Quality and quantity of humic-like and fulvic-like acids entrapped in biochars – The effect of various forestry feedstock and pyrolysis temperature of biochars

Abstract

The objective of the study was to analyze the effect of the pyrolysis temperature (PT) and feedstock on the quality and quantity of humic-like (HLAs) and fulvic-like (FLAs) acids entrapped in the biochars (BioCs) produced from leaves, litter, and straw of forest habitat at different temperatures. Extraction methods, 3DEEM fluorescence and UV–vis spectroscopies enabled to track the changes of HLAs and FLAs concentration, humification degree, lignin type compounds, biological index, aromaticity/aliphaticity, molecular weight and quality of fluorophores. Additionally, the feedstock and PT effect was discussed for the properties of the BioCs and the water extractable organic matter (WEOM). The results showed that the PT increase caused increase in the pH, ash, surface area and negative charge of the BioCs. Woody feedstock resulted in the BioCs with lower ash content, while the BioCs derived from straw revealed low content of organic matter and carbon but high surface area. The feedstock effect was not clear on structural parameters of HLAs and FLAs, however the BioC derived from litter at 750 °C placed HLAs in group of the strong humification. Woody parts in feedstock and increasing PT was the reason of lower humic-like substances content. It was relatively high after pyrolysis at 430 °C and decreased rapidly at higher PTs. The PT increase affected structure of organic fractions. For HLAs, thermal transformation lead to increase of the humification, molecular weight and content of aromatic structures. Changes for FLAs revealed opposite trend due to a greater susceptibility to the pyrolytic partitioning of molecules. PT elevation resulted also in decreasing contribution of protein- and aminoacid- like compounds and degradation of structures derived from lignin-rich feedstocks. These changes were accompanied by progressive decrease in the biological index of HLAs revealing gradual destruction of freshly produced compounds of microbiological origin. WEOM was enriched in organic compounds typical for HLAs fraction, and PT effect was similar to this predominant fraction.