Abstract
Improving our knowledge on the relative contribution of irrigation water, precipitation, and groundwater to tree transpiration is necessary for an efficient and sustainable use of water resources in agriculture. For this purpose, we applied deuterium (2H) enriched water to trace the uptake of irrigation water by apple trees under field and pot conditions. 2H-enriched water was supplied to apple trees in an Alpine valley mimicking sprinkler irrigation. Labeled water infiltration in the soil and presence in apple tree shoots was measured over a week. An ancillary experiment using potted trees was performed to elucidate the role of irrigation water after soil saturation with 2H-enriched water. Under field conditions, 2H-enriched water infiltrated to a maximum depth of 0.6 m, where most of the fine roots were present, and mixed with pre-irrigation soil water. Sprinkler irrigation water was taken up by apple trees 2–4 h after its supply and its contribution to the shoot water content increased in the first 24 h, then it leveled off. Tree water absorbed from the enriched soil layer represented on average 48 ± 3% and 26 ± 2% of the total water in shoot axes and leaves, respectively. The results of the pot experiment confirmed the contribution of irrigation water to shoot water and allowed us to speculate that under field conditions groundwater (ca. 0.9 m deep, with capillary rise expected up to 0.6 m depth) did not significantly contribute to tree water uptake. Results indicate that a large fraction of shoot water (52–74%) did not derive from recent soil water uptake, suggesting a rather limited water mixing within tree organs.