nitrous oxide was mainly due to a decrease
in the amount of synthetic fertilizer used in the state. The changes in emissions from other sources where
extremely small. The total emissions for each source of N2O is
located in the Agriculture appendix.
Data were unavailable for to estimate many potential sources of
nitrous oxide Agriculture appendix. This
lack of data means there could be a substantial percentage error in the
estimates of nitrous oxide emissions. Also,
the estimates of emissions from fertilizers are uncertain.
There are many factors that affect N2O emissions from
fertilizer use, including soil pH and temperature, for which data are not
available. The Workbook
multiplier that combines many variables into one factor, was used here.
Of course, since emissions of N2O make up an extremely small
portion of the total greenhouse gas emissions in Rhode Island, this data gap
introduces very little uncertainty into the overall inventory estimates.
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