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Biochar'', or charcoal intended for use as a soil amendment, has received great attention in recent years as a means of enhancing carbon sequestration and soil properties in agricultural systems. Here we address the potential for biochar use in the context of forest restoration, reviewing relevant experimental studies on biochar use in forest ecosystems, the properties of chars generated from wood waste material, and available data on tree growth responses to biochar. To our knowledge the earliest mention of char use as a soil amendment is actually specifically in the context of forest restoration (in the 1820s in Scotland). Wood waste biochars have an unusual set of properties that suggest their applicability in a forest restoration context: namely, high recalcitrance promoting long-lasting effects, retention of cations, anions, and water, in the soil, sorptive properties that can reduce bioavailability of a wide range of toxic materials, and relative ease of production from locally available feedstocks. A meta-analysis of recent studies on biochar responses of woody plants indicates a potential for large tree growth responses to biochar additions, with a mean 41 % increase in biomass. Responses are especially pronounced at early growth stages, and appear to be higher in boreal and tropical than in temperate systems, and in angiosperms than conifers; however, there is high variability, and field studies are few. The properties of biochars also vary greatly depending on feedstock and pyrolysis conditions; while this complicates their use, it provides a means to design biochars for specific restoration situations and objectives. We conclude that there is great promise for biochar to play an important role in a wide variety of forest restoration efforts, specifically as a replacement product for other forms of organic matter and liming agents.

Recent meta-analyses of plant responses to biochar boast positive average effects of between 10 and 40%. Plant responses, however, vary greatly across systems, and null or negative biochar effects are increasingly reported. The mechanisms responsible for such responses remain unclear. In a glasshouse experiment we tested the effects of three forestry residue wood biochars, applied at five dosages (0, 5, 10, 20, and 50 t/ha) to a temperate forest drystic cambisol as direct surface applications and as complete soil mixes on the herbaceous pioneers Lolium multiflorum and Trifolium repens. Null and negative effects of biochar on growth were found in most cases. One potential cause for null and negative plant responses to biochar is plant exposure to mobile compounds produced during pyrolysis that leach or evolve following additions of biochars to soil. In a second glasshouse experiment we examined the effects of simple leaching and heating techniques to ameliorate potentially phytotoxic effects of volatile and leachable compounds released from biochar. We used Solid Phase Microextraction (SPME)–gas chromatography–mass spectrometry (GC-MS) to qualitatively describe organic compounds in both biochar (through headspace extraction), and in the water leachates (through direct injection). Convection heating and water leaching of biochar prior to application alleviated growth inhibition. Additionally, growth was inhibited when filtrate from water-leached biochar was applied following germination. SPME-GC-MS detected primarily short-chained carboxylic acids and phenolics in both the leachates and solid chars, with relatively high concentrations of several known phytotoxic compounds including acetic acid, butyric acid, 2,4-di-tert-butylphenol and benzoic acid. We speculate that variable plant responses to phytotoxic organic compounds leached from biochars may largely explain negative plant growth responses and also account for strongly species-specific patterns of plant responses to biochar amendments in short-term experiments.

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Abstract:The combination of biochar (BC) with compost has been suggested to be a promising strategy to promote plant growth and performance, but although "synergistic" effects have been stated to occur, full-factorial experiments are few, and explicit tests for synergism are lacking. We tested the hypothesis that a combination of BC and spent mushroom substrate (SMS) has a positive synergistic effect on plant growth and physiological performance in a nutrient-limited growing media. A greenhouse experiment with a full factorial design was conducted using mixed-wood BC (3.0 kg·m −2 ) and SMS (1.5 kg·m −2 ) (the combination was not co-composted) as organic soil amendments for the annual Abutilon theophrasti and the perennial Salix purpurea. Several measurements related to plant growth and physiological performance were taken throughout the experiment. Contrary to the hypothesis, we found that the combination of BC + SMS had neutral or antagonistic interactive effects on many plant growth traits. Antagonistic effects were found on maximum leaf area, above-and belowground biomass, reproductive allocation, maximum plant height, chlorophyll fluorescence, and stomatal conductance of A. theophrasti. The effect on S. purpurea was mostly neutral. We conclude that the generalization that BC and compost have synergistic effects on plant performance is not supported.