Carbon Sequestration

Carbon Sequestration

According to the Intergovernmental Panel on Climate Change, carbon dioxide (CO2 ) concentrations in the atmosphere have increased 32% since 1750.  Moreover, the IPCC states that human activity is the cause of such increases, primarily through the burning of fossil fuels which release CO2 .  Because CO2 is one of the atmospheric gases that reduce the amount of outgoing terrestrial energy from escaping into space, increased levels of this gas will ultimately lead to an increase in the earths temperatures.  Many scientists are seeking methods to remove atmospheric carbon and store it elsewhere in a stable state, thereby offsetting the human addition of carbon to the atmosphere.  Using plants to do this is referred to as terrestrial carbon sequestration.  Through the process of photosynthesis, carbon dioxide is removed from the atmosphere and stored as carbon in biomass.  If net primary production of these ecosystems exceeds decomposition, then these systems become at the very least a short term sink for carbon.

Area sampled delineated by ring

The first two carbon sequestration studies were initiated in 2006 and 2007.  Objectives were to quantify the carbon storage potential of extensive green roofs and to evaluate the effect that species has on carbon flux. The first was performed on eight roofs in Michigan and four in Maryland, ranging from one year to six years in age.  All twelve green roofs were comprised primarily of Sedum species and substrate depths ranged from 2.5 cm to 12.7 cm.  Above-ground plant material was harvested in the fall of 2006.  On average, these roofs stored 162 g Cm -2 in above-ground biomass.  The second study was conducted on the roof of the Plant and Soil Sciences Building on the MSU campus. Twenty plots were established on 21 April 2007 with a substrate depth of 6.0 cm.  In addition to a substrate only control, the other plots were sown with a single species of Sedum (S. acre, S. album, S. kamtshaticum, or S. spurium).  Species and substrate depth represent typical extensive green roofs in the United States.  Plant material and substrate were harvested seven times across two growing seasons.  Results at the end of the second year showed that above-ground plant material storage varied by species, ranging from 64 g Cm -2 (S. acre) to 239 g Cm -2 (S. album), with an average of 168 g Cm -2 .  Below-ground biomass ranged from 37 g Cm -2 (S. acre) to 185 g Cm-2 (S. kamtschaticum) and averaged 107 g Cm -2 .  Substrate carbon content averaged 913 g Cm -2. with no species effect, which represents a sequestration rate of 100 g Cm -2 over the 2 years of this study.  The entire extensive green roof system sequestered 375 g Cm -2 in above- and below-ground biomass and substrate organic matter.

Complete results are published in:

Getter, K.L. D.B.Rowe, G.P. Robertson, B.M. Cregg, and J.A.Andresen.  2009.  Carbon sequestration potential of extensive green roofs.  Environmental Science and Technology 43(19):7564-7570.

Plug of plant biomass and substrate pulled within ring

Carbon Sequestration

Another study was initiated in 2009 that compared the carbon content of nine in ground and four green roof landscape systems of varying complexity to determine their potential to sequester carbon.  Soil or substrate samples were analyzed prior to planting in 2009 and soil/substrate, below- and above-ground biomass were analyzed at the end of the 2010 and 2011 growing seasons. Landscape systems examined at ground level included (1) Kentucky bluegrass lawn, (2) native prairie mix, (3) succulent rock garden consisting of Sedum, (4) woody ground covers, (5) herbaceous perennials and grasses, (6) deciduous shrubs, (7) broad-leaf evergreen shrubs, (8) narrow-leaf evergreen shrubs, and (9) vegetable and herb garden.  In addition, four of these landscape systems were duplicated on green roof platforms (10) extensive green roof consisting of Sedum, (11) extensive green roof consisting of a native prairie mix, (12) extensive green roof consisting of herbaceous perennials and grasses and (13) extensive green roof consisting of vegetable and herb plants.

Following three seasons, the in-ground broad leaf evergreen shrubs (78.7 kg/m 2 ), in-ground herbaceous perennials and grasses (68.7 kg/m 2 ), herbaceous perennials and grasses on the green roof (67.70 kg/m 2 ), in-ground deciduous shrubs (65.7 kg/m 2 ), and needle leaf evergreen shrubs (62.91 kg/m 2 ) had stored more carbon content than the other landscape systems. The Sedum and prairie green roofs contained less carbon than their counterpart in-ground landscape systems, suggesting that although green roofs do sequester a small amount of carbon, greater benefit can be achieved in ground level landscape systems. Management practices will influence net carbon sequestration and the permanence of the carbon sequestered in all systems.

Complete results are published in: )

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