Getting to the root of the matter: Water-soluble and volatile components in thermally-treated biosolids and biochar differentially regulate maize (Zea mays) seedling growth

Abstract

The use of thermally treated biomass, including biochar, as soil amendments can improve soil fertility by providing nutrients, stable C and improving soil water-holding capacity. However, if the degree of carbonization is low, these soil amendments can lower crop productivity as a result of high salinity or organic compounds. The overall effect of these soil amendments is mediated by complex relationships between production conditions, soil properties and environmental conditions. This study aimed to 1) characterize the physiochemical properties and organic compounds released by three soil amendments (softwood biochar or pyrogenic carbonaceous biosolids), 2) determine the effects of these amendments on maize (Zea mays) seedling productivity, and 3) relate properties of these amendments to effects on maize seedling productivity under controlled environment conditions. Physicochemical properties and mobile organic compounds (water-soluble and volatile organic compounds were determined. The amendments were tested in maize germination and greenhouse experiments. Chemical fingerprinting of volatile and water-soluble compounds revealed over 100 mobile organic species. Increasing treatment temperature from 270 to 320°C reduces phytotoxicity of pyrogenic carbonaceous biosolids soil amendments. Water-soluble components of pyrogenic carbonaceous biosolids produced at 270°C (inorganic N, Na and/or organic compounds) were associated with reduced maize seedling productivity. Volatile components of pyrogenic carbonaceous biosolids produced at 320°C were associated with improved maize seedling productivity; nitrogen uptake was increased in spite of smaller root systems as a result of increased mineralization of soil or amendment N and/or uptake of organic N compounds. These results suggest that pyrogenic carbonaceous biosolids have potential benefits to provide plant nutrients when the amount of organic and inorganic species are limited during early growth stages, under greenhouse conditions. Future studies should examine these effects under field conditions to confirm whether controlled environment results translate into effects on yield.

Biochar Volatile Matter and Feedstock Effects on Soil Nitrogen Mineralization and Soil Fungal Colonization

“…Guerena et al [69] found that the removal of VM with an acetone extraction in low-temperature biochar resulted in an increase in plant growth, which was attributed to phytotoxicity effects of the chemical constituents in the VM rather than bioavailable C effects on N availability. There are other examples of chemicals in VM that can have toxic effects on soil microbes [70] and plant growth [71].…”

Comparative Assessment of Biochar Stability Using Multiple Indicators

“…A similar trend was noted in another report [2] when the temperature was increased to 750 • C, resulting in the loss of aromatic groups due to the high-temperature. As an organic amendment to improve soil nutrition, the use of the low-temperature (400 • C) biochars having easily-biodegradable carbons would be favorable for plant growth by stimulating soil microorganisms [19,20]. On the other hand, biochars pyrolyzed at 600 • C and enriched with recalcitrant chemical composition would be more suitable not only for carbon sequestration via the use in composting or soil amendment but also for N 2 O reduction during composting [2].…”

Pyrolytic biochars from sunflower seed shells, peanut shells and Spirulina algae: their potential as soil amendment and natural growth regulators

“…Moreover, shoot growth was promoted in all treatments, and biochars with greater WSOCs content (350-500 °C) exhibited longer shoots than those obtained at higher temperatures (550-650 °C), suggesting that WSOCs could be involved in the enhancement of plant growth. Backer et al [9] also showed that VOCs and WSOCs from biosolids (270 °C-10 min, 320 °C-20 min) and softwood chips (500 °C, slow pyrolysis) did not affect maize seed germination and shoot and root lengths after a 4-d incubation period, and even a stimulatory effect on shoot length was observed by the VOCs emitted by one of the biosolids (320 °C).…”

“…The presence of volatile organic compounds (VOCs) and water-soluble organic compounds (WSOCs) in biochars has been associated with both positive and negative biological effects (including plants, microorganisms and aquatic organisms) [9,14,64,69]. For instance, Deenik et al [20] reported reduced germination of radish and corn seeds when subjected to high volatile matter charcoal extracts of macadamia nut shell (430 °C).…”

“…Volatile organic compounds (VOCs) and particularly other potentially toxic elements and small organic compounds can be of further importance, since most of them remain in the solids during biochar production [14,79] and could be easily leached when biochar is incorporated to the soil due to precipitation, or even by the presence of the water vapour-saturated atmosphere that is often present in the soil matrix [11,29,39,81]. Studies on the effects of these leachates in bioassays have yielded mixed results and have been shown to depend on feedstocks, the thermo-conversion processes involved and production parameters [3,9,18,20,50,51]. Alburquerque et al [4] found that biochar water extracts (10% w/v) from five lignocellulosic agricultural and forest wastes increased seed germination of sunflower relative to the controls, with calculated germination indexes above 60%, typical of non-phytotoxic materials.…”