Overestimation of indirect agricultural N2O fluxes from the groundwater due to neglect of nitrate attenuation

To estimate N₂O fluxes from groundwater due to N leaching from agricultural soils, an empirical parameter had been determined from the ratio of dissolved N₂O-N to NO₃^--N (EF5(1), leading to a first estimate of IPCC-EF5g of 0.015 in 1998. While the data-set of dissolved N₂O was steadily growing, the IPCC-EF5g was first lowered to 0.0025 in the 2006 guidelines, but raised back to 0.006 in the IPCC2019 guidelines based on a more recent review (Tian et al 2019). But it had previously been shown that the concept of EF5(1) must overestimate indirect N₂O fluxes because it related N₂O measured at a certain sampling point in groundwater to the NO₃^- concentration at that point (Well and Weymann, 2005). This neglects the fact that some of the NO₃^- leached to the groundwater surface is typically consumed by denitrification. Studies measuring N₂O together with excess-N₂ from denitrification and residual NO₃^- have shown that this overestimation can be highly relevant (Weymann et al., 2008). An alternative emission factor EF5(2) was thus proposed as the ratio between dissolved N₂O-N and initial NO₃^--N, where the latter was calculated from the sum of excess N₂ and residual NO₃^--N. Neglecting NO₃^- reduction in the EF5g concept had been justified by the wide lack of excess-N₂ data in groundwater (Tian et al, 2019). But NO₃^- consumption in groundwater can also be estimated from the difference between NO₃^--N in leachate calculated from N budgets and the residual NO₃^--N in groundwater monitoring wells. For Germany, these data are widely available and could be used to correct current estimates of indirect N₂O fluxes.

Here we present recalculations of indirect N₂O fluxes using the EF5(2) concept. For the dataset of Tian 2019, we select data from regions where we can assume typical ranges of N fertilization together with the default IPCC factor for N leaching and typical ranges of seepage rates to estimate initial NO₃^--N at the groundwater surface. For Germany, we use respective data from inventories and spatial models. For both cases, indirect N₂O fluxes based on EF5(1) and EF5(2) are compared. For Germany, we also estimate the lowering of currently reported indirect N₂O fluxes with those based on EF5(2). We conclude that there is need for new research on indirect N₂O fluxes from groundwater globally to avoid overestimation of this source.

 

References:

 

IPCC, 2019: 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories, edited by E. Calvo Buendia, K. Tanabe, A. Kranjc, J. Baasansuren, M. Fukuda, S. Ngarize, A. Osako, Y. Pyrozhenko, P. Shermanau, and S. Federici, IPCC, Switzerland. 

Tian, L., Y. Cai and H. Akiyama (2019), Environmental Pollution 245: 300-306.

Well, R. and D. Weymann (2005), 4th International Symposium on non-CO2/ greenhouse gases (NCGG-4), science, control, policy and implementation, Utrecht, Netherlands, 4-6 July 2005: 129-136.

Weymann, D., R. Well, H. Flessa, C. von der Heide, M. Deurer, K. Meyer, C. Konrad and W. Walther (2008). Biogeosciences 5(5): 1215-1226.