Age- and climate- related water use patterns of apple trees on China's Loess Plateau

The Loess Plateau of China is a major apple-cultivating region, but much of the Plateau is water-limited, and the expansion of apple-growing is putting pressure on soil water resources. Plants' water consumption patterns have been intensively studied to facilitate formulation of robust agricultural strategies, but previous studies have generally applied indirect methods to characterize their water use. Moreover, the few studies that have applied direct (isotopic) methods have mostly focused on shallow (0-200 cm) soil layers, usually in stands of a single age or single climatic region. To avoid these limitations, we have investigated the primary water sources of apple trees of three ages (10, 15 and 22 years) in semiarid and semihumid climatic regions of the Plateau using both natural stable isotopic signatures (delta H-2 values) and injections of (H2O)-H-2 into deep soil layers. We found that water content in apple orchards' soil decreased with increases in depth and stand age, and was higher in the semihumid area than in the semiarid area. Nevertheless, patterns of apple trees' water uptake from shallow (0-300 cm) soil layers were similar in the two climatic regions and the main water sources became shallower with increases in stand age. However, water uptake from deep (400-500 cm) soil layers was also detected, particularly in the blossom and young fruit stage in apple orchards of the semiarid area. Moreover, older trees absorbed more water from these layers than younger trees in the semiarid area (but not in the semihumid area) throughout the growing season. Excessive consumption of deep soil water inevitably results in deep soil drying and severely threatens the sustainability of apple cultivation. Our work suggests that it is necessary to take actions (e.g. supplementary irrigation, landscaping and mulching combinations) to reduce the proportion of deep soil water used by apple trees to prevent the development of dried soil layers. It also highlights the need to assess uptake patterns of plants at multiple developmental stages and ages to identify times when and places where interventions may be required or most effective.