GM Crops: The First Ten Years - International Service for the ...

GM Crops: The First Ten Years
As well as soil cultivation other key factors influencing the rate of SOC sequestration include the
amount of crop residue, soil type and soil water potential. The optimum conditions for soil
sequestration are high biomass production of both surface residue and decaying roots that decompose
in moist soils where aeration is not limiting.
The adoption of NT systems has also had an impact on other GHG emissions. For example,
methane and nitrous oxide which are respectively 21 and 310 times more potent than carbon
dioxide. For example, Robertson, 2002 and Sexstone et al., 1985, suggested that the adoption of
NT to sequester SOC could do so at the expense of increased nitrous oxide production where
growers increase the use of nitrogen fertilizer in NT crop production systems.
Robertson et al (2000) measured gas fluxes for carbon dioxide, nitrous oxide and methane and
other sources of global warming potential (GWP) in cropped and unmanaged ecosystems over the
period 1991 to 1999 and found that the net GWP was highest for conventional tillage systems at
114 grams of carbon dioxide equivalents per square metre/year compared with 41 grams/ha for an
organic system with legumes cover and 14 grams/ha for a no-till system (with liming) and minus
20 grams/ha for a NT system (without liming). The major factors influencing the beneficial effect of
no-till over conventional and organic systems is the high level of carbon sequestration and reduced
use of fuel resulting in emissions of 12 grams of CO2 equivalents m-2 year-1 compared with 16
grams in conventional tillage and 19 grams for organic tillage. The release of nitrous oxide in
terms of carbon dioxide was equivalent in the organic and NT systems due to the availability of
nitrogen under the organic system compared with the targeted use of nitrogen fertiliser under the
NT systems.
Using IPCC emission factors, Johnson et al (2005) estimated the offsetting effect of alternative
fertiliser management and cropping systems. For a NT cropping system that received 100 kg N
per ha per year (net from all sources), the estimated annual nitrous oxide emission of 2.25 kg N per
ha per year would have to increase by 32%-97% to completely offset carbon sequestration gains of
100-300 kg per ha per year.
Estimating the full actual contribution of NT systems to soil carbon sequestration is however, made
difficult by the dynamic nature of the soil sequestration process. If a specific crop area is in
continuous NT crop rotation, the full SOC benefits described above can be realised. However, if
the NT crop area is returned to a conventional tillage system, a proportion of the SOC gain will be
lost. The temporary nature of this form of carbon storage will only become permenant when
farmers adopt continuous NT systems which, itself tends to be dependant upon herbicide based
weed control systems.
Where the use of GM crop cultivars has resulted in a reduction in the number of spray passes or the
use of less intensive cultivation practices this has provided and continues to provide for a permanent
reduction in carbon dioxide emissions.
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