Phytotoxicity and hormesis in common mobile organic compounds in leachates of wood-derived biochars.

Although addition of pyrolyzed organic materials (biochars) to soil generally results in increased growth and physiological performance of plants, neutral and negative responses have also commonly been detected. Toxicity of organic compounds generated during pyrolysis, sorbed by biochars, and then released into the soil solution, has been implicated as a possible mechanism for such negative effects. Conversely, water-soluble biochar constituents have also been suggested to have "hormetic" effects (positive effects on plants at low concentrations); however, no specific compounds responsible have been identified. We investigated the relative phytotoxicity-and possible hormetic effects-of 14 organic compounds common in aqueous extracts of freshly produced lignocellulosic biochars, using seed germination bioassays. Of the compounds examined, volatile fatty acids (VFAs: acetic, propionic, butyric, valeric, caproic, and 2-ethylbutyric acids) and phenol, showed acute phytotoxicity, with germination-based ED50 values of 1-30 mmol L-1, and 2-ethylbutyric acid showed ED50 values of 0.1-1.0 mmol L-1. Other compounds (benzene, benzoic acid, butanone, methyl salicylate, toluene, and 2,4-di-tert-butylphenol) showed toxic effects only at high concentrations close to solubility limits. Although phytotoxic at high concentrations, valeric and caproic acid also showed detectable hormetic effects on seedlings, increasing radicle extension by 5-15% at concentrations of ~ 0.01-0.1 mmol L-1. These data support the hypothesis that VFAs are the main agents responsible for phytotoxic effects of lignocellulosic biochar leachates, but that certain VFAs also have hormetic effects at low concentrations and may contribute to positive effects of biochar leachates on early plant development in some cases. Supplementary Information: The online version contains supplementary material available at 10.1007/s42773-024-00339-w.

Biochar mitigates negative effects of salt additions on two herbaceous plant species.

Addition of pyrolyzed biomass ("biochar") to soils has commonly been shown to increase crop yields and alleviate plant stresses associated with drought and exposure to toxic materials. Here we investigate the ability of biochar (at two dosages: 5 and 50 t ha(-1)) to mitigate salt-induced stress, simulating road salt additions in a factorial glasshouse experiment involving the broadleaved herbaceous plants Abutilon theophrasti and Prunella vulgaris. Salt additions of 30 g m(-2) NaCl to unamended soils resulted in high mortality rates for both species. Biochar (Fagus grandifolia sawdust pyrolyzed at 378 °C), when applied at 50 t ha(-1) as a top dressing, completely alleviated salt-induced mortality in A. theophrasti and prolonged survival of P. vulgaris. Surviving A. theophrasti plants that received both 50 t ha(-1) biochar and salt addition treatments showed growth rates and physiological performance similar to plants without salt addition. Biochar treatments alone also substantially increased biomass of P. vulgaris, with a ∼50% increase relative to untreated controls at both biochar dosages. Biochar did not significantly affect photosynthetic carbon gain (Amax), water use efficiency, or chlorophyll fluorescence (Fv/Fm) in either species. Our results indicate that biochar can ameliorate salt stress effects on plants through salt sorption, suggesting novel applications of biochar to mitigate effects of salinization in agricultural, urban, and contaminated soils.

Aquaponic and Hydroponic Solutions Modulate NaCl-Induced Stress in Drug-Type Cannabis sativa L.

The effects of salt-induced stress in drug-type Cannabis sativa L. (C. sativa), a crop with increasing global importance, are almost entirely unknown. In an indoor controlled factorial experiment involving a type-II chemovar (i.e., one which produces Δ9-tetrahydrocannabinolic acid ~THCA and cannabidiolic acid ~ CBDA), the effects of increasing NaCl concentrations (1-40 mM) was tested in hydroponic and aquaponic solutions during the flowering stage. Growth parameters (height, canopy volume), plant physiology (chlorophyll content, leaf-gas exchange, chlorophyll fluorescence, and water use efficiency), and solution physicochemical properties (pH, EC, and nutrients) was measured throughout the experiment. Upon maturation of inflorescences, plants were harvested and yield (dry inflorescence biomass) and inflorescence potency (mass-based concentration of cannabinoids) was determined. It was found that cannabinoids decreased linearly with increasing NaCl concentration: -0.026 and -0.037% THCA·mM NaCl-1 for aquaponic and hydroponic solutions, respectively. The growth and physiological responses to NaCl in hydroponic-but not the aquaponic solution-became negatively affected at 40 mM. The mechanisms of aquaponic solution which allow this potential enhanced NaCl tolerance is worthy of future investigation. Commercial cultivation involving the use of hydroponic solution should carefully monitor NaCl concentrations, so that they do not exceed the phytotoxic concentration of 40 mM found here; and are aware that NaCl in excess of 5 mM may decrease yield and potency. Additional research investigating cultivar- and rootzone-specific responses to salt-induced stress is needed.

Dose-dependence of growth and ecophysiological responses of plants to biochar.

Charcoal is a ubiquitous legacy of wildfire in terrestrial systems that often contributes to rapid revegetation following disturbance; the use of charcoal soil amendments, or "biochars", to promote plant growth has received recent research attention and increasing applied use. Despite its widespread use, well-resolved quantitative estimates of dose-response relationships for biochar effects on plant growth are nonexistent, and studies of biochar dosage effects on plant ecophysiology are minimal. We investigated the effects of biochar dosage on plant growth and ecophysiology in a glasshouse experiment involving two common early-successional plants, Abutilon theophrasti and Trifolium repens. Plants were grown in disturbed temperate soils with increasing dosages of wood biochars: 0, 2, 4, 6, 8, 10, 20, 30, 40, 50 t/ha. We measured leaf-level gas-exchange traits (Amax, gs, WUE), chlorophyll concentration, and leaf area growth throughout the experiment. At the end of the experiment, we measured biomass, foliar nutrition, and soil properties (pH, EC, C and N). Responses of biomass and physiological traits were highly dose-dependent, followed primarily unimodal forms, and differed in some traits between species. Increases in the uptake of K, P, and Mg, were responsible for accelerated growth. Biochars also generally increased the concentration of micronutrients, especially B. As a result, nutrient stoichiometry shifted substantially: in A. theophrasti, biochars increased C:N, P:N, and K:N ratios, suggesting nitrogen dilution or induced deficiency at higher dosages. This work supports the general hypothesis that ecophysiological responses to biochar are dose-dependent and driven mainly by changes in nutrient availability. Additional work is necessary to understand the broader ecological impacts of heterogeneity in soil pyrogenic C levels to succession and ecosystem function.

Thermal treatment and leaching of biochar alleviates plant growth inhibition from mobile organic compounds.

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.