Optimizing green water use to increase food security


With estimates of the world population reaching nine billion by 2050, an increase in food production will become vital. There are limited options for expanding crop production for a growing population, though. Both land available for conversion to fields and freshwater available for irrigation, also called blue water, are dwindling. But another color of water is an untapped resource that holds promise for agriculture – green water.

Green water is the water in soil that is potentially available for plants to take up, use, and release to the atmosphere. To be used by plants, green water must pass through the rhizosphere, the region of soil found close to and influenced by roots. A paper, published recently in the 10th anniversary issue of Vadose Zone Journal, describes how rhizospheres and positive plant-soil feedbacks can lead to more efficient use of green water, a sometimes overlooked resource for crops.

hands holding soil and plant roots

“There is as much green water flowing through plants in the ecosystems of the world as there is blue water flowing in all the rivers that go into the ocean.” says Garrison Sposito, professor at the University of California, Berkeley and author of the paper. “Yet all the public attention in the past has been paid to the charismatic blue water, while green water has been its ‘Cinderella,’ quietly doing the world’s work without fanfare.”

Green water is clearly an important resource. But of all the green water potentially available to plants, as much as 70% can be lost through evaporation or subsurface runoff. That leaves as little as 30% for transpiration under some conditions. Transpiration is termed “productive green water flow” since water used for transpiration leads to plant growth. If, for example, 85% of available green water could be used for transpiration, and therefore become productive, crop yields could as much as triple in some parts of the world.

Better understanding of the plant-soil feedback loop could lead to more productive green water use, says Sposito. For example, healthy soils with plenty of nutrients lead to healthy plant root systems that will take up more water for transpiration than smaller root systems. These plants will grow larger and create lush canopies that in turn provide shade and decrease evaporation from the soil. More green water is then available for transpiration that will stimulate more crop growth.

Beneficial microorganisms regulate key processes that also aid crop growth, such as the transformation of organic matter into plant nutrients.  Within the rhizosphere, those microbes can be especially abundant. A synergistic relationship between plant roots and microbes is therefore necessary for optimal plant growth. Additionally, roots in conjunction with soil microbes can produce a mixture of organic compounds called mucilage. Mucilage increases the water-holding capacity of the soil, making more green water available to plant roots.

Therefore, plants that establish rhizospheres with optimal numbers of beneficial microbes can impact water availability and crop production. “Plant roots can drive the biological evolution of rhizosphere microorganisms,” says Sposito. “We humans can direct this evolution by developing crops that enhance the beneficial functions of microbes within the rhizosphere.”

Another tactic that holds promise is “planting” beneficial microbes in the soil. Beneficial microbes can be introduced to soils in two ways – either by creating an environment that attracts the microbes or by directly placing microbes into the soil. Directly placing microbes, also called inoculation, has been used to enhance nutrient uptake, and researchers believe it may also help with water use. Microbes that fight off plant pathogens are also well known.

Researchers are currently finding ways to capitalize on plant-soil feedbacks and optimize the community of soil microorganisms in the rhizosphere. That work is a key ingredient in the recipe for better utilizing green water and expanding crop production. As blue water availability decreases, Sposito and others in the field feel that now is the time for the “Cinderella” of freshwater resources – green water – to take its place in the spotlight.

View the abstract at http://dx.doi.org/doi:10.2136/vzj2013.02.0041.



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