Distribution Of Black Carbon In Forest Soils Of Great Xing'an Mountains And Carbon Sequestration Potential Of The Soils

The forest soil organic carbon(SOC)storage is huge in Great Xing'an Mountains,and meanwhile the black carbon(BC)stock is also considerable on account of frequent fire disturbance.In this paper,the SOC and BC in the forest were quantified,the distribution of SOC and BC in different particle-size fractions were analyzed,the correlation between SOC and BC and the importance of BC in soil carbon pool were described,the soil carbon sequestration potential was estimated based on the mineral-bonded organic carbon pool as the soil carbon saturation capacity,and the soil carbon saturation theory was discussed.Soil particle-size fractionation was utilized with centrifugation method,and mineral-bonded organic carbon was analyzed with a physical method and a chemical one,too.BC was determined with the oxidation method using potassium dichromate.Results show that:(1)SOC expressed obviously accumulation in surface soil,surface SOC content and density accounting for about 70.6%and 58.9%in soil profile,respectively.By comparison,brown coniferous forest soil has the highest SOC content,however,its SOC density was the lowest because of the thin soil horizon.SOC density in grey forest soil was the biggest based on its large turnover of soil organic matter in subsoil and its deep soil depth.SOC proportion in different particle size fractions was in the order of silt>clay>fine sand>coarse sand.Although most SOC was localized in silt fraction,the correlation between SOC and silt content was non=significant.While,there was a remarkable relation between SOC and clay content,which illustrated that clay mineral provided effective chemical protection for SOC.Physical fractionation method overestimated mineral-bonded organic carbon,because it was found that not all the organic carbon in mineral-bonded matter were bonded with minerals.In general,SOC storage in soil profile was about 16.47 kg·m-2,of which mineral-bonded organic carbon stock accounting for around 45%.(2)BC content and density decreased with soil depth,while the ratio of BC to SOC showed an increasing trend.By comparison,the soil under semi-humid conditions would sequester more BC.BC proportion in different particle size fractions was in the order of clay>silt>fine sand>coarse sand.Although surface BC was mostly enclosed in clay and silt fractions,the correlation between BC and clay or silt content was not conspicuous,which elucidated that the biochemical recalcitrance was the main stabilization mechanism for surface BC.While with the soil depth increasing,soil mineral plays an important role in chemical protection of BC.The rising ratios of BC to SOC manifested that surface BC migrated into deep horizon vertically,and the climbing ratios was attributed to the preservation of BC in fine sand and silt fractions,which by reason of the interaction between bigger BC particles and smaller clay minerals.In mineral-bonded organic matter,the ratio of BC to organic carbon was about 24.9%,which indicated that BC is the essential component of soil stable carbon pool.Meanwhile,the significantly correlation between BC and SOC clarified that the importance of BC in soil organic carbon accumulation.In addition,the ratio of BC to organic carbon reached to 26.3%in particulate organic matter,which demonstrated that particulate organic carbon is not invariably labile as is commonly understood.(3)The surface soils were highly carbon saturated,reaching up around 95.8%in saturation,while carbon saturation declined with soil depth,down to 19.5%in bottom soil,which illustrated that the soil deep in profile could store a considerable amount of SOC.Based on calculation,carbon sequestration potential of the soil deep in profile could be 1.91 times than the current SOC storage,while the potential of surface soil accounted only for about 2.7%of current SOC stock.At present,the carbon saturation theories are all based on the silt and clay mineral and small aggregate protected organic carbon,however,this study revealed that the biochemically inert BC can also localize in labile particulate organic matter,which manifested that the role of particulate organic carbon in soil carbon saturation concept needs to be reconsidered.In view of the existence of BC,the forest soil carbon sequestration potential would be higher than the estimated one in Great Xing'an Mountains.