Evaluation of the relative importance to carbon sequestration and nutrient cycling of root exudation, and turnover of roots and mycorrhizas

维持土壤功能和碳储存是可持续农业的关键目标。大部分碳、氮和磷通过活根的渗出和死根及其相关共生体的周转返回土壤。因此，植物固定的碳以二氧化碳的形式返回大气的速率和营养物质以植物可利用的形式再循环的速率是紧密耦合的。然而，对三种最重要的碳和养分地下通量（根渗出物和根和菌根周转）的定量分离在很大程度上仍未实现。这是由于对碳、氮和磷化合物及其聚合物（如蛋白质、核酸、纤维素、木质素和几丁质）输送到土壤的命运的了解有限，以及缺乏适当的技术来测量输送速率。因此，对这些通量缺乏了解是理解和模拟土壤中大气C的储存以及控制养分循环的因素的主要障碍。我们建议利用最近的技术进展来确定：(1)温带永久草地根和菌根在周转过程中向土壤输送的C、N和P的相对数量和类型。(2)土壤微生物利用通过这些过程输送到土壤中的各种形式的碳的速率和途径，以及这如何控制碳以二氧化碳的形式返回大气。(3)与植物C矿化转化为CO2同步的是N和P向植物可利用形态的回归。(4)最先进的技术测量的根和菌根周转量的估计值与更传统的方法相比如何。

Maintaining soil function and carbon storage represent key objectives in sustainable agriculture. Most C, N and P is returned to soil via exudation from live roots, and turnover of dead roots and their associated symbionts. Consequently, both the rate at which C fixed by plants is returned to the atmosphere as CO2 and the rate at which nutrients are recycled to plant-available forms are tightly coupled. However, quantitative separation of the three most important below-ground fluxes of C and nutrients (root exudation and root and mycorrhizal turnover) remains largely unachieved. This is due both to limited understanding of the fate of the range of C, N and P compounds and their polymers (e.g. protein, nucleic acids, cellulose, lignin and chitin) delivered to soil, and to a lack of suitable techniques to measure rates of delivery. Consequently, poor knowledge of these fluxes is a major impediment to understanding and modelling the storage of atmospheric C in soils and the factors controlling nutrient cycling. We propose to utilise recent technological advances to determine: (1) The relative quantities and types of C, N and P delivered to the soil in turnover of roots and mycorrhizas in temperate permanent grassland. (2) The rate and route of utilisation by soil microbes of the various forms of C delivered to the soil by these processes and how this controls the delivery of C back to the atmosphere as CO2. (3) How synchronous with mineralisation of plant C to CO2 is the return of N and P to plant-available forms. (4) How estimates of root and mycorrhizal turnover measured by state-of-the-art techniques compare with those from more traditional approaches.