Measuring and modelling greenhouse gas fluxes between agricultural soils and the atmosphere

土壤是全球碳循环的主要组成部分，其有机碳含量约为1500pg (1pg = 1gt = 1015g) (Batjes, 1996)，约为植被的3倍，大气中的2倍。通过光合作用，植物将二氧化碳（CO2）转化为有机形式的碳，并通过呼吸作用将一些碳释放到大气中。残留在植物组织中的碳通过它们的根添加到土壤中，并在植物死亡和分解时成为凋落物。这些碳以土壤有机质的形式储存在土壤中。碳可以在土壤中储存数千年，或者迅速以二氧化碳的形式释放回大气中。气候、植被类型、土壤质地和排水都会影响碳在土壤中储存的数量和时间长度。因此，土壤在维持全球碳循环平衡方面发挥着重要作用。然而，由于农业的集约化和机械化，今天土壤的碳含量比几百年前要小。农业实践通过两个主要途径耗尽了土壤有机碳库：1。通过收获和移除作物，减少凋落物中返回土壤的碳量。过度使用破坏土壤的耕作方式，增加了土壤有机质的分解速度，导致土壤中二氧化碳的释放增加。2019年6月，英国政府在法律上承诺，到2050年实现温室气体“净零”排放。农业部门约占英国温室气体排放量的10%。因此，实现净零排放将对农业和农业社区构成重大挑战。然而，土壤也可以通过从大气中吸收或“隔离”碳来帮助减缓气候变化。这可以通过改变管理做法来实现，例如减少耕作、减少休耕期、提高动物粪便和作物残留物的利用效率，以及在主要经济作物之间种植覆盖作物。土地利用变化可带来额外收益，例如种植树木和树篱，以及通过调整肥料施用量和方法减少一氧化二氮（N2O）排放。但是，需要改进土壤与大气之间碳、氮和温室气体通量变化的测量、监测和验证，以便进行定量经济和政策分析。目前，将土壤碳、土地利用和气候数据结合起来创建模型，以估计与农场管理做法变化有关的温室气体通量的变化。然而，许多基于效率的温室气体缓解措施所提供的绝对缓解潜力仍然存在不确定性。因此，有必要增加和改进一系列耕地轮作的温室气体测量，以获得更高的精度。该项目旨在提高我们对控制农业土壤（耕地、草地和户外猪）温室气体通量时空变化的因素的理解。特别是，根据您的特定研究兴趣，学生可以解决以下目标的组合：1。量化农业土壤中土壤有机碳和氮储量。确定作物残茬向土壤中碳和氮的输入以及肥料和有机改良剂的施用。使用涡流相关通量塔和静态室量化水、碳和氮的陆地-空气通量。通量塔还将允许将二氧化碳通量作为温室气体进行量化。在陆地表面将安装一系列的项圈，以允许直接测量N2O和甲烷通量。这将提供有关温室气体通量在不同土地利用方式之间以及在生长季节随时间变化的信息。iv一系列耕作轮作和未来气候下的温室气体通量模式。

Soil is a major component in the global carbon cycle, containing about 1500 Pg (1 Pg = 1 Gt = 1015 g) of organic carbon (Batjes, 1996), which is about three times the amount in vegetation and twice the amount in the atmosphere. Through photosynthesis, plants convert carbon dioxide (CO2) into organic forms of carbon and return some to the atmosphere through respiration. The carbon that remains in plant tissue is added to the soil through their roots and as litter when plants die and decompose. This carbon is then stored in the soil as soil organic matter. Carbon can remain stored in the soil for millennia, or be quickly released back into the atmosphere as CO2. Climate, vegetation type, soil texture and drainage all influence the amount and length of time carbon is stored in the soil. Therefore, soils play a major role in maintaining a balanced global carbon cycle. However, the carbon content of soil is smaller today than a few hundred years ago owing to the intensification and mechanization of agriculture. Agricultural practices have depleted soil organic carbon pools by two main routes:1. Reducing the amount of carbon returned to the soil in litter by harvesting and removing the crop.2. Excessive use of tillage practices which breaks up the soil, increasing the decomposition rate of soil organic matter which leads to an increase in the release of CO2 from the soil.In June 2019, the Government legally committed the UK to reaching 'net-zero' greenhouse gas (GHG) emissions by 2050. The agriculture sector accounts for approximately 10% of the UK's GHG emissions. Therefore, achieving net-zero will pose significant challenges for farming and the farming communities. However, soils can also help mitigate climate change by absorbing or 'sequestering' carbon from the atmosphere. This can be achieved through changes in management practices, such as reduction in tillage, reducing fallow periods, improving efficiency of animal manure use and crop residue use, and planting cover crops between main cash crops. Additional gains can come from land use change, such as planting trees and hedges, and reducing nitrous oxide (N2O) emissions by modifying fertiliser application rates and methods. However, improvements in measuring, monitoring and verifying changes in carbon, nitrogen and GHG fluxes between the soil and atmosphere are needed for quantitative economic and policy analysis. Currently, data on soil carbon, land use and climate is combined to create models that estimate the change in GHG fluxes related to changes in farm management practices. However, uncertainty persists on the absolute mitigation potentials offered by many efficiency based GHG mitigation measures. Therefore there is a requirement to increase and refine GHG measurements from a range of arable rotations for greater accuracyThe project aims to improve our understanding of the factors that control the spatial and temporal variability in GHG fluxes from agricultural soils (arable, grassland and outdoor pigs). In particular, according to your particular research interests, the studentship could address a combination of the following objectives:i. Quantify soil organic carbon and nitrogen stocks within agricultural soilsii. Determine inputs of carbon and nitrogen to soil from crop residues and application of fertiliser and organic amendmentsiii. Quantify Land-Air fluxes of water, carbon and nitrogen using Eddy covariance flux towers and static chambers. The flux towers will also allow quantification of carbon dioxide (CO2) fluxes as greenhouse gases (GHGs). A series of collars at the land surface will be installed across the fields to allow direct chamber measurement of N2O and methane fluxes. This will provide information on how GHG fluxes vary between land uses and also through time during the growing season. iv Model GHG fluxes under a range of arable rotations and future climates.