Croplands Research Group Video Presentations on Nitrous Oxide, September 2015
Jane Johnson opens the workshop to welcome participants, and Alan Franzluebbers introduces the Global Research Alliance and presents USDA travel funds to awarded participants.
Rodney Venterea uncovers the mathematical and statistical methods behind diffusion theory, and how they can be used to theoretically/empirically calculate nitrous oxide flux in the soil in chamber-based measurements. Current flux-calculation methods Hutchinson & Mosier (HM), revised HM (HMR), quadratic (QR), and linear regression (LR) differ in accuracy and precision, without one method proving significantly preferable. Recommendations are to use 4 sampling points; conduct a preliminary soil analysis where possible, and apply the additional chamber bias correction (CBC) method if the analysis is accurate; use HMR if lateral diffusion is suspected; and avoid the LR (low accuracy) and HM (low precision) methods.
Neville Millar presents issues with manual chamber sampling to help account for the temporal variability found in N2O emissions in agricultural cropping systems, by looking at experimental design and sampling strategies. Recommendations vary for different types of projects; general suggestions include sampling times (approx. 1 minute per vial) and numbers (15-20 chambers per sampler per sampling event) and the importance of maximising the probability of detecting flux differences across treatments.
Laura Cardenas introduces a greenhouse gas emissions case study to improve methodology in the grasslands/arable areas as part of the UK Greenhouse Gas Inventory. Most N2O in UK agriculture is a result of leakage and run-off from manure management systems. Nine sampling sites were selected across the UK, covering different fertiliser types, manure types and grazing use. Large differences were found between treatments and blocks, the largest fluxes were detected in grassland soils, error was reduced when greater numbers of chambers were used, and spatial variability overlaid all other bias. The results have been published and are being used with mathematical models to develop new emissions factors for the UK.
Curtis Dell gives a number of recommendations on chamber measurement protocols for accessing greenhouse gas events. Chambers should be at least 10-15cm high, as large as practicable, composed of non-reactive materials, properly sealed and insulated, and have a vent and a rubber septa. A minimum of 3 sampling points with 6-8 replicates is required, and zero time sampling can be most convenient when taken above the soil. Sampling time differences between chambers should be recorded, and samples transferred to vials as soon as possible. Sampling frequency and spacing can depend on the project, and the type of gas chromatograph used should be selected ahead of time for suitability.
Michel Cavigelli suggests novel approaches to interpolating N2O flux between episodic sampling points. Automated chambers can provide a useful baseline for the accuracy of manual sampling methods. Periodic sampling is likely to miss many peak flux events. Episodic sampling yields similar results to automated sampling, though it requires more commitment to the project. As biological processes are not linear, Cavigelli suggests using an exponential decay function rather than a linear method to interpolate between samples taken episodically. Brian Davis then summarizes his episodic sampling project, where huge between-replicate variability and unpredictable peak timing were found; and best practices are suggested for the storage and usage of sampling vials.
Philippe Rochette summarises the history of chamber (and Micronet) measurements, then makes recommendations for chamber deployment strategies to quantify the impact of management practices on soil N2O emissions. He stresses that the success of an experiment can be related to decisions made before field activities. Scientific goals and site selection (soil type and spatial variability) should be set according to limitations of chamber techniques and available resources. During the experiment, Rochette recommends adapting the chamber design and deployment protocol to expected impacts of management practices on spatial and temporal variability in soil N2O production and emission. The impacts of soil tillage and N amendments on N2O emission are illustrated.
Gordon McArthur shows the preliminary results from INGOS studies recently carried out on the extent of chamber disturbances on flux measurements from a variety of chamber types in use around the world. Using a measurement tank and replicate sampling, chambers were tested for disturbances of wind, chamber proximity, chamber leakage, and manual syringe sampling. Results varied widely over the many different chamber designs. Greatest disturbances were found where chambers were manually syringe sampled, rectangular or square in shape, piston-sealed around the outside of the collar, and lacking a vent tube (or the vent tube was not shielded). Chamber proximity appeared to create no disturbance.
Jane Johnson introduces the USDA-ARS Greenhouse gas Reduction through Agricultural Carbon Enhancement network (GRACEnet) that was initiated in 2002 to identify and quantify agricultural strategies to enhance soil carbon storage and minimize greenhouse gas emissions. Mark Liebig introduces the Managing Agricultural Greenhouse Gas network (MAGGnet) as part of the Croplands Research Group of the GRA.
This session is concluded with an open discussion forum to end the workshop.