Root development of fodder radish and winter wheat before winter in relation to uptake of nitrogen

• Fodder radish is more efficient than winter wheat in tightening the N cycle.
• All methods for winter wheat showed a decrease in root density/intensity with depth.
• For fodder radish root results of the three methods was not all in agreement.
• Preferential growth along the minirhizotron tube for fodder radish was a problem.

The nitrate (N) present in soil at the end of autumn is prone to leach during winter and spring in temperate climates if not taken up by plants. In Denmark catch crops are used as a regulatory tool to reduce N leaching and therefore a shift from winter cereals to spring cereals with catch crops has occurred. Quantitative data is missing on N leaching of a catch crop compared to a winter cereal in a conventional cereal-based cropping system. The aim of the study was to investigate whether fodder radish (Raphanus sativus L.) (FR) would be more efficient than winter wheat (Triticum aestivum L.) (WW) at depleting the soil of mineral nitrogen (Nmin) before winter. A secondary aim was to study the agreement between three different root measuring methods: root wash (RW), core break (CB) and minirhizotron (MR). The third aim of the was to correlate the N uptake of FR and WW with RLD. An experiment was made to see if and how root growth was affected by the minirhizotron tube. The experiments were conducted on a Danish sandy loam soil. From September to November the amount of soil Nmin decreased from 49 kg N ha−1 to 14 kg N ha−1 under FR and increased from 28 kg N ha−1 to 44 kg N ha−1 under WW. A test of correlations between root measuring methods showed that there was only a significant positive correlation for FR between CB and RW (R2 = 0.77) and for WW between CB and MR (R2 = 0.26). We conclude that FR is more efficient than WW at tightening the N cycle in the autumn by a greater depletion of soil Nmin. From the comparison of root methods, we conclude that root growth in the subsoil was overestimated for FR by the MR method. This was due to preferential root growth along the MR tubes. The root densities/intensities measured with the three root measuring methods were not directly comparable.