Robertson, G.P., E.A. Paul, and R.R. Harwood
Presented at the ASM at Snowbird (2000-08-02 to 2017-12-05 )
Agriculture plays a major role in the global fluxes of the greenhouse gases CO2, N2O, and CH4. Efforts to mitigate the atmosphere’s growing global warming potential (GWP) requires knowledge about all three gas fluxes under a variety of management strategies, as well as knowledge about historical fluxes from unmanaged systems. Over an 8 – 10 year period we measured soil C and N2O and CH4 fluxes in a replicated series of 4 annual cropping systems, 2 perennial cropping systems, and 4 types of unmanaged communities with different successional histories. Soil carbon accumulated in the no-till and organic-based cropping systems (0.03 and 0.01 kg C /m2/y, respectively), as well as under perennial crops and successional vegetation. Overall fluxes of N2O and CH4 did not differ among annual cropping systems (ca. 3.8 g N2O-N /ha/d and -1.8 g CH4-C /ha/d), but N2O production was substantially lower and methane oxidation substantially higher in the unmanaged systems, with average fluxes of -9.4 g CH4-C /ha/d and 1.2 g N2O-N /ha/d in the late successional sites. GWP analysis shows that in no-till systems the mitigation potential of soil C sequestration (-110 g CO2 equivalents /m2/y) was halved by the GWP of N2O flux (64 g CO2 equivalents /m2/y), and halved again by the GWP of N-fertilizer use (27g CO2 equivalents /m2/y). In the low-input and organic systems, C sequestration mitigation potentials (-29 to -40 g CO2 equivalents /m2/y) were completely offset by the GWP of N2O emissions (58-69 g CO2 equivalents /m2/y). CH4 oxidation contributed little to the mitigation of GWP in any of the cropped or early successional systems. High total mitigation potentials of the perennial crops (ca. 110 g CO2 equivalents /m2/y) and the early successional systems (-211 g CO2 equivalents /m2/y) were due in part to relatively low N2O fluxes. The development of mitigation strategies based on ecosystem management need to include considerations of changes in N2O and CH4 fluxes in addition to changes in C storage.
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