<st week of Arb Notes, I'd like to address a topic relevant to the Arb and the now ever-present conversation about carbon emissions and credits. Do restored prairies sequester significant amounts of carbon? As you may know, Carleton has about 160 acres of high quality restored prairie in the lower Arb. Small-scale prairie restoration began at Carleton in the 80s with a student-driven initiative to restore Hillside Prairie. Large-scale efforts to restore fields previously under corn-soy agricultural rotation began in earnest in 1995 under the direction of Mark McKone and Myles Bakke, and has continued to the present. Sections of up to 20 acres of agriculture are restored each year. By restoring this large expanse of high quality prairie, Carleton has created habitat for many plant, animal and fungal species threatened by the decline of tallgrass prairie (only 1% of pre-settlement tallgrass prairie is left worldwide!). But what about carbon sequestration? With rising atmospheric CO2 levels, does returning this agricultural land to prairie help our global carbon situation as well? The detailed comprehension of this is still not understood, but it is generally thought that restored prairie has carbon sequestration potential.
In agricultural systems, carbon is mostly present in the tissues of plants (which are then removed for human or animal consumption). Tilling doesn’t allow a soil organic matter layer (think decaying plants) to accumulate. In addition, the soil microbial community, which turns dead plant matter into soil organic carbon (SOC) (which is stored longer in the soil), is greatly affected by agricultural practices. So in general, “traditional” agricultural practices don’t favor of carbon storage. In restoring carbon-degraded land to tallgrass prairie we see a few important changes. First, because plants aren’t removed annually there is an increase in aboveground carbon stocks (in plants). As a prairie matures, a significant proportion of its biomass (60-80%) is in the underground root system, meaning that below ground plant carbon stocks increase with time. It takes more time however (up to a century) to re-gain the soil microbial community that turns this plant material to SOC and thus to begin significant accumulation of SOC. As an added note, the carbon released in management burning of prairies is thought to be more-than balanced by the great increases in growth (and thus carbon sequestration) seen in those burned units that year. Although this doesn’t sound perfectly optimistic, a recent study by Matamala et al. (2008) concluded that restored tallgrass prairie soils can sequester 3.5 kg C/m2 (50% of steady state soil fluxes) in the first 100 years, making this a pretty reasonable carbon sequestration strategy. Besides potential use in carbon sequestration strategies (prairie restoration is not on the carbon trading market so far) diverse tallgrass prairie also has the potential to contribute biomass to cellulosic biofuel generation, a possibility being researched by David Tilman at UMinn.