近百年来湖泊有机碳与无机碳埋藏响应流域开发的协同变化——以石林喀斯特地区为例

Synchronic changes of lake organic and inorganic carbon burial in response to catchment development over the past century:A case study of Shilin Karst area

有机碳和无机碳的流域输出是湖泊碳埋藏的重要驱动因子,而喀斯特地区无机碳循环具有反应迅速且对人类活动影响敏感的特点.在流域开发持续增强的背景下,喀斯特地区湖泊有机碳和无机碳的来源、含量与埋藏通量可能会出现同步变化的协同模式.本文以云南省石林喀斯特地区流域土地利用类型不同的两个中型湖泊(长湖、月湖)开展对比分析,通过对沉积物钻孔的土壤侵蚀强度(磁化率)、流域外源输入(C:N比值)、水动力(粒度)、营养盐(总氮、总磷)、藻类生产力(叶绿素色素)等代用指标的分析,结合监测数据和历史资料重建了两个湖泊环境变化的近百年历史,并定量识别了有机碳和无机碳埋藏响应流域开发的变化特征与协同模式.沉积物磁化率和C:N比值结果揭示了流域地表侵蚀和外源输入的阶段性特征,同时总氮和总磷含量记录了长湖和月湖营养水平上升的长期模式.在流域森林覆被较高(33.43%)的长湖中,全岩和有机质C:N比值分别与磁化率信号呈显著正相关(r=0.95和0.89,P<0.001),且与无机碳和有机碳含量呈显著负相关(r=-0.94,P<0.001和r=-0.52,P=0.01),反映了森林植被退化时流域碳输出的减少对沉积物碳含量的影响.而在流域耕地覆被为主(60.98%)的月湖中,全岩和有机质C:N比值与磁化率信号呈显著负相关(r=-0.54,P<0.01和r=-0.67,P<0.001),且全岩C:N比值与无机碳含量(P=0.15)无显著关系,反映了耕作强度的增加可能促进了水体富营养化和内源输入的增强.在两个湖泊中,营养水平的上升和内源生产力的增加促进了有机碳含量的快速增加.进一步分析表明,近百年来长湖有机碳与无机碳含量变化的同步性明显(r=0.54,P<0.001)而在月湖中无显著关系(P=0.20,P>0.05).两个湖泊中沉积通量的变化均与全岩C:N比值呈显著正相关(r=0.48和0.45,P≤0.001),且有机碳与无机碳埋藏通量均呈现同步变化的显著特征(r=0.72和0.85,P<0.001).其中长湖的无机碳埋藏通量显著高于有机碳埋藏通量,而月湖的有机碳埋藏通量略高于无机碳埋藏通量,反映了岩溶地区流域外源输入和水体富营养化的差异性驱动影响.在流域开发增强的梯度下,森林退化会降低流域碳输出的负荷,而农业扩张和水体富营养化会促进藻类生长和内源有机碳的累积.因此,流域土地利用和水体营养水平对湖泊无机碳和有机碳埋藏变化的长期轨迹和协同模式产生了重要影响,对喀斯特地区的碳库评估需要考虑无机碳循环的重要性.

Catchment-scale exports of organic and inorganic carbon plays an important role in affecting the lake sediment carbon burial.Inorganic carbon cycling in Karst landscapes is sensitive to human activities due to the rapid kinetics of carbonate dissolution.As with intensive watershed development,the source,content and burial loading of organic and inorganic carbon in Karst region may show a synchronous change.This study selected two medium-sized lakes(Lake Changhu and Lake Yuehu) with contrasting land uses patterns in the Shilin Karst area of central Yunnan for sediment surveys.With the proxies analyses for soil erosion(i.e.magnetic susceptibility(MS)),catchment allochthonous input(C:N ratio),lake hydrology(particle size),nutrient levels(total phosphorus and nitrogen) and primary production(i.e.algal pigments),together with monitoring data and historical records,the history of limnological changes and catchment development over the last century for both lakes were reconstructed.Furthermore,long-term variation and synchronic pattern of sediment organic carbon(OC) and inorganic carbon(IC) burial in the context of catchment development were quantified.The sediment MS signal recorded the fluctuation in soil erosion intensity and a general pattern of long-term nutrient enrichment in both lakes.In Lake Changhu,with a higher land cover of forest(33.43%),the bulk and organic C:N ratios were positively related with the MS signal(r=0.95 and 0.89,P<0.001) but showed a negative relationship with sediment inorganic and organic carbon content(r=-0.94,P<0.001 and-0.52,P=0.01),respectively,reflecting a significant impact of catchment carbon input.In Lake Yuehu,with a current land cover dominated by the cropland(60.98%),the bulk and organic C:N ratios were negatively related with the MS signal(r=-0.54,P<0.01 and-0.67,P<0.001) and the bulk C:N ratio showed no relationship with the sediment inorganic content(P=0.15),reflecting that the cropland expansion may have enhanced lake eutrophication and autochthonous input.In both lakes,nutrient enrichment and algal production enhanced the rapid OC accumulation.Additional analyses showed that there existed a significantly synchronous change between OC and IC content in Lake Clanghu(r=0.54,P<0.001) but not in Lake Yuehu(P=0.20,P>0.05).In both lakes,sediment loading was positively related with bulk C:N ratio(r=0.48 and 0.45,P≤0.001),and there also existed a significantly synchronous change between OC and IC loadings(r=0.72 and 0.85,P<0.001).Furthermore,the IC burial rate was significantly higher than that of OC in Lake Changhu,while IC consistently lower than OC in the nutrient-rich Lake Yuehu,reflecting differential forcing of vegetation degradation and lake eutrophication.With the intensive development in watershed,forest degradation can reduce the loading of catchment carbon export,while agricultural expansion and eutrophication would promote the growth of algae and the OC accumulation rate.Therefore,land use and lake nutrient levels can significantly impact the long-term trajectory and synchronic variation of sediment OC and IC burial.Thus,IC cycling should be considered in the regional assessment of carbon stock in karst areas.