Application of the denitrification-decomposition model to predict carbon dioxide emissions under alternative straw retention methods

Straw retention has been shown to reduce carbon dioxide (CO2) emission from agricultural soils. But it remains a big challenge for models to effectively predict CO2 emission fluxes under different straw retention methods. We used maize season data in the Griffith region, Australia, to test whether the denitrification-decomposition (DNDC) model could simulate annual CO2 emission. We also identified driving factors of CO2 emission by correlation analysis and path analysis. We show that the DNDC model was able to simulate CO2 emission under alternative straw retention scenarios. The correlation coefficients between simulated and observed daily values for treatments of straw burn and straw incorporation were 0.74 and 0.82, respectively, in the straw retention period and 0.72 and 0.83, respectively, in the crop growth period. The results also show that simulated values of annual CO2 emission for straw burn and straw incorporation were 3.45 t C ha(-1) y(-1) and 2.13 t C ha(-1) y(-1), respectively. In addition the DNDC model was found to be more suitable in simulating CO2 mission fluxes under straw incorporation. Finally the standard multiple regression describing the relationship between CO2 emissions and factors found that soil mean temperature (SMT), daily mean temperature (T mean), and water-filled pore space (WFPS) were significant.

Life cycle assessment of greenhouse gas emissions from irrigated maize and their significance in the value chain

The life cycle assessment component of this multi-institutional project determined greenhouse gas emissions in pre-farm, on-farm and post-farm activities involved in the use of maize for the manufacture of corn chips. When the emissions were expressed in terms of carbon dioxide-equivalents (CO2-e), pre-farm emissions comprised ~6% of the life cycle emissions, on-farm activities comprised ~36% and post-farm activities accounted for ~58% of life cycle greenhouse gas emissions. We used one 400 g packet of corn chips as the functional unit. The single largest source of greenhouse emissions was the emission of nitrous oxide on the farm as a result of fertiliser application (0.126 kg CO2-e per packet). The next largest was electricity used during the manufacture of the corn chips (0.086 kg CO2-e per packet). The manufacture of the packaging (box plus packet, being 0.06 kg CO2-e) was the next largest source and then the oil for frying the corn chips (0.048 kg CO2-e per packet). Greenhouse gas emissions from fertiliser application were primarily nitrous oxide (N2O), which has a global warming potential of 310 kg CO2-e/kg N2O. In typical irrigated farm systems, these emissions, when converted to CO2-e, are almost three times more than the greenhouse gas emissions that result from energy used to pump water. However, pumping irrigation water from deep bores currently produces greenhouse gas emissions that are almost three times those from irrigation using surface waters. Greenhouse gas emissions from the use of tractors on typical farms are about one-third of the emissions from pumping water. Farm management techniques can be used to increase soil carbon and reduce greenhouse gas emissions. If farms that currently burn stubble were to implement stubble incorporation then, in the absence of other changes to the supply chain, they will achieve a 30% reduction in emissions from ‘cradle to farm-gate’. In absolute terms, when the soil carbon dioxide is included (even though soil carbon dioxide in this instance is not counted as a greenhouse gas in national and international greenhouse gas inventories), our measurements indicate that carbon dioxide and greenhouse gas emissions from farms that produce maize using stubble incorporation are 56% lower than emissions from farms that burn their stubble. The pre-farm and on-farm operations add $0.40 value per kg of CO2-e greenhouse gas emitted. Post-farm processing added $2 value per kg of CO2-e greenhouse gas emitted. Processing maize for corn chips emitted more greenhouse gases than processing the same amount of corn for starch or ethanol.