Effects of shoot removal and soil water content on root respiration of spring wheat and soybean

Abstract

In an effort to separate soil respiration into root-derived respiration and soil-microbe-derived respiration under field conditions, removal of the shoot was investigated as a tool to determine the individual contributions. Spring wheat ( Triticum aestivum L. cv Longchun 8139) and soybean ( Glycine max L. cv Tianchan 2) were grown in the field under a movable rain shelter and subjected to three water regimes: (1) well-watered, (2) moderate drought stress, and (3) severe drought stress. Roots from spring wheat and soybean plants that had either been left intact or subjected to shoot excision were monitored for the efflux of carbon dioxide, the influx of oxygen, and total soluble carbohydrate (TSC), malic acid, and citric acid contents. The root respiration rates of shoot-excised and intact plants ranged from 0.88–2.32 and 1.09–2.72 μmol CO 2 m −2 s −1 for wheat and 0.42–1.66 and 0.52–2.03 μmol CO 2 m −2 s −1 for soybean, respectively, indicating that although shoot removal had an adverse effect on root respiration, this technique can be used to measure the root-derived respiration under field conditions. The times of shoot removal that had the most dramatic effects on the gas flux rates in plants subjected to well-watered, moderate drought, and severe drought stress treatments were 3, 2, and 1 h for spring wheat, and 3, 1, and 1 h for soybean, respectively. Both crops showed significantly positive relationships between soil water content and TSC content, and between TSC content and the time when the root respiration of the shoot-excised plants differed significantly from that of plants with intact shoots. These results suggest that root respiration should be measured within 3 h after shoot excision when the plants are under the optimal growth conditions, although the maximum elapsed time should be reduced as the degree of environmental stress increases. Regardless of the water availability, the root respiration should be measured as soon as possible after the shoot is removed, since by the time the root respiration rate has stabilized, it differs greatly from the rate observed shortly after the shoot has been clipped.