Young Trees Share Soil Water with Wheat in an Alley-Cropping System in a Wet Crop Year: Evidence from 2H2O Artificial Labeling

Despite numerous studies investigating competition and/or facilitation for soil water resources in alley-cropping systems (AC), the share of water at the early stage of tree establishment in AC has been poorly examined. This work aimed to explore the water share between crops and trees after four years of tree establishment in AC using deuterium labeling tracer. The investigations were carried out at the Ramecourt AC experimental site, in northern France, according to a randomized block design with three replications. In mid-spring, we injected 300 mL of 10 % deuterated water at 30, 50 and 100 cm soil depths at a 1.5 m distance from a reference tree (Alnus glutinosa or alder, Carpinus betulus or hornbeam, Prunus avium or wild cherry) in an AC, in a pure-forest control plot with ryegrass (FC) and in a randomly chosen area in monocrop wheat control (CC) plot. The tracer uptake was monitored by collecting tree leaves and wheat and ryegrass (RG) whole-plant samples every two weeks in labeled and unlabeled areas. For deuterium natural abundance analyses, the global mean delta H-2 was significantly lower for wheat (-44 +/- 4 parts per thousand) than for RG (-27 +/- 6 parts per thousand) and trees (-20 +/- 3 parts per thousand). The plant species, the block and the plant water content (PWC) accounted for 38 % of the delta H-2 variability (p < 0.01). This result indicated that the most active sites of plant water uptake were different between species. The mean wheat delta H-2 was 2481 +/- 523 parts per thousand, 715 +/- 218 parts per thousand, and 133 +/- 68 parts per thousand at the 30, 50 and 100 cm labeling depths, respectively. The value was significantly higher in the AC (2883 +/- 585 parts per thousand) than in the CC (1131 +/- 274 parts per thousand) only at the 30 cm labeling depth. For trees, the delta H-2 of labeled samples remained negative, unlike wheat. Particularly in the AC, alder and wild cherry presented significantly a higher delta H-2 at 8 and 28 days after labeling, respectively, from 50 and 100 cm labeling depths, compared to the H-2 natural abundance. We concluded that trees and wheat take up their water in the upper soil layer, but in the AC, trees favor wheat water uptake in the topsoil and can flexibly shift their water source to the deep layer in the case of low water availability in the upper soil layer.