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Agriculture's Impact on Global Warming and Potentials for Greenhouse Gas Mitigation
More than half of the conterminous U.S. land base is used for agriculture. The impact of changes in agricultural management on the regional environment can be correspondingly huge: a relatively minor change in soil carbon storage or greenhouse gas production, for example, can have enormous impact when played out over millions of hectares.
For the past ten years scientists at the KBS LTER site have studied fluxes of the major, naturally occurring greenhouse gases–CO2, methane, and nitrous oxide–in a variety of cropped and natural ecosystems. A recent report in Science showed that different cropping strategies can have markedly different global warming potentials. Ten years of research showed that on average, conventional cropping methods had an annual global warming potential (GWP) of 114 CO2- equivalents m-2. Careful analysis showed that nitrous oxide production was responsible for more than half of this potential, with the remaining half the net effect of commercial fertilizer, lime, and fuel use. The KBS analysis also showed that almost all of the GWP impact of these activities could be mitigated substantially by various management strategies. No-till cultivation led to soil carbon storage almost equal to the total GWP cost of other activities. Planting leguminous cover crops to reduce the need for nitrogen fertilizer and agricultural lime also substantially reduced cropping system GWP. Taken together, analysis suggests that not only could row-crop agriculture be GWP-neutral, but crop production could help to mitigate greenhouse gas production in general–sufficient even to offset the annual increase in U.S. GWP from the emission of fossil fuel CO2. Comparison of cropland to natural communities at KBS showed further that ecosystems that had been abandoned from agriculture 10-50 years previously had a higher mitigation potential (a more negative GWP) than any of the cropping systems and greater even than late successional forest. Putting marginal cropland into conservation easements could thus be an additional strategy for mitigating greenhouse gas production elsewhere in the economy.
Many of these processes change slowly. Any change in soil carbon change over less than a decade is very hard to detect. Changes in the annual fluxes of nitrous oxide and methane are equally difficult to detect reliably, in this case because of year-to-year climatic variability.
GWP analysis was not anticipated when the measurements were first initiated. But an ecosystem approach to taking measurements and a commitment to long-term sampling meant that when crucial measurements were needed for complete GWP analysis–measurements of fuel use and soil nitrogen levels, for example - they were available. Detection of such trends at an LTER site means also that experimentation can be initiated in the context of continuing long-term measurements.