土壤腐殖物质特征及其对有机碳长时间尺度稳定性的指示：以陕西洛川黄土剖面为例

Characteristics of humic substances in soil and its implication to long-term stability of organic carbon: A case of Luochuan loess profile

在长时间尺度上研究土壤中腐殖物质的组成和腐殖化特征对于正确评价土壤有机质的稳定性具有重要意义。文中对洛川黄土/古土壤剖面S8以上的17个黄土和古土壤样品中的胡敏酸(HA)、富里酸(FA)和胡敏素(HM)进行定量研究,并对HA和FA的E465、E665值等进行了测定。结合该课题组对矿物组成、总有机碳(TOC)及其各组分含量等的研究结果,围绕腐殖物质特征对有机碳长时间尺度稳定性的指示意义等问题展开了讨论。讨论结果表明:(1)古土壤中腐殖物质各组分含量大小的大致顺序是HM>HA>FA,黄土中为FA>HM>HA;古土壤的HA和HM含量高于黄土,而FA的含量低于黄土;随着埋藏时间的增加,古土壤中HA、FA、HM,以及黄土中FA和HM含量均不同程度地显示出逐渐降低的趋势;黄土/古土壤中HA和FA达到平衡需要20多万年的时间,HM则需要70万年以上。(2)古土壤的HA/FA总体上大于1,并且它们均随着埋藏时间的增加而逐渐降低(S6层例外);黄土的HA/FA低于古土壤;无论是黄土还是古土壤,FA的E4/E6比值均随着埋藏时间的增加而降低,即FA的腐殖化程度增加;C/H、O/C比值说明,古土壤的HA和FA之缩合度和氧化度随着埋藏时间的增加而略微增加;古土壤和黄土中HS演化的总体方向可能为FA→HA→HM(→干酪根)。(3)HA、HA+FA和HA+FA+HM与TOC之间具有很好的相关性(r为0.89、0.94和0.96)。(4)HA、HA+FA和HA+FA+HM均与高岭石含量呈正相关关系(r为0.62、0.57和0.65),说明黄土/古土壤中腐殖物质的稳定性主要与高岭石有关。文中结果对于深化研究黄土/古土壤的有机地球化学特征,以及深刻认识土壤有机质在长时间尺度上的稳定性具有一定的参考价值。

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 F465 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 paleosoi 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/F4 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.