Composting of Biochars Improves Their Sorption Properties, Retains Nutrients During Composting and Affects Greenhouse Gas Emissions After Soil Application.

Biochar application to soils has been suggested to elevate nutrient sorption, improve soil fertility and reduce net greenhouse gas (GHG) emissions. We examined the impact of composting biochar together with a biologically active substrate (i.e., livestock manure-straw mixture). We hypothesized that composting would alter the biochar’s surface chemistry, sorb nutrients from the compost, and impact the resulting GHG production after its application to soil. We studied the sorption of copper (Cu (II)) to two biochars (a charcoal and a gasification coke, both derived from wood) in order to relate sorption to char properties before and after a composting period of 84 days. Furthermore, we quantified changes in nutrient contents after 175 days of composting and the resulting net GHG production of soil-biochar mixtures in incubation experiments. Composting increased the biochar´s sorption affinity as well as its capacity for Cu(II). Differences in the strength of sorption were observed between the two biochars: Cu⁺² sorption to gasification coke was largely irreversible, whereas sorption to charcoal showed higher reversibility. During composting the biochars sorbed significant amounts of organic matter and nutrients. We also observed elevated amounts of soluble nitrogen (inorganic and organic), soluble organic carbon, plant available phosphorus and plant available potassium for both types of biochar following composting. Soils incubated together with composted biochar showed: i) reduced net methane oxidation, ii) no effects on CO₂ emissions, and iii) increased nitrous oxide (N₂O) emissions. Abundance of benzene polycarboxylic acids (BPCA) as molecular markers for the biochar’s aromatic character was not altered by composting. Composted biochar can be utilized as a remediation tool to reduce toxic Cu(II) concentrations in contaminated soils. The strength of Cu(II) sorption will depend on the particular biochar’s properties. Furthermore, using biochar as a bulking agent for composting could reduce the leaching of organic matter and nutrients, as these components are sorbed to the biochar in a plant available form. Composting of biochar could therefore improve its properties as a soil fertilizer and conditioner. However soils treated with composted biochars revealed, as a consequence of a higher N availability, increased N₂O emissions. This connection may have an adverse effect on an overall GHG balance.