Land-use change is an important driver of soil-atmosphere gas exchange, but current greenhouse-gas budgets lack data from urban lands. Field comparisons of urban and non-urban ecosystems are required to predict the consequences of global urban-land expansion for greenhouse-gas budgets. In a rapidly urbanizing region of the U.S. Great Plains, we measured soil-atmosphere exchange of methane (CH4) and nitrous oxide (N2O) for one year in replicated (n = 3) urban lawn, native shortgrass steppe, dryland wheat-fallow, and flood-irrigated corn ecosystems. All soils were net sinks for atmospheric CH, but uptake by urban, corn, and wheat-fallow soils was half that of native grasslands (-0.30 +/- 0.04 g C(.)m(-2.)yr(-1) [.mean +/- I SE]). Urban (0.24 +/- 0.03 g N(.)m(-1.)yr(-1)) and corn (0.20 +/- 0.02 g N(.)m(-2.)yr(-1)) soils emitted 10 times more N2O to the atmosphere than native grassland and wheat-fallow soils. Using remotely sensed land-cover data we calculated an upper bound for the contribution of lawns to regional soil-atmosphere gas fluxes. Urban lawns occupied 6.4% of a 1578-km(2) study region, but contribute up to 5% and 30% of the regional soil CH4 consumption and N2O emission, respectively, from land-use types that we sampled. Lawns that cover small portions of the landscape may contribute significantly to regional soil-atmosphere gas exchange.
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