普定岩溶水-碳循环模拟试验场水体双碳同位素特征(δ13C-Δ14C)与碳足迹

河流输送到海洋的溶解无机碳(DIC)和有机碳(OC)受自然和人为双重因素的影响。了解DIC和OC的年龄、来源和转化,有助于掌握全球碳收支和提高现在以及未来自然和人类对河流碳循环影响的估算精度。本研究以普定岩溶水-碳循环试验场泉(地下水)-池(地表水)耦联系统为研究对象,利用双碳同位素(¹³C- ¹⁴C)方法,结合水生植物生长和传统水文地球化学特征,揭示了地下水-地表水系统中DIC和颗粒有机碳(POC)的来源及其转化机制。研究发现:(1)泉-池系统中DIC和POC的Δ¹⁴C具有相同的变化趋势,泉水中Δ¹⁴C值低于池水中Δ¹⁴C值,反映后者可能有“较年轻”的CO₂的加入;(2)池水水化学和碳同位素变化由土地利用类型和池中水生植物共同控制;(3)池水中颗粒有机碳(POC)浓度明显高于泉水,且其Δ¹⁴C值表现出与沉水植物和DIC的一致性(表观年龄均为3 200900 a),说明池水POC主要源于池中水生植物光合作用利用了碳酸盐风化产生的老碳(DIC),使新形成的有机质在表观年龄上“偏老”;(4)池水水体内源有机碳对水体POC的贡献在75%以上,内源有机碳通量(以C计)在250 t·km^-2·a^-1至660 t·km^-2·a^-1之间,相对于其他土地利用类型,草地对应的地表水系统具有最大的内源有机碳占比和通量,指示了沉水植物控制型浅水水体初级生产对有机碳循环的重要作用。综上,我们认为在岩溶区通过土地利用调整来调控水生植物群落对于增加碳汇具有重要潜力。

Dual carbon isotope (δ13C-Δ14C) characteristics and carbon footprint in the spring-pond systems at the Puding Karst Water-Carbon Cycle Test Site

The dissolved inorganic carbon (DIC) and organic carbon (OC) transported to the ocean by rivers are affected by natural and anthropogenic factors. Understanding the influence of these factors on the age, source, and transformation of DIC and OC can help constrain global carbon budget and improve the assessments of natural and human impacts on the riverine carbon cycle. The present study uses dual carbon isotopes (δ¹³C-Δ¹⁴C) of DIC, particulate organic carbon (POC), and aquatic plants with traditional hydrogeochemical characteristics in the spring (groundwater) and spring-fed pond (surface water) systems at the Puding Karst Water-Carbon Cycle Test Site to identify the relavent carbon sources and carbon transformation processes. The results showed that (1) the Δ¹⁴C of DIC and POC in the spring-pond systems displayed the same trend, that is, the Δ¹⁴C value is lower in the spring water than in the pond water, indicating likely a younger CO₂ invasion of the pond system. (2) The changes in water chemistry and carbon isotope of pond water were controlled by different land use and aquatic plants. (3) The OC concentrations were significantly higher in pond water than in spring water, and the Δ¹⁴C values of POC were close to those of submerged plants and DIC (apparent ages of 3200-900 years). Such finding indicates that the POC in the pond water was mainly derived from aquatic photosynthesis, which involved old carbon (DIC)—produced by the weathering of carbonate rocks—as carbon source to generate new autochthonous OC (AOC) with negative Δ¹⁴C values and “old” apparent age. (4) The contribution percentage of AOC to POC in surface water was more than 75%. Meanwhile, the AOC flux in the spring-pond systems was between 250-656 t C km^-2 a^-1, and the surface water system of grassland contributed the most AOC and had the highest AOC flux relative to other land use. Such finding indicates the important role of primary production in the organic carbon cycle in shallow water controlled by submerged plants. Overall, the regulation of aquatic plant communities through land use adjustment in karst areas can have major impact on the increasing of carbon sink capacity in these areas.