New method could improve drought resilience by pinpointing where plants get their water
A team of U.S. National Science Foundation-supported researchers developed a new chemical fingerprinting framework to trace water from soils to plants, revealing how plants use water under different environmental conditions.
In drought conditions, plants do not use all available water equally. The researchers' new method allowed them to identify the exact water source the plant will use, helping researchers to better understand how resilient the plant is to dry conditions. Similarly, under wetter conditions or during floods, researchers can use the method to determine whether the extra water supports plant growth or contributes to runoff and groundwater sources.
Tracing water sources is useful for the agricultural sector, in particular for improving irrigation strategies. For example, rather than simply tracking total soil moisture, farmers could benefit from knowing how much plant-available water is actually accessible to crops during droughts.
A mismatched isotope mystery
For years, researchers noticed a puzzling pattern in the stable isotope ratios, or chemical fingerprints, of the water inside plants compared to that of water sources in nearby soils. Very often, measurements that were expected to match were, in fact, mismatched.
These isotope "offsets" raised doubts about the approach's accuracy within the research community. Some researchers thought that plants may even be changing the isotopic composition of water.
To figure out what was going on, the team observed data from 110 studies across 212 field sites worldwide and analyzed isotope offsets using possible water sources.
Instead of looking at two isotopic signatures for soil water and one for plant water, the team separated soil water into three groups and plant water into two groups. The change allowed researchers to identify the specific water pool feeding the plants. Once they did, then the isotopic signatures began to match.
The three soil waters:
- Gravitational soil water moves easily through soil, like the water that drains from the bottom of just-watered houseplants.
- Plant-available soil water remains in the soil for uptake, like the water a houseplant eventually absorbs and uses for growth.
- Hydroscopic soil water is tightly bound to soil particles, like the sparse amount of water a neglected houseplant clings to in an attempt to survive.
The two plant waters:
- Sap flow water is taken in by plants and "breathed" out through evapotranspiration.
- Bulk xylem water is drawn out of the soil and incorporated into the plants' systems.
Using this method, researchers successfully matched the stable isotope ratios of plant-available soil water and sap flow water, thereby tracing the exact source of the water the plant used.
By rethinking how water pools are defined and measured, a clearer, more consistent framework may emerge, helping reconcile decades of conflicting observations.