Author

S K. Nolan

Year

2018

Degree Name

BEnviSci Hons

Department

School of Earth & Environmental Sciences

Advisor(s)

Stephen Wilson

Abstract

Nitrous oxide (N2O) is an extremely potent long-lived greenhouse gas that also significantly depletes stratospheric ozone. The global concentration of atmospheric N2O has been increasing significantly, through sources such as synthetic nitrogenous fertilizers in agricultural soils, which has prompted more research in the area in the hope of mitigating its effects upon future climate change. This work aimed to refine regional estimates of N2O emissions from southeast Australia over the period 1993 – 2016 and to look for evidence of the major meteorological and human land-use processes that affect them. This was done by studying the ratio of N2O to radon enhancements from a subset of 205,000 observations from the Cape Grim Baseline Air Pollution Station in Tasmania, that were selected for times when the air stream had passed over the southeast Australian mainland.

The average N2O fluxes for southeast Australia over the period 1993 – 2016 was estimated to be 135 ± 35 kgN km-2 yr-1, which agrees with estimates for the region for earlier time periods. The N2O emissions were found to be significantly increasing, from a yearly average of 107 kgN km-2 yr-1 in 1993 to 179 kgN km-2 yr-1 in 2016, which is consistent with the increasing nitrogenous fertilizer application on southeast Australian farms. The annual cycle of N2O emissions, which were difficult to discern in earlier work, shows two clear maxima: one in spring and another larger peak in autumn. Although monthly precipitation was found to be weakly correlated with the interannual variability of N2O emissions, it appears to have an influence on the sub-annual emissions from soils when accompanied by increasing temperatures during the spring growing season. The autumn emission peak is likely the result of the combination of irrigation and timing of nitrogenous fertilizer application after periods of dry soils in summer.

The magnitude, increasing trend and annual cycle confirm that agricultural land-use activities are having a significant impact upon N2O emissions from southeast Australia. Given the long lifetime of atmospheric N2O and its role in stratospheric ozone depletion, this underlines the importance of mitigating emissions from agricultural soils. This study also underscores the advantages of using a regionally-integrating “top-down” approach such as the radon-calibrated flux technique to estimate regional emissions, particularly in refining the spatial heterogeneity of emissions that is difficult to capture with bottom-up approaches.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.