中国科学院水利部水土保持研究所机构知识库

摘 要

 土壤有机碳（SOC）作为土壤重要的组成成分之一，在保持土壤肥力和维持土壤健康方面扮演着极为重要的角色。黄土丘陵区在植被不断恢复的过程中，土壤有机碳库储量也随之发生了变化。不同小流域因其植被恢复的侧重点不同，恢复的效应可能也不同，是否会对土壤有机碳库的稳定性造成不同影响还需进一步研究。因此本研究选取黄土丘陵区4个不同小流域（坊塌流域、纸坊沟流域、董庄沟流域及杨家沟流域）为研究对象，将土壤有机碳库按照周转时间划分为：土壤活性碳库、土壤缓效性碳库及土壤惰性碳库，通过野外采样、室内培养及实验测定，采用结构方程模型进行综合解释，探究不同小流域内土壤各有机碳库的含量变化特征、周转时间、分解速率及其影响因素，辨析不同小流域土壤有机碳库含量、稳定性变化特征及其主要的影响因素，以期为黄土丘陵区植被恢复过程中土壤固碳效应的评估提供一些实践指导和数据支撑。其主要结论如下：

  （1）不同小流域之间，自然恢复下的董庄沟流域土壤有机碳的稳定性较其他三种小流域更强。农业种植为主型下的坊塌流域，0-20cm土层活性有机碳库、缓效性有机碳库及惰性碳库所占比例高于20-40cm土层，且所占比例的顺序为：惰性碳库>缓效性碳库>活性碳库；整体上，各植被恢复措施下的缓效性有机碳库及惰性碳库所占比例均高于撂荒地，而活性碳库则低于撂荒地。植被恢复为主型下的纸坊沟流域，0-20cm和20-40cm土层活性碳库和缓效性碳库所占比例整体上均高于撂荒地，惰性碳库所占比例则只有表层的高于撂荒地，三库所占比例表现为缓效性碳库>惰性碳库>活性碳库。自然恢复下的董庄沟流域和人工恢复下的杨家沟流域，土壤有机碳各库所占比例顺序均为：惰性碳库>缓效性碳库>活性碳库。

  （2）不同小流域之间，自然恢复下的董庄沟流域活性碳库周转时间最慢（3.47-20.16d），其土壤有机碳的活性最低，稳定最高，而农业种植为主型下的坊塌流域土壤活性有机碳库周转最快（3.98-8.40d），其稳定性较差。土壤有机碳分解动态特征呈现出一致的规律，即分解的前期阶段，分解速率较快，约在5天之内达到最大值，随后分解速率逐渐降低，直到最后处于相对的稳定状态。农业种植为主型下的坊塌流域、植被恢复为主型下的纸坊沟流域、自然恢复下的董庄沟流域，表层的活性有机碳库周转时间高于下层；植被恢复为主型下的纸坊沟流域、人工恢复下的杨家沟流域及自然恢复下的董庄沟流域，表层的缓效性碳库周转时间均较短；惰性碳库的周转时间主要受当地年平均气温的影响，而纸坊沟流域和坊塌流域邻近，董庄沟流域和杨家沟流域毗邻，因此，坊塌流域和植纸坊沟流域的土壤惰性碳周转时间接近一致（325.34a），而董庄沟流域与杨家沟流域的土壤惰性碳周转时间保持一致（336.81a）。

  （3）不同小流域下，受植被类型、坡度、降水等影响，土壤理化性质的变化不同。SOC含量在人工恢复下的杨家沟流域最高（6.88-11.56g.kg^-1），农业种植为主型恢复下的坊塌流域最低（2.83-6.16g.kg^-1）。与人工恢复下的杨家沟流域相比，自然恢复下的董庄沟流域SOC含量较低，与农业种植为主型下的坊塌流域相比，植被恢复为主型下的纸坊沟流域SOC的含量较高。土壤含水率变化范围在植被恢复为主型下的纸坊沟流域最大，农业种植为主型恢复下的坊塌流域次之，人工恢复下的杨家沟流域最低；0-20cm土层的土壤容重均低于20-40cm土层；就土壤pH而言，自然恢复下的董庄沟流域和人工恢复下的杨家沟流域土壤pH较为接近，而农业种植为主型下的坊塌流域和植被恢复为主型下的纸坊沟流域的土壤pH较为接近，且相比董庄沟和杨家沟流域，坊塌流域和纸坊沟流域的pH更高。土壤全氮（TN）含量的变化规律表现为自然恢复下董庄沟流域的TN含量最高（0.28-1.10g.kg^-1），人工恢复下的杨家沟流域次之（0.29-0.71g.kg^-1），植被恢复为主型下纸坊沟的最低（0.15-0.0.32g.kg^-1）。不同小流域下土壤全磷（TP）的含量差异性较小。

  （4）不同小流域下，杨家沟流域的植物各器官碳含量最高，董庄沟流域的次之，坊塌流域的最低；除杨家沟流域外，其它3个小流域内植物各器官的碳含量均表现为叶片>枝>根。董庄沟流域的地上生物量最高，纸坊沟流域的次之，坊塌流域和杨家沟流域的最低。就地下生物量而言，4个小流域内表层的地下生物量均高于下层；且表层和下层，杨家沟流域和董庄沟流域，地下生物量较高，纸坊沟流域居中，坊塌流域最低。

  （5）黄土丘陵区丘陵沟壑区的土壤微生物丰富度高于高塬沟壑区，而微生物量则表现为高塬沟壑区高于丘陵沟壑区。董庄沟流域的土壤微生物量碳（MBC）和微生物量氮（MBN）含量较高，杨家沟流域的MBC和MBN较低。不同小流域之间土壤微生物多样性指数的变化差异较小，而以农业种植恢复为主型下的坊塌流域和植被恢复为主型下的纸坊沟流域所属的丘陵沟壑区与人工恢复下的杨家沟流域和自然恢复下的董庄沟流域所属的高塬沟壑区之间的土壤微生物多样性指数差异较大，尤其是Shannon和Simpson指数，均表现为坊塌流域和纸坊沟流域高于董庄沟和杨家沟流域。

  （6）4个小流域在植被恢复后土壤团聚体水稳定性均有所提高，其中自然和人工恢复下土壤团聚体水稳性提高效果较为显著，且各小流域内，土壤团聚体水稳定性与SOC含量密切相关。坊塌流域和纸坊沟流域内，各植被恢复措施下土壤团聚体组成均以微团聚体为主，而董庄沟和杨家沟流域内各植被恢复措施下则以大团聚体为主；与人工恢复下的杨家沟流域相比，自然恢复下的董庄沟流域大团聚体比例更高。整体看，杨家沟流域土壤的抗侵蚀能力最强（K值为0.1678-0.1970）。相关性分析可知，土壤含水率、容重pH及全磷对各粒径土壤团聚体所占比例、平均重量直径（MWD）及土壤可蚀性（K值）的影响不同，而土壤有机碳和全氮含量对各粒径土壤团聚体所占比例、MWD及K值的影响大致相同。

  （7）不同小流域下，土壤有机碳库含量及其稳定性主要受MBC含量、SOC含量、大团聚体所占比例及土壤真菌和细菌丰富度的影响。其中土壤活性碳库的稳定性主要受MBC含量、SOC含量、0.25mm-0.5mm土壤团聚体所占比例、MWD值及K值的影响较大；土壤缓性碳库的稳定性主要受>5mm团聚体所占比例和土壤含水率的影响较大，且前者起促进作用，后者起减缓作用；土壤惰性碳库的稳定性主要受TN、TP、MBC含量、pH、0.5-5mm团聚体所占比例及土壤容重（BD）的影响，且除BD外，其它指标均起到促进作用。

  （8）自然恢复下的董庄沟流域土壤有机碳的稳定性较农业种植恢复为主型的坊塌流域、植被恢复为主型的纸坊沟流域及人工恢复下的杨家沟流域更强，因此其土壤固碳能力也比其他三个小流域更强。微生物多样性和土壤团聚体对土壤有机碳物理保护作用对土壤有机碳库稳定性的影响较植被生物量及各器官含碳量更为显著。

   ABSTRACT

    As one of the important compositions of soil, soil organic carbon plays an important role in maintaining soil fertility and soil health. In the process of vegetation restoration in Loess Hilly Region, soil organic carbon pool also changed. Different small watersheds may have different effects due to their different goals, so it needs to be further studied whether they will have different impacts on the stability of soil organic carbon pool. This study selected four small watersheds with different vegetation restoration patterns (Fangta watershed, Zhifanggou watershed, Dongzhuanggou watershed and Yangjiagou watershed) as the research objects. And the soil organic carbon pool is divided into: active soil organic carbon pool (ASOC), slow soil carbon organic carbon pool (SSOC) and passive soil organic carbon pool (PSOC) according to the mean residence time (turnover time). Through field sampling, indoor cultivation and experiment determination, we used the structural equation model (SEM) to analyze the changes of the contents of soil organic carbon pools, residence time, decomposition rate and their influencing factors under different vegetation restoration patterns. Exploring the characteristics of soil organic carbon pool capacity, stability and main influencing factors under different vegetation restoration patterns, we hope to provide some practical guidance and data support for evaluating the effect of vegetation restoration on soil carbon sequestration in Loess Hilly Region. The main conclusions are as follows:

   (1) Among the different small watersheds, the stability of soil organic carbon in  Dongzhuanggou watershed with natural restoration manner were stronger than that of the other three watersheds. Under Fangta watershed with farming manner, the proportion of ASOC, SSOC and PSOC in the 0-20cm soil layer was higher than that in the 20-40cm soil layer, and the order of their proportion was PSOC > SSOC > ASOC; On the whole, the proportion of SSOC and PSOC under kinds of vegetation restoration measures was higher than the abandoned farmland, but the proportion of ASOC was lower than the abandoned farmland. In the case of Zhifanggou watersheds with vegetation restoration manner, the proportion of ASOC and SSOC in 0-20cm and 20-40cm soil layers was higher than that in abandoned farmland. The proportion of PSOC was higher than that of abandoned farmland only in the surface layer, and the proportion of three pools showed SSOC > PSOC >ASOC. Under natural and artificial restoration (Dongzhuanggou and Yangjiagou watersheds), the proportion of three soil organic carbon pools was: PSOC> SSOC> ASOC.

   (2) Among different small watersheds, the turnover time of ASOC under natural recovery (Dongzhaunggou watersheds) was the slowest (3.47-20.16d). Therefore, the activity of soil organic carbon in natural restoration pattern was lowest and the stability of it was highest. However, the turnover rate of ASOC under farming (Fangta watershed) was highest (3.98-8.40d), and its stability was relatively poor. The dynamic characteristics of soil organic carbon decomposition showed consistent rules, that is, in the early stage of decomposition, the decomposition rate was increasing and then reached the maximum value about 5 days, after that, the decomposition rate gradually decreased until finally it was in a relatively stable state. The turnover time of ASOC in the surface layer was higher than that in the sublayer under farming (Fangta watershed), vegetation restoration (Zhiganggou watershed) and natural restoration (Dongzhuanggou watershed). Under the conditions of vegetation restoration (Zhiganggou watershed), artificial and natural restoration (Dongzhuanggou and Yangjiagou watersheds), the turnover time of SSOC in the top soil layer was shorter than sub soil layer. The turnover time of PSOC is mainly affected by local annual average temperature. Because Zhifanggou basin is near to Fangta basin and Dongzhuangdou valley was near to Yangjiagou valley, the turnover time of PSOC under farming and vegetation restoration keep similar (325.34a) but the turnover time of PSOC under natural and artificial restoration keep similar (336.81a).

  (3) In different small watersheds, the variation of soil physical and chemical properties was different because of vegetation types, slope, precipitation, etc. The content of SOC was the highest (6.88-11.56 g.kg^-1) in Yangjiagou watershed (artificial recovery) and the lowest in Fangta watershed (farming). Compared with artificial restoration (Yangjiagou watershed), the SOC of natural restoration (Dongzhuanggou watershed) was lower, and SOC under vegetation restoration (Zhifanggou watershed) is higher than that under farming (Fangta watershed). The range of soil moisture content in Zhifanggou watershed was largest and it was lowest in Fangta watershed. The soil bulk density in the 0-20cm soil layer was lower than that in the lower 20-40cm soil layer. In terms of soil pH, the soil pH under natural restoration and artificial restoration was relatively close, while the soil pH under farming and vegetation restoration was relatively close, and the soil pH under farming and vegetation restoration was relatively high. The variation of TN showed that the content of TN under natural restoration was the highest (0.28-1.10 g.kg^-1), followed by artificial restoration (0.29-0.71g.kg^-1), and the lowest under vegetation restoration (0.15-0.0.32g.kg^-1). There was little difference in contents of TP among different vegetation restoration patterns.

   (4) On the whole, the carbon content of plant organs was the highest under artificial restoration, followed by natural restoration and the lowest under farming. Except the artificial recovery, the carbon content of each organ for the other three restoration patterns was leaves> branch > root. The order of aboveground biomass was Dongzhaunggou watershed (natural restoration) > Zhifanggou watershed (vegetation restoration) > Fangta watershed (farming). The underground biomass of the surface layer was higher than that of the bottom layer among different recovery patterns. Under artificial and natural restoration (Yangjiagou and Dongzhaunggou watersheds), the underground biomass was higher, the vegetation restoration (Zhifanggou watershed) was the second and lowest under farming (Fangta watershed).

    (5) The richness of soil microorganism in hilly-gully region was higher than the tableland gully region, while microbial biomass of hilly-gully region was lower than the tableland gully region. Among the four different small watersheds, the content of MBC and MBN were higher in Dongzhaunggou watershed and lower in Yangjiahou watershed. There was little difference in the variation of soil microbial diversity index between Fangta watershe with farming manner and Zhifanggou watershed with vegetation restoration manner, as well as between Dongzhuanggou watershed with natural restoration manner and Yangjiagou watershed with artificial restoration manner. However, there was bigger difference in the variation of soil microbial diversity index between the hilly-gully region (farming and vegetation restoration) and tableland gully region (natural and artificial restoration), especially Shannon and Simpson index, which showed the former was higher than the latter.

   (6) The water-stability of soil aggregates under natural and artificial restoration (Dongzhaunggou and Yangjiagou watersheds) were higher than that of vegetation restoration and farming (Zhifanggou and Fangta watershed). And the water-stability of soil aggregates was closely related to the content of SOC. Under farming and vegetation restoration (Fangta and Zhifanggou watershed), the soil aggregates of all vegetation restoration measures consisted of microaggregates mainly, while under natural and artificial restoration (Dongzhaunggou and Yangjiagou watersheds), macroaggregates were mainly parts for the soil aggregates. Compared with Yangjiagou watershed, the proportion of macroaggregates was higher in Dongzhaunggou watershed. Compared with abandoned areas, the water stability of soil aggregates under various vegetation restoration measures was improved in both the surface and the lower layers, and the improvement effect was significant under natural and artificial restoration. On the whole, the ability of soil resistance to erosion (K value is 0.1678-0.1970) under artificial restoration (Yangjiagou watershed) was strongest. Correlation analysis showed that soil moisture content, bulk density, pH and TP had different effects on the proportion of soil aggregates, MWD and K values, while SOC and TN had similar effects on the proportion of soil aggregates, MWD and K values.

    (7) In different small watersheds, the soil organic carbon capacity and its stability were mainly affected by the content of MBC, SOC, the proportion of macroaggregates and the richness of soil fungi and bacteria. The stability of ASOC was mainly affected by the content of MBC and SOC, the proportion of 0.25mm-0.5mm soil aggregates, the value of MWD and K; The stability of SSOC was mainly affected by the proportion of >5mm aggregates and soil moisture content, and the former played a promoting role while the latter played a moderating role. The stability of PSOC was mainly affected by the content of TN, TP and MBC, pH, the proportion of 0.5-1mm, 1-2mm, 2-5mm soil aggregates and BD, among which, except BD, other indicators played a promoting role.

    (8) The stability of soil organic carbon in Dongzhaunggou watershed under natural restoration was stronger than other three restoration patterns. Therefore, the ability of carbon sequestration under the natural restoration was strongest. Microbial diversity and the physical protection of soil aggregates on soil organic carbon had more significant effects on soil organic carbon stability compared with vegetation biomass and carbon content in various plant organs.