Evaluation of contrasting buffer features within an agricultural landscape for reducing sediment and sediment- associated phosphorus delivery to surface waters

There are a variety of buffering features within the landscape that can be used to trap sediment and associated contaminants such as phosphorus (P), thereby helping to reduce sediment and P delivery to watercourses. Astroturf mats (n = 136) were placed within contrasting buffer features at nine sites [mid-field hedges (two sites), edge-of-field grass strips (six sites) and channel wetlands (one site)] within the River Parrett basin in England. Sediment was recorded on the mats at seven of the sites during the 18-month sampling period. At the other two sites either there was insufficient erosion or sediment by-passed the mats. At the seven sites where mats collected sediment, there was a considerable range in sediment deposition over the 18-month sampling period with site-average values (based on all mats at a site) ranging from 0.02 ± 0.06 to 1.15 ± 1.88 g cm )2 ; the average for all 136 mats was 0.41 ± 1.08 g cm )2 , or approximately 0.27 g cm )2 year )1 . Most of the sediment collected on the mats (n = 60) was sand-sized (>63 lm) material. The site-average total-P content of the <63 lm fraction of the deposited sediment ranged between 616 and 1938 mg kg )1 (average 890 mg kg )1 ). About half of all the mats that collected sediment were from the front of the buffers. Comparison of the sediment in the buffer features with topsoil from the contributing upslope fields suggests that the buffers trap coarser sediment with lower P concentrations, than the contributing topsoil. This suggests that the finer fraction, enriched in total-P, may be passing through the buffers towards river channels. Comparison between sites indicates that sediment deposition within buffers is greater at sites with steeper slopes, erodible soils and certain types of land use, such as maize for silage, reflecting the greater soil erosion and sediment transfers in these fields. The location and careful design of buffer features is a key factor in their effectiveness.