Nitrogen emissions in dairy pastures peak in the springtime

The nitrogen emissions from boreal dairy pastures vary depending on the time of year and ley rotation. The emissions peak in spring, when the nitrates accumulated in the soil over the previous summer and winter leach with the melting snow.

Research Scientist Kirsi H. Saarijärvi of MTT Agrifood Research Finland charted the nitrogen emissions caused by intensive grazing in her doctoral dissertation. The topic has not been previously investigated to this extent in boreal conditions.

The experiments were conducted in a 0.7-hectare lysimeter field at MTT Agrifood Research in Maaninka, Finland. A total of 12 basins are placed underneath the field surface, and analyses of the leachates in these basins reveal the nutrient cycle in the pasture. A comparable pasture-size lysimeter field is not known to exist anywhere else in Europe.

The research also involved measuring the ammonia evaporated from the dung and urine of cows by using a chamber process.

Saarijärvi found that much more nitrogen is left behind on a pasture than in silage cutting fields, since the nitrogen outputs from the pasture are much lower than from silage cutting fields. Most of the nitrogen leaving the pasture is absorbed in the milk produced by the cows grazing in the pasture.

From an ecological perspective, the nitrogen leaching from the soil is the most significant load from dairy pastures.

"The second largest hazard to the environment arises from the ammonia (NH₃) evaporating from cow dung and urine, but its volumes have been previously overestimated. The significance of nitrogen loss in surface runoff is low in the conditions prevailing in Finland,"  Saarijärvi says.

Nitrous oxide is also emitted into the air from the entire field area, but its emissions are also lower than previously assumed.

Saarijärvi stresses that measurements must be carried out throughout the four-year ley rotation in order to gain a representative view of the nitrogen emissions. The nitrogen pulse is especially strong in springtime following pasture renewal.

"The nitrogen accumulated in the ploughed layer is released after pasture renewal. Emissions are smaller in the first years of the rotation, as the grassroots and soil microbes actively bind nitrogen,"  Saarijärvi explains.

She found that the springtime nitrogen load is increased since the microbial activity in the soil is not entirely shut down for the winter but merely slows down. Therefore, nitrous oxide resulting from microbial processes in winter is emitted in large quantities as the soil thaws during the spring.

The nitrogen load is greatest in the areas where cattle spend most of the time in the pasture. The watering facility area functions as a congregation area, and it suffers treading damage that destroys the vegetation and soil pore structure, adding to local nitrogen losses.

The nitrogen emissions from dairy pastures can be reduced by several fairly simple techniques. Saarijärvi notes that emissions can be cut by reducing fertilisation, for example. A white clover mixture, which binds nitrogen directly from the air, can replace fertilisation in good soil conditions.

"The use of protein supplementation for cows should be reduced and the grazing season can be shortened slightly in the autumn. The hot spots of nitrogen cycling can be relieved by moving the watering facility area before the soil around it becomes damaged,"  Saarijärvi suggests.

Where possible, it is best to sow the grass in the spring and to use a catch crop that effectively binds the nutrients.