Comparative study on the distribution of soil organic matter and humic substances in the profile of different cultivated soils in Songnen Plain

Cheronzem, black soil (thin layer), black soil (thick layer) and aeolian soil were selected to study the distribution of soil organic carbon (SOC), humic acid (HA), fulvic acid (FA) and humin (HM) content, which is of significance to accurately assess the soil quality and ability of C sequestration in those farming soils. Compared to topsoil (0 - 20 cm), SOC in 80 - 100 cm of cheronzem, black soil (thin layer), black soil(thick layer) and aeolian soil  decreased by 21.7%, 26.9%, 41.4%, 23.6%, while pH increased from 5.2, 5.9, 6.1, 4.8 to 7.5, 7.2, 8.4, 7.6 in cheronzem, black soil (thin layer), black soil (thick layer) and aeolian soil, respectively. HA content declined by 34.9%, 40.4%, 46.6%, 11.4% in cheronzem, black soil (thin layer), black soil (thick layer) and aeolian soil, respectively. Similarly, FA decreased by 52.2%, 36.2%, 73.3%, 59.0%, and HM decreased by 32.2%, 18.3%, 37.2%, 30.7% (P<0.05) . Interestingly, HA/FA ratio increased by 42.9%, 161%, 101% (P<0.05) from 0 - 20 cm to 80 - 100 cm in cheronzem, black soil (thick layer) and aeolian soil, respectively. However, △lgK of HA decreased by 20.6%, 29.9%, 34.9%, 40.3%, while that of FA decreased by 21.8%,17.1%, 21.8%, 18.9% in cheronzem, black soil (thin layer), black soil (thick layer) and aeolian soil (P<0.05), respectively. SOC and humic substances uniformly decrease in different soil profiles, the average SOC, pH, humic substances and △lgK in black soil (thick layer) are higher than other three soils.