六盘山林区几种土地利用方式对土壤中可溶性有机碳浓度影响的初步研究

 测定分析了六盘山林区典型天然次生林（杂灌林、山杨（Populus davidiana）和辽东栎（Quercus liaotungiensis）林）、人工林（13、18和25年华北落叶松（Larix principis_rupprechti））、农田和草地中穿透雨、凋落物淋溶液、土壤溶液和渗漏水溶液及地下水和降雨中可溶性有机碳 (DOC)的浓度。结果显示：5～10月，雨水中DOC浓度为0.80～1.60mg•L－1，地下水中在2.43～7.66 mg•L－1。9～10月，穿透雨中DOC浓度为1.78～15.20 mg•L－1，其中天然次生林和人工林比农田和草地中高，这些DOC与地表凋落物碳年形成量正相关显著。凋落物浸泡24 h后淋溶产生的DOC浓度为12.30～64.79 mg•L－1，占凋落物碳贮量比例不足1%；浓度方面，天然次生林比农田和草地分别高400%和153%，人工林比农田和草地分别高194%和50%；比例方面，农田和草地比天然次生林分别高79%和98%、比人工林分别高180%和210%，这些DOC浓度与落叶、小枝、碎小物和腐解物碳贮量的正相关显著。9～10月,0～20 cm土层溶液中DOC浓度为7.88～88.44 mg•L－1，占土壤有机碳密度的比例不足0.1%，它们随土层加深而下降，其中天然次生林和人工林中下降幅度比农田和草地中大；浓度方面，天然次生林、人工林比农田和草地中高，差异主要在0～10 cm土层；比例方面，天然次生林DOC比例比农田和草地中低，人工林比它们高，差异主要在0～10 cm土层；这些DOC浓度与土壤湿度及凋落物层碳贮量正相关显著。5～10月，0～40 cm土层渗漏水中DOC浓度为5.76～58.84 mg•L－1，天然次生林、人工林比农田和草地高，差异主要在0～10 cm土层；它们随土层加深而下降，其中天然次生林和人工林下降幅度比农田和草地中大。这些差异可能由土地利用引起的植被和土壤性质改变及其对水文过程的影响所致，说明陆地生态系统中DOC浓度受土地利用变化的影响较大。

DISSOLVED ORGANIC CARBON CONCENTRATIONS IN SOIL UNDER DIFFERENT LAND USES IN THE LIUPAN MOUNTAIN FOREST ZONE

Dissolved organic carbon is an important component of the carbon balance of terrestrial ecosystems and source of energy, carbon, and nutrient transfer from terrestrial to aquatic ecosystems. Land use changes caused by human activities have had major impacts on terrestrial ecosystem carbon cycles, including impacts of soil DOC dynamics. Hence, it is important to understand the impacts of land use changes on DOC for assessing the impacts of land use change on the carbon cycle. Over the last several centuries, extensive areas of native vegetation in the Liupan Mountain Forest Zone have been replaced by croplands or rangelands, whereas in recent decades former arable lands and rangelands have been afforested. However, the impacts of these land_use changes on the terrestrial ecosystem carbon cycle are unclear, especially on the soil DOC dynamics. In order to assess the impacts of land_use changes on soil DOC dynamics, we measured the concentrations of DOC in precipitation, subsurface water, soil leachate, detrital leachate solutions, throughfall, and water percolating through the soil in plots with the same elevation, exposure, and soil types but with different vegetation types. A natural secondary forest dominated by Querces liaotungensis, Populus davidiana, or brushwood, a cropland and rangeland derived from destruction of the natural secondary forests, and a 13, 18 and 25 year old larch plantation, Larix pricipis_rupprechtii, afforested on former croplands or rangelands were studied. Our results showed that concentrations of DOC in precipitation and subsurface water from May to October was 0.80 -1.60 mg•L－1 and 2.43-7.66 mg•L－1, respectively. From September to October, the concentration of DOC in throughfall was 1.78-15.20 mg•L－1 which was higher under natural forests or plantations than in croplands or rangelands and positively correlated with annual production of aboveground detritus (p=0.05). The concentration of DOC in detrital leachate solutions, derived from detritus submerged in water for 24 h, was 12.30-64.79 mg•L－1 and its fraction was not more than 1%. The concentration of detrital DOC was 400% and 153% higher under natural forests than cropland and rangeland, respectively, and 194% and 50% higher under plantation than cropland and rangeland, respectively. Its fraction was 79% and 98% higher in the cropland and rangeland than the natural forest, respectively, and 180% and 210% higher in the cropland and rangeland than the plantation, respectively. The concentration of DOC in detrital leachate was positively correlated with aboveground detrital carbon storage of leaf, branch, humus and litter (p=0.05, n=184). The concentration of DOC in soil solutions in the 0-20 cm deep soil layer was 7.88-88.44 mg•L－1 and its fraction was not more than 1%. The concentration of soil solution DOC was higher under the natural secondary forest or the plantation than cropland or rangeland, and its fraction was lower under natural forest than cropland or rangeland but higher under the plantation than natural secondary forests. The difference of the soil solution DOC concentration or fraction between the natural secondary forest or plantation and rangeland or cropland was greater at 0-10 cm soil depth than 10-20 cm soil layer. The change of this DOC concentration with soil depth was greater under the natural secondary forest or plantation than cropland or rangeland. The concentration of this DOC was positively correlated with detrital carbon storage and soil water content (p=0.05), and soil water content was the main factor that influenced the concentration of DOC in soil solutions. The concentration of DOC in soil percolation water was 5.76-58.84 mg•L－1 which was higher under the natural forest or plantation than cropland or rangeland. The difference in this DOC concentration between the natural secondary forest or plantation and rangeland or cropland was greater in the 0-10 cm than 10-40 cm soil layer. The change of this DOC concentration with soil depth was greater under the natural secondary forest or plantation than cropland or rangeland. These differences were ascribed to the differences in vegetation and soil properties that resulted from changes in land use and their consequent impacts on hydrological processes. The results of this study indicate that changes in land use have large impacts on terrestrial DOC concentrations.