Introduction
Pollution of water by intensive farming continues to be cause for concern for the physicochemical and ecological health of freshwaters (Mateo-Sagasta et al., 2017). Contaminants moving from agricultural land into freshwater systems include fine-grained sediment (Pulley and Collins, 2019), deliberately applied compounds, e.g., fertilizers such as urea (Gilbert et al., 2005), ammonium nitrate (Burt et al., 2011), phosphorus (Haygarth et al., 2005) and other products such as pesticides (Syafrudin et al., 2021). In turn, these emissions not only degrade water quality but also impact detrimentally on freshwater ecology across all trophic levels (Collins et al., 2011; Kemp et al., 2011; Jones et al., 2012a,b; Jones et al., 2014).
Buffer strips have been utilised as a means of reducing the movement of pollutants from agricultural land into watercourses for many years (Barling and Moore, 1994; Hickey and Doran, 2004). The form of the vegetation may take that of a grass verge at the edge of the field where no targeted planting of chosen species is undertaken and natural colonisation is allowed to determine the dominant form of vegetation. Alternatively, targeted planting of specific grasses and woody plants has been utilised to vegetate buffer strips, with consequent effects on landscape aesthetics, biodiversity and interactions with the local watercourse and its ecology (Cole et al., 2020). Choices of the type of plants that can be deliberately planted within a buffer strip range from herbaceous grasses and forbs to small woody shrubs with multiple stems to taller woody tree species. The physiognomy of the plants may affect the runoff, the movement of pollutants including fine-grained sediment or both (Roberts et al., 2012). The interaction-potential the buffer strip has for removing pollutants from the runoff leaving the field upslope may thus change depending on the form of planting used to vegetate the buffer strip. Here, the form of planting chosen may affect the ability of the buffer strip to remove a priority pollutant within a local area, and as a result, some degree of potential exists to optimise buffer vegetation to ameliorate specific local concerns over particular pollutants or, alternatively, to address multiple issues (Stutter et al., 2012).
Water pollution and flooding events associated with the movement of agricultural run-off have been reduced due to the interaction of water and vegetation within buffer strips. However, the ability of a buffer strip to provide such services continuously may be reduced or lost over time if the buffer strip becomes saturated with fine-grained sediment or nutrients (Valkama et al., 2019). To alleviate the potential for saturation of nutrients, planned removal of buffer strip vegetation can be implemented. For grass buffer strips, mowing and/or grazing can reduce the standing crop within the strip and cause compaction by trafficking or trampling. Access to strips may negate the possibility of using machinery in some circumstances (e.g., steeply sloped land), and refusal by grazing animals to consume standing vegetation may affect the amount of vegetation removed. The age of a grass dominated buffer strip may need to be considered if grazing animals are the only option available to reduce the standing crop. Woody plants can be harvested for their timber within a planned management system, and act as a means of both removing nutrients from the strip as well as reinvigorating plant growth rates, and thus facilitating the further removal of nutrients entering the buffer strip.
In England, implementation of water pollution interventions on farms, including buffer strips, is driven by a combination of regulation, incentivization in the form of agri-environment schemes and the delivery of on-farm advice for win-wins. Here, improved uptake rates by farms can be encouraged by robust scientific evidence on the efficacy of buffer strips for controlling runoff and pollution losses. Existing work examining the efficacy of buffer strips for environmental good has focussed on both external and internal factors (Eck, 2000). The former encompass the phase (i.e., particulate, dissolved) and delivery pathway (i.e., surface, subsurface) of the incoming pollution, whereas the latter include buffer width and vegetation cover. Advice delivery has tended to focus on buffer width in the case of internal controls since this is the easiest component of management to influence via farm management and existing evidence on varying efficacy for runoff and water pollution reduction, including width, can be readily extracted from a number of comprehensive reviews (e.g., Barling & Moore, 1994; Hickey and Doran, 2004; Dorioz et al., 2006; Kay et al., 2009; Collins et al., 2009). Beyond buffer strip width, the existing evidence on the effects of different vegetation cover remains less easy to generalise. Some work suggests that for the same buffer strip width, different vegetation cover impacts efficacy for pollution control by at most 20% (Dorioz et al., 2006). Other studies report very limited or no effect of vegetation cover (e.g., Schmitt et al., 1999; Uusi-Kamppa et al., 2000). In other cases, the results of investigations comparing herbaceous and woody vegetation in buffer strips report both a lack of (Syversen, 1995; Daniels and Gilliam, 1996) and detectable (Cooper et al., 1986; Parsons et al., 1994) differences in pollution reduction, with the latter suggesting better performance by herbaceous cover.
Given the above context, the new study detailed herein was undertaken to assess the impact of three different vegetated buffer strips on runoff and sediment loss to contribute to the evidence base. The research project was planned to provide replicated evidence on buffer strip efficacy and to engage multiple stakeholders with this evidence given the ongoing inclusion of buffer strips in agricultural policy in the UK and beyond. This paper reports the preliminary results for the efficacy for reductions in sediment loss using our new dataset.