Nitrous oxide emissions from cropping soils in the Western Australian grainbelt: a 10-year perspective

Key messages

• Ten years of field-based research has shown soil nitrous oxide (N2O) emissions from nitrogen fertiliser application to Western Australian cropping soils are very small (<0.12% of fertiliser N applied) compared to international standards (1.0% of fertiliser N applied).
• The highest daily N2O emissions occurred in response to summer rain rather than in response to fertiliser N applications.
• Liming soil may decrease soil N2O emissions further in some instances, while increasing soil organic C may increase emissions to some extent. Including grain legumes in cropping rotations did not enhance soil N2O emissions in the growing season or post-harvest.
• Quantify direct soil N2O emissions from Western Australian cropping soils in response to fertiliser N application and the growth of grain legumes.
• Investigate strategies for lowering soil N2O emissions from Western Australian cropping soils.

Aims

• Quantify direct soil N2O emissions from Western Australian cropping soils in response to fertiliser N application and the growth of grain legumes.
• Investigate strategies for lowering soil N2O emissions from Western Australian cropping soils.

Introduction

Nitrous oxide is a potent greenhouse gas (GHG) that is emitted from soils in response to a range of biological and abiotic processes (Butterbach-Bahlet al. 2013). For example, nitrifying microorganisms convert soil ammonium (NH4+) to nitrate (NO3-) under aerobic conditions, which may result in N2O formation as a by-product. Denitrifying soil organisms sequentially reduce nitrogen oxides (e.g., NO3-) to nitric oxide (NO), N2O and finally di-nitrogen gas (N2) with incomplete conversion resulting in emission of N2O.

On-farm soil N2O emissions from cropping enterprises include ‘direct’ emissions from applying fertiliser N, mineralisation of crop residues and mineralisation of soil organic matter. ‘Indirect’ soil N2O emissions result from ammonia volatilisation, N leaching, and N run-off from soil. The research presented in this paper focusses on direct emissions from applying fertiliser N and the growth of grain legumes.

Correctly accounting for soil N2O emissions is critical when calculating on-farm GHG emissions. For example, a GHG life cycle assessment of the production and delivery of 1 tonne of wheat from the central grainbelt to port found using regional-specific data for direct soil N2O emissions decreased GHG emissions by 60% compared to using the international default value (Biswaset al 2008).

Accurately measuring soil N2O emissions is challenging as emissions vary spatially across the paddock and can vary from day-to-day in response to environmental conditions (e.g., rainfall, temperature) and farm management practices (e.g., fertiliser N application) (Butterbach-Bahlet al 2013). Consequently, long-term, multi-year field studies that measure soil N2O emissions throughout the year are recommended for determining annual soil N2O emissions (Dorichet al., 2020). This is most often achieved by using manual (static) or fully automated N2O chambers (Cloughet al 2020).

Experiment approach and design

Soil N2O emissions were measured at Cunderdin, Wongan Hills and Buntine, for 2 to 4 years depending on the location (Table 1). Soil types ranged from deep sands (Buntine, Wongan Hills) to duplex soils (Cunderdin). The experimental design varied depending on the research question at each location, but all study sites included three replicated plots. In experiments investigating the effect of fertiliser N application on soil N2O emissions, granular urea was applied at a range of application rates (0–100kg N/ha; Table 1) depending on the crop and year. For specific details about the approach and experiment design at each study site refer to the associated published papers (Table 1).

Measuring soil nitrous oxide emissions

Soil N2O emissions were measured in each treatment plot using automated field-based chambers that measured emissions on a sub daily basis. Briefly, the system consisted of a gas chromatograph fitted with an electron capture detector for N2O analysis, an automated sampling unit for collecting and distributing gas samples, and six to 12 chambers (one per treatment plot) depending on study location. Chambers (500mm x 500mm) were placed on metal bases inserted into the ground, and fitted with a top that could be automatically opened and closed. Four bases were located in each treatment plot to enable the chambers to be moved to a new position every week so as to minimise the effect of chambers on soil properties and plant growth. The height of the chambers was progressively increased to accommodate crop growth, with a maximum height of 950mm. The chambers were programmed to open if the air temperature in the chamber exceeded a set value or if it rained while the chambers were closed. The automated gas sampling unit enabled N2O emissions to be monitored continuously, providing up to eight (hourly) fluxes per day.

Analysis of data

Daily N2O emission for each plot were calculated from the mean of the hourly losses for that day. Annual or cumulative fluxes for each plot were calculated by integrating hourly losses over time for each plot before calculating the average for each treatment. The emission factor (EF; %) for the application of fertiliser N to the soil was calculated for each treatment at each study site by subtracting the mean annual N2O emission (kg N/ha/yr) of the plots that received no fertiliser N, from the mean annual N2O emission of the plots that received fertiliser N, divided by the total fertiliser N applied (kg N/ha/yr), and finally multiplying by 100. A general linear model was used to determine if treatments affected annual soil N2O emissions. Post hoc pair-wise comparisons of means were made using LSD (significance level of 5%).

Results

Nitrous oxide emissions and emission factors

Annual nitrous oxide emissions reported from WA cropping soils fertilised with N were small (0.02–0.27kg N/ha/yr) (Table 1). The greatest annual N2O emissions occurred where organic matter (OM) had been incorporated regularly into the surface soil for nine years with the aim of increasing soil carbon content (Barton et al 2016). The proportion of fertiliser N emitted as N2O on annual basis from WA cropping soils ranged from 0.1 to 0.12% depending on the study location, with values significantly less than the international emission factor of 1%.

Across all sites and treatments, daily soil N2O emissions ranged from -8 to 24g N2O-N/ha (data not shown). Greatest daily losses at all study sites, and in all years, occurred in response to rain during the summer-autumn fallow. During these periods the soil was warm, and rain enhanced soil N mineralisation, thus increasing the availability of mineral N to soil microorganisms in the absence of active plant growth. Soil N2O emissions during the summer fallow period represented 55 to 85% of the annual N2O emission depending on the location and year.

Table 1. Annual nitrous oxide emissions (N/ha/yr) and emission factors (%) for Western Australia dryland cropping soils in response to applying nitrogen fertiliser or growing grain legumes

^aorganic matter, OM; ^bapplied as urea; ^cnot determined as there was no nil fertiliser N treatment.

Land management effects on soil nitrous oxide emissions

Including grain legumes in cropping rotations did not enhance N2O emissions during or between growing seasons (Bartonet al 2011; Bartonet al 2013b). International research had suggested there was a risk that including grain legumes in cropping rotations may increase the risk of soil N2O emissions (Stehfest & Bouwman, 2006). Research conducted at Wongan Hills supported more recent observations that under similar climatic and management regimes, N2O fluxes from legume cropping systems will not necessarily be greater than emissions from N fertilised non-legume crops. Nitrous oxide emissions during the growth phase of a lupin crop (0.002kg N2O-N/ha/yr) at Wongan Hills was a third of that measured during the growth phase a wheat crop (0.006kg N2O-N/ha/yr).

Liming acid soils may offer an opportunity for decreasing soil N2O emissions from N fertilised cropped soils. Applying lime decreased annual N2O emissions from a N fertilised wheat–wheat rotation during a two-year study at Wongan Hills by decreasing the contribution of N2O emissions following summer-autumn rainfall; however, it had no effect in the lupin-wheat rotation (Bartonet al 2013b). A subsequent 15N laboratory study found that increasing soil pH only decreased N2O emissions from an acidic soil when the losses resulted from nitrification (Bartonet al 2013a).

Increasing soil organic matter by incorporating chaff into the surface soil over a nine-year period, increased N2O emissions at Buntine, but also increased grain yields and mineral N (Bartonet al 2016). However, N2O were still low by international standards (<0.12% of the fertiliser N applied), and consistent with values reported for Cunderdin (Table 1). Greatest losses after two years occurred from soil amended with organic matter that received 100kg N/ha each growing season (Table 1). Increasing soil organic matter benefits soil health and crop yields, so land management practices that increase soil organic matter should be encouraged. Instead, N fertiliser practices should be optimised so as to minimise soil N2O emissions.

Conclusions

Ten years of in situ measurements from various sites across the Western Australian grainbelt found annual N2O emissions were small (0.02–0.27kg N/ha/yr) from cropping soils, representing <0.12% of applied fertiliser N. Greatest daily N2O emissions tended to occur in response to summer-autumn rain, and when the soil was fallow.

Liming decreased cumulative N2O emissions from a wheat-wheat rotation at Wongan Hills by 30% by lowering the contribution of N2O emissions following summer-autumn rainfall events but had no effect on N2O emissions from a lupin-wheat rotation. Increasing soil organic C at a long-term study site at Buntine enhanced soil N2O emissions, but losses were still low (0.12% of applied fertiliser N) by international standards. Including grain legumes in cropping rotations did not enhance soil N2O emissions during the growing season or post-harvest, at Cunderdin and Wongan Hills.

Using soil N2O data that is relevant to Western Australian cropping systems is critical when calculating on-farm GHG emissions. Accurately accounting for soil N2O emission will also ensure key sources of on-farm GHG emissions are correctly identified, and that effective mitigation strategies can be implemented.

Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support.

D. Donovan and D. Gatter are thanked for maintaining the automated gas chambers and field sites. R. Buck and C. Swain are thanked for coordinating soil and plant analyses. This research was supported by funding from the Australian Greenhouse Office, Australian Government’s Climate Change Program, Australian Government Department of Agriculture and GRDC. In-kind support was provided by the Cunderdin Agricultural College, Liebe Group, Department of Agriculture, Karlsruhe Institute of Technology, The University of Western Australia, and Western Power.

References

Barton, L., Butterbach-Bahl, K., Kiese, R., Murphy, D., 2011. Nitrous oxide fluxes from a grain-legume crop (narrow-leafed lupin) grown in a semi-arid climate. Glob. Change Biol. 17, 1153-1166.

Barton, L., Gleeson, D.B., Maccarone, L.D., Zúñiga, L.P., Murphy, D.V., 2013a. Is liming soil a strategy for mitigating nitrous oxide emissions from semi-arid soils? Soil Biol. Biochem. 62, 28-35.

Barton, L., Hoyle, F.C., Stefanova, K.T., Murphy, D.V., 2016. Incorporating organic matter alters soil greenhouse gas emissions and increases grain yield in a semi-arid climate. Agric. Ecosyst. Environ. 231, 320-330.

Barton, L., Kiese, R., Gatter, D., Butterbach-Bahl, K., Buck, R., Hinz, C., Murphy, D.V., 2008. Nitrous oxide emissions from a cropped soil in a semi-arid climate. Glob. Change Biol. 14, 177-192.

Barton, L., Murphy, D., Butterbach-Bahl, K., 2013b. Influence of crop rotation and liming on greenhouse gas emissions from a semi-arid soil. Agric. Ecosyst. Environ. 167, 23–32.

Barton, L., Murphy, D.V., Kiese, R., Butterbach-Bahl, K., 2010. Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate. Glob. Change Biol Bioenergy 2, 1-15.

Biswas, W.K., Barton, L., Carter, D., 2008. Global warming potential of wheat production in Western Australia: A life cycle assessment. Water Environ. J. 22, 206-212.

Butterbach-Bahl, K., Baggs, E., Dannenmann, M., Kiese, R., Zechmeister-Boltenstern, S., 2013. Nitrous oxide emissions from soils: how well do we understand the processes and their controls? Philosophical Transactions of the Royal Society B 368, 20130122.

Clough, T.J., Rochette, P., Thomas, S.M., Pihlatie, M., Christiansen, J.R., Thorman, R.E., 2020. Global Research Alliance N2O chamber methodology guidelines: Design considerations. J. Environ. Qual. 49, 1081-1091.

Dorich, C.D., De Rosa, D., Barton, L., Grace, P., Rowlings, D., Migliorati, M.D.A., Wagner-Riddle, C., Key, C., Wang, D., Fehr, B., 2020. Global Research Alliance N2O chamber methodology guidelines: Guidelines for gap‐filling missing measurements. J. Environ. Qual. 49, 1186-1202.

Li, Y., Barton, L., Chen, D., 2012. Simulating response of N2O emissions to fertiliser N application and climatic variability from a rain-fed and wheat-cropped soil in Western Australia. J. Sci. Food Agric. 92, 1130-1143.

Stehfest, E., Bouwman, L., 2006. N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions. Nutr. Cycling Agroecosyst. 74, 207-228.

Contact details

Louise Barton
The University of Western Australia
35 Stirling Highway, Nedlands, Western Australia 6009
Ph: 08 6488 2543
Email: louise.barton@uwa.edu.au