黄土丘陵区植被恢复的土壤碳水效应

黄土高原大规模植被恢复显著影响了这一区域土壤水分和有机碳(SOC),从而影响其承载的土壤水源涵养和固碳服务。明确深层土壤水分和有机碳对植被恢复的响应特征是当前黄土高原地区生态水文与生态系统服务研究的一个重要科学问题,其中植被类型以及生长年限是这一过程的重要影响因素。然而,目前关于深层土壤有机碳和土壤水分对植被恢复的响应及二者关系的研究较少。通过对陕北典型黄土丘陵区不同植被类型和生长年限下0—5 m土壤水分与有机碳的监测,分析了深层土壤水分和有机碳对植被恢复的响应及其特征。研究发现:(1)植被恢复后0—5 m土层均出现水分亏缺,土壤水分亏缺在表层1 m最低,2—3 m最高;对于不同恢复方式,林地土壤水分亏缺在恢复至21—30a时显著高于前一阶段(11—20a),而在恢复31a后水分开始恢复,而灌木、草地土壤水分亏缺程度则随恢复年限延长不断增加。(2)林地、灌木、草地0—5 m平均土壤有机碳含量为1.97、1.77、1.72 g/kg;林地土壤固碳量随恢复年限的增加而增加,并且在恢复20a时固碳量与对照农田相比出现净增;灌木土壤固碳量随恢复年限先增加后降低;草地土壤固碳量则随退耕年限增加呈下降趋势并且低于对照农田。(3)表层0—1 m土壤水分随恢复年限增加变化不显著,深层土壤水分则随恢复年限增加显著降低;相比而言,随恢复年限增加,土壤有机碳随年限的变化在各层土壤中均不显著。深层土壤水分与土壤有机碳呈现显著的正相关,且土壤有机碳的增加速率低于土壤水分,研究认为,深层土壤固碳与土壤水分关系密切,且深层土壤固碳需要充足水分参与。深层土壤水分亏缺可能限制植被细根的发展,使深层土壤有机碳输入减少。

Response of soil moisture and soil organic carbon to vegetation restoration in deep soil profiles in Loess Hilly Region

Large-scale vegetation restoration in the Loess Plateau has had a major impact on the soil moisture content and soil organic carbon (SOC), as well as soil water conservation and carbon sequestration services in this region. The response of deep soil moisture and SOC contents to vegetation restoration raised serious concerns in current studies regarding eco-hydrology and ecosystem services in the Loess Plateau. However, limited studies have examined the responses of deep SOC and soil moisture along with their coupling relationships to vegetation restoration in this critical area. In this study, vertical distribution (0-5 m depth) of SOC and soil moisture contents in different human-introduced vegetation types (grassland, shrubland and forestland) and restoration ages in a typical loess hilly watershed were analyzed, and soil moisture and SOC contents in cropland were measured as controls. Our analyses showed that:(1) Soil moisture deficit was observed in different human-introduced vegetation types. The surface soil layer (0-1 m depth) had the lowest soil moisture deficit, and the 2-3 m depth layer had the highest soil moisture deficit. In forestland, soil moisture deficit was significantly greater in the 21-30a stage than that in the previous stage (11-20a), and subsequently decreased after 31 years of growth. However, soil moisture deficit in shrubland and grassland increased subsequent to abandonment of croplands. (2) The mean SOC contents of forestland, shrubland and grassland in the 0-5 m depth profile were 1.97, 1.77, and 1.72 g/kg, respectively. Soil carbon sequestration in forestland increased with increasing restoration ages, and exhibited net gain in the 20a stage compared with cropland. Carbon sequestration in shrubland increased initially, but then decreased with increasing restoration age. In contrast to forestland and shrubland, carbon sequestration in grassland decreased with increasing restoration age, and was lower than that of control farmland for all restoration stages. (3) Soil moisture content in the surface layer (0-1 m depth) did not exhibit significant changes with increasing restoration age. However, soil moisture content significantly decreased in deep soil layers with increasing restoration age. Nevertheless, no significant correlation was found between levels of SOC and restoration age at any soil layers tested. In deep soil profiles, soil moisture content and SOC exhibited significantly positive correlations. Furthermore, the rate of increase of SOC was lower than that of soil moisture content. These findings indicated that deep soil carbon sequestration was closely related to soil moisture content. The soil carbon sequestration process may require adequate soil moisture at greater soil depth. Deep soil moisture deficit may restrict carbon sequestration in deep soil layers by constraining fine root development.