黄土高原地区土壤有机碳和无机碳储量及含量特征
Stocks and content of organic and inorganic carbon in soil of the Loess Plateau region
第一作者:
责任编辑: 蒋实
收稿日期: 2022-12-3 修回日期: 2024-02-26
基金资助: |
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Received: 2022-12-3 Revised: 2024-02-26
土壤碳库在调节全球碳平衡和减缓温室气体方面有着重要的地位,估算土壤碳储量对评价陆地生态系统碳循环具有重要意义。利用研究区土地质量地球化学调查获取的土壤碳数据,采用“单位土壤碳量”方法,估算了西北地区各层土壤全碳、有机碳和无机碳储量,分析了不同土壤、土地利用和地形地貌类型下的土壤有机碳和无机碳中碳含量特征。结果表明,研究区上下全层(0~2.0 m)土壤中累计求得总碳10 099.4 Mt,表层(0~0.2 m)总碳1 224.8 Mt,上层(0~1.0 m)5 345.9 Mt,下层(1.0~2.0 m)4 753.5 Mt,各层中总碳含量以无机碳为主,占比自上而下逐渐增大,有机碳主要集中在表层。无机碳含量高值区主要分布在青海湟水谷地、甘肃陇中、陕西北部和宁夏南部黄土高原等地区,有机碳高值区主要分布在祁连山一带。风沙土具有最低的表、深层有机碳、无机碳和总碳含量;黑垆土和黄绵土具有最高的表层无机碳含量;黑毡土具有最高的表层有机碳含量;黑垆土具有最高的深层有机碳含量,且具有最高的表层和深层土壤总碳含量。森林具有最高的表层和深层有机碳及表层总碳含量;草原具有最高的表层无机碳和总碳含量;耕地具有最高的深层无机碳含量;裸地具有最低的无机碳、有机碳和总碳含量。山地具有最高的表层和深层土壤有机碳和总碳含量;黄土具有最高的表层和深层无机碳含量;平原总体介于黄土和山地之间。高海拔地区具有极高的有机碳含量。
关键词:
Soil carbon pools play a significant role in regulating global carbon balance and mitigating greenhouse gases. Hence, estimating soil carbon stocks is critical for assessing the carbon cycle in terrestrial ecosystems. Based on the soil carbon data obtained from the land quality geochemical survey in the study area, this study estimated the stocks of total, organic, and inorganic carbon of various soil layers in Northwest China using the unit soil carbon amount (USCA) method. It analyzed the content characteristics of organic and inorganic carbon in soil under different soil, land use, and topographic types. The results of this study are as follows: (1) All the soil layers at depths ranging from 0 to 2 m in the study area exhibited total carbon of 10 099.4 Mt, including 1 224.8 Mt in the topsoil layer (0~0.2 m), 5 345.9 Mt in the upper soil layer (0~1.0 m), and 4 753.5 Mt in the lower soil layer (1.0~2.0 m). Inorganic carbon predominated in all the soil layers, with its proportion gradually increasing from top to bottom, whereas organic carbon was principally concentrated in the topsoil layer; (2) The high-value areas of inorganic carbon content were primarily distributed in the Huangshui Valley of Qinghai Province, and the Loess Plateau region covering the Longzhong area of Gansu Province, northern Shaanxi Province, and southern Ningxia Province. In contrast, the high-value areas of organic carbon content were chiefly distributed in the Qilian Mountains; (3) The aeolian sandy soil exhibited the lowest organic, inorganic, and total carbon contents in the topsoil and deep soil layers. The dark loessial soil and the loessal soil showed the highest inorganic carbon content in the topsoil layer. The dark felty soil and the dark loessial soil displayed the highest organic carbon contents in the topsoil and deep soil layers, respectively. Additionally, the dark felty soil had the highest total carbon content in the topsoil and deep soil layers; (4) Forests exhibited the highest organic carbon content in the topsoil and deep soil layers, and the highest total carbon content in the topsoil layer. Grasslands showed the highest inorganic and total carbon contents in the topsoil layer. Cultivated land had the highest inorganic carbon content in the deep soil layer. Bare land manifested the lowest inorganic, organic, and total carbon contents; (5) Mountains displayed the highest organic and total carbon contents in the topsoil and deep soil layers. Loess had the highest inorganic carbon content in the topsoil and deep soil layers. Plains showed intermediate carbon contents generally between those of loess and mountains. Besides, high-altitude areas manifested extremely high organic carbon content.
Keywords:
本文引用格式
段星星, 刘小龙, 韩宝华, 阿地来·赛提尼亚孜, 金梦婷, 刘彤.
DUAN Xing-Xing, LIU Xiao-Long, HAN Bao-Hua, Adilai·Saitiniyazi , JIN Meng-Ting, LIU Tong.
0 引言
工业革命以来,由于化石燃料的大量使用和土地利用方式的变化,大气中的CO2浓度快速升高[1],以2×10-6/年的速度持续增加,从工业化前的280×10-6上升至2005年的379×10-6,按此速度增长,到21世纪末有可能达到700×1
在全球气候变化背景下,国内有不少学者在国家或区域尺度上依据全国土地质量区域地球化学调查资料数据对土壤碳储量及其分布[12]、固碳潜力[13]、固碳调控以及土地利用变化与土壤有机碳关系[14]等方面进行了研究。Li等[15]在2007年对中国土壤有机碳和无机碳储量进行估计,预测储量分别为83.8 Pg和77.9 Pg。赵宁等[16]研究表明,近50年间中国陆地生态系统碳汇整体呈上升趋势,年平均为0.213±0.030 Pg。Wu等[17]通过研究人类活动对我国北方碳库的影响,发现人类活动造成土壤无机碳库1.6 Pg的损失。西北黄土高原表层土壤有机碳储量为78.56 Mt, 其储量呈上升趋势[18]。高原冻土区土壤有机碳储量约16.5 Pg, 土壤无机碳储量约14 Pg[19]。尽管已有大量土壤碳储量的研究,但数据资料精度及估算方法的不同,学者们所得研究结果也相差较大。加之自然因素和人类活动的影响,土地利用方式的复杂多样,土壤碳储量在水平空间上表现出的空间变异性较为复杂,以致难以准确获得土壤碳信息,使得同一地区的研究结果存在较大差异。同时,由于深层土壤取样工作较为困难,特别是在中、大尺度的研究中,有关土壤有机碳和无机碳空间变异的研究大都集中于表层30 cm以浅土壤中[20],忽略了对深层土壤有机碳和无机碳的垂直分布特征的研究,对于深层土壤碳库估算还存在很多不确定性。土壤碳储量在空间上的分布直接反映了其土壤肥力的分布特征,是植物健康生长的重要保障[21]。因此,准确评估土壤碳储量是评价区域土壤碳循环对全球气候变化反馈效应的基础。
本文选择黄土高原地区为研究区,利用土地质量地球化学调查获取的土壤碳数据,分析了不同土壤类型、土地利用方式及地形地貌中表层、上层和下层土壤碳含量分布特征,估算了表层、上层和下层土壤全碳、有机碳和无机碳储量,为研究区土壤碳库的评价、区域尺度土壤碳库和碳平衡研究提供科学参考和基础数据。
1 材料与方法
1.1 研究区概况
研究区位于黄土高原部分地区,范围西起青海省祁连山,东至太行山,南至秦岭,北抵黄河,涵盖了青海东部、甘肃中东部、陕西中部地区以及宁夏平原,总面积约67.8万km2(图1)。研究区位于中纬度的东部季风区,西风带南部,其中西部和北部属于暖温带—中温带半干旱区,降水量为150~250 mm;东部和南部属暖温带半湿润区,年降水量为600~750 mm,形成夏季、秋季多雨,冬季、春季干旱少雨,平均温度3.6~14.3 ℃,冬季严寒,夏季暖热的气候特点。区内地势西高东低,自西向东波状下降,总体地形地貌景观格局以山地、丘陵、高原、盆地、台地等相间分布。土壤类型主要有黄绵土、灰钙土、褐土、栗钙土、黑垆土、黑毡土、新积土、风沙土、黑钙土、灌淤土、灰褐土、潮土、盐土等;主要土地利用类型为耕地、草地、森林、裸地等。黄土高原是世界上被侵蚀最严重的地区之一[22],黄土沉积区表面已经变得支离破碎[23]。
图1
1.2 数据来源
本文数据来源于西北地区1∶25万土地质量地球化学调查,采用双层网格化方法进行点位布点和采样,表层土壤每1 km2采集1个样,深度0~0.2 m;深层土壤每4 km2采集1个样点,深度1.0~2.0 m,累计采集表层土壤样品43 298件和深层土壤样品10 974件(见图1)。土壤样品干燥后过20目筛,表层土壤以每4 km2分析1个组合样,深层土壤以每16 km2分析1个组合样。根据《多目标区域地球化学调查规范(1∶250 000)》要求,土壤样品中的有机碳采用重铬酸钾容量法分析,全碳采用X射线荧光光谱法分析。按照分析方法准确度和精密度要求,采取严格的质量检查措施。为了有效控制系统分析误差,对分析质量采用全国质量监控制度控制。土地利用类型数据源自https://doi.org/10.5281/zenodo.4417810[24],土壤类型源自
1.3 数据处理
通过对一定区域内单位土壤碳量进行求和,计算得到土壤碳储量,单位土壤碳含量采用网格化计算单元,按照奚小环等[25]提出的可更加逼近真实含量的指数法模型法求取土壤碳储量。
1)表层和深层土壤平均碳含量(S)计算公式为:
①表层土壤(0~0.2 m)平均碳含量:
②深层土壤(1.5~2.0 m)平均碳含量:
式中:TC表、TC深代表表层、深层土壤对应的土壤总碳、无机碳或有机碳含量,%。
2)单位碳含量(USCA)和碳密度(SCD)计算公式为:
①表层土壤(0~0.2 m)单位碳含量:
②上层土壤(0~1.0 m)单位碳含量:
③下层土壤(1.0~2.0 m)单位碳含量:
④上下全层土壤(0~2.0 m)单位碳含量:
式中:104为换算系数;ρ为土壤容重,t/m3;TC为土壤总碳、无机碳或有机碳含量,%;d为土壤深度,其中d1取表层土壤中心深度0.1 m,d2取深层样采样深度2.0 m,d3取1.0 m;USCA单位为t。按照深层土壤样品16 km2内组合为1件样品分析,表层样品通过前期数据处理后,与深层样品数据形成一一对应关系,因此以16 km2为最小统计单元后,即可求得研究区各层土壤有机碳总储量。
3)一定区域内土壤碳储量(SCR)计算公式为:
式中:USCAi为第i个统计单位土壤碳含量,t;n为土壤碳储量统计范围内单位土壤碳量加和个数。
2 结果与分析
2.1 土壤碳储量特征
据统计,研究区上下全层(0~2.0 m)土壤中累计求得总碳10 099.4 Mt,其中有机碳储量3 125.7 Mt,无机碳储量6 973.7 Mt,以无机碳为主,占69.1%;表层土壤(0~0.2 m)中累计求得总碳1 224.8 Mt,其中有机碳储量519.0 Mt,无机碳705.7 Mt,以无机碳为主,占57.6%;上层土壤(0~1.0 m)中累计求得总碳5 345.9 Mt,其中有机碳储量1 887.4 Mt,无机碳储量3 458.5 Mt,以无机碳为主,占64.7%;下层土壤(1.0~2.0 m)中累计求得总碳4 753.5 Mt,其中有机碳储量1 238.3 Mt,无机碳储量3 515.25 Mt,以无机碳为主,占74.0%。如图2所示,碳储量中无机碳大于有机碳,自上而下有机碳占比下降,无机碳占比上升。
图2
图2
研究区土壤各层中有机碳和无机碳储量占比
Fig.2
The percentage of organic carbon and inorganic carbon reserves in different soil layers of the study area
2.2 土壤碳含量分布特征
图3
图3
研究区表层(a)和深层(b)土壤无机碳含量分布
Fig.3
Distribution of inorganic carbon content in surface (a) and deep (b) soil layers of the study area
图4
图4
研究区表层(a)和深层(b)土壤有机碳含量分布
Fig.4
Distribution of organic carbon content in surface (a) and deep (b) soil layers of the study area
2.3 不同土壤类型碳含量特征
研究区土壤类型占比见图5,其中以黄绵土为主,占比32.6%。统计不同土壤类型中有机碳、无机碳和总碳含量(图6),发现有机碳含量在表、深层土壤中均呈现较大幅度变化,表层土壤有机碳含量依次为:黑毡土>黑钙土>灰褐土>栗钙土>褐土>黑垆土>新积土>黄绵土>潮土>灌淤土>盐土>灰钙土>风沙土;深层土壤有机碳含量依次为:黑毡土>黑钙土>灰褐土>栗钙土>黑垆土>褐土>黄绵土>灌淤土>新积土>灰钙土>盐土>潮土>风沙土。无机碳在同类型土壤间,表、深层中含量变化幅度不大,但在不同类型土壤间存在较明显的变化,表层为:黑垆土>黄绵土>灌淤土>栗钙土>新积土>盐土>灰钙土>灰褐土>黑毡土>黑钙土>潮土>褐土>风沙土;深层为:黑垆土>黄绵土>栗钙土>灌淤土>新积土>灰钙土>褐土>盐土>灰褐土>黑钙土>潮土>黑毡土>风沙土。总碳受有机碳和无机碳含量变化的影响,表层为:黑毡土>黑钙土>灰褐土>栗钙土>黑垆土>黄绵土>灌淤土>新积土>褐土>盐土>灰钙土>潮土>风沙土;深层为:毡土>灰褐土>黑钙土>黑垆土>栗钙土>黄绵土>灌淤土>新积土>褐土>灰钙土>盐土>潮土>风沙土。
图5
图6
图6
研究区不同土壤类型碳含量(单位:%)
Fig.6
Carbon content of different soil types in the study area (unit:%)
如图6所示,研究区内风沙土具有最低的表、深层有机碳、无机碳和总碳含量,其中表层有机碳、无机碳和全碳含量分别是全区有机碳、无机碳、全碳含量的21.2%、40.9%、32.4%,深层有机碳、无机碳和全碳含量分别是全区对应含量的28.0%、53.5%、47.5%。黑垆土和黄绵土具有最高的表层无机碳含量,分别达1.63%和1.60%;黑毡土具有最高的表层有机碳含量,达到3.96%,是全区表层有机碳含量平均值的4倍;黑垆土具有最高的深层有机碳含量;黑毡土同时具有最高的表层和深层土壤总碳含量。
2.4 不同土地利用类型碳含量特征
表1显示,研究区以草地和耕地为主,表层土壤无机碳含量依次为草地>耕地>森林>裸地,有机碳含量为森林>草地>耕地>裸地,总碳含量为森林>草地>耕地>裸地;深层土壤无机碳含量为耕地>草地>森林>裸地,有机碳含量为森林>草地>耕地>裸地,总碳含量为草地>耕地>森林>裸地。森林具有最高的表层和深层有机碳及表层总碳含量;草原具有最高的表层无机碳和总碳含量;耕地具有最高的深层无机碳含量;裸地具有最低的无机碳、有机碳和总碳含量。
表1 研究区不同土地利用类型有机碳、无机碳和总碳含量
Table 1
土地利 用类型 | 样品数 | 表层土壤含量/% | 深层土壤含量/% | ||||
---|---|---|---|---|---|---|---|
无机碳 | 有机碳 | 总碳 | 无机碳 | 有机碳 | 总碳 | ||
草地 | 5904 | 1.414 | 1.116 | 2.530 | 1.432 | 0.475 | 1.907 |
耕地 | 3662 | 1.379 | 0.825 | 2.205 | 1.501 | 0.351 | 1.852 |
森林 | 792 | 1.163 | 1.502 | 2.665 | 1.103 | 0.564 | 1.667 |
裸地 | 175 | 0.932 | 0.648 | 1.580 | 0.968 | 0.314 | 1.282 |
2.5 不同地形地貌碳含量特征
表2显示,研究区地形地貌中以黄土地貌为主,黄土、山地和平原具有不同的碳含量,山地具有最高的表层和深层土壤有机碳和总碳含量;黄土具有最高的表层和深层无机碳含量;平原总体介于黄土和山地之间。研究区以中海拔地区为主,高海拔地区具有极高的有机碳含量,表层达到4.208%,深层1.604%。
表2 研究区不同地形地貌有机碳、无机碳和总碳含量
Table 2
地形 地貌 | 样品数 | 表层土壤含量/% | 深层土壤含量/% | ||||
---|---|---|---|---|---|---|---|
无机碳 | 有机碳 | 总碳 | 无机碳 | 有机碳 | 总碳 | ||
黄土 | 4628 | 1.551 | 0.745 | 2.296 | 1.637 | 0.342 | 1.979 |
山地 | 2534 | 1.298 | 1.884 | 3.182 | 1.231 | 0.782 | 2.013 |
平原 | 2859 | 1.262 | 0.959 | 2.222 | 1.348 | 0.352 | 1.700 |
高海拔 | 405 | 1.159 | 4.208 | 5.367 | 0.942 | 1.604 | 2.546 |
中海拔 | 4703 | 1.299 | 0.634 | 1.932 | 1.338 | 0.266 | 1.604 |
低海拔 | 1420 | 1.211 | 0.848 | 2.059 | 1.515 | 0.269 | 1.784 |
3 讨论
3.1 无机碳含量影响因素
黄土高原是东亚季风的产物,由冬季风搬运上风向西北干旱区的粉尘物质堆积形成[30],同时受夏季风挟带的降雨改造[31];黄土高原土壤类型以黄绵土为主,初始粉尘中含有大量被风力搬运而来的碳酸盐矿物,土壤中无机碳含量主要受原始粉尘中碳酸盐输入通量和后期成壤改造作用影响。青海湟水谷地、甘肃陇中、陕西北部和宁夏南部黄土高原等地区降雨量低,植被稀疏,风化作用较弱,无机碳保存较好,因此表、深层无机碳含量均比较高。毛乌素沙地是我国面积最大的沙地[32],以固定、半固定沙丘为主,物质主要来自近源鄂尔多斯高原基岩风化产生的碎屑物质,和黄河上游物质不存在明显的物源联系[33],以中沙和细沙为主,累计占比介于81.0%~95.2%,基本不含黏土[34],因此区内具有极低的无机碳含量。学者认为流经青藏高原东北缘的黄河将大量碎屑物质搬运到银川盆地[33],因此盆地内沿黄河两岸具有比较高的表层无机碳含量。关中盆地受长期的农业活动灌溉影响,良好的土壤水分条件和密集的植物根系有利于碳酸氢盐的形成,从而加速了碳酸盐的溶解,特别是在雨季,表层无机碳受到较强的风化淋滤作用,使其遭受强烈的淋溶而离开表层土壤, 其中一部分在下伏黄土中淀积下来,使得深层无机碳含量高,尤其是渭河上游和秦岭山前地区水分条件更好,无机碳更容易通过地表和地下径流迁移,最终汇入黄河[35]。
3.2 有机碳含量影响因素
有机碳受到一系列自然因素和人为因素影响[36]。地形和土地利用方式是影响土壤有机碳库变化的重要因素[36⇓-38]。土壤有机碳与土壤质量和植物生产力有着内在的联系[39]。不同地形地貌中的山地和不同土地利用类型中森林分别具有最高的表层、深层土壤有机碳含量,表层有机碳高值区主要分布在祁连山一带山地区,表层和深层土壤有机碳含量分别达到1.884%和0.782%,是黄土和平原地区的2倍左右,该区海拔相对高,气温相对低,控制着植物残留物的碳输入和输出之间的平衡[40],抑制了有机碳的分解[41],土地利用类型多为天然森林和草地,具有高覆盖度和丰富根系,为土壤提供了大量的腐殖质和分泌物,且人为扰动小,使得有机碳累积。黄土地貌中表层和深层有机碳含量仅为0.745%和0.342%,黄土高原土壤侵蚀严重,有利于有机碳的去除,尤其是斜穿黄土高原中部的年降雨量400~500 mm的地区[42]。毛乌素沙地以风沙土为主,缺少可以为有机质提供物理保护的粉砂和黏土[43-44],有利于土壤向大气碳流失[36],风沙土具有所有土壤类型中最低的表、深层有机碳含量,植被以稀疏的荒漠草原为主,生物量低,有机碳输入量低,同时也因风蚀作用,有机碳以凋落物和细颗粒形式被去除[45]。银川盆地和关中盆地耕地区内,耕作过程会加速有机碳的分解和侵蚀,使土壤失去的有机碳进入大气中,从而增加大气二氧化碳[46]。耕地表层和深层有机碳含量分别仅为森林的54.9%和62.2%,与相关研究显示土地利用变化会导致30%~45%的损失基本一致[47-48],与不合理的耕作,破坏土壤大团聚体,降低土壤有机质稳定性,以及随后增加土壤温度和通气量,加速有机质的矿化[49]有关,也与耕地在收获过程中去除植物残留可以显著减少有机碳的输入有关[45]。
4 结论
1)研究区表层和深层碳含量具有基本类似的分布规律,无机碳含量高值区主要分布在青海湟水谷地、甘肃陇中、陕西北部和宁夏南部黄土高原等地区,有机碳高值区主要分布在祁连山一带。上下全层(0~2.0 m)土壤中累计求得总碳10 099.4 Mt,表层(0~0.2 m)1 224.8 Mt,上层(0~1.0 m)5 345.9 Mt,下层(1.0~2.0 m)4 753.5 Mt。
2)区内以黄绵土为主,风沙土具有最低的表、深层有机碳、无机碳和总碳含量。黑垆土和黄绵土具有最高的表层无机碳含量;黑毡土具有最高的表层有机碳含量;黑垆土具有最高的深层有机碳含量。黑毡土同时具有最高的表层和深层土壤总碳含量。
3)区内土地利用类型以草地和耕地为主,森林具有最高的表层和深层有机碳及表层总碳含量;草原具有最高的表层无机碳和总碳含量;耕地具有最高的深层无机碳含量;裸地具有最低的无机碳、有机碳和总碳含量。
4)区内黄土、山地和平原具有不同的碳含量,山地具有最高的表层和深层土壤有机碳和总碳含量;黄土具有最高的表层和深层无机碳含量;平原总体介于黄土和山地之间。高海拔地区具有极高的有机碳含量。
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