Dairy farm CH4 and N2O emissions, from one square metre to the full farm scale

The greenhouse gas emissions from agricultural systems contribute significantly to the national budgets for most countries in Europe. Measurement techniques that can identify and quantify emissions are essential in order to improve the selection process of emission reduction options and to enable quantification of the effect of such options. Fast box emission measurements and mobile plume measurements were used to evaluate greenhouse gas emissions from farm sites. The box measurement technique was used to evaluate emissions from farmyard manure and several other potential source areas within the farm. Significant (up to 250 g CH4 m(-2) day(-1) and 0.4 g N2O m(-2) day(-1)) emissions from ditches close to stables on the farm site were found. Plume emission measurements from individual manure storages were performed at three sites. For a manure storage with 1200 m3 dairy slurry in Wageningen emission factors of 11 +/- 5 g CH4 m(-3) manure day(-1) and 14 +/- 8 mg N2O m(-3) manure day(-1) were obtained in February 2002. Mobile plume measurements were carried out during 4 days at distances between 30 and 300 m downwind of 20 different farms. Total farm emissions levels ranged from 14 to 95 kg CH4 day(-1) for these sites. Expressed as emission per animal the levels were 0.7 +/- 0.4 kg CH4 animal(-1) day(-1) for conventional farms. For three farms that used straw bedding for the animals 1.4 +/- 0.2 kg CH4 animal(-1) day(-1) was obtained. These factors include both respired methane and emission from manure in the stable and the outside storages. For a subset of these farms the CH4 emission was compared with monthly averaged model emission calculations using FarmGHG. This model calculates imports, exports and flows of all products through the internal chains on the farm using daily time steps. The fit of modelled versus measured data has a slope of 0.97 but r2 = 0.27. Measurements and model emission estimates agree well on average, for large farms within 30%. For small farms the differences can be up to a factor of 3. CH4 emissions during winter seem to be underestimated.