Can biochar help adapt agriculture to a hotter, dryer climate?

Shelby Hoglund

2018-2021 FFAR Fellow

University of Arizona

  • Soil Health

Let’s talk about desert agriculture. Warning: you may feel thirsty. Agriculture in the arid Southwestern United States is productive year-round, with conditions that permit crops to grow the entire year. But with a predicted hotter, dryer climate looming in the near future, desert agriculture faces challenges.

My research addresses climate change adaptation through soil health. A key component of healthy soil is its organic matter. Organic matter—the part of the soil made up of decomposing plant and animal residue—is particularly important in arid environments because it acts like a sponge to hold water. The water content in soil affects microbial activity, which plays a key role in soil nutrient cycling. Soil microorganisms unlock nutrients for plants to absorb, increasing productivity.

Soils with a very low amount of organic matter will retain little water because the “sponge” is missing. The USDA Natural Resources Conservation Service calculated that increasing soil organic matter by just 1% can increase the amount of water that soil can retain by 25,000 gallons per acre!

Okay, I’m sold! Where can I get this soil organic matter?

Well, animal manures and compost are high in organic matter and make an excellent soil amendment. However, those materials may only last a few years in soils because microorganisms enjoy feasting on them and the extreme desert climate either blows them away in the wind or cooks them with heat and UV radiation.

But we have an alternative. There is a material made from organic waste that will stay in the soil for much longer—decades to centuries instead of a few years—while also benefiting water-holding capacity, organic matter content and nutrient retention. This material is biochar. Biochar is created by recycling organic waste through a process that heats up the material to temperatures as high as 1,000 °C. The intense heat occurs in a chamber without oxygen, which means that about half the carbon (a key ingredient in organic matter) becomes a stable material rather than turning into carbon dioxide; the carbon remains carbon, while also producing liquid hydrocarbon byproducts that can be used as fuel.

But how much biochar do farmers need to apply in order for their soil to retain more water? How much is too much? What other benefits or detriments exist? I am studying these effects and will share the results with farmers. I have several field sites where I am quantifying the effects of adding biochar in different amounts to arid agricultural soils in southern Arizona to understand changes in variables important for crop growth such as soil moisture levels, soil microbial activity and pH.

One field site is at a grape vineyard in Sonoita, Arizona, where the focus is to increase plant-available nutrients by calling on agriculture’s biggest team of volunteers: soil microorganisms. How do we get them to come out and help?

We can provide a “sponge” that holds water for them for the dry periods between irrigation. We hypothesize that adding biochar to the soil will help the soil retain irrigation water for longer. At another field site, I am growing wheat in a field with plots that contain either co-composted biochar or a blend of mature compost with biochar. Why wheat? Wheat is commonly grown in southern Arizona following alfalfa and prior to cotton. Many biochar studies observe the boost that biochar can provide to crop yield. This field study is a bit different and somewhat harsher to the plants: I plan to irrigate the field so that plots have a 50% irrigation deficit, 25% irrigation deficit, or no deficit (100% of the normal irrigation). Over time, I will compare the microbial activity in the soil, the soil moisture content and the wheat plant response between the different plots. This work can help us determine how biochar can maintain crop yield while reducing farmers need to irrigate.

My research goals are becoming a reality thanks to the support of my advisor, Dr. Joseph Blankinship, my industry sponsor, Arizona Vignerons Alliance and the FFAR Fellows program.

Photo: Vineyard in Sonoita, AZ with biochar application near rows of vines, March 2019.
Vineyard in Sonoita, AZ with biochar application near rows of vines, March 2019.
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