Optimizing chamber methods for measuring nitrous oxide emissions from plot‐based agricultural experiments

Abstract

Nitrous oxide emissions (N 2 O) from agricultural land are spatially and temporally variable. Most emission measurements are made with small ( 1 m 2 area) static chambers. We used N 2 O chamber data collected from multiple field experiments across different geo-climatic zones in the UK and from a range of nitrogen treatments to quantify uncertainties associated with flux measurements. Data were analysed to assess the spatial variability of fluxes, the degree of linearity of headspace N 2 O accumulation and the robustness of using ambient air N 2 O concentrations as a surrogate for sampling immediately after closure (T 0 ). Data showed differences of up to more than 50-fold between the maximum and minimum N 2 O flux from five chambers within one plot on a single sampling occasion, and that reliability of flux measurements increased with greater numbers of chambers. In more than 90% of the 1970 cases where linearity of headspace N 2 O accumulation was measured (with four or more sampling points), linear accumulation was observed; however, where non-linear accumulation was seen this could result in a 26% under-estimate of the flux. Statistical analysis demonstrated that the use of ambient air as a surrogate for T 0 headspace samples did not result in any consistent bias in calculated fluxes. Spatial variability has the potential to result in erroneous flux estimates if not taken into account, and generally introduces a far larger uncertainty into the calculated flux (commonly orders of magnitude more) than any uncertainties introduced through reduced headspace sampling or assumption of linearity of headspace accumulation. Hence, when deploying finite resources, maximizing chamber numbers should be given priority over maximizing the number of headspace samplings per enclosure period.

Reducing nitrous oxide emissions by changing N fertiliser use from calcium ammonium nitrate (CAN) to urea based formulations

“…If the slope of the linear regression line fit to the multiple sampling points was significantly different to zero (i.e. P<0.05) this group were assigned to the "non 14 zero flux" group; This group was further tested for the significance of the quadratic term for describing the relationship between N 2 O flux and time; if the quadratic term was not significant the relationship was assumed to be linear as per Chadwick et al (2014). A generalised linear mixed modelling approach was used to test for a fertiliser N treatment effect on annual N 2 O-N loss using the PROC GLIMMIX procedure of SAS 9.3.…”

“…N 2 O fluxes were measured over a 12 month period using the closed, static chamber technique (Chadwick et al, 2014) with the chamber design consistent across the three sites. The stainless steel chambers consisted of a chamber base measuring 0.4m x 0.4m wide and 0.1m high which was inserted into the ground to a depth of ≥5 cm, with a corresponding lid of height 0.1m.…”

“…Ambient air samples acted as the time zero (T 0 ) N 2 O estimate for all headspace N 2 O calculations, as per Chadwick et al (2014) who showed that using ambient air as a surrogate for individual chamber T 0 headspace samples did not result in any consistent bias in calculating fluxes. Gas samples were injected into 12 ml (HB) or 7 ml (JC and MP) preevacuated glass vials with double-wadded PTFE/silicone septa (Labco, UK) which were over pressurised during storage.…”

Contribution of litter layer to soil greenhouse gas emissions in a temperate beech forest

“…Concentration changes over time were determined with quadratic best-fit equations for CO 2 and N 2 O, and an exponential best-fit equation for CH 4 . Greenhouse gas fluxes were discharged if regression coefficients (r 2 ) were below 0.70 for CH 4 and N 2 O, and below 0.90 for CO 2 (Barton et al 2008;Chadwick et al 2014;Unteregelsbacher et al 2013). Positive fluxes represent net GHG emissions, negative fluxes represent net GHG uptake.…”

Soil resilience and recovery: rapid community responses to management changes

“…Emissions were measured using the static chamber technique (Mosier 1989), with three chambers (each covering 0.16 m 2 ) randomly allocated per plot to account for spatial variability. Chambers were closed to allow headspace accumulation of N 2 O for 40 min before gas samples were taken from each chamber and stored in pre-evacuated vials (Chadwick et al 2014). Initial chamber concentration was assumed to be the same as for ambient air, for which ten samples were taken on each sampling occasion at chamber height.…”