Abstract:

Understanding the compositions of humic substances (HS) and its humification in soil on a longterm scale are important to evaluate the stability of soil organic matter. In this study, humic acid (HA), fulvic acid (FA) and humin (HM) in 17 samples of loess and paleosol from above S8 of the Luochuan loess/paleosol profile were analyzed quantitatively. The values of E465 and E665 of both HA and FA were also measured. By combining the contents of HA, FA and HM with the results of mineral compositions and total organic carbon, the characteristics of HS in soil and its implication to longterm stability of organic carbon were focused. The results indicated the following four aspects. (1) The order of the contents of HS in most of paleosol samples is HM>HA>FA, whereas that in loess is FA>HM>HA, with higher HA and HM but lower FA in paleosol than in loess; along with the increase in burial time, HA, FA, and HM in paleosol and FA and HM in loess decrease gradually to different extent; finally HA and FA in both loess and paleosol will not decrease till older than 200 ka, whereas HM till older than 700 ka. (2) HA/FA ratios in most paleosol samples are larger than 1 and decrease gradually with increased burial time (with an exception of S6); HA/FA ratios in loess are lower than those in paleosol; E4/E6 ratios of FA in both loess and paleosol decrease with increased burial time, indicating an increased humification degree; meanwhile, C/H and O/C ratios suggest that the condensation and oxidation degrees of HA and FA in paleosol increase slightly with increased burial time. The results in (1) and (2) indicate that the evolution direction of HS in both paleosol and loess may be as FA→HA→HM (→kerogen). (3) There are good correlations of TOC with HA, HA+FA, and HA+FA+HM (r=0.89, 0.94, and 0.96, respectively). (4) There are also good correlations of kaolinite content with HA, HA+FA, and HA+FA+HM (r=0.62, 0.57, and 0.65, respectively)，indicating that the stability of HS in both loess and paleosol might be relevant with kaolinite. These results shed further light on organic geochemistry of loess and paleosol and on longterm stability of organic carbon in soil.

Key words: humic acid, fulvic acid, humin, humification degree, clays, loesspaleosol