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.
Highlights
In the face of the dual challenge to mitigate global climate change and ensure food security for a growing global population, crop management strategies that build soil organic matter (SOM) have received considerable attention [1,2,3,4]
Given the emphasis that biochar proponents have placed on biochar stability in soil, we focused this review on that aspect of biochar, but reported benefits of biochar include increased soil fertility and water holding capacity, increased crop production, and remediation of contaminated soils
N We lack the field studies that are needed to understand with confidence how biochar production and application affects whole-system greenhouse gas (GHG) balance
Summary
In the face of the dual challenge to mitigate global climate change and ensure food security for a growing global population, crop management strategies that build soil organic matter (SOM) have received considerable attention [1,2,3,4]. Claims 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, 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
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