A Systematic Review of Biochar Research, with a Focus on Its Stability in situ and Its Promise as a Climate Mitigation Strategy

Abstract

BackgroundClaims about the environmental benefits of charring biomass and applying the resulting “biochar” to soil are impressive. If true, they could influence land management worldwide. Alleged benefits include increased crop yields, soil fertility, and water-holding capacity; the most widely discussed idea is that applying biochar to soil will mitigate climate change. This claim rests on the assumption that biochar persists for hundreds or thousands of years, thus storing carbon that would otherwise decompose. We conducted a systematic review to quantify research effort directed toward ten aspects of biochar and closely evaluated the literature concerning biochar's stability.FindingsWe identified 311 peer-reviewed research articles published through 2011. We found very few field studies that addressed biochar's influence on several ecosystem processes: one on soil nutrient loss, one on soil contaminants, six concerning non-CO2 greenhouse gas (GHG) fluxes (some of which fail to support claims that biochar decreases non-CO2 GHG fluxes), and 16–19 on plants and soil properties. Of 74 studies related to biochar stability, transport or fate in soil, only seven estimated biochar decomposition rates in situ, with mean residence times ranging from 8 to almost 4,000 years.ConclusionsOur review shows there are not enough data to draw conclusions about how biochar production and application affect whole-system GHG budgets. Wide-ranging estimates of a key variable, biochar stability in situ, likely result from diverse environmental conditions, feedstocks, and study designs. There are even fewer data about the extent to which biochar stimulates decomposition of soil organic matter or affects non-CO2 GHG emissions. Identifying conditions where biochar amendments yield favorable GHG budgets requires a systematic field research program. Finally, evaluating biochar's suitability as a climate mitigation strategy requires comparing its effects with alternative uses of biomass and considering GHG budgets over both long and short time scales.

Substitution of peat moss with softwood biochar for soil-free marigold growth

“…Assuming a conservative aerobic decomposition rate for peat in substrates of 5% per annum (Cleary et al, 2005), within one century of mining and use in soil-free substrates 95% of mined peat would be expected to revert from a C sink to source (CO 2 ). In contrast, high-temperature BCs are thought to generally exhibit lower decomposition rates than undecomposed or humified biomass such as compost and peat (Woolf et al, 2010) and exhibit centennial to millennial residence times (Gurwick et al, 2013). The molar O:C = 0.36 for the BC in this study corresponds to a half-life of 100-1000 years (Spokas, 2010), suggesting that one century after production and use in soil-free substrates, at least 50% of C in the softwood BC in this study would be converted to CO 2 .…”

Pyrolysis Dynamics of Biomass Residues in Hot-Stage

“…In spite of the fact that the consequences of adding biochar to soils are still not fully understood (Spokas et al 2012;Gurwick et al 2013), this soil amender is expected to improve the productivity and environmental quality of agroecosystems similar to Amazonian Dark Earths (Novotny et al 2009). Research has demonstrated that biochars play a significant role in mitigating soil greenhouse gas emissions (Case et al 2012;Zhang et al 2013;Nelissen et al 2014), while others studies show the opposite (Troy et al 2013) or neutral effects (Mukome and Parik 2013;Suddick and Six 2013).…”

Improving Farming Practices for Sustainable Soil Use in the Humid Tropics and Rainforest Ecosystem Health

“…Large-scale application of biochar in agriculture has been proposed as a promising technology for long-term C-sequestration and underground storage [104,105]. At the same time, much research and many (though not all) experimental applications of charcoal have demonstrated positive effects of black carbon on soil physical properties, including pore space; bulk density; water retention and infiltration [102,106,107]; soil chemical characteristics, such as a long-term increase in the cation exchange capacity [108,109], nutrient retention [110], improved P availability [111,112] (but frequently reduced N-availability [113], and reduced methane and nitrous oxide emissions [114][115][116] (but see [117,118]). Biochar applications have also been reported beneficial for soil biology, such as increases in mycorrhizal fungi [119] and in biological N 2 -fixation [120] (but see [121] and increases in overall diversity of the microbiota [103,122,123], despite the possible toxic nature of some biochar compounds [124].…”