Characteristics Of Organic Carbon Compositions And Their Carbon Isotopes In Soils In Central-south Hunan Province,China

The soil organic carbon?SOC?in the earth's surface constructs is the most active carbon pool,which is closely relative to the global climate change.The study on different SOC composition and fractions,and their carbon isotopic compositions in soils under different parent soil-forming rocks and different soil land-use conditions is of great significance and importance to further understand the stability of SOC distribution in soils,and to make out the possible controlling factors on such SOC distribution.In this study,soil samples were collected from forest,paddy field,and upland in black shale,granite,and basalt distribution regions in central-south of Hunan Province.An optimized H₂SO₄ sequential hydrolysis procedure was used to extracted labile fraction I?LOC I?,labile fraction II?LOC II?,recalcitrant organic carbon?ROC?;and the stable carbon isotope technique was used to measure their carbon isotope.Based on the distribution characteristic of different SOC fractions and their carbon isotope in topsoil?0?20 cm?and subsoil?20?70 cm?under different parent materials and land uses,we further analyzed the effects of parent materials and land uses on each SOC fraction.The main findings and conclusions are as follows:The contents of total SOC in soils derived from granite were highest among different parent materials.The contents of total SOC in forest,paddy fields,and upland soils derived from granite were 4.25?25.9 g kg^–1(the mean value is 11.2 g kg^–1),7.03?24.2 g kg^–-1(the mean value is 14.8 g kg^–1),5.30?17.1 g kg^–1(the mean value is 11.4 g kg^–1),respectively.The contents of total SOC were generally decreased with depth under different parent materials and land uses.The contents of total SOC in paddy soils were highest among different land uses.The results showed that the soil organic carbon content was affected by soil parent material,land uses and soil depth.The storages of total SOC were calculated according to soil organic carbon content,bulk density,and soil thickness.The results showed that the total SOC storage in soils derived from granite was highest among different parent materials.And the total SOC storages in topsoil and subsoil derived from granite were 4.61 kg m^–2 and 5.14 kg m^–2,respectively.But the total SOC storage in paddy soil was the highest among different land uses.And the total SOC storages of topsoil and subsoil in paddy field were 4.64 kg m^–2 and 6.19 kg m^–2,respectively.These findings indicated that subsoil also plays an important role in the storage of soil organic carbon,and there is an obvious correlation between total organic carbon storage and its content.In studied regions,the contents of LOC I,LOC II,and ROC were significantly correlated with total SOC contents?R²=0.744?0.982,P<0.05?,indicating that the contents of each SOC fraction were determined by the contents of total SOC.The LOC I,LOC II,and ROC in soils derived from black shales,granite,basalt accounted for 29.9?47.1%?3.76?12.0%?45.0?68.0%in total SOC,respectively.The higher proportion of ROC means the more stability of SOC,implying the SOC is harder to be decomposed.The distribution of LOC I,LOC II,and ROC under different parent materials and land uses showed non-significant differences between topsoil and subsoil.The results showed that the LOC I,LOC II,and ROC storage in soils derived from granite was highest among different parent materials,and the LOC I,LOC II,and ROC storages in topsoil derived from granite were 1.53 kg m^–2,0.27 kg m^–2,and 2.49 kg m^–2,respectively;in subsoil were 1.98 kg m^–2,0.28 kg m^–2,and 2.65 kg m^–2,respectively.But the LOC I,LOC II,and ROC storage in paddy soil was the highest among different land uses,and the LOC I,LOC II,and ROC storages of topsoil in paddy field were 1.43 kg m^–2,0.25 kg m^–2,and 2.67 kg m^–2,respectively;in subsoil were1.98 kg m^–2,0.32 kg m^–2,and 3.57 kg m^–2,respectively.In general,the storages of each SOC fraction ordered as following:ROC>LOC I>LOC II.This means that the recalcitrant fraction is the main carbon pool in SOC pool under different parent materials and land uses.Furthermore,in topsoil,the storages of LOC I and LOC II were significantly affected by parent materials and the storages of ROC were significantly affected by parent materials and land uses.While,in subsoil,the storages of each SOC fraction were mainly controlled by land uses.In general,the ?¹³C values of LOC,ROC and total SOC under different parent materials and land uses ordered as following:LOC>total SOC>ROC.This might be caused by the differences in composited materials and stabilities between LOC and ROC.The results indicated that the?¹³C values of LOC,ROC and total SOC had a similar variation tendency in the soil profiles.Except for the soil profiles of forest derived from basalt,the?¹³C values of LOC,ROC and total SOC increased with increasing soil depth.The ? value,which is the slope of the fitting curve for the logarithmic value of total SOC content and the?¹³C value of total SOC,can be used to reflect the decomposition rate of soil organic matter.The lower the?value means the higher decomposition rate of soil organic matter.The?value of forest among different parent materials were following the order:basalt>granite>black shales.This finding suggested that the decomposition rate of soil organic matter in forest soils derived from black shales were highest.Therefore,the contribution rate of organic carbon oxidation to atmospheric CO₂ in forest soils derived from black shale is the highest,which is worth paying much more attention.