Ammonia loss from a sub-tropical pasture, Gatton, Queensland, 2013

NH3 measurements by passive samplers The Leuning ammonia samplers were placed at five heights (0.2, 0.5, 0.8, 2.2 and 2.9 m) at the centre of each treatment circle (30 m diameter). Two masts with the same heights as the treatment circles were established for background measurements. Background measurements were made upwind and downwind of the treated circles, at least 50 m from the edge of the closest circle. Samplers were changed twice per day (0800-1700 and 1700-0800 hrs) for a week after fertiliser application, then daily for another week and every 2-3 days for the third and final week. Trapped ammonia was eluted from the NH3 samplers on site using MilliQ water. Eluted samples were placed in the freezer and then transported under refrigeration back to the University of Melbourne and analysed with the Skalar San++ segmented flow analyser.

The Leuning et al. 1985 ammonia samplers

N2O snapshots measurement N2O fluxes were measured by closed static chambers (50 x 50 m square, 25 cm height) for the three treatments plus background with five replications/treatment (total of 20 chambers). The chambers were placed at a separate location in the background area within the pasture site. This was to avoid interference of air movement and disturbance to the circular treatment area caused by the N2O work. N2O gas samples were collected on days 1, 2, 3, 4, 5, 6, 7, 15, 16 and 20 after the treatments were applied. On each sampling day, gas samples (20 mL) were collected at 0, 30 and 60 minutes after chamber closure using a gas-tight syringe, transferred into pre-evacuated exetainers, transferred back to the University and analysed by gas chromatography.

Gas chromatograph

Soil mineral N Soil samples (0-10 cm) were collected daily for four days after fertiliser application, then every second day for the following two weeks using a 2.5 cm id corer. Ten soil cores were collected per quarter of the treatment circles, following the same transect from one edge to the middle at each sample time. Each set of 10 samples was composited, with a subsample then taken for analysis. Additional background soil was collected from the areas between the treatment circles and composited. Collected soil was immediately dried at 40°C, sieved (<2mm) and extracted with 2M KCl-PMA. The extract was filtered through a Whatman No. 42 filter paper and analysed for urea, NH4+ and NO3- on the Skalar San++ segmented flow analyser.

Skalar San++ segmented flow analyser

15N recovery Microplots (circular; 25 cm diameter by 25 cm height) were established on the background areas of the site with three replications. 15N-enriched granular urea (~10% atom excess) was applied to the microplots at a rate of 40 kg N/ha. Inhibitor treatments were applied to the 15N granular urea immediately after they were placed into the microplots with application of 2 uL/granule. The treatments were done in triplicates and were; 1) 15N granular urea + water 2) 15N granular urea + NBPT @ 2 L/tonne as Lockdown (to make Green UreaNV equivalent) 3) 15N granular urea + DMPP (40.7%) @ 3 L/tonne (to make Urea with ENTEC equivalent) At the end of the experiment, the entire core was removed from site and split into 0-10 and 10-20 cm sections. Additional soil was collected from beneath the plots, with 3 composite cores (2.5 cm i.d) taken from 20-35 cm. Plant (above and below-ground (0-10 cm layer)) samples were collected (together with reference soil and plant samples). All samples are in the final stages of processing with preparation for grinding prior to analysis, and will be analysed for total N and 15N enrichment by isotope ratio mass spectrometry. Samples collected were dried at 40°C and 60°C for soil and plant samples, respectively, until constant weight. Plant samples were ground using a tissue-lyser. Total sample weights were recorded to enable calculation of recovery. Soil samples were crushed into 2 mm pieces, homogenized, and subsampled for fine grinding using a pulveriser. Another subsample was dried at 105°C for moisture content.

Isotope ratio mass spectrometer

Biomass Pasture biomass cuts were taken from ten replicates within the treatment circles and background area at the beginning and completion of the NH3 measurements. Collected samples were weighed and subsampled with the samples dried at 60°C for 72 hrs immediately after collection.

Oven