江河源冻土区土壤碳氮空间分布特征及其影响因素

江河源区是我国高寒生态安全屏障的重要区域,冻土的长期存在使其形成低温冻结环境,弱化了土壤微生物活性,抑制了土壤有机质的矿化过程,因而其近地表浅层土壤碳氮含量高。然而,土壤碳氮含量对不同冻土分区和环境因素响应的空间分异规律尚不清楚。为此,针对江河源4个不同冻土区(季节冻土区、岛状多年冻土区、不连续多年冻土区、片状连续多年冻土区)共11个样点进行植被样方调查、土壤分层采样。在分析碳氮含量的基础上,探讨了年均地温(MAGT)、活动层厚度(ALT)、海拔(ASL)、土壤深度(SD)、植被特征及土壤pH对土壤有机碳(SOC)、全氮(TN)、碳氮比(C/N)的影响。结果表明:(1)SOC、TN、C/N在片状连续多年冻土区最高,在季节冻土区最低,且与年均地温负相关,和海拔正相关;(2)江河源区SOC、TN、C/N随土壤深度的增加而降低,自表层至40cm深度整体下降幅度分别为58.45%、36.96%、17.01%;(3)SOC、TN、C/N与植被覆盖度(FVC)显著正相关(P≤0.05),与土壤pH值显著负相关(P≤0.01);(4)冗余分析表明,土壤pH、MAGT、ALT、SD、FVC是影响江河源区SOC、TN、C/N空间分布的关键因素。研究结果可为厘清气候趋暖条件下江河源区土壤碳氮空间分异规律及多年冻土热稳定性对土壤碳氮排放的影响提供科学基础,同时也有助于预测多年冻土区土壤碳氮空间变化。

Spatial distribution of soil organic carbon and total nitrogen contents in association with permafrost variability in the Source Areas of the Yangtze and Yellow Rivers

The Source Areas of the Yangtze and Yellow Rivers (SAYYR), which is located in the transitional zone from predominantly continuous permafrost to seasonally frozen ground, is one of the most important cold regional ecological barriers in China. The negative temperature and frozen environment facilitate the long-term existence of frozen ground, accordingly inhibiting soil microbial activities and the mineralization process of soil organic matter, resulting in high carbon and nitrogen contents in the near-surface shallow soils. However, it is still unclear how the soil carbon and nitrogen content respond to the spatial distribution pattern of the variability in the thermal state of permafrost and environmental factors. In this study, eleven plots were investigated at four different typical kinds of frozen ground (seasonally frozen ground, island permafrost, discontinuous permafrost, and predominantly continuous permafrost) in the SAYYR. We conducted plant quadrat survey and soil sampling from different depths. After analyzing the soil carbon and nitrogen content, we explore the distribution of soil organic carbon (SOC), total nitrogen (TN), C/N, and key factors that influence the distribution of soil organic carbon (SOC), total nitrogen (TN), and C/N. The environmental factors include mean annual ground temperature (MAGT), active layer thickness (ALT), altitude (ASL), soil depth (SD), vegetation characteristics, and soil pH. The results show that: (1) the contents of SOC, TN, and C/N were positively correlated with altitude but negatively correlated with MAGT, being characteristics of an obviously vertical pattern, with the highest values coming from the predominantly continuous permafrost region, and the lowest values coming from the seasonally frozen ground region; (2) With the increase of soil depth, the overall decreasing rates of SOC, TN, and C/N were 58.45%, 36.96%, and 17.01%, respectively; (3) The SOC and TN contents were positively correlated with fractional vegetation (FVC) (P≤0.05), but significantly negatively correlated with soil pH (P≤0.01); and; (4) The redundant analysis showed that soil pH, MAGT, ALT, SD, and FVC in permafrost were the dominant environmental factors for the spatial distribution and depth patterns of SOC, TN, and C/N. Our study provides data support for clarifying the spatial distribution characteristics between SOC, TN, and C/N, and the determinations of the thermal stability of permafrost on soil carbon and nitrogen emission under climate warming in the SAYYR in the future. Thus, our study will also shed light on the prediction of soil carbon and nitrogen change in permafrost region, which is one of the biggest carbon pools of great uncertainty.