Forms And Conversions Of Organic Carbon And Its Structural Characteristics In Ancient Paddy Soils In Liyang Plain

Soil organic matter is an important part of the global carbon cycle.The ancient buried paddy soil has been closed for a long time which is significantly different between the ancient conditions"with ploughing with fire and weeding with flooded water"and the mode of modern mechanized cultivation,tillage application of chemical fertilizers,however,the influence of the above soil formation process on organic matter is still not fully understood.In this study,different forms of organic carbon in different layers of paddy soil at Shanlonggang archaeological section of the Liyang Plain were detected.The section of different combinations of humus components in buried ancient paddy soil and modern cultivated paddy soil were compared and analyzed;Moreover,many kinds of spectroscopy and spectral methods were used to analysis the humic acid（HA）and fulic acid（FA）composition,element composition and structure characterization of them;X ray diffraction and SEM technique were used to investigate the mineral composition,morphology and elemental composition of the clay complexes.The main conclusions were as follow:The content of organic carbon（SOC）in the profiles of PA,PB,and PC at Shanlonggang Site ranged from 5.10 to 10.85 g/kg,all of which gradually decreased with the increase of soil depth.However,the SOC content of buried ancient paddy soil was less than that of modern cultivated paddy soil.The SOC content of different diameter aggregate showed as:（5-2mm）>（2-1mm）>（1-0.25mm）>（<0.25mm）;Compared to the modern farming paddy soils,the SOC content distribution in buried ancient rice soil was more uniform.（1）The contents of the SOC,EOOC,WSOC and AOC in each soil layer of buried ancient paddy soil were relatively uniform,and they were all lower than modern cultivated paddy soil,and the plough layer was particularly significant;Compared with modern cultivated paddy soil,the mineralization cumulative amount and the mineralization rate were lower in the buried ancient paddy soils.The potential mineralization was proportional to the content of SOC,EOOC,WSOC,AOC and pH,and was inversely proportional to soil depths.（2）The vibrations in the infrared spectrum of the ancient paddy soil was mainly composed of those of halogen and inorganic compounds,and some others of functional groups liked olefins,aromatics and hydroxyl compounds.In the infrared spectrum of the original red soil,the absorbance was lower with the ancient paddy soil than with the modern paddy soil at the wave band 3620～1631cm^-1,but higher with the former than with the latter at the wave band of 1631～470cm^-1.In the infrared spectrum of the soil with organic matter removed,the ancient paddy soil was lower than the modern paddy soil in absorbance at all wave bands.First-ordered reciprocal（FD value）of the infrared spectrum showed that the ancient paddy soil didn’t vary much from the modern paddy soil;After organic matter was removed from the ancient paddy soil,peaks in its infrared spectrum were all weakened,especially FD peaks.（3）Compared with modern cultivated paddy soil,the content of humus of different forms in buried ancient paddy soil was lower and tended to be steady.The average content of combined humus of the buried ancient paddy soil is loose combined state>tight combined state>stable combined state,and were all less than that of modern cultivated paddy soils.（4）Excepting for the content of clay（<2μm）,other grade organic-mineral complexes were less than modern cultivated paddy soil.The results of X-ray diffraction analysis showed the contents of kaolinite and illite in buried ancient paddy of the clay organic-mineral complexes were higher than modern cultivated paddy soil,but vermiculite was the opposite.The results of scanning electron microscopy（SEM）analysis showed the percentage of element weight of clay organic-mineral complex of the PC profile were Fe<Al<Si<O,O of clay organic-mineral complex of A1 layer in buried ancient soil was significantly lower than A2 layer in modern cultivated paddy soil,but Fe,Si and Al of those were slightly higher than A2 layer in modern cultivated paddy soil.（5）The C/H and O/C atomic ratios of HA to FA in in two paddy soils were the largest in the plough layer of buried ancient paddy soil,and the average C/N and O/C atomic ratios of HA were significantly smaller than those of FA.The results of ¹³C NMR analysis showed that the organic carbon in two paddy soils both are aliphatic.The aromaticity of the HA and FA in of the modern and the buried ancient paddy soils both decreased with the increase of soil depth.It can be concluded that the degree of condensation and oxidation of HA and FA of buried ancient paddy soils were greater than that of modern cultivated paddy soils.The HA and FA of buried ancient paddy soils presented lower aromaticity than that of modern cultivated paddy soils and its structure was simpler.In general,the characteristics of the soil organic carbon and structure of the humic substances in paddy soils were analyzed,which was a long-term evolution process of paddy soil in the plain area of the middle and lower reaches of the Yangtze River since the civilization of human cultivation.The results of this study provide new insights into revealing the effect of secondary paddy soil formation on the spatial distribution,structural characterization,and stability mechanisms of humus constituents in ancient paddy soils.