Effects of land use patterns on the characteristics of Fourier Infrared Spectroscopy of organic carbon within aggregates in a Mollisols

Soil aggregates provide physical protection for soil organic carbon (SOC), and are the main sites for SOC sequestration.The carbon content and its chemical components in soil aggregates are vital important for evaluating the capacity of carbon stabilization.Based on a long-term field experiment with different land use patterns (Cropland, Grassland, Forestland and Bareland) established in the National Observation Station of Hailun Agro-ecology System in 1985, the study investigated the effects of long-term different land use patterns on soil aggregates stability and organic carbon characters within aggregates fractionated by wet sieving and dry sieving methods.The chemical components of organic carbon within aggregates and bulk soil were measured by Fourier Infrared Spectroscopy methods.After 35 years of different land use patterns, the contents of SOC and total nitrogen (TN), aggregate stability and chemical components within aggregates changed significantly, with the largest changes being observed in grassland soil.Compared with the cropland soil, the contents of SOC and TN, and relative peak area of aliphatic-C increased in grassland and forestland.The proportion of >0.25 mm aggregate and mean weight diameter were the highest in grassland.Compared to grassland and forestland, the contents of SOC and TN, and relative peak area of aliphatic-C were lower in bareland and cropland, < 0.25 mm aggregates dominating there, with a lower mean weight diameter and carbon sequestration capacity.The physical stability and water stability of soil aggregates under dry sieving and wet sieving methods were further compared, respectively.Grassland improved the physical stability and water stability of soil aggregates significantly.The physical stability of forestland aggregates decreased significantly, while the water stability of bareland aggregates decreased significantly.Correlation analysis and principal component analysis demonstrated that the aliphatic-C groups in 1 420 cm^-1 and 2 920 cm^-1 were dominated in grassland and forestland soils, while the aromatic-C group in 1 630 cm^-1 was dominated in bareland and cropland soils.The results showed that converting cropland to forestland or grassland is an effective measure to improve the carbon sequestration capacity of soil aggregates, which can increase soil structure stability by enhancing the stability of soil aggregates, improve the quantity of and stable functional groups of OC within aggregates, and finally enhance the carbon sequestration capacity of soil.The proportions of aliphatic-C groups in soil macroaggregates, are more sensitive to changes of land use patterns, would be considered as a sensitive indicator to SOC changes.