Grazing and nitrogen fertilisation increases nitrous oxide emissions from grasslands

By Jørgen E. Olesen, Dmitri Chatskikh, Jørgen Berntsen and Nick Hutchings, Danish Institute of Agricultural Sciences, Dept. of Agroecology

Climate change caused by anthropogenic emissions of greenhouse gases is one on the greatest global environmental threats. Agriculture contributes to the emissions of greenhouse gases, in particular methane (CH4) and nitrous oxide (N2O). Agriculture in Denmark contributes to 18 pct. of the national greenhouse gas emissions.

Model for emission of nitrous oxide

Nitrous oxide is primarily formed in the soil as a result of bacterial nitrogen transformations. This occurs both in connection with nitrification of ammonium to nitrate and in connection with denitrification of nitrate to dinitrogen (N₂).

The formation of nitrous oxide is affected by climate and soil, and - not least – by crop choice and fertilisation. Higher temperatures will increase the bacterial turnover in general and thus the formation of nitrous oxide. For denitrification in particular the process increases with increasing soil water content. However, the most important factor for nitrous oxide emissions are normally the content of soil mineral nitrogen (ammonium and nitrate).

These effects have been incorporated in the FASSET simulation model. The models estimates of nitrous oxide emissions have been compared with measurements from grasslands in Denmark, Finland and England. The model explained up to 89 percent of the variation in measured annual nitrous oxide emissions.

Nitrous oxide from grasslands

Grasslands may constitute an important source for nitrous oxide emissions. This is a result of both the high rates of fertiliser applied in grasslands and because the animals during grazing deposit urine and faeces that adds to the available nitrogen.

The FASSET model has been used to estimate the emissions from grassland used for either grazing or cutting. To simplify calculations a pure ryegrass was used and it was fertilised with mineral nitrogen only. The calculations were repeated for 30 different climate-years, and the average annual cycle of nitrous oxide emissions are shown in Figure 1.

The emissions of nitrous oxide increase after each fertilisation event. The increase is greatest in mid-summer, where the soil temperature is high and the grass growth may be limited by water shortages. The large recycling of nitrogen in the grazed system leads to a considerably higher level of nitrous oxide emissions. This higher level of nitrous oxide emissions is evident even during spring before the grazing starts, which is a residual effect from the grazing during the previous year.

Fertilisation increases nitrous oxide emissions

The model estimates showed considerably higher nitrous oxide emissions from loamy soils compared with a sandy soil (Figure 2). The soil aeration is generally better on the sandy soils and this reduces the risk of nitrous oxide emissions, because conditions for denitrification are more rarely present.

The nitrous oxide emissions increased with increasing nitrogen fertilisation. The rate of increase was greater at higher fertilisation rates. This was primarily an effect of a larger addition of mineral nitrogen to the soil at higher nitrogen fertilisation. The increase in nitrous oxide emissions at higher fertiliser rates was particularly large for the loamy soils and under grazing.

The model results presented do not account for the uneven spatial distribution of urine and faeces. However, preliminary results show that this does not have any major effects on the total nitrous oxide emissions.

The large effect of grazing on nitrous oxide emissions shows the importance of ensuring a proper regulation of the protein intake by the grazing animals. A high protein intake will increase the nitrogen excretion in urine and faeces and thus increase not only the emissions of nitrous oxide, but also the losses through ammonia volatilisation and nitrate leaching.

The role of organic farming

Denmark has ratified the Kyoto Protocol, which commits to a 21 percent reduction in the national emissions of greenhouse gases during the period 1990 to 2010. The focus on emission reduction measures in agriculture has so far been on the reduction of nitrogen fertiliser use. In the national emissions inventory, nitrous oxide emissions from fertiliser use are estimated as 1.25 percent of the applied amount of nitrogen. The model results presented here show that this is a much too simple approach for estimation of the actual emissions.

There are large differences between soil types in the emissions, and the proportion of fertiliser nitrogen emitted as nitrous oxide is smaller at lower nitrogen rates. This non-linear response of nitrous oxide emissions to nitrogen input may make organic farming an interesting option for reducing greenhouse gas emissions, because the nitrogen input is lower in organic farming. There is, however, need for further studies to document this.