Biochar application to soil has attracted extensive attention worldwide due to its carbon (C) sequestration and fertility-enhancing properties. However, the lack of biochar accumulation in highly disturbed agroecosystems challenges the perceived long-term stability of biochars in soil. This 9-year field experiment was conducted in rice paddy fields to understand the temporal degradation of biochars produced from two contrasting feedstocks (rice straw vs. bamboo) at a high temperature (600 °C). Obvious physical alterations, surface oxidation, and transformation of condensed aromatic C occurred in biochars in the disturbed paddy field with frequent redox cycles. Increase in O/C atomic ratio, levels of high-temperature-sensitive degradable components, H/C ratio, and linear alkyl-C content were observed, which were indicative of time-dependent molecular changes and degradative transformation of biochars. Biochar degradation was characterized by the loss of labile C at an early stage and the degradation of aromatic C at a later stage. Based on the massive loss of C content in biochars (10.3–11.8%) and considerable degradation of aromatic C (5.0–8.7%) in 9 years, we argue that current biphasic C dynamic models probably overestimate the stability of biochars in agroecosystems such as rice paddy fields. Long-term field experiments (>5 years) are required to assess biochar's potential for C sequestration. This study provides long-term field data regarding the temporal changes in biochar physicochemical properties, which may facilitate the development of a robust assessment scheme on the long-term persistence of biochars in agroecosystems.
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