Effective nitrogen (N) management is crucial for optimizing crop yields while reducing the risk of leaching. A study from the Department of Agroecology at Aarhus University conducted a field experiment to investigate how cattle slurry affects nitrogen leaching from clover grass in organic crop rotations. 

"The experiment was based on two six-year organic cattle crop rotations, with two and four years of clover grass, respectively, where the clover grass was fertilized with increasing N amounts in the form of cattle slurry," explains Assistant Professor Henrik Thers from the Department of Agroecology at Aarhus University. 

Cattle slurry doesn't affect nitrate leaching 

The experiment consisted of plots treated differently. Some plots with clover grass received nothing, while others received 100, 200, and 300 kg total nitrogen per hectare in the form of cattle slurry. The idea was to investigate how the addition of cattle slurry affects nitrogen leaching and crop yields in subsequent crops. 

The results of the study indicate that even at the highest amount of cattle slurry, there was no significant impact on leaching. 

"We monitored nitrogen leaching continuously to see the role played by different amounts of cattle slurry. But whether you provide zero or up to 300 kg total N per hectare, it doesn't affect leaching, which is at a low level," says Henrik Thers. 

Retains nitrogen in the soil 

Furthermore, the study shows that the accumulated nitrogen balance in clover grass fields increases, especially at the highest fertilization levels. This raises the question of whether nitrogen accumulates in the soil and contributes to higher yields in subsequent crops. 

"We could actually see that the accumulated nitrogen results in higher yields in the crops following the clover grass in the organic crop rotations we studied," explains Henrik Thers. 

Especially in the two most fertilized treatment groups at 200 and 300 kg nitrogen per hectare, there was an accumulation in the soil leading to higher yields subsequently. According to the researchers, these higher yields can be attributed to a long-term effect of the applied slurry, not just an effect of the latest dose of fertilizer. 

"It is fundamentally interesting because if the effect was short-lived, it could be due to residues of slurry still present in the soil. But if it's due to a more long-term effect, the nitrogen from the slurry must have been bound in the soil's nitrogen pool to be released later," explains Henrik Thers, adding that it may be difficult to distinguish between the short- and long-term effects, but the results indicate that the long-term effect of the slurry has the greatest impact on the subsequent crop yields. 

Careful fertilization management is crucial 

The study highlights the need for careful fertilization management and nitrogen handling in agricultural practices. While clover grass itself proves to be resilient to cattle slurry application, Henrik Thers also emphasizes that the rotations studied are particularly robust. 

"The rotations we've worked with here are particularly good. They are what I call 'closed,' meaning there's always green on the fields, ensuring no leaching occurs. The only time we recorded leaching at the higher end of the scale was in one of the rotations where mechanical weed control was carried out in the autumn. Even though rye was sown immediately after, leaching still occurred; the rye simply couldn't grow big fast enough. Therefore, it's important if you have these organic clover grass rotations to consider how you treat them to ensure they don't become 'leaky' if you want to achieve this positive effect on both the environment and yield. And that applies to three years, and probably more, after plowing the clover grass," he explains.

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