Controls on the spatial distribution of natural pipe outlets in heavily degraded blanket peat

Natural soil pipes are recognised as a common geomorphological feature in many peatlands, and they can discharge large quantities of water and sediment. However, little is known about their morphological characteristics in heavily degraded peat systems. This paper presents a survey of pipe outlets in which the frequency and extent of natural soil pipes are measured across a heavily gullied blanket peat catchment in the Peak District of northern England. Over a stream length of 7.71 km we determined the occurrence and size of 346 pipe outlets, and found a mean frequency of 22.8 km−1 gully bank. Topographic position was an important control on the size and depth of pipe outlets. Aspect had a large influence on pipe outlet frequency, with southwest and west- facing gully banks hosting more than 43% of identified pipe outlets. Pipe outlets on streambanks with signs of headward retreat were significantly larger and closer to the peat surface compared to pipe outlets that issued onto uniform streambank edges. We suggest that larger pipe frequencies are observed on gully banks that are more susceptible to desiccation cracking, and propose that future peatland restoration works could prioritise mitigating against pipe formation by revegetating and reprofiling south and west facing gully banks

Aquatic carbon concentrations and fluxes in a degraded blanket peatland with piping and pipe outlet blocking

Soil piping is an important agent of erosion in many environments, including blanket peatlands. Peatland restoration that aims to reduce erosion has mainly focussed on revegetation and blocking ditches and gullies, rather than reducing erosion from natural soil pipes. However, little is known about the contribution of pipeflow to the fluvial carbon budget of degraded blanket peatlands and whether it is possible to moderate it. In a heavily degraded blanket bog, dissolved and particulate organic carbon (DOC and POC), and water colour, from two catchments were compared before and after half of the pipe outlets in one catchment were blocked. One blocked pipe was monitored for discharge and water quality both pre- and post-blocking as new pipe outlets had formed around the blocked outlet. Both pre- and post-blocking, maximum concentrations of DOC and POC were markedly higher in pipe-water than stream-water, with ratios of 1.2 (pre) and 1.3 (post) for DOC, and 4.8 (pre) and 8.8 (post) for POC, rendering pipe-to-stream transfer more effective for DOC than POC due to the deposition of POC close to pipe outlets. The increase in DOC and POC flux post-blocking in both catchments was near-identical, suggesting pipe outlet blocking was ineffective in reducing fluvial carbon export from pipe networks. Extrapolation of pipe fluxes to catchment scale showed pipes potentially contribute c. 56% of DOC exported by the stream, and that more POC was produced by pipes than was exported by the stream. Our work highlights that pipes need to be considered when seeking to reduce fluvial carbon export in degraded blanket peatlands