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Aeration shifts microbial balance toward nitrifying bacteria
Soil health is a hot topic around the world, and new research is demonstrating the significant impact of irrigation on soil health–not just in keeping crops alive, but in creating a hospitable environment for the countless microbes that enable plants to thrive.
It is no secret that roots need air as well as water to survive–that is clear to anyone who has seen plants dying in a flooded field. However, we are only starting to understand the complex interactions among microbes in the soil that mineralize bound nutrients, convert fertilizers into forms that can either be used by plants or leaked into the environment, and help plants take up the nutrients they need to survive and maximize their production of food, feed, fuel and fiber.
Research by a team of scientists at California State University, Fresno in the U.S. and Memorial University of Newfoundland, Canada, showed that aerating irrigation water promotes a shift toward bacteria that convert ammonia into plant-available nitrate rather than microbes that are more likely to produce volatile nitrous oxides that waft away into the atmosphere.
Shift to Luxurious Growth Conditions
Dr. Dave Goorahoo of Cal State, Fresno,whoheadedtheresearch team–whichalsoincludedJosue Samano-Monroy, Dr. Florence Cassel Sharma and Touyee Thao at Cal State, Fresno; Dr. Adrian Unc and Crystal McCall at Memorial University of Newfoundland and Dr. Govind Seepersad at the University of the West Indies in Trinidad and Tobago–presented the results at the Irrigation Association show in the U.S. last December.
In his presentation, Goorahoo noted, “…bacteria dominance indicates a likely shift to more luxurious growth conditions.”
Goorahoo and his colleagues studied clay soils from a vegetable field near Mendota, California, in the state’s highly fertile Central Valley. Treated soils had been irrigated for five years with subsurface drip tape that was aerated with Venturi injectors. The injectors use the flow of irrigation water to draw in air and mix it with the flow to form what Goorahoo describes as “an air/water slurry.” Untreated soils on the same farm were irrigated through subsurface drip tape without the injectors.
Using sophisticated DNA analysis techniques, the research team measured the balance of nine genes in the soil samples, each associated with a specific type of bacterium or Archaea fungus. The soil irrigated with aerated water had a higher proportion of bacteria known to fix nitrogen in the soil into a form usable by crops, while the un-aerated plots had a higher ratio of nitrate-reducing bacteria that convert nitrite into nitrous oxide (N2O)–a potent greenhouse gas–and NOx compounds.
“While AirJection irrigation did not have a significant impact on nitrogen fixation or ammonia oxidation, the practice of adding aerated water via the buried drip line did have a significant impact on denitrification genes, suggesting lower NOx production potential and thus likely to increase the availability of nitrate in the root zone,” Goorahoo told the Irrigation Association audience.
The presence of more plant-available nitrate in the root zone as well as healthier roots to channel it into the plant is likely to improve nutrient use efficiency (NUE) and reduce nitrate leaching, Goorahoo added.
“This might be hypothesized to enhance nitrogen use efficiency potential with AirJection,” he said, “and with the judicious water management within the root zone, plant nitrate uptake can be enhanced with a potential reduction in nitrate leaching.”
The team’s microbial research is a fitting follow-up to Goorahoo’s findings in earlier yield trials with aerated subsurface drip systems in the same area.
In2013,headdressedanIrrigation Association conference with results from eight years of field trials on a large produce farm with 1,500 acres of AirJection-equipped buried drip systems. The farm recorded a 23-percent average increase in cantaloupe yield with aerated water, as well as increases in honeydew, sweet corn and pepper yields. A 2008 paper Goorahoo presented to the association highlighted yield increases in California coastal strawberries of 18.3 percent #1-grade fruit and 6.9 percent of #2-grade fruit in aerated plots, in addition to larger root systems in aerated peppers and increases in the size and weight of aerated cantaloupes. In another California study, tomato yields rose 21 percent with aerated irrigation water in normal soils and 38 percent in saline soil.
Goorahoo also cited a 2013 study in Australia’s CSIRO Journals by Central Queensland University researchers demonstrating a 10-percent yield increase in cotton on heavy clay soils, which are prone to waterlogging and oxygen shortage during irrigation events. Using injectors to bring air levels in the water up to 12 percent by volume, the Australian team noted higher moisture extraction rates–an indicator of water use efficiency and root activity–greater root mass and weight, more light interception (canopy), and enhanced root function. With the yield increase, the scientists calculated that the aeration system would pay off in less than four years.
Goorahoo noted that subsurface drip aeration systems have been studied in Spain, Egypt, Italy, Japan and China on crops ranging from top-dollar vegetables to silage corn. In addition to Venturi injectors, some researchers have studied the addition of hydrogen peroxide to irrigation supplies to oxygenate the water. However, the injector approach is chemical-free, nearly maintenance-free, and requires no personal protective equipment or hazardous material storage.
The injectors in Goorahoo’s research utilize the Venturi effect, which describes the low pressure zone created when a stream of water passes through a specially shaped tube whose diameter narrows, then expands again. That low pressure effect creates a vacuum that is used to draw air into the stream.
Dr. Srikanth Pathapati, Director of Computational Fluid Dynamics Analysis and Engineering at MazzeiInjectorCompanyinBakersfield, California, pointed out that Venturi injectors must be very carefully designed and constructed to optimize both injection and mixing activity.
“The physics behind Venturi injectors can be altered by very small differences in the shape of the device,” Pathapati noted. “We design, model and precisely construct our injectors to optimize not only how much air the injector can pull in, but how effectively it can shear the bubbles. That shearing action thoroughly mixes the gas and liquid so the irrigation system can deliver water with high levels of dissolved oxygen rather than just entrained bubbles. Design, materials and quality control are extremely important to getting a high-quality injector.”
Goorahoo continues to research the impacts of aerating irrigation water. He said aerated irrigation water’s impacts on soil microbial activity, crop performance in saline soils, rooting characteristics of various crops, pest resistance, nutrient use efficiency and water use efficiency are all areas ripe for exploration.
In the meantime, he noted, “venturi injectors can increase root zone aeration and add value to growers’ investmentinSDI[subsurfacedripirrigation]”–important news for irrigation suppliers and farmers facing the current challenging agricultural economy.