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Enabling better global research outcomes in soil, plant & environmental monitoring.

Water potential & Soil-Plant-Atmosphere Continuum

Author: Michael A. Forster, 2013, ICT International

Scientists, engineers and land managers now have the ability to continuously monitor the soil-plant-atmosphere continuum (SPAC). This fundamentally crucial aspect of plant physiology has to date been beyond technological capabilities to measure. But now ICT International has advanced technology which can continuously measure the soil, plant and atmospheric environment.

The number of hypotheses, theories, and models which can now be tested in plant physiology, and in particular plant-water relations, has increased exponentially.

The SPAC is defined as the movement of water from the soil, through the plant and to the atmosphere along an interconnected film of liquid water (Lambers et al 2008).

The SPAC is fundamental to the hydrologic cycle, the ability of plants to photosynthesize, and therefore to most life on earth. Understanding the SPAC is crucial in plant physiology studies.

Water movement through the SPAC is driven by the passive movement of water generated by an energy gradient. The energy gradient is created by a difference in water potential from high potential in the soil, to a gradually lower potential in the plant and the atmosphere. Figure 1 outlines the water potential gradient along the SPAC for a hypothetical tree growing in a mild environment and well-watered soil. Technology previously available to scientists could not practically or conveniently measure water potential along the SPAC.

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Soil water potential, difficult at the best of times, was measured with instruments such as tensiometers and thermal matric sensors in the field, or with a pressure chamber extractor in the lab.

Plant water potential was measured destructively with a plant water status console (pressure bomb). Leaves were harvested from the plant at various times of the day and measurements made.

To create pre-dawn values, or diurnal curves, let alone a week of continuous data, required dedicated and costly staff. The atmosphere was the easiest to measure as all that was needed was a good quality weather station.

ICT International can now provide a solution for continuous monitoring in soils, plants and the environment.

Figure 2 outlines the placement of various sensors along the SPAC for continuous monitoring. The soil is monitored with the SMM Soil Moisture Meter and the MP306 Soil Moisture Sensors, configured to measure both volumetric water content and water potential. The roots, trunk, branches and leaves of the plant can be continuously monitored with the PSY1 Psychrometer.

The atmosphere is measured with the advanced weather station, the AWS Automatic Weather Station. The AWS has the ability to measure air water potential, as well as other variables such as Vapour Pressure Deficit (VPD) and evapotranspiration.

Figure 2 displays real data as collected from a Banksia spinulosa shrub, growing in Armidale, New South Wales, Australia.

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The data in Figure 2 clearly shows a water potential gradient from high potential in the soil, to lower potentials in the trunk and canopy, and extremely low potential in the atmosphere.

Figure 3 is 7 days of continuous data from the same plant. Sunday was a hot day followed by Monday which was cloudy and cool. Throughout the week, temperatures progressively increased to beyond 35°C before a slightly cooler Saturday. Late on Saturday a storm passed over the monitoring site, with rainfall reaching 43.4 mm.

At the start of the monitoring period the soil was purposely near-saturated with an irrigation system. A MP306 Soil Moisture Sensor, connected to a SMM Soil Moisture Meter, was installed to continuously monitor water potential at a depth of 15 cm. Throughout the week, soil water potential gradually declined to approximately -0.5 MPa before heavy rainfall late on Saturday afternoon. The trunk PSY1 Psychrometer was installed approximately 10 cm from the soil surface. The midday value ranged between -0.8 and -1.2 MPa during the week.

The canopy PSY1 Psychrometer was installed approximately 30 cm from the distal tip of a foliated stem. The canopy water potential reached as low as -5 MPa on the very hot Friday.

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The atmosphere, or air, water potential was extremely negative, reaching -160 MPa, demonstrating how dry the atmosphere is. The main feature of Figure 3 is the gradient of water potential from the soil to the atmosphere.

Figure 4 shows the daily sum of water potential for the soil, trunk, canopy and air. This is a useful way to summarise the continuous data from Figure 3 which can then be used for further statistical analysis of experimental treatment groups.

Figure 4 clearly shows that the atmosphere is overwhelmingly drier than the soil and the plant. Figure 4 also suggests that the canopy of this plant is tightly coupled with atmospheric water potential over this measurement period. A longer measurement period, with differing treatments such as an imposed drought, or differing plant varieties such as a plant with greater fine root biomass, would provide even more interesting data and insights into the SPAC.

The ability to continuously measure environmental variables is a powerful tool. As shown in Figures 3 and 4, scientists, engineers and land managers now have the ability to continuously monitor the SPAC. Such ability will lead to many more questions which will need answers.