Acid-Precipitable Component of Alkali Extracts from Straw Exhibits Higher Chemical Activity than that from Straw-Derived Biochar

Abstract

Alkali extracts (AE) of straw (CS) and straw-derived biochar (Bc) serve as artificial humic substances (HS), demonstrating their potential for soil improvement. However, limited research exists on the differences between these extracts and soil-derived HS, and the distinctions between the extracts of CS and Bc, to inform the selection of precursor materials for artificial HS. In this study, AE were extracted from CS and Bc using the traditional alkaline extraction method. Their yields and compositions were compared in relation to soil HS. Their structural features were also analyzed subsequently, focusing on the acid-precipitable component (AP) with reference to humic acid (HA) using techniques such as elemental composition analysis, Fourier transform infrared spectroscopy, and 13C nuclear magnetic resonance spectroscopy among others. The results indicated that: (1) The AE yields from CS and Bc were 136 and 5.8 g kg−1, respectively, with the extraction rate from CS being 35 times higher than that of Bc. (2) In both CS and Bc, the AP accounted for 37–39% in AE, which were significantly lower than soil’s 62%. These components exhibited lower molecular condensation and hydrophobicity than soil HA. (3) The AP of CS had an H/C ratio of 1.132 and a hydrophobic carbon/hydrophilic carbon ratio of 0.176, while for Bc, these ratios were 1.083 and 7.043, respectively. Which indicates that AP of CS exhibited greater chemical activity. These findings offer valuable insights and foundational data for understanding artificial HS and selecting optimal materials to mitigate environmental problems caused by soil degradation.

Keywords: Straw, biochar, Alkali extracts, Acid-precipitable component, Structural features