The overall effects on crops growing under wind farms appear positive. He has led a team of plant and soil scientists who have been looking into the effects since 2009. They started their work after seeing more wind farms and turbines in Iowa. The new land use was financially positive for landowners, but researchers wondered if the same was true for farmers growing crops.

It’s unusual, because we’re continuing the previous land use and we’re adding another. We’re sort of double-cropping, because these can be thought of as two forms of energy production. The Chinese do this when they plant soybeans between horticultural crops. We’re planting turbines.

If the turbines change the micro-climate for corn and soybeans, the team wanted to learn if that’s a big enough change to be measured and what the potential impacts are. Wind blowing across a corn or soybean field without turbines creates a certain turbulence that carries moisture from the transpiring crop. The moisture rises into the atmosphere and pulls down cooler drier air. At night the wind is calmer and the land cools.

Turbines take some of the wind energy, slowing it but increasing its turbulence so it interacts with the crop more. That possibly increases evaporation from the crop or moves carbon dioxide down into the crop.

The biggest changes are at night. That’s because during the day there’s a lot of chaotic turbulence, just because the sun is heating the surface and the wind is gusty. At night when it becomes calm, the crop cools. If it’s a humid night there will be dew formation. By adding turbines it a little more like daytime. So dew formation is delayed and it may start to evaporate sooner.

Because fungus and mold like a wet environment, the shorter the wet period the less favorable for growth of potential pathogens. In the fall the shorter wet period could speed harvesting because farmers typically need to wait for soybeans to dry in the morning.

Another factor is that turbines bring warmer air down to interact with cool air near the surface. Throughout the wind farm the surface is a little warmer, which inhibits dew formation. Satellites can measure surface temperatures. There are little dots across the state of Iowa marking every wind farm because they’re slightly warmer than the surrounding area. So we know it has an effect that’s large enough to be seen there.

Another plus is air-pressure fluctuation measured around wind turbines. There is a lot of carbon dioxide in the top few feet of soil — as much as two or three times what is in the air. The movement of air by wind turbines pumps air down. The movement draws carbon dioxide out of the soil so more is available to the plant for photosynthesis. The air moving downward also creates more plant movement, which, in turn, increases sunlight penetrating the dense crop canopy.

On the negative side is the tendency of higher temperatures occurring at night in areas with wind farms. During the day corn takes in solar energy and carbon dioxide to make plant material. At night it cools and gives back some of the carbon dioxide. it gives up more if it’s warmer. So the nighttime warming of the turbines is not a totally good thing. Nighttime temperatures have been increasing during the past 40 years and are becoming a limiting factor for crop yields.

But overall crops grown in wind farms seem to benefit. There are three ways the crop is being fertilized — from air, soil and increased photosynthesis. We measured increased carbon dioxide uptake during the day, with increased respiration at night. But during the course of the day there was more uptake. So as far as the impact of the turbines on the carbon dioxide processes and the photosynthesis process in the near vicinity of the turbines, it’s a net gain.

The team would like to look at the impact of wind movement through a wind farm as it slows and tends to move up. That could create clouds if the air is warm and moist — and potentially create rain.

We don’t know if wind farms are a preferential location for cloud formation or something that’s going to provide more rain in an area beyond the wind farm. We have some preliminary measurements that suggest that this is a real effect. Theoretically yes, there should be an effect. But we don’t know if it’s large enough to be measured or to be important.

Email gstakle@iastate.edu or call 515-294-9871 for more information.