Biochar as a Climate Change Impact Mitigation Tool
Global warming as a result of increasing concentrations of greenhouse gases (GHGs) (CO2, CH4, and N2O) calls for all-out measures to reduce emissions and increase capture of GHGs. Thus, there is great impetus to reduce GHGs emission from the soil, which could be either a sink or source of GHGs depending on the landuse, management, and soil type (Zwieten et al. 2010).
Tropical primary forest soils are generally sinks of CH4 and sources of CO2. Conversion of primary forests to plantations, agroforestry, and/or agriculture significantly alters the soil physical and biological microenvironments, which might revert the soil from a sink to a source of GHGs. Since CH4 has 25 times greater warming potential than equivalent mass of CO2, landuse change reducing soil CH4 influx will have serious impacts on mitigating global warming (Forster et al. 2007). Therefore, the development of tools and techniques to mitigate GHGs emission from soil is very important.
Biochar (activated carbon produced from pyrolysing biomass) application to soil has been shown to increase plant growth and ameliorate degraded soils (Zwieten et al. 2010). It usually adds pulses of P and K to the soil and increases soil CEC, pH, and aeration (Thomas and Gale 2015). While it has the potential to sequester carbon and improve soil properties, a few recent studies have found that biochar also has the potential to reduce GHGs emission from soil (Sackett et al. 2014).
Biochar addition generally reduces soil bulk density, increases pH, moisture, and enhances soil microbial activities(Zwieten et al. 2010); we, therefore, expect that addition of biochar will increase CH4 uptake. CO2 efflux related to biochar addition could mainly be affected by metabolic activity of roots, free-living and symbiotic heterotrophs, plant’s resource allocation responses, labile C, and soil moisture and temperature (Peng and Thomas 2010, Peng et al. 2007). Biochar generally promotes plant growth, leading to higher autotrophic and heterotrophic respiration, which brings on higher CO2 efflux. If addition of biochar, however, reduces fine root production, it will lead to a reduction in CO2 efflux. Since tropical forest soils are P and K – limited, we expect the plants to show high above ground growth responses to biochar addition and thus reduce CO2 efflux. In the tropics, rapid mineralization of the labile biochar C fraction by biotic and abiotic sources might increase short-term CO2 efflux. Biochar addition likely increases soil temperature, which might increase soil respiration, and consequently CO2 efflux. Similarly, biochar increases soil water holding capacity and with high moisture availability we might expect increased CO2 efflux from increased low roots and microbial activities. All these responses are highly dependent on diurnal and annual variability of rainfall, air temperature, and many other soil characteristics. There is remarkable confusion and lack of studies on biochar effects soil greenhouse gas fluxes across tropical systems, especially at the landscape level.
Bangladesh is considered as one of the most climate-change vulnerable countries. If biochar can reduce GHGs emission from soil while facilitating plant growth, it would be a great mitigation tool to fight climate change on the frontline.
To understand the mechanism and potential of biochar in reducing soil greenhouse gas emission (mainly CO2 and CH4), we will set up 20 x 20 m plots across a gradient of forest disturbances (i.e., primary and secondary forests, agroforestry, and agriculture) in and around Khadimnagar National Park, Bangladesh. Three replicates of control (no biochar) and 10t/ha biochar will be applied to the plots. In each plot 10 systematically placed 10cm radius permanent PVC collars will be installed to measure soil CO2 and CH4 fluxes using LGR Ultraportable Greenhouse Gas Analyzer. We hypothesized that biochar addition will increase CO2 efflux in short term and reduce in long term while increasing soil CH4 uptake.