Nitrate leaching from organic and conventional crop production farms

Summary in the original language of the document

Farm accounting data from the Institute of Food Economics and from Central Agricultural Registers in Denmark were used to define the import of nitrogen (N) to farmed fields on conventional and organic arable farms to 129 and 51 kg N ha-1 yr-1, respectively. Based on the recorded distribution of crops, a generalised crop rotation was defined for each of the two farming systems. The crop rotation for the organic farm had a high share of spring cereals and additionally 20% grass-clover in the rotation, whereas the conventional rotation was dominated by cereals, primarily winter cereals.
The FASSET model was used to simulate the N balance of these two crop rotations. This included N leaching and change in the soil organic N pool. All simulations were performed for three soil types, representing the variation in Danish soils. These three soil types were further subdivided into low and high N fertility, representing a low and a high content of organic N in the soil.
The model results showed that the organic arable crop rotation had a higher field N surplus than the conventional rotation, 61 versus 40 kg N ha-1 yr-1. Since there was a higher increase in the soil N stock in the organic crop rotation, this difference in field N surplus did not result in differences in N leaching between the two systems. There was an average annual increase in soil organic N content of 13 kg N ha-1 for the organic crop rotations, whereas there was a decline of 6 kg N ha-1 for the conventional rotation. This is primarily caused by differences in crop choice.
The effect of a number of management options were analysed using the model, including cover crops, straw incorporation, reduced manure application and changes in the crop rotations. These analyses showed that use of cover crops were clearly the most effective measure to reduce N leaching, with an effect at rotation level of 9 kg N ha-1 yr-1. However, the full potential for growing cover crops cannot be used in practice due to the need for controlling perennial weeds, which often requires intensive harrowing in the autumn after harvest of the cereal crops. Such an intensive tillage during autumn may increase N leaching. Other measures like straw incorporation had a minor effect on N leaching, but a marked effect on soil organic N content. Straw incorporation thus increased soil N content by 14 kg N ha-1 yr-1 compared with the baseline scenario.
Based on the simulations it was concluded that there were no differences in N leaching between conventional and organic arable crop production systems. However, the N leaching from conventional crop production was simulated to have declined by 4 kg N ha-1 yr-1 due to the implementation of measures in the Danish Action Plan for the Aquatic Environment. Prior to the implementation of this action plan, N leaching from conventional arable farming was therefore slightly higher than from organic farming.