Cross-season legacy effects of climate extremes on alpine soil microbial communities: resilience, regimes shifts and biogeochemical cycles

土壤中微生物种类繁多，是地球运行所依赖的生物地球化学循环的重要驱动力。这些土壤微生物群落很容易受到人类活动引起的环境变化的影响，包括土地利用和气候变化。然而，我们对环境变化导致的微生物群落的变化如何影响它们所支持的全球生物地球化学循环的理解很差。大多数研究只调查环境变化的一个方面，加剧了这种缺乏知识的情况：在现实世界中，人类的行动正在对环境造成多个同时的变化，包括预计将随着持续的气候变化而变得更加强烈和频繁的气候极端事件。这就是这项建议的意义所在：了解不同季节的极端气候如何结合在一起影响土壤微生物群落及其支持的关键生物地球化学循环。我们在高山生态系统中做到了这一点，这些生态系统覆盖了地球表面的大部分，支持了高水平的生物多样性，并为人类提供了一系列服务，包括储存大量的碳、养分和水。此外，山区受到气候和土地利用变化的相当大的威胁。例如，山区发生气候变化的速度几乎是北半球平均水平的两倍，导致冬季降雪减少，从而减少了地面的隔热，增加了冻融活动，而夏季降雨量减少，导致长期干旱。然而，这两个因素是如何结合在一起影响土壤微生物群落的，人们对此知之甚少。此外，山区的农业也在发生变化，许多山区放弃了传统的放牧做法，导致矮小灌木侵占高山地带。简而言之，山区处于全球变化的前线，但对微生物群落和它们所执行的生物地球化学过程的后果仍然知之甚少。虽然土壤微生物通常能够抵抗或从单个环境扰动中恢复，但它们可能无法承受快速连续发生的多个冲击。冬季降雪减少，紧随其后的是严重的夏季干旱，这可能会使土壤微生物群落及其执行的养分循环功能进入替代状态。当这种转变发生时，土壤微生物群落的结构及其循环关键养分的能力将永久改变。令人担忧的是，我们不知道在什么条件下微生物群落会发生这种制度变化，也不知道土地利用做法的变化是否会影响结果。我们也不知道这对全球生物地球化学循环意味着什么，考虑到高山土壤中储存的大量碳，以及氮循环对农业活动的重要性，这是令人惊讶的。该建议正面解决了这一问题，测试了积雪减少和夏季干旱如何影响土壤微生物群落的多样性和功能，以及对这些未被研究的生态系统中生物地球化学循环的影响。我们将解决以下迄今尚未探索的问题。一种极端气候是否会损害土壤微生物群落及其支持的生物地球化学循环抵御进一步干扰并从中恢复的能力？在夏季干旱的频率和严重程度如何，微生物群落会进入另一种状态？灌木的侵蚀是否抑制了不同季节极端气候对土壤微生物群落的综合影响？通过测试这些问题，我们将获得对微生物群落的结构、功能和动力学的新的、变革性的理解，以及这如何与生物地球化学循环联系起来，我们将在一个研究不足和脆弱的自然生态系统中持续和快速的环境变化的背景下进行现场研究。

Soil contains a vast diversity of microbial that is an important driver of the biogeochemical cycles on which the functioning of earth depends. These soil microbial communities are vulnerable to environmental changes caused by human actions, including land use and climate change. However, our understanding of how shifts in microbial communities resulting from environmental change influences the global biogeochemical cycles they support is poor. This lack of knowledge is exacerbated by most studies investigating only one facet of environmental change: in the real world, human actions are causing multiple and simultaneous changes to the environment, including climate extremes that are expected to become more intense and frequent with on-going climate change. This is what this proposal is about: understanding how seasonally distinct climate extremes combine to impact soil microbial communities and the crucial biogeochemical cycles that they support. We do this in high mountain ecosystems, which cover a large part of the Earth's land surface, support high levels of biodiversity, and provide a host of services for humankind, including the storage of vast amounts of carbon, nutrients and water. Moreover, mountains are under considerable threat from climate and land use change. Climate change, for example, has been taking place in the mountains at almost double the rate of the northern hemisphere average, leading to less snow in winter, which reduces insulation of the ground and increases freeze-thaw activity, and less rain in summer, which causes prolonged drought. Yet, how these two factors combine to affect soil microbial communities, and is poorly understood. Furthermore, farming in mountains is also changing, with traditional grazing practices being abandoned in many mountains areas, which is causing encroachment of dwarf shrubs into the alpine zone. Put simply, mountain areas are at the front line of global change, but the consequences for microbial communities and the biogeochemical processes that they perform remain poorly understood. Whilst soil microbes can often resist or recover from individual environmental perturbations, they may not be able to withstand multiple shocks happening in quick succession. Reduced snow in winter, followed immediately by severe summer droughts may tip soil microbial communities, and the nutrient cycling functions they perform, into alternative states. When such a shift occurs, the structure of the soil microbial community, and its capacity to cycle key nutrients will be permanently altered. Alarmingly, we do not know under which conditions such regimes shifts in microbial communities will occur, or whether changes in land-use practices will affect the outcome. Nor do we know what it will mean for global biogeochemical cycles, which is surprising considering the vast amounts of carbon stored in alpine soils, and the importance of nitrogen cycling for agricultural activities. This proposal tackles this issue head on, testing how reduced snow cover and summer drought affects the diversity and function of soil microbial communities and the consequences for biogeochemical cycles in these understudied ecosystems. We will tackle the following so far unexplored questions. Does one type of climate extreme impair the ability of soil microbial communities, and the biogeochemical cycles they support, to resist and recover from further perturbations? At what frequencies and severities of summer drought do microbial communities tip into an alternative state? Does encroachment of shrubs dampen the combined effect of seasonally distinct climate extremes on soil microbial communities? By testing these questions, we will gain novel, transformative understanding of the structure, function, and dynamics of microbial communities, and how this links to biogeochemical cycling, and we will do so in situ in the context of ongoing and rapid environment change in an understudied and vulnerable natural ecosystem.