林业科学  2019, Vol. 55 Issue (5): 11-19   PDF    
DOI: 10.11707/j.1001-7488.20190502
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文章信息

卫玮, 党坤良.
Wei Wei, Dang Kunliang.
秦岭南坡林地土壤有机碳密度空间分异特征
Spatial Variation of the Density of Soil Organic Carbon in Forest Land on the Southern Slope of Qinling Mountains
林业科学, 2019, 55(5): 11-19.
Scientia Silvae Sinicae, 2019, 55(5): 11-19.
DOI: 10.11707/j.1001-7488.20190502

文章历史

收稿日期:2017-08-18
修回日期:2019-04-12

作者相关文章

卫玮
党坤良

秦岭南坡林地土壤有机碳密度空间分异特征
卫玮, 党坤良     
西北农林科技大学林学院 杨凌 712100
摘要:【目的】研究陕西秦岭南坡林地土壤有机碳密度空间分异特征,为秦岭林区土壤有机碳科学管理提供理论依据。【方法】在陕西秦岭南坡不同林区(洋县长青、佛坪县龙草坪、太白县太白山、宁陕县宁东和宁陕县宁西)及立地条件(海拔、坡向、坡位和坡度)设置样地,通过调查、取样和测定,采用差异性检验分析不同立地因子对土壤有机碳密度(tC·hm-2)的影响,并通过逐步回归分析量化各因子对土壤有机碳密度影响的相对重要性。【结果】秦岭南坡林地土壤有机碳密度均值为125.41 tC·hm-2(52.60~307.36 tC·hm-2),在0~10,10~30和30~60 cm土层分别为59.04,41.65和24.73 tC·hm-2,分别占土壤有机碳总密度的47.07%,33.21%和19.72%;秦岭南坡不同林区土壤有机碳密度差异较大,表现为龙草坪(143.55 tC·hm-2)>宁东(138.37 tC·hm-2)>宁西(134.09 tC·hm-2)>太白山(109.29 tC·hm-2)>长青(90.22 tC·hm-2);土壤有机碳密度随海拔升高先增后降,在海拔800~1 200 m最低(平均90.24 tC·hm-2),在海拔2 000~2 400 m最大(平均166.43 tC·hm-2),当海拔高于2 400 m后下降(平均132.51 tC·hm-2);阴坡土壤有机碳密度(127.23 tC·hm-2)稍高于阳坡(123.25 tC·hm-2);土壤有机碳密度随坡度增大而降低,由147.52 tC·hm-2减至87.06 tC·hm-2;土壤有机碳密度在下坡位(166.36 tC·hm-2)大于中坡位(129.43 tC·hm-2)和上坡位(77.14 tC·hm-2)。【结论】秦岭南坡林地土壤有机碳密度存在显著的区域差异,并随海拔升高先升后降,在各海拔间和不同坡位间均差异显著(P < 0.05)或极显著(P < 0.01),但在阴坡与阳坡间无显著差异。逐步回归分析表明,坡位和海拔是影响土壤有机碳密度差异的主导立地因子。与我国其他主要林区相比,陕西秦岭南坡林地土壤有机碳密度处于较高水平,在我国森林土壤碳库中具有重要地位,应加强管理。
关键词:秦岭南坡    土壤有机碳密度    空间分异    立地条件    回归分析    
Spatial Variation of the Density of Soil Organic Carbon in Forest Land on the Southern Slope of Qinling Mountains
Wei Wei, Dang Kunliang     
College of Forestry, Northwest A & F University Yangling 712100
Abstract: 【Objective】The patterns of spatial variation of the density of soil organic carbon (SOC) in forest land was studied on the southern slope of Qinling Mountains in Shaanxi Province, which provided a theoretical basis for the scientific management of SOC in Qinling Mountains.【Method】Sample plots were set up in different forest regions (Changqing of Yang County, Longcaoping of Foping County, Taibai Mountain of Taibai County, Ningdong of Ningshan County and Ningxi of Ningshan County) and different site conditions (elevation, slope aspect, slope position and slope) on the southern slope of the Qinling Mountains. Investigation, sampling and determination were carried out. Difference tests were used to analyze the effects of different site factors on SOC density (tC·hm-2). Stepwise regression analysis was employed to quantify the relative importance of each factor on the variation of SOC density.【Result】The average SOC density was 125.41 tC·hm-2(52.60-307.36 tC·hm-2)on the southern slope of Qinling Mountains, which were 59.04, 41.65, and 24.73 tC·hm-2 in 0-10, 10-30, and 30-60 cm soil layers respectively, accounting for 47.07%, 33.21% and 19.72% of the total SOC density, respectively. The SOC density varied greatly among different regions on the southern slope of Qinling Mountains, showing Longcaoping of Foping County (143.55 tC·hm-2), Ningdong of Ningshan County (138.37 tC·hm-2), Ningxi of Ningshan County (134.09 tC·hm-2), Taibai Mountain of Taibai County (109.25 tC·hm-2), and Changqing of Yang County (90.22 tC·hm-2). The SOC density firstly increased and then decreased with increasing altitude. The SOC density (90.24 tC·hm-2) was the lowest at 800-1 200 m above sea level, and the SOC density (166.43 tC·hm-2) was the highest at the 2 000-2 400 m above sea level. When the altitude was higher than 2 400 m, the SOC density decreased, with an average of 132.51 tC·hm-2. The SOC density (127.23 tC·hm-2) of shady slope was slightly higher than that (123.25 tC·hm-2) of sunny slope. The SOC density decreased from 147.52 to 87.06 tC·hm-2 with the increase of slope. The SOC density in the down slope (166.36 tC·hm-2) was higher than those in the middle (129.43 tC·hm-2) and up-slope (77.14 tC·hm-2).【Conclusion】The variations of SOC density among different forest regions were significant. The trend was increased first and then decreased with the increase of elevation. Significant differences (P < 0.05) or extremely significant differences (P < 0.01)of SOC density appeared in elevation ranges and different slope position. However, the difference of SOC density was not significant between shady slope and sunny slope. Stepwise regression analysis showed that the slope position and elevation were the dominant topographic factors that affect the SOC density. Compared with other forest areas in China, the SOC density in forest land on the southern slope of Qinling Mountains was at a higher level in the country. The SOC on the southern slope of Qinling Mountains plays an important role in the soil carbon pool of forest ecosystem in China and the management should be strengthened.
Key words: southern slope of Qinling Mountains    soil organic carbon density    spatial characterization    site conditions    analysis of stepwise regression    

土壤有机碳既是评估土壤质量的关键指标,也是影响土壤营养物质分解和碳循环的重要因素(Liao et al., 2006Pan et al., 2009Martin et al., 2011)。立地因子通过影响植被固碳和微生物活性而显著影响土壤有机碳密度(Hao et al., 2002Ziadat et al., 2005),同时对土壤理化性质和空间变异也有重要影响(Kitchen et al., 2003De Deyn et al., 2008Sharma et al., 2011)。因此,揭示立地因子对森林土壤有机碳密度的影响规律,是认识和分析全球碳循环问题的关键(Tan et al., 2004)。

秦岭既是我国生物多样性最丰富区域之一,也是我国中部的重要生态屏障,在保障区域生态安全及缓解气候变化方面的作用和意义非常重要(刘康等,2004赵庆云等,2014)。郭建明等(2010)发现海拔2 800 m以上的秦岭太白红杉(Larix potaninii var. chinensis)林土壤有机碳密度随海拔增加呈下降趋势,北坡大于南坡;侯琳等(2008;2009)发现秦岭火地塘天然次生油松(Pinus tabulaeformis)林土壤有机碳密度随坡度增加而减小。这些研究对了解秦岭林区土壤有机碳特征具有一定积极意义,但因涉及林分类型单一,海拔范围较窄,研究区域范围较小,其结果很难准确反映秦岭南坡土壤有机碳密度的总体状况、空间分异特征及其与立地因子间的关系。本研究通过对秦岭南坡较大空间尺度的野外调查、采样和分析,探索土壤有机碳密度空间分异特征,揭示其空间差异及随立地因子的变化规律,以期为秦岭林区土壤有机碳科学管理提供理论依据。

1 研究区概况

秦岭位于我国中部,东西长约1 500 km,南北宽100~150 km,海拔1500~2 500 m,主峰太白山海拔3 767 m。为全面了解陕西秦岭南坡林地土壤有机碳密度的总体状况及随空间区域、立地因子的变化规律,研究区域设在秦岭南坡植被保存较好的太白县太白山、洋县长青、佛坪县龙草坪、宁陕县宁西和宁陕县宁东五大林区(107°17′—108°36′E,33°14′—34°05′N),各研究区基本概况见表 1

表 1 研究区概况 Tab.1 Survey of study region
表 2 秦岭南坡林地土壤有机碳密度总体特征 Tab.2 Characteristics of soil organic carbon density in forest land on the southern slope of Qinling Mountains
2 研究方法 2.1 样地设置和调查

利用GPS确定样地位置,并记录各样地的海拔、坡度、坡位和坡向。按不同立地条件(海拔、坡向、坡度、坡位)设置114块20 m×20 m样地,其中太白山40块,长青18块, 龙草坪11块,宁西20块,宁东45块。

2.2 土壤取样及测定

在每块样地的四角及中心各设一个土壤剖面。共挖掘土壤剖面570个,其中太白山100个,长青90个,龙草坪55个,宁西100个,宁东225个。按土壤自然发生过程分0~10,10~30和30~60 cm三层取样,并记录各土层厚度。然后将土样密封保存,带回室内分析。用环刀法测定土壤密度,土样风干后挑出植物根和大于2 mm的石砾,用排水法测定石砾体积含量;另取风干土样磨碎过200目筛,用Liquic TOCⅡ型碳分析仪测定土壤有机碳含量。

2.3 立地因子划分

按秦岭南坡地形条件,并参考《西北主要树种培育技术》(罗伟祥等,2007)的森林立地划分方法,划分立地条件等级。海拔(X1)分为:800~1 200(X11),1 200~1 600(X12),1 600~2 000(X13),2 000~2 400(X14)和2 400~2 800 m (X15);坡向(X2)分为阳坡(正南、西南、东南和正西,X21)和阴坡(正北、东北、西北和正东,X22);坡度(X3)分为缓坡(5°~15°,X31)、斜坡(15°~25°,X32)、陡坡(25°~35°,X33)、急坡(35°~45°,X34)和险坡(>45°,X35);坡位(X4)分为上坡位(地形图上距坡顶1/3处及以上坡面,X41)、中坡位(上坡位与下坡位之间,X42)和下坡位(地形图上距坡顶2/3处及以下坡面,X43)。

2.4 数据处理

土壤有机碳密度计算公式(唐朋辉等,2016)如下:

${\rm{SO}}{{\rm{C}}_{\rm{d}}} = \sum\limits_{i = 1}^n {\frac{{\left({1 - {\theta _i}} \right) \times {c_i} \times {d_i} \times {\rho _i}}}{{10}}} 。$ (1)

式中:SOCd为整个土壤剖面有机碳密度(tC·hm-2);n=3;θi为第i层>2 mm的石砾体积含量(%);cidiρi为第i土层的土壤有机碳含量(g·kg-1)、土壤厚度(cm)和土壤密度(g·cm-3)。各样地的土壤有机碳密度取5个土壤剖面的均值。

利用SPSS18.0软件进行数据分析。采用t检验阐明不同区域和立地的土壤有机碳密度差异特征;通过多元逐步回归分析,揭示影响土壤有机碳密度的主导立地因子。

3 结果与分析 3.1 土壤有机碳密度总体状况

秦岭南坡林地土壤有机碳密度的均值为125.41 tC·hm-2,最小值和最大值分别为52.60和307.36 tC·hm-2,变异系数达0.42,说明该区土壤有机碳密度空间差异较大(表 2)。0~10,10~30和30~60 cm土层土壤有机碳密度分别为59.04,41.65和24.73 tC·hm-2,占总量的47.07%,33.21%和19.72%,且三者间显著差异(P<0.05),表明0~10和10~30 cm土层是构成土壤有机碳密度的主体。

3.2 土壤有机碳密度区域差异

不同林区的土壤有机碳密度表现(表 3)为龙草坪(143.55 tC·hm-2)>宁东(138.37 tC·hm-2)>宁西(134.09 tC·hm-2)>太白山(109.29 tC·hm-2)>长青(90.22 tC·hm-2)。t检验表明,长青与宁西林区差异显著(P<0.05)、长青与宁东和龙草坪均差异极显著(P<0.01),而其他林区间无显著差异。

表 3 秦岭南坡不同林区土壤有机碳密度及t检验结果 Tab.3 t-test result and soil organic carbon density in different region on the southern slope of Qinling Mountains
3.3 立地因子对土壤有机碳密度的影响 3.3.1 海拔

土壤有机碳密度与海拔关系密切,其随海拔升高先升后降(表 4)。在海拔800~1 200 m,土壤有机碳密度最小,平均为90.24 tC·hm-2;随海拔升高,土壤有机碳密度增加,在海拔2 000~2 400 m达最大,平均为166.43 tC·hm-2;当海拔超过2 400 m时,土壤有机碳密度降低。t检验表明,海拔800~1 200 m的土壤有机碳密度与海拔1 200~1 600 m以外的其他海拔土壤有机碳密度均差异极显著(P<0.01);海拔1 200~1 600 m与海拔2 000~2 400和2 400~2 800 m土壤有机碳密度均差异极显著(P<0.01);海拔1 600~2 000与2 000~2 400 m差异极显著(P<0.01),但与2 400~2 800 m无显著性差异;海拔2 000~2 400与2 400~2 800 m差异显著(P<0.05)(表 4)。

表 4 秦岭南坡海拔对土壤有机碳密度的影响及t检验结果 Tab.4 t-test result and effect of elevation on soil organic carbon density on the southern slope of Qinling Mountains
3.3.2 坡向

阴坡土壤有机碳密度(127.23 tC·hm-2)稍高于阳坡(123.25 tC·hm-2),但差异不显著(表 5)。说明在水热条件相对优越的秦岭南坡,坡向不是其主要影响因素。

表 5 秦岭南坡坡向对土壤有机碳密度的影响及t检验结果 Tab.5 t-test result and effect of slope aspect on soil organic carbon density on the southern slope of Qinling Mountains
3.3.3 坡度

土壤有机碳密度随坡度增大逐渐减小(表 6)。t检验表明(表 6),缓坡和斜坡均与险坡差异显著(P<0.05),而其他坡度间差异均不显著。

表 6 秦岭南坡坡度对土壤有机碳密度的影响及t检验结果 Tab.6 t-test result and effect of slope on density of soil organic carbon on the southern slope of Qinling Mountains
3.3.4 坡位

下坡位、中坡位和上坡位土壤有机碳密度分别为166.36,129.43和77.14 tC·hm-2,三者之间均差异极显著(P<0.01)(表 7)。

表 7 秦岭南坡坡位对土壤有机碳密度的影响及t检验 Tab.7 t-test result and effect of slope position on soil organic carbon density on the southern slope of Qinling Mountains
3.4 主要立地因子

以海拔、坡度、坡位和坡向为自变量,以土壤有机碳密度为因变量,进行逐步回归分析。对坡向和坡位进行量化,正北、东北、西北、正东、东南、正西、西南和正南8个坡向分别赋值为1,2,3,4,5,6,7和8(陈亮中,2007许明祥等,2011李旭等,2016唐朋辉等,2016),以样地距坡顶距离占坡面长度的百分数作为坡位值。利用确定系数大小判断不同立地因子的相对重要性。

表 8中,R2是指除该变量外的其他变量进行逐步回归的确定系数;ΔR2是指在其他变量的基础上增加该变量时回归方程确定系数的增量,其值越大说明此变量越重要;Rp2为偏确定系数,是指新加入回归方程的变量所解释部分占缺少该变量的回归方程未能解释部分的比例大小;Radj2是调整确定系数,是指回归方程中的所有自变量对因变量变异的解释能力(姜航等,2015)。由R2、ΔR2Rp2可知,坡位对土壤有机碳密度的影响最大,能独立解释变异的47.4%;其次是海拔,两者合计能解释变异的49.3%。坡度与坡向的偏相关系数分别为-0.029和0.034,相关性均不显著,未能进入回归方程。说明坡位和海拔是影响秦岭南坡林地土壤有机碳密度的主要立地因子。

表 8 立地因子对土壤有机碳密度逐步回归分析结果 Tab.8 Stepwise regression analysis of site factors on soil organic carbon density
4 讨论 4.1 秦岭南坡与我国主要林区土壤有机碳密度比较

我国各林区因植被、气候和立地条件等的不同,土壤有机碳密度差异较大。西双版纳和海南岛尖峰岭热带雨林土壤有机碳密度分别为74.41和110.29 tC·hm-2(刘鹏飞,2013郭晓伟,2015),亚热带四川大巴山森林土壤有机碳密度为117.07 tC·hm-2(钟吉安,2009),暖温带黄土高原中部和子午岭森林土壤有机碳密度分别为73.70和105.23 tC·hm-2(刘伟等,2011;杨晓梅等,2017),寒温带大小兴安岭森林土壤有机碳密度为192.80~220.60 tC·hm-2(魏亚伟等,2013)。可以看出,陕西秦岭南坡林地土壤有机碳密度平均值(125.41 tC·hm-2)高于热带、亚热带和暖温带地区,而低于寒温带地区。一方面表明秦岭南坡林地土壤有机碳密度处于较高水平;另一方面也说明温度对林地土壤有机碳密度有重要影响,是导致不同林区土壤有机碳密度差异的主要原因。土壤有机碳密度既是反映土壤肥力的重要参数(薛志婧等,2015),也是反映土壤碳储能力的主要指标。秦岭南坡林地土壤有机碳密度反映其土壤肥力较高,碳储能力较强,在我国森林土壤碳库中具有重要地位。

4.2 秦岭南坡不同林区土壤有机碳密度差异的原因

在秦岭南坡不同林区,土壤有机碳密度除长青与宁西、宁东和龙草坪存在显著(P<0.05)或极显著(P<0.01)差异外,其余林区间均无显著差异,其原因在于长青林区地处秦岭南坡南缘,海拔总体较低、年平均温度较高,人口数量和密度较大(本区人口44.7万),人为干扰严重,且本区为20世纪80年代采伐后天然更新的次生林,林分质量不高,森林覆盖率在90%以下,郁闭度在0.80左右,故土壤有机碳密度相对较低;相比之下,宁东、宁西和龙草坪林区均处秦岭南坡中段腹地,海拔较高,年均温度较低,且这些林区是秦岭南坡植被保存最为完好的区域,其原始性、自然性较高,质量较好,森林覆盖率在98%以上(赵德怀,2006),同时这些林区人烟稀少(林区人口9万人),人为干扰较小,故其土壤有机碳密度不仅较高,且各林区间土壤有机碳密度也较为一致。

4.3 海拔对土壤有机碳密度的影响

秦岭南坡土壤有机碳密度随海拔升高呈先升后降的单峰型变化曲线,这与前人研究结果(陈海滨等,2010李龙等,2014)一致。海拔对温度有重要影响(赵明月等,2014),在低海拔区域,温度高,有机质分解快,且人口密度大,人为干扰强,故土壤有机碳密度较低。随海拔升高,温度降低,微生物分解受到抑制,导致有机碳积累,同时人口密度减小和人为干扰降低,植被质量提高,故土壤有机碳密度随海拔升高而增大。当海拔升至一定高度时,虽温度进一步下降,但生长期缩短,加之林分类型发生变化,由中山阔叶林、针阔混交林变为亚高山针叶林,归还土壤的凋落物量减少,其林下微气候也存在较大差异(杨万勤等,2007),同时林分郁闭度降低,质量变差,土层变薄,故导致土壤有机碳密度下降。由此可见,土壤有机碳密度随海拔的变化既受温度影响,也受林分类型制约。秦岭太白红杉林地土壤有机碳密度随海拔增加呈减小的趋势(郭建明等,2010),是因为太白红杉林分布在海拔2 600~3 200 m,与本研究结果并不矛盾。

4.4 坡位对土壤有机碳密度的影响

本研究表明,坡位对土壤有机碳密度影响显著,下坡位土壤有机碳密度高于中坡位及上坡位,这与张苏峻等(2010)的研究结论一致。与上、中坡位相比,下坡位在重力和淋洗作用下,土层较厚,土壤有机质聚集(姜航等,2015),故有机碳密度较大。唐朋辉等(2016)研究表明,秦岭南坡红桦(Betula albo-sinensis)林土壤有机碳密度受坡位影响不显著,是因为红桦林在秦岭南坡多分布在地形较平缓地段,而本研究涉及空间范围较大,样地设置考虑了各种坡位条件,因此二者结果并不矛盾。

4.5 坡向和坡度对土壤有机碳密度的影响

本研究表明,秦岭南坡的土壤有机碳密度表现为阴坡高于阳坡。这与许多研究结果(张鹏等,2009舒洋等,2013)一致。与阳坡相比,阴坡光照时间短,温度低,微生物分解速率小,利于土壤有机碳累积。但秦岭南坡优越的水热条件显著削弱了其在坡向间的差异程度。

土壤有机碳密度与坡度呈显著负相关,这与许多前人研究结果(姜航等,2015周鑫等,2016)一致。坡度越大,土层越薄,重力和淋洗作用越强,越不利有机质积累,土壤有机碳密度就越低(侯琳等,2008)。因此,土层厚度和重力作用是导致土壤有机碳密度在坡度差异的主要原因。

4.6 主导因子

逐步回归分析结果表明,坡位和海拔是影响土壤有机碳密度的主导因子,其直接原因是二者导致土层厚度和水热条件发生改变,是影响土壤有机碳密度差异的主要因素(赵明月等,2014郭晓伟等,2015),但二者只能解释土壤有机碳密度变异的49.3%,说明主导因子是相对的。

5 结论

陕西秦岭南坡林地土壤有机碳密度平均为125.41 tC·hm-2,与我国其他林区相比处于较高水平,但存在显著的区域和立地差异:龙草坪最高(143.55 tC·hm-2),长青最低(90.22 tC·hm-2);土壤有机碳密度随海拔升高而增加,在海拔高于2 400 m后开始下降;除海拔800~1 200与1 200~1 600 m、1 200~1 600与1 600~2 000 m、1 600~2 000与2 400~2 800 m土壤有机碳密度无显著差异外,其余海拔范围间均存在显著(P<0.05)或极显著(P<0.01)差异;阴坡土壤有机碳密度稍高于阳坡,但差异不显著;土壤有机碳密度随坡度增大而减小,缓坡和斜坡与险坡差异显著(P<0.05);土壤有机碳密度随坡位下降而升高,下、中、上坡位间差异均极显著(P<0.01)。坡位和海拔是影响秦岭南坡土壤有机碳密度的主导因子。

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