Center for Ecosystem Science and Society, Northern Arizona University, Flagstaff, AZ, USA.
Biological Sciences, Northern Arizona University, Flagstaff, AZ, USA.
Glob Chang Biol. 2021 Mar;27(6):1293-1308. doi: 10.1111/gcb.15481. Epub 2020 Dec 24.
Almost half of the global terrestrial soil carbon (C) is stored in the northern circumpolar permafrost region, where air temperatures are increasing two times faster than the global average. As climate warms, permafrost thaws and soil organic matter becomes vulnerable to greater microbial decomposition. Long-term soil warming of ice-rich permafrost can result in thermokarst formation that creates variability in environmental conditions. Consequently, plant and microbial proportional contributions to ecosystem respiration may change in response to long-term soil warming. Natural abundance δ C and Δ C of aboveground and belowground plant material, and of young and old soil respiration were used to inform a mixing model to partition the contribution of each source to ecosystem respiration fluxes. We employed a hierarchical Bayesian approach that incorporated gross primary productivity and environmental drivers to constrain source contributions. We found that long-term experimental permafrost warming introduced a soil hydrology component that interacted with temperature to affect old soil C respiration. Old soil C loss was suppressed in plots with warmer deep soil temperatures because they tended to be wetter. When soil volumetric water content significantly decreased in 2018 relative to 2016 and 2017, the dominant respiration sources shifted from plant aboveground and young soil respiration to old soil respiration. The proportion of ecosystem respiration from old soil C accounted for up to 39% of ecosystem respiration and represented a 30-fold increase compared to the wet-year average. Our findings show that thermokarst formation may act to moderate microbial decomposition of old soil C when soil is highly saturated. However, when soil moisture decreases, a higher proportion of old soil C is vulnerable to decomposition and can become a large flux to the atmosphere. As permafrost systems continue to change with climate, we must understand the thresholds that may propel these systems from a C sink to a source.
全球陆地土壤碳（C）的近一半储存在北极永久冻土区，那里的空气温度上升速度是全球平均水平的两倍。随着气候变暖，永久冻土融化，土壤有机物质更容易受到微生物的分解。富含冰的永久冻土的长期土壤变暖可能导致热喀斯特的形成，从而导致环境条件的变化。因此，植物和微生物对生态系统呼吸的相对贡献可能会因长期土壤变暖而发生变化。利用地上和地下植物材料以及年轻和年老土壤呼吸的天然丰度 δC 和 ΔC，采用混合模型来划分每个源对生态系统呼吸通量的贡献。我们采用了分层贝叶斯方法，该方法结合了总初级生产力和环境驱动因素来约束源贡献。我们发现，长期的永久冻土变暖实验引入了一个土壤水文学成分，该成分与温度相互作用，影响老土壤 C 的呼吸。在较温暖的深层土壤温度下，由于土壤较潮湿，老土壤 C 的损失受到抑制。当土壤体积含水量在 2018 年与 2016 年和 2017 年相比显著下降时，主要的呼吸源从地上植物和年轻土壤呼吸转移到老土壤呼吸。老土壤 C 呼吸的生态系统呼吸比例最高可达 39%，与湿润年份的平均值相比增加了 30 倍。我们的研究结果表明，当土壤高度饱和时，热喀斯特的形成可能会减缓老土壤 C 的微生物分解。然而，当土壤水分减少时，更多的老土壤 C 容易分解，并可能成为大气中的一个大通量。随着永久冻土系统继续随着气候而变化，我们必须了解可能促使这些系统从碳汇转变为源的阈值。
