CO2 emissions from fossil fuel use

In mechanized farming, fossil fuel is used both to power farm implements and to manufacture and transport fertilizers, pesticides and machinery. Adopting NT usually conserves energy by eliminating energy-intensive tillage and reducing the wear on tillage equipment. The amount of energy saved depends on the previous tillage intensity. For a subhumid site in western Canada, Zentner et al. (2004) found that NT, compared to CT, reduced on-farm fuel and lubricants by 25-31%. In wetter regions where the intensity of CT systems is higher, NT may reduce tillage-related on-farm fossil energy use by up to 60% (West and Marland, 2002).

Tillage accounts for only 30% or less of total energy use, so energy saved by reducing tillage can easily be offset by increased herbicide and particularly by fertilizer inputs. At sites in western Canada, for example, energy savings from reduced tillage intensity were completely offset by increased amounts of fertilizer N and herbicides (Zentner et al., 1998). In the USA, West and Marland (2002) reported that energy gains due to reduced tillage were negated by increased amounts of fertilizer N and herbicides for maize, but not for winter wheat or soybean.

A critical question, clearly, is whether increased use of fertilizer or herbicides is needed to maintain crop yields under NT. According to Elliot and Coleman (1988), herbicide requirements were actually lower in NT than in CT, after the initial transition. At two sites in western Canada, yields in cereal monocultures receiving equivalent rates of nitrogen were lower in NT than in CT (Nyborg et al., 1995), but at other sites in this region, yields under NT were similar to, or higher than, those under CT, particularly when rotations included pulse crops (Carefoot et al., 1990; Izaurralde et al., 1995). Kupusta et al. (1996) reported that maize yields under NT were similar to those under CT when equivalent rates of nitrogen were broadcast, but were slightly lower under NT when starter nitrogen was used.

If soil carbon increases upon adopting NT, higher nitrogen inputs might be needed, at least in the short-term, assuming that the carbon/nitrogen ratio of organic matter is constant. The increased requirement for nitrogen inputs should diminish as soil carbon levels reach steady state. Many factors interact to determine how tillage affects yield, and an assumption that nitrogen requirements will be higher for NT than for CT may not always be justified, especially in the long term. If crop yields can be maintained without increased use of fertilizer N, NT can often significantly reduce CO2 from fossil fuel use.

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