Fodder radish is one of the most effective and most cultivated nitrogen catch crops in Denmark, and there are several reasons for this. It is characterized by rapid growth and deep roots that have a high capacity to take up nitrogen from deep soil layers.

But how does the amount of nitrogen available to the plant in the upper soil layers affect fodder radish root growth and nitrogen absorption?

Researchers from Aarhus University have taken a closer look at this.

- Inorganic nitrogen in the deep soil layers leaches more easily into the groundwater, as catch crops with roots restricted to the upper soil layers, such as ryegrass, cannot reach it. On the other hand, catch crops from the Brassicaceae family (crucifers), such as fodder radish,  often have roots as deep as 2 meters in late autumn, but we lack knowledge about how the amount of available nitrogen in the upper soil layers affects root growth and the ability to absorb nitrogen from the deep soil layers. We have now gained insight into this, says Associate Professor Hanne Lakkenborg Kristensen from the Department of Food Science.

Root growth is crucial - and can be influenced by soil type

Previous research in the area points in different directions.

For example, an older study has shown that nitrogen uptake in maize is reduced below a depth of 1.2 meters when the nitrogen concentration in the upper soil layers is high, while another recent study has shown that there is no difference in nitrogen uptake by babyleaf rucola, when nitrogen fertilizer is added late in the season and when no nitrogen fertilizer is added. Rucola belongs to the Brassicaceae family, like fodder radish, and has roots as deep as 1.4 meters when harvested a second time on the same plant.

Root growth is important for catch crops to work as intended, but if we take a step back, the soil type is also believed to play a role in root growth:

- The texture of the soil is believed to influence root development as the soil's clay content, penetration resistance and porosity seem to influence root growth. For example, one study shows that chicory roots can reach 2.5 meters depth in sandy clay soil, while another study using other methods only showed a root depth of 0.75 meters in coarse sand. However, a third study shows that beetroot roots grow deeper than 1.5 meters in both sandy clay soil and coarse sand, says Hanne L Kristensen:

- So, it is an area that needs to be further examined. The various studies have been carried out using different methods, and under different conditions of field management, climate and geography, and this makes it difficult to compare the results.

Studies in the field and greenhouse

The researchers from Aarhus University have carried out both field and greenhouse experiments to understand how the soil's nitrogen availability and type affects the root growth and nitrogen uptake of fodder radish.

The field experiment consisted of four different treatments, each with a combination of two different nitrogen levels (high and medium) and two different nitrogen depths (0.6 meters and 1.2 meters). The latter took place by injecting the stable nitrogen isotope ¹⁵N late in the autumn (November). To mimic a high level of inorganic nitrogen in the upper soil layers, nitrogen fertilizer was added to the soil in early September - just over two weeks after sowing the fodder radish.

The greenhouse experiment consisted of eight different treatments with the same nitrogen levels and depths as in the field experiment, but on two different soil types, sandy loam (Årslev on Midfunen, Denmark as in the field experiment) and loamy sand (Foulum near Viborg, Denmark), which were packed in 2 m long plastic tubes, where the fodder radish was grown.

Root growth affected - root depth unaffected

The roots were examined thoroughly in each of the two experiments and from here, it was possible to conclude that root growth was influenced by nitrogen availability.

The amount of roots was negatively affected by the high nitrogen level in the upper soil layers in the field. It was sufficient for the plant to produce few roots for nitrogen uptake in depth. However, this was not the case in the greenhouse, where, on the contrary, there were more roots at the high nitrogen level. This difference could be attributed to the general nitrogen nutrition status of the fodder radish, which was lower in the greenhouse, at a level that limited growth.

- The depth of the root system was not affected by the nitrogen availability of the soil, neither in one soil type nor the other, and neither in the field (1.8 m depth) nor in the greenhouse. A number of other factors can affect the root depth, such as the water content of the soil and the soil type, so this may have played a role in the greenhouse trial, where the root depth was 0.5 m deeper in the sandy loam compared to the loamy sand, Hanne L Kristensen says.

Nitrogen absorption reduced in deep soil layers

With high nitrogen availability in the upper soil layer, the ability of the fodder radish to absorb inorganic nitrogen from the deep soil layer was reduced in late autumn:

- This was shown both by the fact that the catch crop in the field with high nitrogen availability generally absorbed less nitrogen from deep soil layers, and by the fact that the roots both in the field and in the greenhouse absorbed less nitrogen per unit of root length. As for the field experiment, it may be explained by the lower nitrogen requirement of fodder radish, which was shown by a higher nitrogen concentration in the plant biomass, as well as by the lower amount of roots in the deep soil layers. The nitrogen uptake per unit of root length was as much as 38 % lower with high nitrogen availability than with medium availability in the deep soil layers, says Hanne L Kristensen.

Zooming in on nitrogen uptake at medium nitrogen availability – after all, this is the most common situation for catch crops – another very interesting observation was made. Nitrogen uptake was almost twice as high in the deep soil layer as found at medium depth in late autumn. This suggests that the plant has a great potential to absorb inorganic nitrogen deep in the soil, thus making it a  particularly effective catch crop in situations where deeply placed nitrogen has been left behind:

- The difference may be explained by the fact that the younger roots in the deep soil layers are simply more active in terms of absorbing nitrogen than the older roots further up at the medium soil depth at this point in the growing season of the catch crop, says Hanne L Kristensen:

- The study has increased our basic understanding of the interaction between the catch crop and growing conditions and emphasizes fodder radish as a good catch crop for nitrogen to the benefit of groundwater and the climate.