Effect of deep and shallow root systems on the dynamics of soil inorganic N during 3-year crop rotations

Unused inorganic nitrogen (Ninorg) 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 Ninorg 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 deep-rooted 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 Ninorg 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 Ninorg was retained over two winter seasons. The retention in the deeper soil layers allowed Ninorg to be taken up by succeeding deep-rooted main crops or catch crops. The effects of crop rooting depth on Ninorg in the subsoil layers from 1.0 to 2.5 m were striking. White cabbage reduced Ninorg 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 Ninorg leached after previous crops, and catch crops can be grown before shallow-rooted crops to lift the deep Ninorg up to layers where these crops have their roots.