Differential effects on water and nitrogen uptake from deep soil layers by increasing crop rooting depth : Case studies on chicory and oilseed rape

Summary

Water and nitrogen (N) are two most important factors determining plant growth and development. Adequate supply of these resources is crucial for maintaining yield, meanwhile inefficient use can lead to substantial loss of farm input and potentially to environmental pollution – e.g. N leaching. Deep-rooted crops have been highlighted for enhancing water and nitrogen uptake from deep soil layers. On the other hand, the potential of the increased rooting depth on water and nitrogen acquisition can be modified by the level of water and nitrogen supply. However, due to the lack of techniques to access and study deep roots and their functioning, water and nitrogen supply regulation on deep root growth, deep water, and nitrogen uptake remain unknown.
This thesis discusses current progress and challenges in studying deep water and nitrogen uptake and presents results from three experiments conducted from 2018 to 2020, where the target crops were grown in 4 m tall semi-field rhizotrons. Using a 2H-15N dual-labelling technique and chicory (Cichorium intybus L.) as a target crop, the first study revealed the disparities between water and nitrogen uptake at depths of down to 3.5 m. 2H and 15N fraction in transpiration water and leaf samples showed that root water uptake decreased drastically with the increased depth and reduced root intensity. In contrast, the nitrate uptake from 1.1 and 2.3 m was comparable. Furthermore, at the 1.1 and 2.3 m soil layers, the peak of 15N accumulation was shown after ten days of injection, then decreased afterward. The fraction of 2H-labelled water in transpiration water tended to increase until 20 days of labelling. The disparity of the patterns implied more rapid nitrogen uptake than water uptake at the labelled depths.
After getting a general idea of the similarities and discrepancies between deep water and nitrogen uptake, the effects of water and nitrogen application on water and nitrogen acquisition were investigated in the subsequent years. As with the first experiment, 2H-labelled water and 15N-labelled nitrate were applied at 0.5 and 1.7 m depth to track water and nitrogen uptake. Winter oilseed rape (Brassica napus L.), which can develop roots to more than 3 m during its lifecycle, was used as a model crop in the following experiments. During the experimental periods, oilseed rape developed roots below 2 m and extracted water and nitrogen from there. Neither water nor nitrogen supply altered the subsequent root growth, while the results showed that both high N application and topsoil water deficiency enhanced subsoil water uptake. Unlike water uptake, there was no implication that deep N uptake was sensitive to water supply, whereas the uptake efficiency of labelled-15N was doubled by reduced N supply. The results address the importance of deep roots in acquiring soil VIII resources under sub-optimal conditions such as water and nitrogen deficiency, which should be considered when water and nitrogen management are made.
Overall, the studies revealed that deeper rooting, but not necessarily denser roots in the subsoil, was efficient at exploiting subsoil nitrogen. This potential was comparable under water deficiency and could be stimulated by nitrogen deficiency. Although deep rooting also played an essential role in enhancing water uptake when water deficiency occurred in topsoil layers, the contribution of deep roots to total water uptake can be limited to the increased soil depth and reduced root growth in the subsoil.