Abstract
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.
Highlights
Subjects Agricultural Science, Environmental Sciences, Mycology, Plant Science Keywords Biochar, Mobile organic compounds, Phytotoxicity, Leaching, Thermal treatment, Volatile organic compounds
The negative and null responses of two common forage crop species, ryegrass and clover, to biochar additions were contrary to the overall trend of positive plant growth responses presented in recent meta-analyses (Jeffery et al, 2011; Liu et al, 2013; Biederman & Harpole, 2013; Thomas & Gale, 2015)
Our results strongly suggest that mobile organic compounds from biochar were responsible for this growth inhibition, as heating and leaching biochar before application alleviated this negative response, and addition of leachates alone replicated the negative responses observed
Summary
Subjects Agricultural Science, Environmental Sciences, Mycology, Plant Science Keywords Biochar, Mobile organic compounds, Phytotoxicity, Leaching, Thermal treatment, Volatile organic compounds. Some mobile organic compounds released from biochar in soils are hormetic, i.e. stimulating plant growth at low dosages but inhibiting growth at high dosages (Baldwin et al, 2006; Calabrese, Iavicoli & Calabrese, 2012). When diluted (103–107 times), wood vinegars commonly stimulate plant growth (Mu, Uehara & Furuno, 2003; Mu, Uehara & Furuno, 2004), but remain potent insecticides and fungicides (Yatagai et al, 2002; Velmurugan, Han & Lee, 2009). At higher concentrations these compounds can be herbicidal. To our knowledge phytotoxicity from mobile organic compounds released by biochar has not been tested in plant growth beyond the germination stage, or in plants grown in soil media other than pure sand
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