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Cropping systems comprising winter catch crops followed by spring wheat could reduce N leaching risks compared to traditional winter wheat systems in humid climates. We studied the soil mineral N (N inorg ) and root growth of winterand spring wheat to 2.5 m depth during 3 years. The roots of the winter and spring wheat penetrated the soil at a similar rate (1.3 mm o C day −1 ) and by virtue of its longer growing period, winter wheat reached depths of 2.2 m, twice that of spring wheat (1.1 m). The deeper rooting of winter wheat was related to much lower amounts of N inorg left in the 1 to 2.5 m layer after winter wheat (81 kg N inorg ha −1 less). When growing winter catch crops before spring wheat, N content in the 1 to 2.5 m layer after spring wheat was not different from that after winter wheat. The results suggest that due to its deep rooting, winter wheat may not lead to as high levels of leaching as it is often assumed in humid climates. Deep soil and root measurements (below 1 m) in this experiment were essential to answer the questions we posed.

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Unused inorganic nitrogen (N inorg ) left in agricultural soils will typically leach to deeper soil layers. If it moves below the root zone it will be lost from the system, but the depth of the root zone depends on the crop species grown. In this experiment we studied the effect of 3-year crop sequences, with different combinations of deep-rooted and shallow-rooted crops, on soil N inorg dynamics to 2.5 m soil depth and the possibility of crop utilization of N leached to deep soil layers. We grew ten different crop sequences for 3 years. The crops and catch crops grown were selected to allow different sequences of deeprooted and shallow-rooted crops. Very different rooting depths were obtained, from only 0.5 m (leek), to~1.0 m (ryegrass and barley), 1.5 m (red beet), 2.0 m (fodder radish and white cabbage) and more than 2.5 m by the chicory catch crop. The results showed a significant retention of N inorg within the 2.5 m soil profile from one year to the next, but the retained N had leached to deeper parts of the profile during the winter season. Only little N inorg was retained over two winter seasons. The retention in the deeper soil layers allowed N inorg to be taken up by succeeding deep-rooted main crops or catch crops. The effects of crop rooting depth on N inorg in the subsoil layers from 1.0 to 2.5 m were striking. White cabbage reduced N inorg below 1.0 m with up to 113 kg N ha -1 during its growth. Grown after catch crops, leek and red beet left on average 60 kg N ha -1 less below 1.0 m than leek and red beet grown without a preceding catch crop. We conclude that it is possible to design crop rotations with improved nitrogen use efficiency by using the differences in crop rooting patterns; deep-rooted crops or catch crops can be used to recover N inorg leached after previous crops, and catch crops can be grown before shallow-rooted crops to lift the deep N inorg up to layers where these crops have their roots.

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