The results of a literature study examining quantitative estimates of N 2 O emission rates are presented for a range of land-uses across Europe. The analysis shows that the highest N 2 O emission rates are for agricultural lands compared to forests and grasslands. The main factors regulating these rates are available mineral nitrogen, soil temperature, soil water content and the available labile organic compounds. These controls operate across different time-scales, all must exceed a certain threshold for N 2 O emission to occur. The results support the need for an emission factor function of land-use and climate within models describing nitrogen dynamics in catchments. This would allow the assessment of the net N 2 O emission within catchments in terms of current levels and potential changes associated with climate variability, climate change and land use change.
Nitrous oxide fluxes and denitrification rates were measured in situ over a year at a riparian site in the UK. An exponential relationship was found between denitrification rates and soil moisture, with a sharp increase in denitrification rate at a water-filled pore space of 6080%. Similar relationships were found in other studies compiled for comparison. The present study is unique in measuring denitrification in an intact ecosystem in the field, rather than in cores in the field or the lab. The exponential relationship between denitrification rate and soil moisture, with a threshold at 6080% water-filled pore space (2040% gravimetric moisture), has proven to be comparable across a wide range of ecosystems, treatments and study conditions. Whereas moisture content determines the potential for denitrification, the absolute rate of denitrification is determined by available nitrate (NO 3 ), dissolved organic carbon and temperature. As a first approximation, denitrification rates can be simply modelled by using a general exponential relationship between denitrification potential and water-filled pore space (or volumetric/gravimetric water content) multiplied by a constant value determined by the nitrogen status of the site. As such, it is recommended that the current relationship used in INCA to relate denitrification to soil moisture be amended to an exponential form, with a threshold of approximately 70% for the onset of denitrification.
Abstract. Nitrous oxide (N20 ) emissions were measured weekly to fortnightly between April 2001 and March 2002 from two riparian ecosystems draining different agricultural fields. The fields differed in the nature of the crop grown and the amount of fertiliser applied. Soil water content and soil temperature were very important controls of N20 emission rates, with a 'threshold' response at 24% moisture content (by volume) and 8°e, below which N20 emission was very low. N20 fluxes were higher at the site that had received the most fertiliser N, but N0 3 was not a limiting factor at either site. There was also a ' threshold' effect of rainfall, in which major rainfall events (:::: 10 mm) triggered a pulse of high N20 emission if none of the other environmental factors were limiting. These results suggest the existence of multiple controls on N20 emissions operating at a range of spatial and temporal scales and that non-linear relationships, perhaps with a hierarchical structure, are needed to model these emissions from riparian ecosystems.
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