Changes of labile, stable and water-soluble fractions of biochar after two years in a vineyard soil

Abstract

The long-term stability of biochar in soil is a key factor for carbon storage and agro-environmental benefits in soil applications. Therefore, to better use biochar as both a sustainable soil organic amendment and a carbon sequestration technique, it is necessary to understand the biochar aging process in soil, as biochar characteristics can be altered by a variety of processes that occur in the environment. Reliable chemical characterization can be obtained through thermoanalytical techniques, which are known to provide a trustworthy estimation of the biochar stability in soil. In the present study, the changes of labile and stable fractions of biochar after two years of application in a vineyard field were quantified before and after the amendment. Aged biochar was manually isolated from the soil-biochar mixture, and its chemical properties were measured by elemental analysis, thermogravimetric analysis (TGA) and analytical pyrolysis (Py-GC-MS). Dissolved organic matter (DOM) was analyzed by elemental and UV-vis spectroscopy analysis. A slight decrease of carbon content (−13 ± 5%) after two years was associated with a significant drop of TGA labile (from 26 to 9%) and water-soluble carbon (from 3.1 to 0.8 mg g⁻¹), concomitantly, the pyrolytic indices of aromaticity (e.g. toluene/naphthalene ratio) decreased. These results contrasted with the increase in the H/C and O/C ratios as stability indexes. The TGA stable fraction remained almost unaltered (49 vs. 52%) confirming the consistency of this index for recalcitrance. The results from Py-GC-MS and TGA techniques are consistent in highlighting significant changes in the biochar labile fraction due to the aging process. Instead, no changes were found in the stable fraction between the aged biochar and the fresh one, with important consequences on carbon cycling and soil sequestration.