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交换以及土壤和植被中的碳储存，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。关于极端天气对气候生态系统反馈的影响的研究是有限的，因为它们具有不可预测的性质，而且我们无法预测时空事件。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重大影响。植物根系与构成根际“微生物组”的各种微生物群落有关，其组成和功能在不同的土地利用方式之间存在显著差异，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性的差异。极端天气影响根际微生物组的方式以及气候生态系统反馈的后果几乎一无所知。据预测，气候变化将增加强降雨事件的频率，这可能会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。我们能够研究2012年自然极端降雨事件对根际微生物组的影响，这是有记录以来最潮湿的一年，这是第一次。我们发现，在农林复合系统中，长期饱和减少了有益的外生菌根真菌的丰度和多样性，同时增加了病原真菌和腐生性真菌的丰度。我们假设这些影响将对气候生态系统反馈产生深刻的和以前未被认识到的影响，通过改变植物-土壤碳通量，温室气体排放和植物生产力来运作。我们预计，这些反馈将取决于O2耗竭的程度和寿命（反映季节性时间和降雨持续时间）以及与生态系统参数的相互作用，包括根际微生物组组成，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互动。我们将在林肯郡的潜育土上建立一个完整的土芯和对比土地利用植被的宏观世界，我们早期的工作是在那里进行的;低管理强度的永久性牧场和农林复合经营，中等管理强度的草耕地，高管理强度的耕地。我们将模拟长期（4周）和极端（8周）持续时间的生长季节和非生长季节饱和度。我们将研究饱和处理对温室气体排放和根际微生物组的响应和恢复的影响。我们将专注于频率，丰度和社区的连通性在分类和功能水平。重要的是，我们将以整体的方式考虑社区，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度的反应。通过这种方式，我们将为社区对干扰事件的反应提供新的理解。我们将用一种稳定的碳同位素标记植物，它可以与背景碳区分开来。这将使我们能够研究长期的饱和度对从植物流到土壤中的碳的量的影响，土壤碳或呼吸的比例，微生物生物量中保留的量，以及饱和度对天然土壤碳释放的“启动”的影响。该计划将消除以前被忽视的极端天气对气候根际反馈的影响，使我们对生态系统对极端天气的反应以及土地利用（包括管理强度）在调解这些反应中的作用的基本理解发生逐步变化。

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.