On-Farm
Understanding
On-Farm
Understandingfor
forFarmers
Farmers
Climate Change and GHG emissions
What is soil carbon?
Soil carbon is the common name for carbon held within the soil,
primarily in association with organic matter or darkly coloured
humus found in the top layers of soil. Living organisms and their
remains contain a fair proportion of the element carbon (C).
Some of these remains end up in the soil, and are processed and
decomposed by millions of microorganisms including fungi and
bacteria, as well as by insects and worms. Soil organic matter can
be 50 to 60% C by dry weight, and some of this C can remain
stable in the soil for generations or even centuries. Soil organic
matter helps to maintain soil structure, retain soil moisture,
prevent erosion, and can act as a reservoir for nutrients and as a
source or sink for carbon dioxide (CO2).
Soil carbon
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Cookston soil
Soils contain more carbon than vegetation and the atmosphere
combined and a good level of soil organic matter is good for farming.
Carbon cycle and climate change
http://www.windows2universe.org/earth/climate/images/carboncycle_jpg_image.html
Farming, Food and Health. First
Te Ahuwhenua, Te Kai me te Whai Ora. Tuatahi
Life processes are fuelled by carbon
compounds which are oxidised to CO2 , that in
turn is exhaled by all animals and plants.
Conversely, CO2 is assimilated by plants during
photosynthesis to build new carbon
compounds. The carbon that is retained by
plants and not lost in other ways such as by
grazing pasture may be transferred to the soil
via roots or decomposing plant residues. Soil C
may be returned directly to the atmosphere
from the soil when the soil organic material is
oxidised by decomposition or burning. There is
growing interest in exploring the potential of
soil carbon to act as an atmospheric carbon
sink to offset greenhouse gas emissions.
On-Farm Understanding for Farmers
How much carbon is in the soil?
The amount of organic carbon in soil is a balance
between the inputs of new plant and animal material
and the constant losses as soil carbon is decomposed.
The amount of C in soil varies but soils often contain
more C than vegetation and the atmosphere combined.
It is estimated that in a hectare of grazed dairy pasture,
soil typically contains 80-100 tonnes/C in the upper 30
cm and sometimes considerably more. Factors affecting
how much C a soil can hold include:
•Depth of soil,
•Sand, silt, and clay content,
•Plants and animals present,
•Climate,
•Land use or management.
The carbon content of soil tends to remain stable
unless land use changes such as deforestation or
conversion to cropping take place or there are
substantial changes in land management, such as
increased drainage or cultivation.
Measuring soil carbon change
Soil carbon sequestration could help offset greenhouse
gas emissions but it is difficult to measure soil carbon
content with sufficient accuracy. Even though relatively
small change in soil carbon is important it is difficult to
detect because the soil organic matter is not found
evenly throughout the soil and can be patchy.
Further reading
•Ghani et al 2010: A literature review of soil carbon under
pasture, horticulture and arable land uses – report
prepared for AgMardt and can be downloaded at:
http://www.agresearch.co.nz/science/soilcarbon.asp
•Harry Clark: Are New Zealand pastoral soils increasing the
quantity of carbon they store? http://www.congresosrohr.com/learn/doc/abstract/H_CLARK.pdf
•Parsons AJ & Rowarth JS: Changing carbon in pasture
soils. Presented to Soils 2008, Conference Massey
University, Palmerston North 2008
•Schipper L: Carbon in soils, Science Teacher 2009.
http://earth.waikato.ac.nz/staff/schipper/download/soilc
arbon.pdf
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What factors increase soil carbon in the soil?
The amount of carbon in any soil depends on a
number of factors, and reflects the balance
between accumulation and breakdown. In general,
Soil C content is increased by factors that enhance
photosynthesis and decreased by factors that
enhance soil respiration. Conversely, factors that
decrease photosynthesis or removal of residues will
decrease soil C while decreasing respiration will
potentially increase soil C.
Practices that affect the balance of carbon building
up or being lost from the soil include:
•Type, frequency and intensity of cultivation
,
•Plant residue management e.g. is it burnt,
ploughed in or removed,
•Grazing intensity and rotation of stock,
•Soil fertility,
•Irrigation.
Soil C can be topped up directly by applying organic
material such as manure and composts to the soil.
Key Points
•The C content of soils tends to reach a
steady state unless significant changes in
soil /land use or management take place.
•Soil C content is increased by factors that
enhance photosynthesis and is decreased
by factors that enhance plant and soil
respiration.
•Decreased photosynthesis or removal of
residues will decrease soil C while
decreasing respiration will potentially
increase soil C.