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土壤颜色是土壤分类、气候重建和环境遥感的重要指标。铁氧化物和腐殖质是土壤的两大致色组分,主导了土壤可见光波段的光谱响应特征。本文基于土壤中铁氧化物和腐殖质交叉致色效应不明确的问题,以高岭石(Kao)为基底,选取典型铁氧化物赤铁矿(Hm)和针铁矿(Gt)以及典型腐殖酸胡敏酸(Ha)和富里酸(Fa),基于高分辨率漫反射光谱方法(DRS),系统探讨了不同含量单一致色组分的光谱特征、颜色指数及其二元交叉干扰效应。研究发现,就单一组分的致色效应而言,红色Hm强于黄色Gt,黑色Ha强于棕色Fa;腐殖质加入对铁氧化物致色有明显影响,通常会使可见光波段平均反射率、明度(V)和彩度(C)降低,色调(H)偏黄,但Hm抗干扰能力大于Gt。Lab颜色系统的a*、b*以及DRS的红度(Red%)与黄度(Yellow%)对Hm和Gt含量变化敏感,可作为土壤铁氧化物定量反演的指标,但Ha的加入可导致Hm和Gt估值偏低,Fa对估值的影响较小,干扰形式与Hm和Gt的含量范围有关。基于此,本文给出不同腐殖酸含量条件下土壤Hm和Gt的估算公式,为理解土壤的颜色变化及铁氧化物定量提供重要参考,也为基于环境遥感的大面积铁氧化物调查奠定基础。
Soil color is an important index of soil classification, climate reconstruction and environmental remote sensing. Iron oxides and humus are the two major chromogenic components of soil, which dominate the spectral response characteristics of soil.
This paper is based on the unclear cross-coloration effect of iron oxides and humus in soils and sediments.
We selected kaolinite as substrate, hematite (Hm) and goethite (Gt) as the representatives of iron oxides, humic acid (Ha) and fulvic acid (Fa) as the representatives of humus. The spectral characteristics, color changes and cross-interference characteristics of iron oxides and humus with different contents were discussed by high-resolution Diffuse Reflectance Spectrum (DRS).
It is found that in terms of the chromogenic effect of a single component, Hm is stronger than Gt, and Ha is stronger than Fa. The addition of humus has an obvious effect on the coloration of iron oxides, which usually reduces the Mean Reflectance, Value (V) and Chroma (C), and makes the Hue (H) yellowish. The anti-jamming ability of Hm is greater than that of Gt. The a* of Lab color system and the redness of DRS are sensitive to the change of Hm content. The b* of Lab color system and the yellowness of DRS are sensitive to the change of Gt content. They can be used as indices for the quantitative determination of soil iron oxides. However, the addition of Ha can lead to the underestimation of Hm and Gt, and the addition of Fa has less effect on the estimation depending on the contents of Hm and Gt.
Based on this, the estimation formulas of Hm and Gt mixed with different contents of humic acids are given. It not only helps understand the color change of natural soils but also lays a foundation for iron oxide determination at a large scale based on remote sensing.
土壤颜色是土壤最直观的特征[
铁氧化物与腐殖质是土壤中的两大致色组分[
随着Munsell系统与Lab系统等颜色描述系统的提出,土壤颜色的精确描述成为可能[
Hm和Gt是土壤主要的铁氧化物染色剂,Ha和Fa是土壤主要的致暗腐殖质,而高岭石(Kao)在热带及亚热带富铁土和铁铝土中广泛分布[
为明确不同土壤致色组分的独立光谱特征及其交叉影响,本文采用Kao为基底,将铁氧化物和腐殖质以不同比例混合制成单一组分或二元组分模拟土壤样品。取0.2 g混合样品在玛瑙研钵中磨至200目(75 μm)以下,置于2.54 cm × 7.62 cm × 0.12 cm载玻片上,加入适量去离子水,调制成泥浆状,均匀涂抹至玻璃片5.5 cm × 2.54 cm的区域,置于室温风干待测。
为明确不同致色组分的致色效率,以Kao为基底,分别与致色组分Hm、Gt、Ha和Fa混合,用逐步稀释法制作不同致色组分的单一序列,致色组分含量以2为底指数递增,分别为0、0.78125、1.5625、3.125、6.25、12.5、25、50、100、200和400 g·kg–1,构成纯Hm序列、纯Gt序列、纯Ha序列、纯Fa序列。
为探究腐殖质对铁氧化物致色效应的干扰,以Kao为基底,将不同含量(0、0.78125、3.125、12.5和50 g·kg–1)的腐殖酸序列与不同含量(0.78125、1.5625、3.125、6.25、12.5、25、50、100和200 g·kg–1)的铁氧化物序列两两混合,构成腐殖酸(Ha、Fa)与铁氧化物(Hm、Gt)的二元混合序列。
本研究实验均基于漫反射光谱(DRS),该方法对土壤和沉积物中的铁氧化物灵敏性极高,理想状态下可检测含量低至0.01%的Hm和Gt [
目前适用于土壤与沉积物的颜色描述系统主要包括DRS颜色指数系统、Lab系统、Munsell表色系统及CIE XYZ系统[
Lab系统包括L*、a*、b*三个颜色参数,L*代表亮度,变化于黑(0)和白(100)之间,a*代表红度,变化于红(+100)和绿(-100)之间;b*代表黄度,变化于黄(+100)和蓝(-100)之间[
建立颜色指数与铁氧化物间的回归模型,并从其稳定性和预测能力两个方面对模型进行检验。模型的稳定性采用决定系数
式中,
通过相对误差(Relative Error,RE)评价腐殖质致色组分对铁氧化物致色的干扰程度,不仅可以明确腐殖质组分的干扰程度,还可以明确其干扰形式。
式中,RE为相对误差,预测值是指通过回归模型计算的土壤铁氧化物预测值,真实值为样品真实值。
通过基底Kao及致色组分Hm、Gt、Ha和Fa在可见光波段的原始光谱(
基底和致色组分可见光光谱
Visible spectrum of substrates and chromogenic components
通过基底及致色组分原始光谱计算的颜色指数(
基底及各致色组分的颜色指数
Color index of substrates and chromogenic components
样品 |
DRS颜色指数 |
Lab系统 |
Munsell系统 |
||||||||||||
Vi | Bl | Gr | Ye | Or | Re | Mref | L* | a* | b* | H | V | C | |||
注:Vi、Bl、Gr、Ye、Or、Re和Mref分别代表紫度、蓝度、绿度、黄度、橙度、红度和平均反射率。Note:Vi,Bl,Gr,Ye,Or,Re and Mref represent violet%,blue%,green%,yellow%,orange%,red% and mean reflectance in the visual light band. | |||||||||||||||
Kao | 17.3 | 14.0 | 22.6 | 9.9 | 13.2 | 23.0 | 93.6 | 97.5 | 3.2 | 10.1 | 16.5 | 9.7 | 1.7 | ||
Hm | 6.5 | 5.1 | 8.9 | 8.1 | 21.4 | 50.0 | 14.8 | 39.9 | 30.5 | 35.8 | 9.0 | 3.9 | 7.4 | ||
Gt | 4.0 | 4.5 | 17.8 | 16.1 | 21.4 | 36.1 | 32.4 | 68.5 | 11.6 | 62.0 | 19.9 | 6.7 | 9.2 | ||
Ha | 16.0 | 12.9 | 21.3 | 9.8 | 13.6 | 26.5 | 3.1 | 20.3 | 2.2 | 4.8 | 18.6 | 2.0 | 0.9 | ||
Fa | 17.0 | 13.4 | 21.2 | 9.4 | 13.1 | 25.9 | 5.1 | 26.4 | 2.7 | 4.1 | 15.6 | 2.6 | 0.7 |
通过单一致色组分序列可见光波段的原始光谱发现,土壤模拟样品的反射率随致色组分含量递增而普遍降低,光谱形态从纯Kao基底光谱逐渐过渡至纯致色组分光谱。纯Hm和纯Gt序列光谱反射率随含量增加呈现明显的波段分异,小于550 nm波段的反射率呈现减速下降的趋势,大于550 nm波段呈现加速下降的趋势(
单一致色组分序列可见光光谱
Visible spectrum of single chromogenic component sequences
从基于DRS计算的光谱指数来看,平均反射率(Mref%)(
单一致色组分序列颜色指数
Color index of single chromogenic component sequences
从Lab系统来看,L*的变化规律与基于DRS的Mref%相当(
从Munsell系统来看,H值随Hm增加迅速递减,愈偏向红色(R);随Gt增加先增后减少,先偏黄红(YR)后偏红(R);随Ha增加缓慢增长;随Fa增加相对快速增长(
从DRS参数来看,Hm序列的Mref%(
不同胡敏酸(a~i)和富里酸(j~r)含量干扰下赤铁矿序列的颜色指数变化
The change of color index of hematite sequence under the interference of different humic acid(a~i)and fulvic acid(j~r)contents
从Lab系统来看,Hm序列L*(
从Munsell系统来看,Hm序列的H值(
整体而言,腐殖质对Hm致色具有削弱作用,且Ha的影响远大于Fa,一般使土壤的Mref%、L*和V降低,Red%和a*下降,色调H会从红(R)偏向黄红(YR)。
从DRS光谱指数来看,Ha的加入使Gt序列的Mref%,Red%,Yellow%均降低(
不同胡敏酸(a~i)和富里酸(j~r)含量干扰下针铁矿序列的颜色指数变化
The change of color index of goethite sequence under the interference of different humic acid(a~i)and fulvic acid(j~r)contents
从Lab系统来看,Ha的加入使Gt序列的L*、a*和b*也呈现下降趋势(
从Munsell系统来看,Ha的加入使Gt序列的V值和C值下降(
整体而言,腐殖质对Gt致色同样具有削弱作用,Ha的影响也远大于Fa,颜色指数变化趋势与Hm序列基本相同,一般使土壤的Mref%、L*和V降低,Yellow%和b*下降。
表生铁氧化物是环境敏感矿物,广泛应用于土壤分类和气候重建中。热带亚热带土壤中,Hm和Gt是游离铁的主要组分,Ha与Fa是有机质的重要组分。Hm常在红壤、赤红壤和砖红壤的显色中占据主导,母质含铁高、温度高但相对湿度适中的海南玄武岩风化的砖红壤可使Hm含量最高达120 g·kg–1[
铁氧化物种类与含量测定通常采用X射线衍射法(XRD),但自然铁氧化物含量相对较低,结晶度差,难于直接定量。基于Munsell、Lab等不同颜色系统及漫反射光谱(DRS)的颜色指数广泛应用于土壤与沉积物的铁氧化物定量中。Torrent最早通过Munsell的H、V和C计算出RR(Redness Rating)指数来估算土壤中Hm的含量,并提出Hm与RR正相关(
与此同时,基于分光测色仪、非接触式分光光度仪等仪器,Lab系统颜色指数因其对颜色差异的高敏感度在气候重建中得到广泛应用。杨胜利等[
漫反射光谱(DRS)对于土壤和沉积物中的铁氧化物极为敏感,检测限低于0.01%,获取光谱稳定,对样品无损耗,是目前用来识别土壤铁氧化物的重要手段[
腐殖质作为土壤的重要致色组分,在高腐殖质含量的温带土壤及其表层土壤会干扰铁氧化物相的致色。Baumgardner等[
然而,有机质的致色效应主要通过腐殖质分子体现,铁氧化物的致色效应主要通过Hm和Gt矿物体现,有机质含量、游离铁含量等常用土壤化学指标与土壤光谱和颜色并无机理上的联系。而且,天然土壤的显色还容易受到土壤基底等其他因素干扰。基于此,本文通过模拟土壤,建立了在不同腐殖质含量条件下铁氧化物含量与常用颜色指数之间的关系模型,并计算了均方根误差(RSME)来评估其模型精度(
不同腐殖质含量下颜色指数与铁氧化物含量之间的回归关系
Regression relationship between color index and iron oxide content under different humus content
腐殖质含量Humus content/(g·kg–1) | Lab估算方程Lab estimation equation |
DRS估算方程DRS estimation equation |
||||||
方程 | RMSE | 方程 | RMSE | |||||
注:Ha和Fa分别代表胡敏酸和富里酸含量,Hm和Gt分别代表赤铁矿和针铁矿含量。Note:Ha and Fa represent humic acid and fulvic acid content respectively,while Hm and Gt represent hematite and goethite content respectively. | ||||||||
0 | Hm = 0.162e0.1791a* | 0.96 | 3.82 | Hm = 0.007e0.1939˟Red% | 0.99 | 1.37 | ||
0.78125 | Hm = 0.151e0.1871a* | 0.97 | 3.31 | Hm = 0.005e0.2016˟Red% | 0.98 | 1.07 | ||
Ha | 3.125 | Hm = 0.175e0.1831a* | 0.96 | 3.47 | Hm = 0.005e0.2015˟Red% | 0.97 | 1.57 | |
12.5 | Hm = 0.411e0.1682a* | 0.97 | 2.64 | Hm = 0.009e0.1955˟Red% | 0.97 | 0.95 | ||
50 | Hm = 0.578e0.1844a* | 0.98 | 1.25 | Hm = 0.008e0.2113˟Red% | 0.97 | 0.99 | ||
0 | Hm = 0.162e0.1791a* | 0.95 | 3.82 | Hm = 0.007e0.1939˟Red% | 0.99 | 1.37 | ||
0.78125 | Hm = 0.102e0.1972a* | 0.95 | 3.81 | Hm = 0.003e0.214˟Red% | 0.99 | 1.71 | ||
Fa | 3.125 | Hm = 0.066e0.2079a* | 0.91 | 4.49 | Hm = 0.001e0.2283˟Red% | 0.99 | 3.28 | |
12.5 | Hm = 0.136e0.1898a* | 0.91 | 4.48 | Hm = 0.002e0.221˟Red% | 0.98 | 2.37 | ||
50 | Hm = 0.128e0.1925a* | 0.91 | 4.58 | Hm = 0.001e0.2363˟Red% | 0.99 | 2.07 | ||
0 | Gt = 0.183e0.1017b* | 0.99 | 1.92 | Gt = 5E-05e0.9529˟Yellow% | 0.98 | 0.62 | ||
0.78125 | Gt = 0.181e0.1067b* | 0.99 | 1.38 | Gt = 4E-05e0.9848˟Yellow% | 0.99 | 0.59 | ||
Ha | 3.125 | Gt = 0.259e0.1033b* | 0.98 | 1.35 | Gt = 7E-05e0.9505˟Yellow% | 0.97 | 1.03 | |
12.5 | Gt = 0.406e0.1048b* | 0.94 | 2.70 | Gt = 1E-04e0.9436Y˟ellow% | 0.93 | 1.55 | ||
50 | Gt = 0.539e0.1166b* | 0.95 | 1.83 | Gt = 7E-05e1.0196˟Yellow% | 0.90 | 3.33 | ||
0 | Gt = 0.183e0.1017b* | 0.99 | 1.92 | Gt = 5E-05e0.9529˟Yellow% | 0.98 | 0.62 | ||
0.78125 | Gt = 0.145e0.1056b* | 0.98 | 2.29 | Gt = 4E-05e0.9727˟Yellow% | 0.97 | 1.19 | ||
Fa | 3.125 | Gt = 0.106e0.1097b* | 0.97 | 2.82 | Gt = 2E-05e1.0069˟Yellow% | 0.99 | 1.12 | |
12.5 | Gt = 0.092e0.1155b* | 0.98 | 2.12 | Gt = 2E-05e1.0414˟Yellow% | 0.99 | 2.17 | ||
50 | Gt = 0.061e0.1265b* | 0.98 | 2.12 | Gt = 1E-05e1.086˟Yellow% | 0.98 | 0.48 |
为了进一步明确自然体系中基于颜色指数对铁氧化物估值的偏差,本文通过无腐殖质干扰下(Ha=0,Fa=0)时颜色指数(Red%和Yellow%)与铁氧化物之间建立回归方程,反演有腐殖质干扰时(0.78125 g·kg–1、3.125 g·kg–1、12.5 g·kg–1、50 g·kg–1)的Hm和Gt含量,并计算其相对误差(RE)来衡量腐殖质对铁氧化物反演的干扰程度(
腐殖质对铁氧化物致色的干扰程度及相对误差
The interference degree and relative error of humus to the coloration of iron oxides
通过基于DRS计算的Red%估算Hm,Ha的加入普遍会使Hm估值偏低,干扰范围为- 67.59%~55.57%,仅在Hm < 1.5625g·kg–1时,会使Hm估值偏高(
通过基于DRS计算的Yellow%估算Gt,Ha的加入一般会导致Gt估值偏低,相对误差为-89.69%~48.12%;仅在Gt < 0.78125 g·kg–1时使Gt估值偏高(
本研究对热带亚热带地区基于颜色的土壤及沉积物的铁氧化物定量提供了重要参考,适用于氧化环境下的铁氧化物致色主导的富铁土及铁铝土等强风化土壤,对认识草原土壤和荒漠土壤的颜色变化也有重要启示,但草原土壤和荒漠土壤可能有暗色原生矿物、绿泥石等其他致色矿物干扰[
为明确土壤铁氧化物与腐殖质的交叉染色效应,通过以Kao为基底,Hm、Gt和Ha、Fa为致色端元的二元混合模拟光谱实验发现,Ha对Hm和Gt的致色效应具有削弱作用,一般使土壤的Mref %、L*和V降低,Red%和a*下降,Yellow%和b*下降,不同之处在于Ha使Hm序列的色调H从红(R)偏向黄红(YR),而Ha对Gt序列的H影响因Gt含量出现分异。与此同时,模拟土壤中Lab系统的a*和基于DRS计算的Red%与Hm含量之间、Lab系统的b*和基于DRS计算的Yellow%与Gt含量之间显著正相关,但这种关系会不同程度受到腐殖质干扰,Ha的干扰能力大于Fa,Ha的加入会导致基于颜色指数的Hm和Gt的估值普遍偏低。Fa的加入会使低含量Hm的估值偏高,高含量(> 100 g·kg–1)Hm的估值偏低;Fa的加入会使低含量(< 3.125 g·kg–1)Gt的估值偏高,高含量Gt的估值偏低。
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