Discussion
Reductions in suspended sediment loss, relative to the experimental control, were ordered: willow buffer strips (44%) > deciduous woodland buffer strips (30%) > grass buffer strips (29%). These results for the establishment phase of the buffer treatments therefore suggest that woody treatments improve sediment trapping the most. Clearly, these preliminary results might change as the vegetation treatments mature over time. The zero efficacy in in the first monitoring year across all buffer treatments and in the second monitoring year for the willow and deciduous woodland buffers reflected the soil disturbance associated with the establishment of the three vegetation treatments. The impact of soil disturbance associated with the installation of other sediment mitigation measures on farms, such as channel bank reprofiling and fencing, and has been reported by other studies (Lloyd et al., 2016).For the establishment period monitored in this study, our results suggest that the grass buffer strip treatment matures faster with respect to sediment trapping than the other two woody vegetation treatments. Over time, the potential for buffer strip saturation with trapped fine-grained sediment could be expected to increase.
Direct comparisons of experimental results for buffer strip efficacy are typically compromised by various factors. These include the contrasting climate, soil types, runoff lengths, vegetation types and agricultural practices of research sites. Additional potentially confounding factors include deployment of different research infrastructure and study scales and durations. Nonetheless, it is useful to interpret our new data on buffer strip efficacy for reducing sediment loss in the context of existing evidence. Working on 6 m buffer strips with fescue, shrubs and trees, serving 3% slopes, Borin et al. (2005) reported a sediment trapping efficacy of 93%. Schmitt et al. (1999), comparing 7.5m and 15 m grass, shrub and sorghum buffers, serving a slope of 6.7%, reported a sediment trapping efficacy range of 63-93%. Syversen (1995), testing 3, 10 and 15 m grass and shrub buffer strips serving slopes of 14% and 28%, reported efficacies of 61-91%. Schwer and Clausen (1989) working on 26 m wide grass buffer strips, serving slopes of 2%, reported a sediment trapping efficacy of 95%. Across the existing scientific literature reporting reductions in sediment loads due to buffer strips, the efficacy range is typically 40% to 100% with reductions of >50% commonplace (Dorioz et al., 2006). Given the close functional relationship between fine-grained sediment and phosphorus, efficacy ranges for reductions in particulate phosphorus loads are similar. Our new results for reductions in sediment loss are reasonably well aligned with, although slightly lower, than existing understanding of reductions in sediment loads, although it is important to acknowledge that our study represents the establishment phase of the vegetation treatments. On that basis, the overall efficacies for the study period should be viewed as being underestimates of longer-term performance. Previous work has underscored the potential for reductions in sediment loss to be strongly influenced by deposition of incoming sediment along the upslope leading edge of buffer strips due to the initial reduction in runoff velocity and sediment transport capacity (Ligdi and Morgan, 1995; Pearce et al., 1997). Such edge effects were not observed during our experiment.
Excess sediment loss from agricultural land remains a global issue despite the uptake of best management practices. For England and Wales, for example, such elevated sediment losses due to current structural land cover have been estimated to result in £523M of environmental damage costs annually, with the uptake of best management practices on farms only reducing those societal costs to £462M (Collins and Zhang, 2016). Buffer strips continue to feature in the mix of best management practices implemented on farms to protect water quality and their uptake by farmers can be facilitated by robust evidence on the efficacy for reducing water pollution. Agricultural runoff encountering a buffer strip meets a more porous and rougher surface, resulting in a reduction in runoff velocity and sediment transport capacity. Here, the vegetation cover generates increased resistance to runoff and sediment transport and the root systems increase the permeability of the soil surface, thereby encouraging infiltration and deposition (Magette et al., 1989; Rose et al., 2003).
Buffer strips can also assist in the management of the sediment problem by stabilising and reducing the erosion of riverbanks (Kemper et al., 1992; Bowie, 1995) and by displacing sediment generating land management away from watercourses (Wenger, 1999). The beneficial effects of displacement are often, however, less pronounced on heavy meandering watercourses where channel migration drives bank erosion (Williamson et al., 1992). In England Wales, eroding channel banks have been estimated to contribute 22% of the total fine-grained sediment load delivered rivers and streams (Zhang et al., 2014). The potential beneficial impacts of buffer strips on reducing bank erosion across England and Wales, as well as sediment loss from utilised agricultural land, should therefore be borne in mind given the important role of bank erosion in the excess sediment problem nationally.
When interpreting evidence for buffer strip impacts on sediment loss, it is important to acknowledge various issues which can confound efficacy. Buffer strips can be prone to silting up, especially when soils are saturated (Barfield et al. 1979; Hayes et al., 1979). Under such conditions, deposited sediment is likely to remain unconsolidated and prone to remobilisation, especially when a sequence of extreme storm events occurs or buffer strips are breached by concentrated runoff in preferential flow paths. Sediment trapping by buffer strips is commonly particle size selective with coarser particles preferentially retained (Hayes et al., 1984; Robinson et al., 1996; Hickey and Doran, 2004). Here, particle size selectivity is often buffer width dependent, with narrow 1 m buffer strips only trapping the coarsest particles (Hayes et al., 1979). Vegetation management can influence buffer strip efficacy for reducing incoming sediment loads since, for example, long grass is more prone to lodging, which can permit preferential flow routes and reduced efficacy. Incoming flow mechanisms can influence efficacy for reducing sediment loads with, for example, concentrated flows reducing efficacy (Dillaha et al., 1986; Dosskey et al., 2002; Canqui et al., 2004). At our experimental site, however, pervasive raindrop-impacted saturation-excess overland flow has been identified as a primary mechanism for sediment mobilisation and delivery, rather than concentrated runoff (Pulley and Collins, 2019). Finally, in real-world settings, buffer strips serving agricultural land can be bypassed by field drains (Haycock and Muscutt, 1995; McKergow et al., 2003), meaning that the reductions in sediment loads relate to the surface runoff pathway. In England and Wales, a considerable proportion of farmed land has assisted underdrainage in support of productive agriculture (Robinson and Armstrong, 1988), and field drains represent an important sediment delivery pathway (Deasy et al.,2009; Zhang et al., 2016). This means that more engineered buffer strip solutions will be required to deliver multi-pathway control of sediment pollution in many parts of England and Wales. Such solutions might, for instance, include the cutting back of field drains to permit the construction of artificial wetlands (Lenhart et al., 2016) thereby delivering a ‘treatment-train’ strategy combining buffer strips and wetlands. Where woody vegetation on buffer strips is harvested, the timing of such management activities will be critical to minimise compaction issues since these could reduce sediment trapping efficacy.