Herbicide impacts on soil ecosystem services and determining factors involved, in Uruguayan agricultural production systems

Herbicides are a key component of the agricultural intensification process, being in fact the most important tool to control weeds in agro-ecosystems where direct sowing is practised to prevent soil erosion by water (Coleman et al., 2019). Given the increasing number of resistant weeds, as well as the cost/benefit ratio of herbicides, herbicide use is expected to continue to grow (Maggi et al., 2019). However, herbicides side effects raise serious environmental and human health concerns. The hazard of individual pesticides is tested before their commercialization, to insure they do not pose unacceptable risks to non-target populations and environment. Yet, this hazard/risk assessment usually relies on laboratorial tests, performed at controlled and often reference conditions, with standard species and standard endpoints, and field experiments indicated just under special conditions. Some of these assumptions may not be applicable to protect current production systems where pesticides are used (Topping et al., 2020). Main limitations relate therefore to: Application schemes: herbicides are usually applied in mixtures of different active ingredients to ensure a more effective weed control and/or to prevent further resistance problems. Although their agronomical effect is thoroughly analysed, little is known about side effects of realistic mixtures. Furthermore, different active ingredients alone or combined, are frequently applied (Topping et al., 2020). It has been demonstrated in similar agricultural systems that rate of herbicide application is higher than dissipation, what means that they are being accumulated on soils (Primost et al., 2017). The multiple application factor is sometimes considered to calculate the contaminant concentration, however, the ecotoxicological effect of successive applications on non-target communities may have different results than just testing the accumulated dose. Test conditions: They are combined with diverse stressing conditions (droughts/flood, high/low temperatures, contaminants already present in the soil), which affect biological responses and biological susceptibility to contaminants, most likely resulting in under or over-estimation of risks (Lokke et al., 2013). Endpoints: not only sensitive organisms should be tested, as some additional endpoints, for example ammonia oxidizers and mycorrhizas have been recommended to be included under current testing conditions because they are extremely relevant indicators of soil health and functions (Karpouzas et al., 2016). Considering when to assess the pesticide effects and how long does it last, is also a key factor. Resilience: each systems resilience capacity is different, and relies on the biome present at the time of the exposure. Indeed, in-soil organisms and plants development will be affected differently depending on the previous managements. Several thermal weed control alternatives to herbicides arise in this context, (Coleman et al., 2020). They are not as widely used as herbicides because they consume higher amounts of fossil energy and need costly equipment to be implemented (Upadhyaya et al., 2007). However, some of them are being commercially used, as the weed electrocution method. Its main benefits are: do not produce toxic residues, rapidly control herbicide resistant weeds, preserve soil structure and by affecting directly the root, avoid heating the surrounding soil. No information on their ecotoxicological impacts was found, therefore, this issue is worthy to be addressed. Given this highly complex panorama, it seems that nowadays prognosis studies may not be enough to maintain agroecosystems healthy. If solutions are pursued to reduce herbicides environmental impacts, we must first assess the actual damage, as well as the evolution of soil health after decreasing herbicides input. This work intend to tackle this complex scenario, presenting the following main objectives: i) Design a framework to assess the impacts of mixtures of herbicides on soil health involving a holistic approach. ii) Assess the effects of a frequently applied herbicide scheme, on soil ecosystem services, under a soybean-cover crop rotation. iii) Evaluate how changes in the frequency, quantity and type of herbicide active ingredients application scheme can affect soil functioning. iv) Evaluate the effects of weed electrocution method on soil functioning.