Extreme rainfall: Unravelling the importance of new climate-rhizosphere feedbacks across contrasting land use systems

陆地生态系统通过温室气体（GHG）交换以及土壤和植被中的碳(C)储存，在决定全球气候-生态系统反馈方面发挥着重要作用。越来越多的证据表明，气候变化导致极端天气的频率增加。极端天气对气候-生态系统反馈影响的研究是有限的，因为它们的不可预测性，以及我们无法预测空间和时间上的事件。然而，越来越多的证据表明，极端气候对一系列重要的生态系统功能产生了重大影响。植物根系与构成根际“微生物组”的多种微生物群落有关，这些微生物群落的组成和功能因土地利用而有显著差异，反映了优势植物物种的差异、管理措施的强度以及随之而来的土壤理化特征的差异。极端天气对根际微生物群的影响方式以及对气候生态系统反馈的影响几乎一无所知。据预测，气候变化将增加强降雨事件的频率，这可能通过洪水或地下水上升导致土壤饱和，导致缺氧或缺氧条件，从而促进温室气体的产生，并改变微生物群落。2012年是有记录以来最潮湿的一年，我们第一次在野外研究了自然极端降雨事件对根际微生物群的影响。我们发现，在农林业系统中，长时间的饱和降低了有益的外生菌根真菌的丰度和多样性，而增加了致病真菌和腐养真菌的丰度。我们假设，这些影响将通过改变植物-土壤碳通量、温室气体排放和植物生产力，对气候-生态系统反馈产生深远的、以前未被认识到的影响。我们预计，这些反馈将取决于O2消耗的程度和持续时间（反映降雨的季节时间和持续时间）以及与生态系统参数（包括根际微生物组组成）的相互作用，这些参数根据土地利用系统和管理强度而变化。在这里，我们将首次揭示这些反馈和相互作用。我们将在林肯郡建立宏观环境，包括完整的土壤核心和不同土地利用方式的植被，我们早期的工作就是在林肯郡进行的；低经营强度的永久牧场和农林业，中等经营强度的草料耕地，高经营强度的耕地。我们将模拟生长季节和非生长季节饱和的典型长期（4周）和极端（8周）持续时间。我们将研究饱和处理对温室气体排放的响应和恢复以及根际微生物群的影响。我们将从分类和功能两个层面关注群落的频率、丰度和连通性。重要的是，我们将以整体的方式考虑群落，使我们能够比较真核生物（如真菌、原生生物、线虫）、细菌和古细菌群体对饱和的反应。通过这种方式，我们将对社区对干扰事件的反应提供基本的新理解。我们将用稳定的碳同位素标记植物，这可以与背景C区分开来。这将使我们能够研究饱和对从植物流向土壤的碳量的长期影响，作为土壤碳或呼吸的比例，微生物生物量中保留的碳量，饱和度对原生土壤C释放“启动”的影响。该计划将描述以前被忽视的极端天气对气候-根际反馈的影响，为我们对生态系统对极端天气的响应的基本理解提供一个阶梯式的变化，以及土地利用（包括管理强度）在调节这些响应中的作用。

Terrestrial ecosystems play major roles in determining global climate-ecosystem feedbacks, through exchange of greenhouse gases (GHG), and carbon (C) storage in soil and vegetation. There is mounting evidence that climate change is resulting in increased frequency of extreme weather. Studies of the impacts of extreme weather on climate-ecosystem feedbacks are limited because of their unpredictable nature, and our inability to predict events in space and time. However, evidence is building that climate extremes have significant impacts on a range of vital ecosystem functions.Plant roots are associated with diverse microbial communities which constitute the rhizosphere 'microbiome', the composition and function of which varies significantly between land uses, reflecting differences in the dominant plant species, the intensity of management practices, and consequent differences in soil physico-chemical characteristics. Almost nothing is known of the way in which extreme weather affects the rhizosphere microbiome, and the consequences for climate-ecosystem feedbacks. Climate change is predicted to increase the frequency of intense rainfall events, which can result in soil saturation via flooding or rise in groundwater, leading to hypoxic or anoxic conditions which promote GHG production, and change microbial communities. We were able to study the effect of a natural extreme rainfall event in 2012, the wettest year since records began, on the rhizosphere microbiome, in the field, for the first time. We revealed that prolonged saturation in an agroforestry system reduced the abundance and diversity of beneficial ectomycorrhizal fungi, while increasing abundance of pathogenic and saprotrophic fungi. We hypothesise that these effects will have profound and previously unrecognised impacts on climate-ecosystem feedbacks, operating through altered plant-soil C flux, GHG emissions and plant productivity. We anticipate that these feedbacks will depend on the extent and longevity of O2 depletion (reflecting seasonal timing and duration of rainfall) and Interaction with ecosystem parameters, including rhizosphere microbiome composition, which vary according to land use system and management intensity. Here we will unravel these feedbacks and interactions for the first time. We will set up macrocosms with intact soil cores and vegetation from contrasting land uses on a gley soil in Lincolnshire, where our earlier work was conducted; low management intensity permanent pasture and agroforestry, intermediate management intensity grass-arable, and high management intensity arable. We will simulate growing season and non-growing season saturation typical of long term (4 week) and extreme (8 week) durations. We will investigate the effect of saturation treatments on the response and recovery of GHG emissions and the rhizosphere microbiome. We will focus on the frequency, abundance and connectivity of communities at both taxonomic and functional levels. Importantly we will consider communities in a holistic manner, allowing us to compare responses of eukaryote (eg fungi, protists, nematodes), bacterial and archaeal groups to saturation. In this way we will provide fundamental new understanding of community responses to disturbance events. We will label plants with a stable isotope of C which can be differentiated from background C. This will allow us to investigate the long term effect of saturation on the amount of C which flows from the plant to the soil, the proportion retained as soil C or respired, amounts retained in the microbial biomass, and the effects of saturation on 'priming' of C release form native soil C. The programme will characterise previously overlooked effects of extreme weather on climate-rhizosphere feedbacks, delivering a step-change in our fundamental understanding of the responses of ecosystems to extreme weather, and the role of land use, including management intensity, in mediating these responses.