Digging into the "Gadgil effect": how the competitive balance between fungal guilds affects carbon and nitrogen cycling

森林土壤是解决全球气候变化的一部分，通过固碳（C）来补偿人为CO2排放，这是全球气温上升的主要原因。土壤固碳取决于两个对比过程：土壤有机质（SOM）分解，导致碳损失，和SOM稳定，通过将含碳生物分子转化为耐腐颗粒，导致碳储存。分解过程主要由自由生活的腐养（ST）土壤真菌控制。这些真菌通过分解有机质获得C（能量）用于代谢。另一类土壤生物是根栖共生外生菌根（EM）真菌，它们从与树木的结合中获得C，并探索土壤中的其他营养物质，如氮和磷。由于EM真菌从树木中获得C供应，大多数已经失去了从SOM降解中获得C的酶促能力，因为这对它们的营养来说不是必需的。由于ST和EM真菌都是森林土传生物，它们相互作用和竞争，特别是对土壤养分的竞争。这种竞争性的相互作用可能会导致抑制SOM分解（在EM优势下），因为EM真菌不需要分解SOM的碳，但确实迅速获得土壤养分，如N和P。观察到的影响，导致抑制SOM分解，因为这些真菌类型的竞争被称为“Gadgil效应”。如果这是真的，这种现象控制了土壤分解过程中的二氧化碳释放，从而通过增强土壤中的碳固存为减缓气候变化提供了为数不多的选择之一。然而，ST和EM真菌在C循环中相互作用的机制仍然存在争议，并且知之甚少，证据基础不一致。在我们的项目中，我们建议确定ST和EM真菌之间的相互作用是否可以被忽视的森林土壤生态学的一个组成部分，可以被操纵，以增加土壤固碳在森林中。在世纪的后半叶，因为Gadgil效应被提出，流行的观点是，该组EM或ST真菌作为一个整体。然而，最近的遗传研究表明EM真菌之间具有高度的功能多样性，这表明将它们视为一个组是错误的。这可能很好地解释了为什么科学文献中报道了许多关于加德吉尔效应的相互矛盾的发现。我们对EM真菌作为一个群体在代谢中的丰富功能的新理解开辟了询问并最终确认Gadgil效应存在（或其他）的新途径。我们打算确定的机制，导致改变土壤固碳由于ST和EMF真菌的相互作用：例如，增加某些EM真菌的优势可能会导致增加在森林土壤中的稳定SOM的创建。通过这种方式，土壤真菌可以作为缓解全球气候变化的一个有管理的组成部分。在这个建议中，我们具体解决3个基本问题：（1）干-EM真菌的相互作用在SOM分解和稳定的作用是什么？(2)真菌类群的进化起源和功能生态是否影响它们之间的相互作用对SOM动态的影响？(3)在不同的环境条件下，社区中有哪些机制在促进科学、技术和少数民族之间的互动？我们选择了四个不同的真菌群，每个真菌群由一个EM真菌与最接近的ST物种配对组成。我们设计了一个强大的模型系统，以定量地解决真菌的相互作用对SOM的分解和稳定的功能效果。我们的实验计划包括四个级别的复杂性，依次验证ST-EM真菌相互作用的发现：（1）生长培养基上的真菌培养物，（2）EM真菌与植物共生的微观世界，（3）森林树木幼苗的微观世界，和（4）树种和土壤氮素可用性不同的田间地点。

Forest soils are part of the solutions to global climate change by sequestering carbon (C) to compensate for anthropogenic CO2 emissions, which are the major cause of rising global temperatures. Soil C-sequestration depends on two contrasting processes: soil organic matter (SOM) decomposition which results in C-loss, and SOM stabilisation that results in C-storage by converting C-containing biomolecules into decay-resistant particles. Decomposition processes are primarily controlled by the free-living saprotrophic (ST) soil fungi. These fungi gain C (energy) for their metabolism by decomposing SOM. Another group of soil organisms are root dwelling symbiotic ectomycorrhizal (EM) fungi that obtain C from their association with trees, and explore the soil for other nutrients such as nitrogen and phosphorus. As EM fungi have a C supply from trees most have lost their enzymatic capabilities to acquire C from SOM degradation, as this is not an imperative for their nutrition. As ST and EM fungi are both forest soil-borne organisms they interact and compete, particularly for soil nutrients. This competitive interaction may result in suppression of SOM decomposition (under EM dominance) as the EM fungi do not need to breakdown SOM for carbon but do rapidly acquire soil nutrients such as N and P. The observed effect that causes the suppression of SOM decomposition as these fungal types compete is known as the "Gadgil effect". If true, this phenomenon controls CO2 release from soil during decomposition and consequently offers one of the few options for climate change mitigation through enhanced C sequestration in soils. However, the mechanism by which ST and EM fungi interact in C cycling remains controversial and poorly understood, with an inconsistent evidence-base. In our project, we propose to determine whether the interactions between ST and EM fungi could be a neglected component of forest soil ecology that may be manipulated to augment soil C-sequestration in forests.For the last half of the century, since the Gadgil effect was posited, the prevailing view has been that the group of EM or ST fungi act as a whole. However, recent genetic studies suggest a high functional diversity among EM fungi, indicating it would be wrong to treat them as one group. This may well explain why numerous contradictory findings of Gadgil effect have been reported in the scientific literature. Our new understanding of abundant functions in the metabolism of EM fungi as a group opens new avenues of interrogating and finally confirming the existence (or otherwise) of the Gadgil effect. We intend to identify the mechanisms that lead to alteration of soil C-sequestration due to ST and EMF fungal interactions: e.g. increasing the dominance of certain EM fungi may lead to an increase in the creation of stable SOM in forest soils. In this way, the soil mycota may be able to be harnessed as a managed component in mitigating global climate change. In this proposal, we specifically address 3 fundamental questions: (1) What is the role of ST-EM fungal interactions in SOM decomposition and stabilisation? (2) Does the evolutionary origin and functional ecology of fungal taxa effect their interactions on SOM dynamics? (3) What are the mechanisms operating in communities in regard to ST-EM interactions under different environmental conditions? We have selected four distinct fungal groups, each of them consisting of one EM fungus paired with the closest related ST species. We designed a robust model system to quantitatively address the functional effect of fungal interactions on SOM decomposition and stabilisation. Our experimental plan includes four levels of complexity that sequentially validate the findings of ST-EM fungal interactions: (1) fungal cultures on growth-media, (2) microcosms, where EM fungi are in symbiosis with the plant, (3) mesocosms with forest tree seedlings, and (4) field sites differing in tree species and soil N availability.