The biochemical impact of biochar in soil environments

Abstract

Biochar, a product of thermochemical conversion of biomass, is a way to sequester carbon and mitigate climate change, improve soil agronomic properties and enhance crop production. However, such uses can only be valuable if the biochar does not negatively impact on normal soil microbially mediated processes that are important to soil health. The physical and biochemical characterization of biochar products is therefore important. One of the two central factors that affect the physicochemical properties of biochar is the production temperature (or highest temperature of treatment, HTT); the other being nature of the feedstock. A study of existing literature on biochar research reveals a lack of a systematic and rigorous approach focused on individual feedstock or HTT. It is to fill this gap that this study aimed to rigorously examine: the characteristics of biochar in a systematic way that focusses on a single feedstock source while varying the HTT from two different treatment facilities. It also aimed to assess the impact these biochars had on soil properties to which they (biochars) were added. The specific objectives were: to synthesize from the same feedstock six biochars, three from each of two different production processes (Batch and Continuous) over a range of pyrolysis temperatures and then subject the biochars to rigorous characterization; to investigate the influence of the synthesized biochars on a range of soil processes, and microbial diversities; to assess how the addition of the synthesized biochars to two soil types affects the physicochemical properties of the amended soil and influences plant growth. Sitka Spruce (Picea sitchensis) wood was pyrolysed at 400, 600 and 800oC. Experimental methods used included; titrimetric analyses, combined thermogravimetry – differential scanning calorimetry – quadrupole mass spectrometry, Fourier transform infrared spectroscopy and gas chromatography – mass spectrometry. In order to assess the impact of the biochars on the soil environments, a fully replicated and systematic plant growth trial was done. The post-harvest amended soils were then used to measure soil processes and also determine microbial community diversity against chosen controls. Results obtained from this study showed altered physicochemical properties of the biochars (increases in pH and total organic carbon; decreases in cation exchange capacity and water holding capacity), confirming the first hypothesis that biochemical and physical properties of the biochar are systematically altered with increasing HTT. However, there was very little difference between the properties of the biochars from the different production processes indicating that uniformity can likely be predicted based on HTT. Biochar addition to soil enhanced its basal respiration rate in the low pH soil but suppressed it in the near neutral soil, suppressed denitrification enzyme activity in the near neutral soil and these effects were to some extent affected by HTT. Biochar addition raised the total organic carbon content and lowered bulk density in both the acid and near- neutral soils and also increased the pH in the acid soil but not in the near neutral soil. The significant alteration of these soil properties was also influenced by changing the HTT. Biochar addition also influenced leek growth compared to the controls only in the acid soil. However, altering the biochar HTT had no significant effect on leek growth in both soils.