Alpine Grassland Soil Organic Carbon Stock and Its Uncertainty in the Three Rivers Source Region of the Tibetan Plateau

Alpine grassland of the Tibetan Plateau is an important component of global soil organic carbon (SOC) stocks, but insufficient field observations and large spatial heterogeneity leads to great uncertainty in their estimation. In the Three Rivers Source Region (TRSR), alpine grasslands account for more than 75% of the total area. However, the regional carbon (C) stock estimate and their uncertainty have seldom been tested. Here we quantified the regional SOC stock and its uncertainty using 298 soil profiles surveyed from 35 sites across the TRSR during 2006–2008. We showed that the upper soil (0–30 cm depth) in alpine grasslands of the TRSR stores 2.03 Pg C, with a 95% confidence interval ranging from 1.25 to 2.81 Pg C. Alpine meadow soils comprised 73% (i.e. 1.48 Pg C) of the regional SOC estimate, but had the greatest uncertainty at 51%. The statistical power to detect a deviation of 10% uncertainty in grassland C stock was less than 0.50. The required sample size to detect this deviation at a power of 90% was about 6–7 times more than the number of sample sites surveyed. Comparison of our observed SOC density with the corresponding values from the dataset of Yang et al. indicates that these two datasets are comparable. The combined dataset did not reduce the uncertainty in the estimate of the regional grassland soil C stock. This result could be mainly explained by the underrepresentation of sampling sites in large areas with poor accessibility. Further research to improve the regional SOC stock estimate should optimize sampling strategy by considering the number of samples and their spatial distribution.

青藏高原高寒草原是全球土壤有机碳（soil organic carbon, SOC）库的重要组成部分，但野外实地观测数据不足且空间异质性极强，导致其储量估算存在极大不确定性。三江源地区（Three Rivers Source Region, TRSR）的高寒草原总面积占比超过75%，但目前鲜有研究对该区域的碳储量估算及其不确定性开展验证。本研究依托2006至2008年间在TRSR范围内35个样点采集的298个土壤剖面，量化了该区域的SOC储量及其不确定性。研究结果显示，TRSR高寒草原0~30cm表层土壤的有机碳储量达2.03拍克碳（Pg C），95%置信区间为1.25~2.81 Pg C。其中高寒草甸土壤占区域SOC总估算储量的73%（约1.48 Pg C），但其不确定性占比最高，达51%。若检测草原碳储量10%的偏差，其统计功效低于0.50；若要将统计功效提升至90%以检测该偏差，所需采样点数量约为当前调查样点数的6~7倍。将本研究观测得到的SOC密度与Yang等发布的数据集对应值进行对比后发现，两套数据集的结果具有较好的可比性，但合并两套数据集并未降低区域草原土壤碳储量估算的不确定性。该结果主要可归因于偏远难达区域的采样点覆盖不足。未来若要优化区域SOC储量的估算精度，需结合采样数量与空间分布特征优化采样策略。