Study On Physicochemical Properties And Stability Of Biochar In Paddy Soil

Application of biochar in soil has attracted extensive attention worldwide because of its ability to sequester carbon?C?and improve soil fertility.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 high temperature.In this study,microstructure analysis technologies were used to explore long-term temporal changes in biochar physicochemical properties,which may assist in developing a robust assessment scheme on the long-term persistence of biochars in agroecosystems and its potential for C sequestration.The main conclusions are summarized as follows:?1?Scanning electron microscopy?SEM?,analyses of specific surface area,pore structure were used to characterize the physical structure of biochar.After 9 years of burial,the specific surface area of straw charcoal and bamboo charcoal increased by4.5 times and 1.4 times,and the average pore size was 0.61 times and 0.42 times of the original sample,respectively.?2?Fourier-transform infrared spectroscopy?FTIR?,¹³C nuclear magnetic resonance?NMR?,X-ray photoelectron spectroscopy?XPS?,elemental composition,and thermogravimetric?TG?parameters were used to characterize the chemical properties of biochar.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?increased by40.8%-79.9%?,levels of high-temperature-sensitive degradable components,H/C ratio?increased by 32.2%-57.5%?,and linear alkyl-C content?increased by 1.87 and1.12 times?were observed,which were indicative of time-dependent molecular changes and degradative transformation of biochars.Also,after 9 years of burial,the thermal decomposing components in straw charcoal and bamboo charcoal increased by 18%and 38%,respectively.?3?We identified stage-wise degradation of biochars in the long-term field experiment,biochar degradation was characterized by loss of labile C at the early stage and degradation of aromatic C at the 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,biochar degrades rapidly in episodically flooded wetland paddy agri-systems.We argue that the stability of biochars in agroecosystems such as rice paddy fields is probably overestimated by the current biphasic C dynamic models.