AN INTRODUCTION TO STABILIZATION WEDGES: CAMPUS STORMWATER BASINS AS CARBON SINKS

JMU's projected annual business-as-usual greenhouse gas emissions in 2025 are expected to be 115 gigagrams (Gg) of carbon dioxide equivalents (CO2e). JMU has committed to reducing these emissions by 30% via initiatives such as building retrofits, composting, and renewable energy.

JMU's stormwater basins help reduce nutrient and sediment loading to the Shenandoah River. Although these basins are small (1.2 ha; 0.4% of campus), these constructed wetlands are likely to sequester CO2 as soil organic matter, thereby serving as a “stabilization wedge” by offsetting campus emissions.

To test this idea, undergraduate students in two sections of a “Carbon footprint” class for nonscience majors measured soil organic matter from ~20 basins using a microwave (6 min @ half power)/muffle furnace (16 h @ 400C) combination to estimate soil organic matter as loss-on-ignition from dry soil. Student teams then calculated CO2-sequestration rates in Mg CO2/year after making a number of simplifying assumptions (e.g., dates of basin construction, 0% soil organic matter before basin construction, bulk density ~1 Mg/m3).

Basin soil organic matter concentrations averaged 7.3%, but ranged widely. Volumetrically, these values translate to ~7 kg SOM or 3.5 kg SOC/m2-0.1m, using a Shenandoah Valley-specific empirical relationship (%SOC=0.5*%SOM+0.3, R2=0.92, n=34). JMU’s campus stormwater basins sequester ~100 Mg CO2e/year. Compared to other stabilization wedges, this carbon offset is quite modest. For example, the 35-ha campus arboretum (~110-y oak-hickory forest) sequesters ~260 Mg CO2/y assuming net ecosystem production of 200 g C/m2/y.

On a specific areal basis (Mg CO2e/ha/y), however, stormwater basins have 11-fold greater sequestration rates than the arboretum. Since stormwater basin construction and maintenance costs are $0 when considered from a campus stabilization wedge perspective, the carbon-footprint-return-on-investment exceeds that of all other initiatives. Finally, this exercise provided a hand-on, real-world experience for nonscience majors, most of whom arrived in class with only a limited understanding of the basics of the carbon cycle and global change.