2 EQUIPAMIENTO ACTIVO
4.3.3 COMPARACIÓN Y ELECCIÓN DE LA PROPUESTA
Where p = the moles of NH4+-N or NO3--N in the sample; f = the moles of N in the 15N enriched fertiliser applied; c = atom % of the sample; a = atom % of the 15N enriched fertiliser applied (5.0 atom %); and b = atom % 15N abundance of the control (0.3663).
3.2.5 Nitrous oxide collection and analysis
The lysimeters were affixed with stainless steel gas rings. They were fixed to the top of the lysimeter casing using silicone sealant. The gas rings consisted of a water trough, to create an air-tight seal when the headspace chambers were applied. The headspace chambers (internal volume 0.0176 m3) were each equipped with a three way stopcock attached to a length of rubber tubing, to facilitate headspace gas sampling, and a sealable vent, to ensure air pressure inside the headspace was equilibrated with atmospheric pressure. The chambers were insulated with a thin layer of styrofoam that was attached and sealed with duct tape.
Figure 3.5 Gas ring and headspace chamber on lysimeter.
3.2.5.1 N2O Sampling procedure
A 50 mL sample was drawn into a 50 mL gas tight syringe, and pushed back into the headspace three times to mix the gas inside the chamber. A 10 mL sample was then collected and injected into a previously evacuated 6 mL glass vial (over-pressurising it) fitted with a screw cap and rubber septum (Exetainer; Labco Ltd, High Wycombe, UK). Sampling was executed at time 0, 30 and 60 minutes. Ambient air samples were also collected at these times. Nitrous oxide measurements were undertaken twice per week, for the first two weeks following any fertiliser and/or urine treatment applications, and once per week thereafter. The first N2O measurement was taken on Day 3 (23 Feb 2011) and the final measurement taken on Day 373 (28 Feb 2012) with a total of 59
measurements over the course of the experiment.
In addition to this, N2O samples were collected for 15N analysis using the same technique described above, except, sampling was executed after 180 minutes (3 h), and a 20 mL sample was collected and injected into a previously evacuated 12 mL vial (Exetainer; Labco Ltd, High Wycombe, UK). Measurements for N2O-15N were taken less frequently, once per week for the first two weeks following fertiliser and/or urine application, and once every two weeks thereafter. Samples for 15N2O were only collected from lysimeters receiving 15N fertiliser treatments, as well as two controls receiving no fertiliser or urine (lysimeters #13 and #27).
Three way stopcock
Ventilation cavity
Chamber insulation
The soil surface pH of each lysimeter was measured concurrently with N2O
measurements using a portable pH meter (Mettler Toledo FiveGoTM FG2) with a flat surface pH electrode (Mettler Toledo InLab Surface). Measurement of pH began on Thursday 21 July and continued to 19 January 2012 with a total of 17 measurements.
3.2.5.2 Sample analysis
Analyses of the headspace gas samples for N2O were performed using a gas
chromatograph (model 8610; SRI Instruments, Torrance, CA) interfaced to a liquid autosampler (model 222XL; Gilson, Middleton, WI). The autosampler had been modified for gas analysis by substituting a purpose built (PDZ-Europa, Crewe, UK), double-concentric injection needle. This allowed the entire gas sample to be flushed rapidly from the vial into the gas chromatograph. The GC configuration included two 0.3 cm OD stainless steel columns packed with Haysep Q connected in series, oxygen free dry N carrier gas (40 mL min-1) and a 63Ni electron capture detector at 320˚C. Gas samples were generally analysed within one week of sampling. Immediately prior to analysis, the over-pressurised samples were brought to ambient atmospheric pressure by placing one end of a double ended hypodermic needle just below (0.5 cm) the surface of water in a small beaker, and piercing the vial’s septum with the other end. A brief flow of bubbles was observed in the water, and when this stopped, the gas in the vial was at ambient pressure. Dissipating this excess gas through a water medium gave a visual indication of when the samples were at ambient air pressure and also avoided any potential contamination of the sample with ambient air. Reference gases (0.32 µL L-1, BOC Ltd, Auckland, New Zealand) were prepared using the same equilibration technique as described above. Detection limits for N2O analysis were considered significant if concentrations were 0.01 µL L-1 greater than the ambient concentration.
Analysis of the gas samples collected for 15N analysis was performed using an Isotope Ratio Mass Spectrometer (EA-CF/IRMS; PDZ Europa GSL / 20-20). The samples were analysed for N2O concentration and 15N enrichment. A detailed description of gaseous 15N analysis by IRMS is given in Section 3.2.8. The N2O-15N recovered as a percentage of the total 15N applied as fertiliser was then calculated using Equation 3.2 above.
3.2.5.3 Calculation of gas flux from lysimeters
A total of three gas samples were collected from the chamber headspace of each lysimeter: one at t0 (0 min), t1 (30 min) and t2 (60 min). This enabled the accurate
calculation of the N2O flux using Equations 3.3 and 3.4 described by Hutchinson and Mosier (1981) for both linear and non-linear N2O accumulation in the headspace. The N2O flux (F) was calculated using the following equations:
𝑭𝑭 =�(𝐶𝐶(𝐶𝐶1− 𝐶𝐶0) ÷ (𝐶𝐶2− 𝐶𝐶1) ≤ 1 → 𝑈𝑈𝑈𝑈𝑒𝑒𝑙𝑙𝑙𝑙𝑒𝑒𝑒𝑒𝑎𝑎𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑎𝑎𝑒𝑒𝑙𝑙𝑅𝑅𝑒𝑒 3.3 1− 𝐶𝐶0) ÷ (𝐶𝐶2− 𝐶𝐶1) > 1 → 𝑈𝑈𝑈𝑈𝑒𝑒𝑒𝑒𝑅𝑅𝑒𝑒𝑙𝑙𝑙𝑙𝑒𝑒𝑒𝑒𝑎𝑎𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑎𝑎𝑒𝑒𝑙𝑙𝑅𝑅𝑒𝑒 3.4 𝑭𝑭𝑬𝑬𝑬𝑬.𝟑𝟑.𝟑𝟑 = [𝐶𝐶2[− 𝐶𝐶𝐺𝐺 0) 𝑉𝑉𝑐𝑐𝑃𝑃] 𝐶𝐶ℎ𝑎𝑎𝐶𝐶𝐷𝐷𝑀𝑀𝑁𝑁2 𝑐𝑐 (𝑇𝑇𝐾𝐾 + 𝑇𝑇℃)] 𝐴𝐴𝑐𝑐 𝑒𝑒2 3.3 𝑭𝑭𝑬𝑬𝑬𝑬.𝟑𝟑.𝟒𝟒 = 𝑉𝑉𝑐𝑐(𝐶𝐶1− 𝐶𝐶0) 2 (2𝐶𝐶1− 𝐶𝐶2− 𝐶𝐶0 𝑙𝑙𝑒𝑒 � 𝐶𝐶1− 𝐶𝐶0 𝐶𝐶2− 𝐶𝐶1� 𝑃𝑃 𝐶𝐶ℎ𝑎𝑎𝐶𝐶𝐷𝐷𝑀𝑀𝑁𝑁2 [𝐺𝐺𝑐𝑐 (𝑇𝑇𝐾𝐾+𝑇𝑇℃)] 𝐴𝐴𝑐𝑐𝑒𝑒1 3.4 Where:
F = N2O, flux [g N2O-N ha-1 day-1]
P = Atmospheric pressure [Pa] (101325)
Vc = Chamber volume [m3]
Ac = Chamber area [m2]
Gc = Gas constant [J K-1 mol-1] (8.314)
TK = Absolute temperature at 0˚C [K] (273.15)
T˚C = Air temperature at 5 cm height [˚C]
Cha = Conversion factor m2 to ha (10000)
CD = Minutes per day [min] (1440)
MN2 = Molecular weight of N2O-N [g mol-1] (28.0134)
t0 = Time 0 minutes, start of cover period
t1 = Time 30 minutes
t2 = Time 60 minutes, total cover period
C0, C1, C2 = N2O concentrations at time t0, t1, and t2 [ppmv]
In order to calculate a cumulative N2O flux, a daily flux was calculated for the days where N2O was not measured by integrating the measured daily fluxes and linearly interpolating between each successive measurement (trapezoidal method). A cumulative N2O flux was then calculated by the addition of the measured and integrated fluxes.