Which Is More Important To Biofuels – The Soil Or The Crop?

By James Conca, Contributor
Biofuels are a key component of a clean energy future, and it turns out that the soil is more important than the crop. Kenn Brown and Chris Wren

As an Earth scientist, I was pleased to see it was the soil. I shouldn’t have been surprised. Farmers have known this for centuries.

Soil texture and soil microbes are key. It’s what a farmer does when he reaches down, scoops up some soil in his hands, looks at it, smells it and sighs.

This is especially important if you’re trying to capture carbon from the atmosphere and put it in plants that will produce biofuels for either running cars or running power plants.

Biofuel is fuel derived from living matter called biomass (usually plant matter). Examples of biofuels include, but are not limited to, biodieselethanol, and vegetable oil. Biofuels can be categorized into three different types based on the source of biomass. Since biofuels are obtained from current plant growth, they are considered a renewable source of energy.

Ethanol is the most common biofuel in North America, as most gasoline contains up to 10% ethanol. This fuel is referred to as E10, where the number refers to the percentage of ethanol in the fuel. Flex fuel vehicles are capable of running up to E85. The remaining 15% of fuel must be gasoline as ethanol is harder to ignite in engines.

Most of the carbon contained in soil is in the form of organic matter. The composition of this matter is determined by plants, microbes, and the soil. However, scientists do not fully understand how variation in plant inputs, the structure of soil microbial communities, and the physical and chemical attributes of the soil, interact to influence the chemical makeup of organic matter in soil.

Soil texture influences the local microbiome. Researchers used scanning electron microscopic images of soil particles and aggregates to understand the influence of biofuel crops and site selection on soil organic carbon chemistry. Hofmockel, Johnson and Dohnalkova PNNL

Researchers at PNNL, led by Kirsten Hofmockel, addressed this knowledge gap by coupling microbial characteristics with detailed soil chemistry from two long-term bioenergy research experiments. They found that the texture of the soil was more important than the crop type on the makeup of the soil’s microbial community and the chemistry of the soils’ organic matter.

Plants take carbon from the air, build cells, then release some of the carbon into the soil through roots and root exudates. Some have suggested that growing biofuel crops in marginal soils, which generally have low carbon content, might be a way to remove carbon from the atmosphere and store it below the ground. But that turns out to be incorrect.

Scientists must understand the controls on how organic matter builds up in soils to identify successful strategies that improve soil health and increase carbon storage, especially for potential biofuel crops.

In the PNNL studies, researchers characterized the influence of crop and site on fungal and bacterial community structure, potential enzyme activity, soil carbon chemistry, and total soil carbon and nitrogen concentrations at two long-term biofuel field experiments.

The two sites had both corn and switchgrass crops. One site had predominantly sandy loam soil, the other had predominately silty loam soil. The research found that the crop was less influential on soil microbial community structure and organic matter chemistry than soil type.

Ethanol production from corn is the most common biofuel in North America, as most gasoline contains up to 10% ethanol, and up to 85% in flex fuel vehicles. DOE EERE

Soil type was especially influential on fungal community structure and the chemical composition of relatively persistent carbon. After eight years of no-till management, silty loam soil still contained twice the total carbon and nitrogen as sandy loam soil, with no significant response to biofuel cropping.

The figure below, from Christopher Kasanke, Qian Zhao, Sheryl Bell, Allison Thompson and Kirsten Hofmockel (2020), shows the sharp differences in soil effects. Notice that the soil made all the difference in both crops with respect to carbon and nitrogen retention. The figure key is - total carbon (a) and total nitrogen (b) in sandy corn (light blue), sandy switchgrass (dark blue), silty corn (light red), and silty switchgrass (dark red) in large (circle). Small (triangle) macroaggregates generated using an optimal moisture sieving approach. Plotted values represent mean percent C or N per gram of dry soil.

The bottom panels are micrograph images of the small aggregates in sandy (c) and silty (d) soils, showing how the small structure would influence things like soil and nutrient retention. In sandy soils, there is little microporosity to hold them.

The researchers concluded that this most likely results from enhanced microbial activity in the soils. So the initial site selection is critical to plant–microbe interactions and substantially affects the potential for long-term carbon storage in surface soils under biofuel production.

“Alternatively,” said Hofmockel, “deeply rooted perennial plants used for biofuel production may provide a way to capture carbon deep within the soil profile. The ability of biofuel cropping systems to sequester carbon will depend on how carbon from plant and microbial sources interact with soil minerals. Microbes behave differently deeper in the soil. Deeper-rooted plants present an opportunity to increase carbon inputs and reduce carbon return to the atmosphere. But it all depends on the soil chemistry. Our research continues to dig deeper into the soil profile to understand the soil factors that are most promising for carbon sequestration.”

Figure showing sharp differences in the soil effect on both crops (see text for explanation). The bottom panels are micrographs of small aggregates in sandy (c) and silty (d) soils, the scale bar being 50 micrometers. Kasanke PNNL

Biofuels and fossil fuels (coaloil and natural gas) are both derived from organic matter, but differ in how recently the organic matter died. Fossil fuels come from organic matter that died millions of years ago, whereas biofuels come from recently deceased organic matter.

From a climate change perspective, using biofuels in an engine still releases just as much carbon dioxide as fossil fuels. However, since they're derived from recent biomass which took in the CO2 as it was growing, the CO2 released in combustion causes no net increase in atmospheric carbon, making biofuels close to carbon neutral, aside from the CO2 and N2O costs of agricultural equipment, fertilizer production, transportation and conversion of the biomass.

Conversely, fossil fuels are releasing CO2 that has been stored for eons. Burning them increases the amount of carbon dioxide in the carbon cycle, particularly in the atmosphere and oceans.


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