Production of N2O in grass-clover pastures

Summary

Agricultural soils are known to be a considerable source of the strong greenhouse gas nitrous oxide (N2O), and in soil N2O is mainly produced by nitrifying and denitrifying bacteria. In Denmark, grass-clover pastures are an important component of the cropping system in organic as well as conventional dairy farming, and on a European scale grass-clover mixtures represent a large part of the grazed grasslands. Biological dinitrogen (N2) fixation in clover provides a major N input to these systems, but knowledge is sparse regarding the amount of fixed N2 lost from the grasslands as N2O. Furthermore, urine patches deposited by grazing cattle are known to be hot-spots of N2O emission, but the mechanisms involved in the N2O production in urine-affected soil are very complex and not well understood.
The aim of this Ph.D. project was to increase the knowledge of the biological and physical-chemical mechanisms, which control the production of N2O in grazed grass-clover pastures.
Three experimental studies were conducted with the objectives of:
I assessing the contribution of recently fixed N2 as a source of N2O II examining the link between N2O emission and carbon mineralization in urine patches III investigating the effect of urine on the rates and N2O loss ratios of nitrification and nitrification, and evaluating the impact of the chemical conditions that arise in rineaffected soil
The results revealed that only 3.2 ± 0.5 ppm of the recently fixed N2 was emitted as N2O on a daily basis. Thus, recently fixed N released via easily degradable clover residues appears to be a minor source of N2O. Furthermore, increased N2O emission following urine application at rates up to 5.5 g N m-2 was not caused by enhanced denitrification stimulated by labile compounds released from scorched plant roots. Finally, the increase of soil pH and ammonium following urine application led to raised nitrification rate, which appeared to be the most important factor explaining the high initial N2O emission from simulated urine patches. The results are discussed in relation to the national N2O inventory guidelines issued by the Intergovernmental Panel on Climate Change, and the environmental impact of organic farming practises are also considered. Suggestions for future research are outlined.