Theoretical comparison of advanced methods for calculating nitrous oxide fluxes using non-steady state chambers

Authors: Venterea, Rodney - Rod

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2013
Publication Date: 5/1/2013
Citation: Venterea, R.T. 2013. Theoretical comparison of advanced methods for calculating nitrous oxide fluxes using non-steady state chambers. Soil Science Society of America Journal. 77(3):709-720.

Interpretive Summary: Several flux-calculation (FC) schemes are available for determining soil-to-atmosphere emissions of nitrous oxide (N2O) and other trace gases using data from non-steady-state flux chambers. Recently developed methods claim to provide more accuracy in estimating the true pre-deployment flux (f0) compared with previous methods. This study used numerical modeling to evaluate performance of the NDFE, CBC, and HMR methods for estimating soil N2O efflux. A diffusion-reaction model having fewer restrictions than previous models was used to simulate a range of biophysical conditions affecting chamber data. Two FC methods having similar theoretical foundations but differing in their implementation procedures responded with dramatically different sensitivities to violation of their underlying assumptions. The NDFE method tended to substantially over-estimate f0 as lateral gas diffusion and soil N2O uptake increased. The CBC method was nearly insensitive to these processes except when their magnitude increased above certain levels. The HMR method was also less sensitive than NDFE and entirely insensitive to chamber leakage. Thus, the CBC and HMR methods performed more reliably overall compared with NDFE, but each has their drawbacks. The CBC method utilizes correction factors calculated from measured soil parameters which introduce additional sources of error. Sensitivity analysis showed that soil bulk density was the most critical parameter and that errors increased with longer chamber deployment periods (DP). The HMR method consistently under-estimated f0, and to an increasing extent as DP increased. The results are used to highlight the unique advantages and limitations of each method which need to be considered in selecting among methods and in designing measurement protocols. These findings will be useful to researchers studying greenhouse gas (GHG) emissions from agricultural production systems by promoting the development and implementation of more accurate measurement protocols, and will help scientists and policy-makers in developing more accurate GHG assessments at the field, national, and global scales.

Technical Abstract: Several flux-calculation (FC) schemes are available for determining soil-to-atmosphere emissions of nitrous oxide (N2O) and other trace gases using data from non-steady-state flux chambers. Recently developed methods claim to provide more accuracy in estimating the true pre-deployment flux (f0) compared with previous methods. This study used numerical modeling to evaluate performance of the NDFE, CBC, and HMR methods for estimating soil N2O efflux. A diffusion-reaction model having fewer restrictions than previous models was used to simulate a range of biophysical conditions affecting chamber data. Two FC methods having similar theoretical foundations but differing in their implementation procedures responded with dramatically different sensitivities to violation of their underlying assumptions. The NDFE method tended to substantially over-estimate f0 as lateral gas diffusion and soil N2O uptake increased. The CBC method was nearly insensitive to these processes except when their magnitude increased above certain levels. The HMR method was also less sensitive than NDFE and entirely insensitive to chamber leakage. Thus, the CBC and HMR methods performed more reliably overall compared with NDFE, but each has their drawbacks. The CBC method utilizes correction factors calculated from measured soil parameters which introduce additional sources of error. Sensitivity analysis showed that soil bulk density was the most critical parameter and that errors increased with longer chamber deployment periods (DP). The HMR method consistently under-estimated f0, and to an increasing extent as DP increased. The results are used to highlight the unique advantages and limitations of each method which need to be considered in selecting among methods and in designing measurement protocols.