From N2 fixation to N2O emission in a grass-clover mixture

Summary

In organic dairy farming, a major N input to the plant-soil system comes from biological N2 fixation by pasture legumes, but knowledge is sparse on how much of the fixed N2 is lost from the pastures as N2O. Nitrifying and denitrifying bacteria are the main contributors to the N2O production in soils.
Currently, no contribution from biological N2 fixation in legume pastures is included in the national N2O inventories, partly because of uncertainties in quantifying the N2 fixation in the pastures (Mosier et al., 1998). According to the guidelines issued by The Intergovernmental Panel on Climate Change (IPCC), inventories for N2O emissions from agricultural soils should be based on the assumption that 1.25 % of added N is emitted as N2O (IPCC, 1997). The standard N2O emission factor of 1.25 % could be considerably unrepresentative for biologically fixed N2. Firstly, only a part of the fixed N is mineralised during the lifetime of the crop. Secondly, the release of inorganic N into the soil occurs slowly. A 15N2-tracer-experiment was initiated on grass-clover grown in pots. The aim was to assess:
· the contribution of recently fixed N2 as a source of N2O
· the translocation of N from clover to companion grass
References
IPCC, 1997. Greenhouse gas inventory. Reference manual. Vol. 3. Intergovernmental Panel on Climate Change. Bracknell, UK.
Mosier, A. et al. 1998. Nutrient Cycling in Agroecosystems 52, 225-248.