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Greenhouse-gas budgets for irrigated dairy pasture and a winter-forage kale crop

Laubach, Johannes, Hunt, John E.
Agricultural and forest meteorology 2018 v.258 pp. 117-134
arid lands, biomass, carbon, carbon dioxide, cattle manure, cows, ecosystems, emissions factor, excreta, feed intake, forage crops, gases, global warming, grasslands, grazing, greenhouse gas emissions, greenhouse gases, inventories, irrigation, kale, methane, methane production, nitrous oxide, pasture plants, pastures, soil, winter, New Zealand
Managed grasslands can be net sources or sinks for three major greenhouse gases (CO2, CH4 and N2O). We measured the exchange of these gases for three years over an irrigated, intensively-managed dairy pasture in New Zealand that was grazed ten times per year. We also measured the greenhouse gas (GHG) exchanges over a neighbouring dryland pasture for two years, including its conversion to a winter-forage kale crop in the second year and the grazing of that crop. From the gas exchanges, measurements of the grazed biomass, and estimates of other imports and exports we obtained annual net carbon (C) budgets and GHG budgets for these ecosystems. The irrigated pasture system (excluding cows) removed CO2 from the atmosphere, 423 (±23)gC m−2yr−1 on average, and when considering the other C inputs and outputs it was also a net C sink in each year, gaining 81 (±27)gCm−2yr−1 on average. The net CO2 uptake of the dryland in the conversion year was about half that of the irrigated pasture, and its net C budget was neutral. The irrigated pasture, without grazing cows, emitted CH4 throughout all seasons. These emissions were about 15 times greater than emissions expected just from cow dung; we cannot discern what fractions originated from the soil and the pasture plants, respectively. At the dryland/kale site, CH4 emissions of the same magnitude occurred. The emissions of N2O from the irrigated pasture were 0.68 (±0.026)gNm−2yr−1 on average (± standard error), and about half that from the kale crop. These results agree reasonably well with expected emissions based on the N inputs from fertiliser and excreta, using emission factors from New Zealand’s national GHG inventory; however, it is unclear what fraction of the observed emissions can be considered as non-anthropogenic background fluxes. For the irrigated pasture, the global-warming potential of the N2O emissions (expressed as CO2-equivalent mass) was approximately equal to the net C uptake. Hence, the pasture was offsetting its own N2O emissions. However, CH4 emissions directly from cows (calculated from the cows’ feed intake) were two to three times greater than the N2O emissions, and about six times greater than the pasture’s CH4 emissions. Therefore, the dairy system including pasture and cows was a net GHG source.