Wolfram Buss

Risks and benefits of marginal biomass-derived biochars for plant growth

In this study, 19 biochars from marginal biomass, representing all major biomass groups (woody materials, grass, an aquatic plant, anthropogenic wastes) were investigated regarding their content of available potentially toxic elements (PTEs) and nutrients (determined by NH4NO3-extractions) and their effects on cress (Lepidium sativum) seedling growth. The objective was to assess the potential and actual effects of biochar with increased PTE content on plant growth in the context of use in soil amendments and growing media. It showed that the percentage of available PTEs was highest for biochars produced at the highest treatment temperature (HTT) of 750°C. On average, however, for all 19 biochars, the percentage availability of Cu, Cr, Ni and Zn (<1.5% for all) was similar to the percentage availability reported in the literature for the same elements in soils at similar pH values which is a highly important finding. Most biochars exceeded German soil threshold values for NH4NO3-extractable PTEs, such as Zn (by up to 25-fold), As and Cd. Despite this, cress seedling growth tests with 5% biochar in sand did not show any correlations between inhibitory effects (observed in 5 of the 19 biochars) and the available PTE concentrations. Instead, the available K concentration and biochar pH were highly significantly, negatively correlated with seedling growth (K: p<0.001, pH: p=0.004). K had the highest available concentration of all elements and the highest percentage availability (47.7±19.7% of the total K was available). Consequently, available K contributed most to the osmotic pressure and high pH which negatively affected the seedlings. Although a potential risk if some of these marginal biomass-derived biochar were applied at high concentrations, e.g. 5% (>100tha(-1)), when applied at agriculturally realistic application rates (1-10tha(-1)), the resulting smaller increases in pH and available K concentration may actually be beneficial for plant growth.

Toxicity screening of biochar-mineral composites using germination tests

This study assessed the properties and toxicity (water cress germination trials) of 38 waste-derived, novel biochar-mineral composites (BMCs) produced via slow pyrolysis and hydrothermal carbonization (hydrochars). The biochars were produced from sewage sludge and compost-like output (CLO) by varying the type of mineral additive (zeolite, wood ash and lignite fly ash), the mineral-to-feedstock ratio and the carbonization process. While pure hydrochars completely inhibited germination of water cress, this effect was ameliorated by mineral additives. Seedlings grew best in pyrolysis chars and while wood ash addition decreased plant growth in many cases, 1:10 addition to CLO doubled germination rate. The factors responsible for the phytotoxicity can be attributed to pH, salinity and organic contaminants. Importantly, while pure minerals inhibited germination, conversion of minerals into BMCs reduced their inhibitory effects due to buffered release of minerals. Overall, mineral wastes (e.g., combustion ashes) and waste biomass can be used safely as sources of nutrients and stable organic carbon (for soil carbon sequestration) when converted into specific biochar-mineral composites, exploiting synergies between the constituents to deliver superior performance.

Biochar Phosphorus Release Is Limited by High pH and Excess Calcium

Aside from its use for improving soil properties, biochar is increasingly promoted as a direct nutrient provider for sustainable recycling of waste materials. However, incomplete understanding of the interacting factors that determine P release from biochar may limit the efficiency of P recycling from biochar to soil. In particular, the contrasting pH of biochar and soil need to be considered. In this study, soil-free biochar (rice [ L.] husk, 700°C) extractions were performed under different pH (4.6-9.9) and extractant conditions to test how solution composition affects biochar P release. When solution pH was in the range of 7.6 to 8.6 and excess Ca was present in the solution or in biochar, P release was low-only 1 to 7% of the total P was released compared with ∼20% under most other conditions. Importantly, we demonstrate that biochar total Ca concentration is closely related to P availability ( = 0.76) and could be used to predict biochar P release. The results suggest that for maximum P release, low Ca concentrations in biochar and (soil) solution are needed and/or a pH <7.5 at the soil-biochar interface. This novel understanding will help engineer sustainable biochar fertilizers optimized for P provision.

Standard Biochar Materials

T editorial “Sifting Through the Embers” is topical and nicely identifies some key needs in biochar research. The common issue of insufficient information on biochar production and characteristics to allow for replication of research or to benchmark findings of other researchers is also clearly highlighted. The development of standardized reference materials is put forward as a first step toward addressing this important matter. The UK Biochar Research Centre (UKBRC) at the University of Edinburgh embarked on the development of standard biochar materials in 2012. The set of 12 standard biochar materials (Edinburgh Standard Biochar set) was released to the global research community in 2015 (see Figure 1). These are freely available and are the product of several