Hydraulic Lift by Sagebrush Roots Stimulates Soil Nutrient Cycling and Plant N Uptake.

Hydraulic lift (HL) is the redistribution of water from deep, moist soil layers through plant-root systems and into upper, drier soil layers. Hydraulic lift has been documented in soils of a wide range of ecosystems, but its ecological importance is still under debate. It has been hypothesized that movement of water into dry, but nutrient-rich surface soils could stimulate microbial activity and nutrient cycling, and thus, increase plant acquisition of nutrients during the summer months when deep soil moisture still provides sufficient water for plant growth; however this phenomenon has never been demonstrated. To examine the effects of HL on nutrient cycling in surface soils, we supplemented deep soil moisture (70 to 100 cm soil depth) to mature sagebrush plants at a field site in N. Utah, USA.  In late summer, after 3 mo of deep-water supplementation and HL stimulation, we used a novel ¹⁵N-isotope dilution technique, where ¹⁵NH₃ gas is injected into undisturbed soil around sagebrush plants, to measure gross N-cycling rates in the 0 to 10-cm soil layer. We found that gross N mineralization, net ammonification, and soil respiration rates were faster in the surface soil beneath plants with stimulated HL, compared to control plants (with no deep water supplementation).  ¹⁵N tracer measurements also showed that plant uptake of surface soil N and translocation into developing inflorescences was greater in sagebrush plants with stimulated HL than in control plants. As far as we know, this is the first study to show conclusively that HL can increase soil nutrient cycling and plant acquisition of nutrients, and potentially increase plant fitness.