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

Abstract

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.

Biochar Phosphorus Release Is Limited by High pH and Excess Calcium

“…Therefore, 1 M NH 4 NO 3 –extractable P concentrations and total Ca concentration were determined in 10 different biochars produced at 550°C to investigate the relationship between the parameters. Details of the biochars, analytical procedures, and the results have previously been reported (Buss et al, 2016a,b). The following feedstocks were used: demolition wood, Arundo donax L., sugarcane ( Saccharum spp.)…”

“…An exponential curve of type f = ae − bx was fitted to previously published data for total Ca concentrations in 550°C biochars (Buss et al, 2016b) and percentage P availability based on NH 4 NO 3 extractions (Buss et al, 2016a) using the least‐squares method. All statistical analyses were conducted using SigmaPlot 13 (Systat Software, 2014) and a significance level of p < 0.05.…”

“…However, it remains uncertain how the interactive effect of soil solution composition and pH affects P release from biochar. Furthermore, the percentage P extracted from different biochars varies significantly and is strongly dependent on the type of feedstock used for pyrolysis (Buss et al, 2016a; Shepherd et al, 2017). So far it is unclear which biochar characteristics influence the P release.…”

Toxicity screening of biochar-mineral composites using germination tests

“…1). It is unlikely that heavy metals caused toxicity directly 216 although they will have contributed to the high osmotic potential (Buss et al, 2016b). 217 Salinity / K concentration and pH, however, does not explain the low germination rate 218 caused by addition of CLO and sewage sludge.…”

A case study: what is leached from mallee biochars as a function of pH?

“…Some biochars are known to sorb organic (Buss et al 2016; Mohan et al 2014) and inorganic molecules (Silber et al 2010; Velghe et al 2012), retaining nutrients against leaching losses. These are all important factors in improving crop productivity and soil fertility.…”