Disturbance Affects Relationship between the Nitrogen and Carbon Cycles
Carbon and nitrogen are always on the move. Both elements are versatile – they are constantly being converted from one form to another, and are required by all living things. “Because plants, animals, and microbes also require fixed ratios of the two elements, carbon and nitrogen’s chemical cycles are inherently linked,” says U.S. Forest Service soil scientist Jennifer Knoepp.
Knoepp recently contributed to a study exploring how disturbance affects nutrient cycles in the southern Appalachian Mountains. The study was led by Ashley Keiser, a postdoctoral research associate at Iowa State University, and published in the journal Ecosystems.
“We focused on a part of the nitrogen cycle called nitrification,” says Knoepp. Nitrification occurs when soil microbes convert ammonium into nitrate. Much of the overlap between the carbon cycle and the nitrogen cycle occurs in the soil, in processes conducted by soil microbes.
Microbes break down nutrients, build new compounds for their own growth, and eventually die. Their life cycle is a small but critical part of nitrogen’s biogeochemical cycle through plant and animal tissues as well as the atmosphere, soil, and water. Some steps in the carbon and nitrogen cycles are only conducted by soil microbes, which means that without these organisms, the world as we know it would grind to a halt.
Nitrification rates reflect the amount of carbon and nitrogen-containing compounds, such as ammonium in the soil. Different groups of soil microbes compete for ammonium, and Knoepp and her colleagues hypothesized that the presence or absence of available soil carbon is what tips the balance in favor of one group or the other.
Knoepp and her colleagues compared nitrification rates among 10 southern Appalachian watersheds that had different disturbance histories. Disturbance can affect soil nutrients in many ways. For example, logging removes large amounts of carbon that would have eventually decomposed. After logging, the amount of carbon stored belowground usually declines, while the amount of nitrogen released from the soil usually increases. However, the effect of disturbance on nutrient cycling has not been fully understood, especially at large spatial scales.
The scientists created a set of models that explained patterns in nitrification rates across the landscape. “We found that a history of disturbance at the landscape scale reduces the amount of available soil carbon,” says Knoepp. The amount of carbon in the soil can become so low that carbon shortages – rather than nitrogen shortages – limit microbial growth. The rate of nitrification in disturbed watersheds was strongly related to the rate at which nitrogen re-entered the soil as plants and animals died and decomposed.
Undisturbed watersheds had more carbon available to microbes, and in these forests, the rate of nitrification was not related to the amount of decomposing organic matter. “Our study suggests that past disturbance affects the amount of available soil carbon,” says Knoepp. “Disturbance history plays a role in controlling carbon and nitrogen cycling in the soil.”
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For more information, email Jennifer Knoepp at jknoepp@fs.fed.us