FOREST SOILS ARE IMPORTANT SINKS FOR METHANE, BUT CAN HUMAN USE OF FORESTS REDUCE THEIR EFFICIENCY?

Forest soils are known to act as sinks for atmospheric methane, a major greenhouse gas. Methane is the most abundant organic trace gas in the atmosphere, primarily created by biological processes of microbes (methanogenesis). It plays an important role in global climate change, and forest soils are known to be effective sinks for methane. In soils, methane is primarily utilised by bacteria that oxidize it to produce carbon. However, the effectiveness of soils as sinks is affected by land-use practices such as agriculture, and forest or woodland soils are considered more effective sinks than soils in human landscapes.

Biligiri Rangaswamy Temple Wildlife Sanctuary is located at the confluence of the biologically diverse regions of Western and Eastern Ghats. Until the eighteenth century, these forests were hardly accessible to those residing in the vicinity. They were earlier inhabited by the hunter-gatherer Soliga tribe, who lived on non-timber forest products and practiced shifting cultivation. Shifting cultivation was banned by 1972, when the area was declared as a wildlife sanctuary, and Soligas now practice permanent agriculture. However, they graze cattle and continue to gather non-timber forest products from the sanctuary.

Despite protection, these forests are increasingly affected by anthropogenic influences (animal grazing, forest fires, etc.) of the people living in the vicinity of the sanctuary. These disturbances can be chronic given their impacts on the ecosystem. Grazing of livestock inside the forest encourages the growth of vegetation, in turn encouraging firewood collectors. Intensive grazing by livestock and other anthropogenic activities could pose serious threats to the natural habitat, including to forest soils. Treading and trampling by grazing animals leads to soil compaction, especially in wet tropical conditions. Soil compaction decreases the number of soil pores, in turn decreasing soil aeration. The condition of reduced soil pore volume and increased water-filled pore space reduces the ability of soils to absorb methane. Methane oxidizers in soils are also sensitive to disturbances.

A study in this landscape investigated the variations of soil methane fluxes in disturbed and undisturbed forest sites across four seasons, with forest grazed by cattle regarded as disturbed forest. Gas fluxes at the soil surface were collected from disturbed and undisturbed forest areas, and samples were analyzed for methane using a gas chromatograph in the laboratory. The net methane sink was higher in undisturbed forest than in disturbed forest both overall and in each season. The accumulated methane sink value measured over four different seasons was highest during the monsoon, whereas the lowest value was post monsoon. During the monsoon, grazing activities are lower due to heavy and prolonged rainfall, which could be the possible explanation for higher methane sink values at this time. In contrast, the higher grazing pressure during the post-monsoon season leads to soil compaction due to livestock treading and trampling, thus creating anaerobic conditions in the soil with reduced airfilled pore spaces. This leads to increased methanogenic activity, and could be the possible explanation for the reduced methane sink during the post-monsoon season.

While human practices like agriculture are known to play a role in altering forest sink potentials, this study shows that disturbances due to animal grazing can also reduce soil’s methane oxidation potential, hence reducing the overall methane sink strengths of soils in forestlands.

Nani Raut is a researcher at the Norwegian University of Life Sciences, Norway. nani.raut@umb.no

Illustration: Kalyani Ganapathy

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