Sustainable Restoration of Soil Functionality in PTE-Affected Environments: Biochar Impact on Soil Chemistry, Microbiology, Biochemistry, and Plant Growth

Abstract

Biochar can be useful for the functional recovery of soils contaminated with potentially toxic elements (PTEs), even if its effectiveness is variable and sometimes limited, and conflicting results have been recently reported. To shed some light on this regard, softwood-derived biochar was added at 2.5 (2.5-Bio) and 5.0% w/w (5.0-Bio) rates to an acidic (pH 5.74) soil contaminated by Cd (28 mg kg−1), Pb (10,625 mg kg−1), and Zn (3407 mg kg−1). Biochar addition increased soil pH, available P and CEC, and reduced labile Cd, Pb, and Zn (e.g., by 27, 37, and 46% in 5.0-Bio vs. the unamended soil). The addition of biochar did not change the number of total heterotrophic bacteria, actinomycetes, and fungi, while it reduced the number of Pseudomonas spp. and soil microbial biomass. Dehydrogenase activity was reduced in amended soils (e.g., by ~60 and 75% in 2.5- and 5.0-Bio, respectively), while in the same soils, urease increased by 48 and 78%. Approximately 16S rRNA gene amplicon sequencing and the Biolog community-level physiological profile highlighted a significant biochar impact (especially at a 5% rate) on soil bacterial diversity. Tomato (but not triticale) yield increased in the amended soils, especially in 2.5-Bio. This biochar rate was also the most effective at reducing Cd and Pb concentrations in shoots. Overall, these results demonstrate that 2.5% (but not 5.0%) biochar can be useful to restore the soil chemical fertility of PTE-polluted soils with limited (or null) impact on soil microbial and biochemical parameters.

Mixing Compost and Biochar Can Enhance the Chemical and Biological Recovery of Soils Contaminated by Potentially Toxic Elements

“…The effect of biochar on DHG activity of PTE-contaminated soils is controversial: while some researchers (e.g., [ 3 , 23 ]) observed an increase in DHG following the addition of biochar, others, such as Kaurin et al [ 41 ] and Ali et al [ 42 ], reported its reduction. In our study, the decrease in DHG in biochar-treated soils could be due to a reduced microbial population, which in turn could be due to a decrease in available nutrients (i.e., DOC, exchangeable K and Ca) strongly retained by the biochar [ 7 , 41 , 42 , 43 , 44 ]. On the contrary, the higher content of available nutrients in compost-treated soils (i.e., DOC, available P, total N, and exchangeable K, Ca, and Mg) might be responsible for the increased microbial number, which resulted in higher DHG as previously reported [ 2 , 14 , 41 , 45 , 46 ].…”