Emissions of nitrous oxide from arable organic and conventional cropping systems on two soil types

Summary

Conventional cropping systems rely on targeted short-term fertility management, whereas organic systems depend, in part, on long-term increase in soil fertility as determined by crop rotation and management. Such differences influence soil nitrogen (N) cycling and availability through the year. The main objective of this study was to compare nitrous oxide (N2O) emissions from soil under winter wheat (Triticum aestivum L.) within three organic and one conventional cropping system that differed in type of fertilizer, presence of catch crops and proportion of N2-fixing crops. The study was replicated in two identical long-term crop rotation experiments on sandy loam soils under different climatic conditions in Denmark (Flakkebjerg–eastern Denmark and Foulum–western Denmark). The conventional rotation received 165–170 kg N ha-1 in the form of NH4NO3, while the organic rotations received 100–110 kg N ha-1 as pig slurry. For at least 11 months, as from September 2007, static chambers were used to measure N2O emissions at least twice every calendar month. Mean daily N2O emissions across the year ranged from 172 to 438 µg N m-2 d-1 at Flakkebjerg, and from 173 to 250 µg N m-2 d-1 at Foulum. A multiple linear regression analysis showed inter-seasonal variations in emissions (P<0.001), but annual N2O emissions from organic and conventional systems were not significantly different despite the lower N input in organic rotations. The annual emissions ranged from 54 to 137 mg N m-2, which corresponded to 0.5–0.8% of the N applied in manure or mineral fertilizer. Selected soil attributes were monitored to support the interpretation of N2O emission patterns. A second multiple linear regression analysis with potential drivers of N2O emissions showed a negative response to soil temperature (P=0.008) and percent water filled pore space (WFPS) (P=0.052) at Foulum. However, there were positive interactions of both factors with NO3-N, i.e. high N2O emissions occurred during periods when high soil nitrate levels coincided with high soil temperature (P=0.016) or high soil water content (P=0.056). A positive effect (P=0.03) of soil temperature was identified at Flakkebjerg, but the number of soil samplings was limited. Effects of cropping system on N2O emissions were not observed.