Improving herbicide degradation studies: maintaining soil structure, microbial functioning and rhizosphere effects to reflect natural conditions

理由：实现粮食安全的一个重要组成部分是继续需要现代、安全的农用化学品，以减少杂草、虫害和疾病造成的损失。这需要尽可能最好地了解它们在农业生态系统中的行为和退化情况。目前监管框架下的实地和实验室测试未能提供这一理解，因为它们是在与自然实地条件不同的受控条件下进行的。这样的测试系统早于我们目前对土壤复杂性质的理解，特别是环境扰动、土壤结构、微生物多样性和根际环境的影响。目的：加深对土壤结构、根际和微生物多样性对除草剂降解率的作用的了解，并将这些知识应用于测试系统的设计，以用于预测最佳使用和环境影响。背景：现行测试指南(OECD 307)在评估潜在植保化学品的转化方面存在重大限制。首先，OECD 307规定，在进行培养研究之前，应对土壤进行筛分(2 Mm)；由此产生的结构解体通过一系列机制影响微生物群落的大小、活动和组成：(I)以前位于充满水的团聚体中但现在暴露在空气中的细胞脱水；(Ii)与大团聚体结合的真菌菌丝断裂；(Iii)微生物底物的生物有效性改变，例如以前封闭在团聚体中的土壤有机碳；以及(Iv)微生物种群的空间重组和充满水和空气的途径的连接性改变。第二个重大缺陷是缺乏庄稼。先正达最近使用一种14C标记的除草剂(扑草净)进行的研究表明，包括有活力的作物根系会导致除草剂更快地下降(DT50的50%)，形成更多的不可提取残留物，并被植物吸收到最低限度。在这些不同过程的相对大小和相互作用方面存在着显著的知识差距，需要将更现实的情景纳入除草剂消散研究。目标和方法目标1.量化作物保护产品(CPP)在试验系统和田间转化的关键物理和生物途径和驱动因素。在这一目标下，实验将研究土壤物理干扰、土壤类型和CPP化学对CPP生物转化和土壤微生物(群落规模、组成、活性)、生化(例如可用碳共基质)和生物物理特性(例如连接空气和水的途径；扩散)的影响。目标2.开发保持或重建关键土壤物理性质的方法，以保持土壤微生物群落的功能多样性，使降解率更接近地反映田间测量结果。在我们用X射线CT表征土壤和在微观世界中重建土壤结构方面的经验基础上，我们将专注于确定控制孔-固界面和扩散路径的方法。目的3.为了量化生物物理(土壤结构)、生化(根际沉积)和微生物(根际)对微生物群落的相对重要性，根际过程有助于驱动种植和非种植土壤中CPP的不同行为。将在农业技术土壤健康设施中种植覆盖作物(根据我们的BBSRC工作“利用根对土壤进行生物工程土壤”选择)，以创建现实的测试系统(1立方米)，每个系统在受控的土壤和环境条件下。在最初阶段，我们使用根替代物来提供不同的碳源来测试特定的假设。

Rationale: An essential component of achieving food security is the continuing need for modern, safe agrochemicals available to reduce losses due to weeds, pests and diseases. This requires the best possible understanding of their behaviour and degradation in agricultural ecosystems. The field and laboratory testing under the current regulatory framework fall short on delivering this understanding as they are conducted under controlled conditions that deviate from natural field conditions. Such testing systems predate our current understanding of the complex nature of soil, and notably the influence of environmental perturbations, soil structure, microbial diversity and the rhizosphere. Aim: To develop a deeper understanding of the role of soil structure, the rhizosphere and microbial diversity on herbicide degradation rates, and to apply this knowledge to the design of test systems for use in predicting optimal application and environmental impact. Background: There are significant limitations to current test guidelines (OECD 307) for evaluation of the transformation of potential crop protection chemicals. Firstly, OECD 307 specifies that soil should be sieved (2 mm) prior to incubation studies; the resulting disintegration of structure influences the size, activity and composition of microbial communities through a number of mechanisms: (i) dehydration of cells previously inside water-filled aggregates but now exposed to air; (ii) rupture of macroaggregate-binding fungal hyphae; (iii) altered bioavailability of microbial substrates, e.g. soil organic carbon previously occluded within aggregates; and (iv) spatial reorganisation of microbial populations and the altered connectedness of water- and air-filled pathways. A second significant shortcoming is the absence of crops. Recent work by Syngenta using a 14C labelled herbicide (prometryn) showed that inclusion of viable crop root systems resulted in faster decline of the herbicide (50% of DT50), higher formation of non-extractable residues, and minimal uptake by the plants. There is a significant knowledge gap in the relative magnitude and interaction of these different processes, and a need to incorporate more realistic scenarios into herbicide dissipation studies. Objectives and approachObjective 1. Quantify key physical and biological pathways and drivers of transformation of Crop Protection Products (CPP) in test systems and fields. Under this objective, experiments will examine the effect of soil physical disturbance, soil type, and CPP chemistry on CPP biotransformation and the soil microbial (community size, composition, activity), biochemical (e.g. available carbon co-substrates) and biophysical characteristics (e.g. connected air and water ways; diffusion). Objective 2. Develop methods which maintain or recreate key soil physical properties that retain functional diversity in soil microbial communities, so that degradation rates more closely reflect field measurements. Building upon our experience in characterising soils with X-ray CT and recreating soil structure in microcosms, we will focus on identifying methods to control pore-solid interfaces and diffusive pathways. Objective 3. to quantify the relative importance of the biophysical (soil structure), biochemical (rhizodeposition) and microbial (rhizosphere) effects on microbial communities brought about by the processes in the rhizosphere that contribute to driving differing CPP behaviour in planted versus non-planted soil.Cover Crops (selected based on our BBSRC work 'using roots to bio-engineer soils') will be grown in the Agri-Tech Soil Health Facilities to create realistic test systems (1 m3) each under controlled soil and environmental conditions. At an initial stage we test specific hypotheses using root surrogates to supply different sources of Carbon.