He got his start in forestry research, but when it comes to bioenergy, DeLucia declares himself a “grass guy.” In this interview with reporter Laurie Udesky, he explains why he favors tall grasses over trees for bioenergy.
Laurie Udesky: Why do you prefer tall grasses for feedstock to forests?
Dr. Evan DeLucia: One of the challenges with trees, from my perspective, is that even rapidly growing trees are not producing as much total biomass per unit land area as what we’re seeing with some of these really, really fast-growing grasses. So the bang for your buck just seems to be a little bit greater for the grasses than, say, for plantation forestry.
Could you elaborate on that?
I got into this business because of an increase of CO2 in the atmosphere and a consequent increase in global temperatures, which I think is one of the greatest threats that we face. So finding alternative fuel supplies is, to me, extraordinarily critical.
That said, we’ve been exploring bioenergy/biofuel from plant material. That has a number of benefits – plants are nature’s photocells. They do a very good job of converting CO2 in the atmosphere with a little bit of sunlight into sugars, starch and cellulose that can be used as a fuel supply. The challenge is that biofuels from plant material have lower energy content than fossil fuels. Fossil fuels pack a lot more punch. Per unit mass, fossil fuels have significantly more energy content.
And the lower the productivity of the plant material, the more land you’re going to have to use to achieve a significant level of fossil fuel displacement. So the best thing you can do is select a feedstock that is highly productive. Generally speaking, the less land you use, the fewer environmental problems you’ll face.
Many of the grasses we’ve been looking at – miscanthus, cordgrass, switchgrass – these plants yield more on an annual basis than rapidly growing pine forests. Miscanthus, for example, can yield anywhere between 20 and 40 tons of dry biomass per hectare. A rapidly growing pine plantation in the Piedmont region of North Carolina would yield about 10 tons per hectare.
Do grasses have other advantages as a feedstock?
Perennial grasses have the advantage that you can cut off the tops year after year without disturbing the bottom – in other words, without digging up the root system. Imagine harvesting and replanting trees compared to mowing your lawn. (With grasses) you keep cutting off the top. You leave the bottom in place, the root system intact. Ultimately they’ll have to be replanted, but just when varies by species. Some can go quite a long time – decades, even.
So what that means is when you don’t disturb the soil, you’re giving the soil a chance to build up soil organic carbon. This is another store of carbon that’s below ground, that’s not in the atmosphere. The enormous fibrous root systems in these grasses accumulate in the soil, they die, they turn over. Some of that material is respired back into the atmosphere but some of it stays in the soil. So it’s building a very carbon-rich fertile soil– with good water holding characteristics and good nutrient characteristics.
It also holds additional carbon below ground that would normally be in the atmosphere.
The reason for this is when you till soil, you oxygenate it and you jazz up the microorganisms that decompose organic matter. Regular tillage tends to enhance the rate of oxygenation in surface soils and get that carbon that’s stored there back into the atmosphere.
What else attracted you to tall grasses?
Grasses have a third advantage that I’m enamored with: At the end of the season many grasses – as a matter of fact, most perennial grasses – will move most of the nitrogen and critical nutrients that are in the above-ground tissues to the rhizome, a big root structure below ground. They’ll store it there for use in the next year. That means that many of these perennial grasses require very little, if any, further fertilization to keep them going. So most of the nutrients that they used to do the leaf business of photosynthesis can be recycled year after year.
Overall, 1) the productivity is very high, 2) grasses can be harvested without disturbing the soil, they tend to store a lot of organic carbon below ground, and 3) they retranslate those nutrients very efficiently. Those are the big three that got me going.
Are there potential problems with using trees for bioenergy?
One thing that would be quite a risk would be converting mature forests – particularly the few uncut forests we have here or in tropical parts of the world. It would be quite problematic to harvest those and grow other (energy) crops there. The reason primarily is that a tree takes many, many years to store carbon, and you’re releasing all that carbon into the atmosphere. Now having said that, that carbon was fixed by photosynthesis, so you’re just putting into the atmosphere what was once in the atmosphere, completing the cycle. But you’re just taking what was fixed 50 years ago and releasing it into the atmosphere now. And so that’s boosting atmospheric CO2 today with what was pulled out of the atmosphere yesterday.
The other thing about trees – and this is a challenge for grasses, too – is biodiversity.
Monocultures are not very desirable from an ecological perspective. By definition, they have very low biodiversity.
They do not host the gorgeous array of insects and mammals and birds that you would see in a diverse ecosystem. So one thing we want to be very careful about is converting native forests to a monoculture. Not only do we reduce or eliminate the kind of diverse ecosystem services that that unit of land can provide, but we’re also greatly diminishing the biodiversity on that landscape.
What does “carbon sequestration” mean to you?
Here’s the problem I have with the term “carbon sequestration,” which people generally use to mean carbon storage. Unlike fossil fuels, contemporary plant material, regardless of where it is – whether it’s the stem of a tree or carbon deep in the soil – it’s not stored there. It’s residing there for some period of time, and that period of time could be short or long. And that period of time could be changed by management practices.
There’s this wonderful petty debate that you’ll hear: tree people vs. grass people. I don’t know why we have this distinction (since both feedstocks are useful in different locations). But tree people will say to grass people: “Well, you’re growing these grasses, and the grasses are building huge reserves of carbon in the soil, which is all great, until you till that soil and put it back into beans and corn or whatever you put it into. And then inexorably that carbon will be released back into the atmosphere, so you haven’t stored it!”
Fine, guilty as charged. But then I like to look at the forestry guys and say: "Ok, you have these big, gorgeous trees, and they are essentially one lightning strike away from releasing all of their carbon back into the atmosphere. So just because it’s in wood, it doesn’t mean that it’s stored, either.”