D. Weymann

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

Abstract 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.

Estimation of indirect nitrous oxide emissions from a shallow aquifer in Northern Germany.

Ground water is considered to be an important source for indirect N2O emissions. We investigated indirect N2O emissions from a shallow aquifer in Germany over a 1-yr period. Because N2O accumulated in considerable amounts in the surface ground water (mean, 52.86 microg N2O-N L(-1)) and corresponding fluxes were high (up to 34 microg N2O-N m(-2) h(-1)), it was hypothesized that significant indirect N2O emissions would occur via the vertical and the lateral emission pathway. Vertical N2O emissions were investigated by measuring N2O concentrations and calculating fluxes from the surface ground water to the unsaturated zone and at the soil surface. Lateral N2O fluxes were investigated by measuring ground water N2O and NO3- concentrations at five multilevel wells and at a waterworks well. Negligible amounts of N2O were emitted vertically into the unsaturated zone; most of it was convectively transported into the deeper autotrophic denitrification zone. Only a ground water level fall and rise triggered the emission of N2O (up to 3 microg N2O-N m(-2) h(-1)) into the unsaturated zone. Ground water-derived N2O was probably reduced during the upward diffusion, and soil surface emissions were governed by topsoil processes. Along the lateral pathway, N2O and NO3- concentrations decreased with increasing depth in the aquifer. Discharging ground water was almost free of N2O and NO3-, and indirect N2O emissions were small.

Application of open-path spectroscopic measurement techniques (FTIR) for the up-scaling of greenhouse gas emissions from soils

The path-averaging, multi-component Fourier Transform Infrared (FTIR) absorption spectrometry at an open path of 100 m length is applied for the up-scaling of greenhouse gas (GHG) flux measurements from soil surfaces. For the detection of the emissions of N2O and further GHG from arable field soils a measuring tunnel for controlled enrichment of released gases was installed at the soil surface covering an area of 495 or 306 m2. The concentrations of GHG were measured by FTIR across the whole measuring tunnel. The precision of the FTIR system is discussed to detect the concentration increases during a time period of up to two hours. During a 2-years-time frame the N2O fluxes between the soil and the atmosphere at the agricultural field varied between 1.0 and 21 μg N2O-N m-2 h-1. A non-intrusive emission and flux measurement method at a scale from 100 m up to 27.000 m2 on the basis of the fluxgradient method (0.50 and 2.70 m height above surface) was developed and tested by means of FTIR (N2O and further GHG concentrations) and area averaging meteorological measurements (determination of horizontal winds and friction velocity using acoustic tomography). To detect the concentration gradient between the two heights the precision of the FTIR system is discussed. Two campaigns in October 2007 and June 2008 were performed with this new methodology when wind speeds were low. The measurement errors are discussed and the results compared with the measurement tunnel results that were higher by up to 25 %.