Sub sections:
Mitigating nitrous oxide emissions from soils using pulses and improved nitrogen management
Project Leader: Dr Graeme Schwenke (I&I NSW)
Our project hypothesis is that increasing the ratio of pulses to cereals grown across the grains region will reduce the overall soil emissions of nitrous oxide (N2O) associated with dryland cropping. This reduction in emissions may come about in two ways; firstly nitrogen (N) sourced from legumes may be less prone to loss from the soil, and secondly, N sourced from legumes means less fertiliser N is needed and so less greenhouse gases are produced through fertiliser manufacture.
The project involves measurement and comparison of N2O emissions and N dynamics in three commonly practiced dryland cropping rotation systems typical of northwest NSW, namely; chickpea-wheat, chickpea-sorghum, and canola-wheat. We are also comparing emissions from soil growing chickpea with fababean and fieldpea; two other pulses commonly grown in the region. The non-legumes are grown with nitrogen fertiliser added as per budgeted crop demand, while the pulses are inoculated at sowing with species-specific rhizobium.
The GRDC-funded project officially commenced on 1st May, 2009, although funding was not available until October. Using borrowed funds, we were able to prepare the necessary infrastructure of a dedicated power line, field laboratory, and ultra high purity gas supplies in time for the commencement of the planned field trials. We began the project with a comprehensive soil sampling and characterisation of our study site; a black vertosol on the Tamworth Agricultural Institute property. Soon after, the trials were marked out and the first crops of chickpea, fababean, fieldpea and canola were sown. Concurrent with this sowing was the installation of a set of 6 automatic gas emission sampling chambers in selected plots of the rotation experiment. These chambers were loaned to us by Professor Peter Grace at QUT in Brisbane, while our own set were being constructed. Other plots in this experiment, plus those in the legume species experiment had gas emissions sampled periodically using manual chambers. During the crop’s growth we sampled both soil mineral nitrogen and plant biomass nitrogen monthly. Greenhouse gas emissions were measured continuously (7.5 times per day using automatic chambers) or periodically (at least once a week with manual chambers). Kelly Baker, our project-funded technical officer commenced employment in December 2009
In the 157 days from sowing until harvest, soil under N-fertilised canola emitted a total of 293 g N2O-N/ha. Much of this occurred in the first two months after N application in conjunction with rainfall events that would have stimulated soil nitrification after the urea had hydrolysed to ammonium. Emissions from canola plots equated to 0.37% of the N applied as urea at sowing, although we did not correct for background, i.e. nil fertiliser N. Over the same period the soil under chickpeas emitted only 28.5 g N2O-N/ha. If we consider this as the background N2O emission then the emission factor for N fertiliser under canola during crop growth was 0.33% of that applied. This is close to the Australian emission factor used for accounting emissions from non-irrigated crops, and therefore of similar magnitude to previous research done on soil under cereals. However, those figures are based on 12 months of measurement, including both the crop and the fallow. Rainfall during the cropping period totalled 179 mm, which is approximately two-thirds of the long-term average at Tamworth. Despite this, crop growth was reasonable with canola yielding 1.7 t grain/ha and chickpea 1.3 t grain/ha, although chickpea yields were substantially depressed by insect damage during grain-filling.
Daily N2O emissions ranged from -1.7 to 39.6 g N2O-N/ha/day in the canola (+N) plots and -1.6 to 12.5 g N2O-N/ha/day for chickpea. The higher results were recorded for both crops during a week of rainfall from 26th December 2009 that totalled 134 mm. Most daily emissions however were low, with the canola soil emitting more than the chickpea soil, which was often below detection. Both the range in our results and the low averages were of the same order as other soil N2O emissions measured under dryland wheat crops in Australia.
This research has been presented to a range of audiences through several seminars and site visits, including; university undergraduate and post-graduate students, visiting scientists, and I&I NSW extension agronomists and specialists.