Production of N2O in grass-clover pastures

In organic as well as conventional dairy farming, grass-clover pastures is an important component of the cropping system. This is because grass-clover is an excellent cattle fodder, and because clover has the ability of fixing atmospheric N2. When budgets for N2O emissions are made accord-ing to the IPCC guidelines it is assumed that 1.25 % of added nitrogen is emitted as N2O. This emission factor is used for all nitrogen inputs although the factor relies on experiments with fertilizer and manure, only. The emission factor for biological fixed nitrogen may be lower than 1.25 %, because nitrogen is released only slowly into the soil. However knowledge is very sparse. On the other hand, when the effect of grazing cattle is added the situation might be different. In Denmark organic cattle are supposed to be on grazing fields for at least 150 days a year. Nitrogen returned to the system in urine and dung is likely to locally exceed the needs of the plants and is therefore at risk of being lost as N2O. Thus far, however, there have only been a few detailed estimates of total N2O emissions from grassland livestock productions, and understanding of the factors controlling N2O emissions remains unsatisfactory

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

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

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

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

Production of N2O in grass-clover pastures

In organic as well as conventional dairy farming, grass-clover pastures is an important component of the cropping system. This is because grass-clover is an excellent cattle fodder, and because clover has the ability of fixing atmospheric N2. When budgets for N2O emissions are made accord-ing to the IPCC guidelines it is assumed that 1.25 % of added nitrogen is emitted as N2O. This emission factor is used for all nitrogen inputs although the factor relies on experiments with fertilizer and manure, only. The emission factor for biological fixed nitrogen may be lower than 1.25 %, because nitrogen is released only slowly into the soil. However knowledge is very sparse. On the other hand, when the effect of grazing cattle is added the situation might be different. In Denmark organic cattle are supposed to be on grazing fields for at least 150 days a year. Nitrogen returned to the system in urine and dung is likely to locally exceed the needs of the plants and is therefore at risk of being lost as N2O. Thus far, however, there have only been a few detailed estimates of total N2O emissions from grassland livestock productions, and understanding of the factors controlling N2O emissions remains unsatisfactory