『Summary
　N2O concentrations and denitrification-related factors (NO3, SO4, dissolved organic carbon (DOC) and CO2) were investigated in the surface groundwater of a catchment in northern Germany, the Fuhrberger Field Aquifer (FFA). We sampled 79 plots that were selected according to the three criteria of land use, historical land use conversion (1954-1995) and groundwater level. We sampled three sites within each plot. The sampling depth was 0.5 m below the groundwater surface.
　We found no indication for the occurrence of autotrophic denitrification in the surface groundwater. Heterotrophic denitrification was identified as the main process for N2O accumulation. The variability of N2O concentrations on the plot-scale was extremely high and was poorly explained by the three sampling criteria. Other denitrification-related variables such as NO3, SO4 and DOC were less variable. The selection criteria land use and groundwater level clearly influenced the order of magnitude of N2O concentrations in the surface groundwater. Under arable land, high NO3 concentrations resulted in high N2O concentrations. The surface groundwater under forest and pasture was almost NO3-free and had also very small N2O concentrations. Plots where the distance from the soil surface to the groundwater surface was large (＞1m up to 3.4 m) showed higher N2O concentrations in the surface groundwater than plots where the distance was small (＜1 m). A large distance from the soil surface to the groundwater leads to a longer residence time and more decomposition of DOC in the soil. Consequently the less bioavailable DOC could inhibit the efficiency of the heterotrophic denitrification in the groundwater, yielding more N2O. Elevated organic carbon levels in plots with historic land use conversion (pasture to arable) were very stable and did not influence N2O concentrations. The high within plot variability showed that an upscaling of N2O from the plot-scale to the catchment-scale is possible as long as the groundwater level regime and the land use do not change.

Keywords: Variability; N2O; Denitrification; Land use; Groundwater level; Upscaling』

Introduction
Methods
　Study site
　Selection of sampling sites
　N2O and CO2 sampling and analysis
　DOC, NO3 and SO4 sampling and analysis
　Statistical methods
　　Dispersion variance
　　Separation of data in order to analyse factors which influence the N2O concentrations
Results and discussion
　Spatial variability of N2O concentrations and of denitrification-related factors in the surface groundwater
　Influence of the land use on N2O concentrations and denitrification-related factors in the surface groundwater
　Dispersion variance of N2O concentrations: Variability of N2O concentrations between and within plots
　Correlation of denitrification-related factors with N2O concentrations: a signature for the dominance of autotrophic or heterotrophic denitrification in the surface groundwater
　　SO4
　　NO3
　　DOC
　　CO2
　　Multiple regression analysis
　Does historic land use conversion from pasture to arable land have an impact on the N2O concentrations in the surface groundwater?
　Does the distance from the soil to the groundwater surface affect the N2O concentration in the surface groundwater?
Further research
Conclusions
Acknowledgements
References