Recent research progress on the significance of aquatic systems for indirect agricultural N2O emissions

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A considerable fraction of N applied to agricultural soils is lost to adjacent systems via leaching and runoff. During the flow of this N-load through drainage systems, aquifers, riparian zones, streams and estuaries, N2O is produced, consumed, transported and emitted to the atmosphere. These ‘indirect’ agricultural N2O emissions are considered as a potentially important N2O source. However, their magnitude is still under debate. In this paper, (1) the processes causing ‘indirect’ emissions are summarized; (2) concepts of emission factors are discussed; (3) recent studies supplying indirect emission factors are reviewed; and (4) a potential new approach for evaluating emission factors is presented. The majority of recent data on N2O fluxes from aquifers and drainage systems support the assumption that the IPCC default emission factor for N2O from leached agricultural N in groundwater and drainage ditches (EF5-g) of 0.015 is too high. Recent reports of relatively high N2O emission from riparian areas suggest that future estimates of EF5 should explicitly account for these systems. In this paper, three different types of emissions factors (EF(A), EF(B) and EF(C)) are compared. Among the investigated systems, N2O emission in relation to NO3 −-loading (EF(A)) in riparian buffer zones ranged between 0.02 and 0.06 which is more than one order of magnitude above EF(A) of aquifers, rivers and drainage systems (0.00065 to 0.001). N2O emission in relation to NO3 − consumption within a specific system (emission factor EF(C)) is suitable for classifying the environmental impact of nitrate removal. EF(C) in the riparian buffer zones was one order of magnitude higher compared to aquifers and rivers. This suggests that restoration of riparian buffers to lessen NO3 − discharge to streams and oceans could increase global N2O emission. Isotopic signatures of N2O in aquatic systems are clearly distinct from the signatures of surface emitted N2O. This suggests that it might be possible to validate the contribution of aquatic systems to global N2O emission using isotopic budget calculations.