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

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 approach
Objective 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.