黄土高原典型区域土壤腐殖酸组分剖面分布特征

黄土高原作为典型的气候敏感带和生态环境脆弱区，诸多因素影响着这个区域的土壤有机碳及其组分的分布特征。本文以黄土高原典型区域土壤剖面 0—200 cm土样为对象，分析了土壤腐殖酸、胡敏酸（HA）和富里酸（FA）含量随地理位置及土层深度的分布特征，并进一步探讨了土壤腐殖酸、HA和FA与全氮含量及土壤颗粒组成的关系。结果表明，黄土高原主要类型土壤腐殖酸、HA和FA含量均较低，且存在明显地理位置和土层分异性：从南到北同层次土壤腐殖酸、HA和FA含量均显著降低，同一区域随土层深度增加各组分含量均表现为在 0—40 cm土层明显下降， 40—120 cm土层稍有下降，120 cm土层以下基本稳定；土壤腐殖酸占有机碳比例变化范围为26.6%—54.7%，相对较小，且在整个剖面变化幅度不大，从南向北土壤腐殖酸占有机碳比例有增加趋势；土垫旱耕人为土在 0—40 cm、 40—120 cm和 120—200 cm土层中HA占腐殖酸比例分别为39.8%、49.0%和53.5%，HA/FA分别为0.66、0.96和1.15，黄土正常新成土在以上土层中HA占腐殖酸比例分别为26.3%、33.9%和42.3%，HA/FA分别为0.36、0.51和0.73，干润砂质新成土在以上土层中HA占腐殖酸比例分别为13.4%、37.1%和45.2%，HA/FA分别为0.16、0.59和0.82，说明黄土高原南北主要类型土壤腐殖酸品质总体较差，均属富里酸型土壤，且从南到北腐殖酸品质逐渐下降；土壤腐殖酸、HA和FA均与全氮含量呈极显著线性相关（P<0.01），土壤有机碳、腐殖酸及HA含量与粘粒及砂粒百分含量亦呈高度线性相关（P<0.01）。

Distribution characteristics of humus fraction in soil profile for the typical regions in the Loess Plateau

The Loess Plateau is known as a famous climate-sensitive zone and a fragile ecological environmental belt in China. A variety of environmental factors, such as soil type, geographic location, climate, etc., constrain the accumulation of soil organic matter in this region. Thus, quantifying the spatial distribution of soil organic carbon (SOC) and its fraction is significant for better understanding on soil carbon cycle across the Loess Plateau region. Soil humus, as the major soil organic matter component, is a key indicator for the soil quality and fertility. Yet, the distribution characteristics of soil humus and its fractions across the Loess Plateau have been paid little attention and still remain big uncertainty. In this study, we characterized the vertical distribution of humus and its fractions including Humic Acid (HA) and Fulvic Acid (FA) in the 0-200 cm soil profile along south-north gradient in the Loess Plateau. Then, we further explored the correlations among soil humus, HA, FA, soil total nitrogen, and soil particle composition. An intensive field campaign was conducted to collect 132 soil samples in some typical sites from the south to the north of the Loess Plateau. The results show that the contents of humus, HA and FA, varying among soil types and along soil profiles, are lower than those in other regions of China reported in previous studies. A decreasing gradient from the south to the north of the Loess Plateau is observed for the contents of humus and its fractions. Along the soil profile, the contents of humus, HA, and FA decrease with depth 0-60 cm, slightly decrease with depth 60-120 cm, and remain low and stable below 120 cm soil layers. The fractions of soil humus in SOC increase from the south to the north of the Loess Plateau, ranging from 26.6% to 54.7%, which is lower than that in other regions across China. However, these observed fractions in the same sites have no significant difference in the entire profile. The fractions of HA in soil humus are 39.8%, 49.0%, and 53.5%, respectively, and the HA/FA ratios are 0.66, 0.96, and 1.15, for 0-40 cm, 40-120 cm and 120-200 cm soil profiles, respectively, for Emn-Orthic-Anthrosol soil type. The fractions of HA in humus are 26.3%, 33.9% and 42.3%, respectively, and the HA/FA ratios are 0.36, 0.51, and 0.73, for 0-40 cm, 40-120 cm and 120-200 cm soil profiles, respectively, for Los-Orthic-Entisol soil type. The fractions of HA in humus are 13.4%, 37.1% and 45.2%, and the HA/FA ratios are 0.16, 0.59, and 0.82, for 0-40 cm, 40-120 cm and 120-200 cm soil profiles, respectively, for Ust-Sandiic-Entisol soil type. This study indicates that all typical soil types that were observed from the south to the north of the Loess Plateau are FA-enriched; the humus quality is relatively poor compared to other regions in China and decreases gradually from the south to the north of the Loess Plateau. Finally, the contents of humus, HA, and FA are significantly (P< 0.01) correlated with soil total nitrogen content. On the other hand, the contents of SOC, humus, and HA are significantly (P< 0.01) correlated with soil sand and clay contents.