Not only do they turn animal waste into safe fertilizer, they may also help stem the tide of global warming.
Back in the mid-1990s, a team of North Carolina A&T scientists went to the school’s 500-acre University Farm, leveled an area half the size of a football field, and seeded it with cattails. Then, every day for the next twelve years, they flooded it with wastewater – some 10,000 gallons a day – from a nearby swinehouse. They were looking for a cheap way to process pig manure. And as they now report in the journal Water Science & Technology, they found that and then some.
Pig manure is a bigger problem than you think. Farmers commonly dispose of it by flushing it into a lagoon. The lagoon water can then be used to feed crops; rich in nitrates and phosphates, pig excrement makes for good fertilizer. However, the water is often too rich in fertilizing chemicals–the excess chemicals can seep into the soil and contaminate groundwater.
The wetland at University Farm started out as an experiment to see if nature might provide a way to filter out the excess fertilizer before it reaches the crop fields. Led by North Carolina A&T’s Gudigopuram Reddy, that experiment proved a resounding success almost from the very start. As water flowed over the marshy wetland, the cattails tended to soak up nitrates and phosphates. By adjusting the inflow, Reddy and his colleagues could control the concentrations of chemicals in the outflow.
But more than a decade later, Reddy–now collaborating with a team that includes A&T scientists Johnsely Cyrus and Charles Raczkowski–has discovered that the artificial wetland is also good for something else: capturing and sequestering greenhouse gas. To understand how, let’s take a step back.
In addition to nitrates and phosphates, pig manure contains a boatload of carbon—in the form of sugars, proteins, fats, and other organic molecules. Those molecules are essentially one step removed from becoming harmful greenhouse gas: All it takes is for a microbe to break them down and release them as carbon dioxide. To prevent that from happening, you need to either convert the carbon to a form that microbes can’t easily digest or store it some place that microbes can’t easily access.
Reddy and company’s artificial wetland does both. In the shallow marshy areas, the fast-growing cattails convert small carbon molecules into hard-to-break-down cellulose. And in the oxygen-poor environment near the wetland’s floor, where much of the inflowing carbon settles, microbes can’t thrive.
Soil scientists like to talk about carbon using a pair of fancy terms: lability and recalcitrance. Labile organics are carbons that get consumed quickly, sometimes in a matter of months. Recalcitrant organics, on the other hand, can remain stable for hundreds of years. Part of the magic of the artificial wetland is that it converts labile stuff into recalcitrant stuff—lots and lots of recalcitrant stuff.
How much? Reddy’s team estimates that an acre of artificial wetland sequesters nearly a ton of carbon per year, which is in line with previous studies of naturally occurring wetlands. The lion’s share of that carbon is recalcitrant, stored in a top layer of slowly decaying plant debris or captured as complex organics in the underlying soil.
Have the A&T researchers found a silver bullet that could stop global warming in its tracks? Unfortunately, no. Humans spill a whopping 40 billion tons of greenhouse gas into the air each year. Even a wetland the size of the entire United States would make only a small dent in global greenhouse gas emissions. That said, with the planet’s climate hurtling toward uncharted territory and no comprehensive solution in sight, every little bit surely helps.