三江源高寒草甸下溪流溶解性有机碳的季节性输移特征

Seasonal Variations of Dissolved Organic Carbon Exports in Streams Under Alpine Meadow in the Three Rivers' Headwater Regions,Qinghai-Tibetan Plateau

溶解性有机碳(DOC)在生态系统中起着重要作用,但河流DOC输移动态及其对气候变化和多年冻土退化的响应还不清楚。对青藏高原三江源地区高寒草甸下8条小流域河流于2016年8月至2017年7月进行逐月采集水样,同步测定流量,在室内对DOC浓度进行分析,研究河流DOC浓度和输移通量的逐月变化规律以及对降雨和温度逐月变化的响应。结果表明:(1) DOC的年平均浓度介于4. 05±1. 20～6. 55±2. 86 mg·L^(-1)之间,均值为5. 30 mg·L-1; DOC平均浓度与流域内高寒沼泽草甸(ASM)覆盖面积比例呈显著线性负相关,而与高寒草甸(AM)呈显著线性正相关;此外,有多年冻土发育的流域内河流平均DOC浓度明显高于无多年冻土发育的流域。(2) DOC浓度随季节性气温的变化呈现较大的变异性,在-8～2℃气温回升的过程中,DOC平均浓度随气温的升高呈急剧上升的趋势,在随后2～13℃气温继续升高到最高的过程中,DOC平均浓度又急速降低,而在之后13～-8℃气温下降的过程中,DOC平均浓度呈一个缓慢降低的趋势,并在12月达到最低。(3) DOC平均输移通量也显示出较大的季节差异,其范围为0. 006±0. 000 5～3. 01±0. 74 kg·km^(-2)·d^(-1),均值为1. 12±0. 81 kg·km^(-2)·d^(-1); DOC输移通量与流域内平均径流量显著线性正相关,DOC的输移主要集中在春季融雪期和夏季丰雨期。气候变暖会导致多年冻土活动层厚度增加,因此,温度增加导致DOC输移增加的结果提示,气候变暖可能会增加青藏高原高寒草甸区河流对有机碳输移和释放。

Dissolved organic carbon(DOC)plays an important role in ecosystem.Little is known about the dynamics of riverine DOC exports and their responses to climate change and permafrost degradation.To examine the seasonal variations of concentrations and exports of DOC and their responses to changes of air temperature and precipitation, we collected monthly river water samples from 8 catchments under alpine meadow in the Three Rivers’headwater regions on the Qinghai-Tibet Plateau from August 2016 to July 2017.The discharges in these rivers were monitored, and DOC concentrations of water samples were analyzed in the laboratory.The results showed:(1)The annual average DOC concentrations of these streams varied from(4.05±1.20)mg·L^-1 to(6.55±2.86)mg·L^-1 ,with an mean value of 5.30 mg·L^-1 .The DOC concentrations were significantly negatively correlated with the coverage area of alpine swamp meadow(ASM),but significantly correlated with the coverage of alpine meadow(AM).The average DOC concentrations in the rivers in permafrost regions were significantly higher than those in non-permafrost regions.(2)The DOC concentrations showed a great variations with seasonal changes of air temperature.From the spring to the beginning of summer,DOC concentrations increased sharply.During this period,the monthly mean air temperature increased from -8℃ to 2℃.The DOC concentrations decreased rapidly as the air temperature increased from 2℃ to 13℃.From summer to winter,the temperature decreased from 13℃ to -8℃.Meanwhile,the average DOC concentrations decreased gradually,and the lowest value was recorded in December.(3)The seasonal changes of DOC export varied greatly,ranging from(0.006± 0.0005)kg·km-2·d^-1 and(3.01 ± 0.74)kg·km-2·d^-1 among the rivers,with an average value of(1.12±0.81)kg·km-2·d^-1 .DOC export fluxes were positively correlated with the discharges.The highest DOC export was recorded in the spring snowmelt period and the rainy summer.Since the climate change may lead to a thicker active layer in permafrost regions,our results showed that the DOC export increased along with temperature,indicating that climate warming can cause a higher DOC export in alpine meadow in the permafrost regions on the Qinghai-Tibetan Plateau.