Uncertainties in the fate of nitrogen II: A quantitative assessment of the uncertainties in major nitrogen fluxes in the Netherlands

Enhanced levels of nitrogen in the environment may have several adverse effects, including decreased plant species diversity in ( semi) natural terrestrial ecosystems, eutrophication of surface waters, pollution of ground-water due to nitrate leaching and global warming due to nitrous and nitrogen oxide (N2O and NOx) emissions. To determine the effectiveness of policies aimed at the reduction of emission of ammonia (NH3), N2O and NOx, nitrate (NO3) leaching and nitrogen (N) runoff, it is essential to have information on the fate of nitrogen in both agricultural and non-agricultural soils on a regional and national scale and its inherent uncertainties. In this paper, we quantified the uncertainties in the emission, uptake, accumulation, denitrification, leaching and runoff of nitrogen at a national scale and for specific land use-soil type combinations. Furthermore, we identified which parameters contribute most to the overall uncertainty in the emission of ammonia to the atmosphere and the leaching/runoff to groundwater and surface water. To gain quantitative insight into the propagation of the uncertainty, a model was developed representing all crucial processes in the N chain by simple process descriptions. Uncertainties were quantified for the Netherlands as a whole, including terrestrial systems, both agricultural and non-agricultural land, and aquatic systems. For agricultural and non-agricultural land, plots were distinguished, consisting of a multiple of 500 3 500 m and of 250 3 250 m grid cells, respectively, with unique combinations of soil use, soil type and groundwater table class that were derived from existing digital maps. Model parameters were assigned by using relationships with soil type, groundwater level class and land use. The uncertainty was quantified by means of a Monte Carlo analysis, whereas statistical approaches were used to identify which parameters contribute most to the overall uncertainty of the fate of nitrogen. The 90% confidence interval for the fluxes of N compounds to air, groundwater and surface water (in Gg N.yr(-1)) ranged between 102 and 194 for ammonia emission, between 18 and 51 for N2O emissions, between 32 and 108 for NO3 inflow to groundwater and between 2 and 38 for N inflow to surface water. The uncertainty in NH3 emission was mainly caused by the uncertainty in the NH3 emission fractions for animal manure, whereas the uncertainty in N2O emission was mainly due to the uncertainty in the fractions relating total nitrification and denitrification to N2O emissions. The uncertainty in inflow to groundwater and runoff to surface water was mainly caused by the uncertainty in denitrification in the soil and in upper groundwater and in non-agricultural soils also by the N accumulation in the soil. In view of the need to monitor and evaluate the impact of N reduction policies and measures, it is essential to put more effort in activities yielding a reduction of these large uncertainties, such as additional data gathering and process research under field circumstances.