title: The effects of elevated carbon dioxide on greenhouse gas emissions from a temperate pasture
Creators: name: Brown, ZA
Publisher: name: University of Tasmania
Year: 2020

description: Global surface temperatures have been rising since the pre-industrial era and are predicted to continue rising due to the positive radiative forcing of greenhouse gases in the atmosphere. The atmospheric concentration of carbon dioxide (CO\(_2\)), the gas responsible for most of this warming, has increased since the industrial revolution due to the burning of fossil fuels and land use change. The concentration of nitrous oxide (N\(_2\)O), a potent greenhouse gas with substantially higher global warming potential than CO\(_2\) over a 100-y horizon, has similarly increased due to agriculture and nitrogenous fertiliser use. In addition to warming the earth’s surface, elevated CO\(_2\) (eCO\(_2\)) may change terrestrial carbon (C) and nitrogen (N) cycling due to plant responses to increased C availability. Plants respond to eCO\(_2\) in two ways: through increased biomass production and increased water-use efficiency. These responses may alter the availability of C, N and water in soils, which could affect microbial activity through shifts in substrate availability and soil aerobic status. As many processes mediated by microbes release CO\(_2\) and N\(_2\)O, it is likely that both plant responses to eCO\(_2\) could alter these gas fluxes. Soils are the largest source of CO\(_2\) and N\(_2\)O, making it essential that accurate predictions of future greenhouse gas emissions include the response of these processes to eCO\(_2\). Using a temperate pasture dominated by Lolium perenne, this study measured the impact of three levels of CO\(_2\) concentration (400, 475 and 550 μmol CO\(_2\) mol\(^{-1}\)) on the emission of CO\(_2\) and N\(_2\)O. Additionally, water supply was independently controlled via irrigation treatments (adequate, excess = +20% and limit = -40%) to determine if CO\(_2\)-effects on soil processes were driven by plant production, soil water content or both. Plant biomass was increased under 550 μmol CO\(_2\) mol\(^{-1}\) relative to ambient due entirely to greater root growth but no similar change in production was present under 475 μmol CO\(_2\) mol\(^{-1}\). Plant N requirements were increased under eCO\(_2\) relative to ambient, resulting in additional N immobilised in plantbiomass under eCO\(_2\) and substantially lower soil mineral N availability. Consequently, N\(_2\)O emissions were never higher under eCO\(_2\) than ambient and were often lower. This effect dramatically reduced N\(_2\)O emissions from this pasture with the strongest effect in the week following fertiliser application when emissions were highest. Under eCO\(_2\), soil CO\(_2\) emissions were higher than at 400 μmol CO\(_2\) mol\(^{-1}\), though mean emissions were similar between 475 and 550 μmol CO\(_2\) mol\(^{-1}\). Similarly, the rate of root decomposition in soils under eCO\(_2\) was higher than in control plots as was the loss of soil C. The CO\(_2\)-effect on both CO\(_2\) and N\(_2\)O emissions was independent of soil water content, demonstrating that the CO\(_2\)-effect on plant production and organic matter inputs drove the processes underlying these fluxes. Therefore, eCO\(_2\) had a strong effect on the emission of CO\(_2\) and N\(_2\)O from this temperate pasture. In particular, N\(_2\)O emissions were suppressed by the CO\(_2\)-effect on plant N immobilisation and CO\(_2\) emissions were accelerated due to increased belowground C input. Importantly, the effects of soil water content on these emissions were absent in this system entirely, suggesting that the CO\(_2\)-effect on plant production can have a substantial impact on greenhouse gas emissions from temperate pastures.