Data associated with "Contributions of hypolithic communities to surface soil organic carbon across a hyperarid-to-arid climate gradient"

Carbon (C) in soils accounts for a substantial and dynamic portion of the global C cycle, with concentrations of soil organic carbon (SOC) typically closely linked to vascular plant productivity. However, C fixed by non-vascular photosynthetic organisms may account for a sizeable proportion of SOC in locations where vascular plants are not abundant. In the hyperarid Namib Desert, vascular plant growth is very limited and largely ephemeral. Extreme abiotic conditions even limit establishment of cyanobacterial soil crusts, with photoautotrophic cyanobacteria largely restricted to the undersides of translucent quartz clasts that buffer environmental extremes. The importance of these ‘hypoliths’ in enhancing SOC pools, and how this may vary with climate and clast physical characteristics, remains unknown. We worked across a rainfall and fog gradient in the central Namib to assess quartz clast size and distribution, factors affecting the probability of hypolithic colonization, and the landscape-level influence of hypoliths on surface SOC pools. Clast colonization increased with clast thickness and with increasing mean annual rainfall. SOC and chlorophyll a (a proxy for cyanobacterial biomass) concentrations were greater under colonized quartz clasts than under non-quartz clasts or in bare soil. Landscape-level SOC, estimated by combining SOC concentration with the distribution and colonization of quartz clasts, was greatest at the site with the highest rainfall and lowest at a mid-gradient site with moderate rainfall and fog. Climate change scenarios that promote quartz colonization have the potential to double SOC pools at the mid-gradient site, although SOC changes would be more muted at other sites. Low vascular plant and biocrust cover in the Namib Desert allows hypolithic communities to play an outsized role in SOC pools; a loss of all hypolithic cyanobacteria would lead to 10-20% declines in surface SOC pools. Future climate change has the potential to shift surface SOC if it alters cyanobacterial colonization of quartz.

土壤中的碳（C）是全球碳循环中占比可观且动态变化的组成部分，土壤有机碳（soil organic carbon, SOC）的浓度通常与维管植物生产力紧密相关。然而，在维管植物匮乏的区域，非维管光合生物（non-vascular photosynthetic organisms）固定的碳可能在土壤有机碳库中占据相当大的比例。在超干旱的纳米布沙漠中，维管植物生长极为受限且大多为短命植物。极端非生物条件甚至限制了蓝藻土壤结皮（cyanobacterial soil crusts）的形成，光能自养蓝细菌（photoautotrophic cyanobacteria）大多仅局限于半透明石英岩屑（translucent quartz clasts）的下表面，这类岩屑可缓冲极端环境条件。这类石下生物（hypoliths）对土壤有机碳库的提升作用，以及其作用如何随气候与岩屑物理特征发生变化，目前仍未明确。我们在纳米布中部沿降雨与雾梯度开展研究，以评估石英岩屑的尺寸与分布、影响石下定殖概率的因素，以及石下生物对地表土壤有机碳库的景观尺度影响。研究发现，岩屑定殖率随岩屑厚度与年平均降雨量的增加而提升。定殖了石下生物的石英岩屑下方的土壤有机碳与叶绿素a（chlorophyll a，蓝藻生物量的替代表征指标）浓度，均高于非石英岩屑下方或裸土中的对应值。通过结合土壤有机碳浓度与石英岩屑的分布及定殖情况估算得到的景观尺度土壤有机碳库，在降雨量最高的站点达到最大，而在降雨量与雾量适中的中度梯度站点则最低。若气候变化情景（climate change scenarios）促进石英岩屑的石下定殖，中度梯度站点的土壤有机碳库规模有望翻倍，不过其他站点的土壤有机碳变化幅度会相对平缓。纳米布沙漠中维管植物与生物结皮覆盖率较低，使得石下生物群落对土壤有机碳库发挥着远超其自身占比的作用；若所有石下蓝细菌消失，地表土壤有机碳库将下降10%~20%。若未来气候变化改变蓝细菌在石英岩屑上的定殖情况，则有可能改变地表土壤有机碳库的规模。