Novel soil ecosystems created by natural gas leaks

Pervasive natural gas leaks from urban distribution pipelines, common in most major cities in the United States, provide a unique opportunity to investigate both the impacts of gassed soils on biogeochmical cycling and the novel environment these gassed soils create. Previously unaccounted natural gas leaks in urban environments are quickly gaining attention as a significant source of methane lost to the atmosphere. Surprisingly, roughly 1.4% of the natural gas distributed in the United States from 2005-2008 is unaccounted for (Phillips et al., unpublished data). Current methane mapping projects in urban areas identify significant leaks from underground pipe infrastructure as the source of this ‘missing methane' (Phillips et al., unpublished data). Beyond its contribution to global warming, elevated methane at leak sites is known to have significant effects on above- and below-ground vegetation, as well as on soil composition and soil processes. Characterizing the effects of natural gas leaks on soils and vegetation is particularly important in light of the role of soils, soil biota, and above- and below-ground biomass in the global carbon cycle and the pervasiveness of leaks in the United States. Likewise, investigation of the unique ecological niche created by gassed soils will tell us something about the ability of methanotrophs and/or methanogens to colonize such environments and the adaptability of preexisting microbial communities to methane-rich soils. The central question motivating this research is, what are the effects of methane on soils, soil biota, and vegetation at or near leak sites?

As Boston, Massachusetts is one of the oldest cities in the United States, with an equally aged natural gas distribution infrastructure, it provides an ideal environment in which to investigate the effects of pervasive natural gas leaks on urban ecosystems and subsequent biogeochemical cycling. Mapping projects within the city have already identified numerous leaks of variable size and age (Phillips et al., unpublished data). What's more, preliminary data show that gassed soils can often exceed 90% methane and below 10% oxygen (Phillips et al., unpublished data). Soils exposed to methane at leak sites become desiccated and anoxic, creating a virtually unexplored extremophile domain. Anoxic conditions are ideal for methanogen colonization, while methane-rich conditions create suitable habitat for methanotrophs. Soil at leak sites often appears black and gooey, forming a crust-like substance at the soil's surface. Preliminary analyses of this soil reveal elevated levels of potassium and very low levels of nitrate (NO3-N) (Hendrick and Phillips, unpublished data). While vegetation at leak sites is known to suffer significantly increased rates of mortality, preliminary investigation reveals that methane at leaks sites is able to invade both below- and above-ground tissues. Trees and Ganoderma at leak sites exhibit elevated levels of methane in their tissues and appear to be acting as conduits, moving methane from the soil into the atmosphere (Phillips et al., unpublished data).

Soil respiration, soil leaching, and the ability of soils to support plant and microbial life are all known to be affected by methane-enrichment (Phillips and Ackley, pers. communication). Upon performing further analyses, I expect that soils and vegetation at or near methane leaks will be substantially different from those found at control sites. Within the city of Boston, MA, I hope to answer a number of questions. Specifically, do soils exposed to elevated methane differ in micro- and macro-nutrient content, soil moisture content, soil texture, organic matter, and/or microbial community structure and composition when compared to unexposed soils? What is the magnitude of methane flux from leak sites? What role does vegetation play in moving methane into the atmosphere? Are there correlations with flux intensity and soil/vegetation impacts?

Characterization of the effects of elevated methane on soils, soil biota, and vegetation will inform future carbon budget forecasts on a city-by-city basis. Understanding the impact of methane at leak sites will inform city planners in their efforts to ameliorate affected soils and replant in areas devoid of vegetation. Characterization of the magnitude of fluxes will inform future carbon budgets and identify specific environmental impacts correlated with flux intensity (ie. species-specific tree mortality, unique soil conditions). Natural gas is being hailed as a “bridge fuel” and the next horizon in ‘clean' energy, despite being 25 (IPCC AR4) to 33 times (Shindell et al. 2009; Howarth et al. 2011) more potent as a greenhouse gas than carbon dioxide. In light of increased urbanization and the recent boom in shale gas extraction and development, a full environmental analysis of methane enrichment on soils and vegetation is imperative, and may lead to a more pragmatic treatment of natural gas fuels.