Source to Sea: Soil carbon transport from forested environments to coastal waters (Ref IAP2-18-72)

Scotland is a carbon (C) nation with significant stores held in both terrestrial and marine environments. Scotland’s peatlands, for example, hold Europe’s largest organic carbon (OC) reserves, and recent research has shown that Scotland’s sea lochs are even more effective carbon stores [1,2]. At the land-ocean interface terrestrial and marine C is closely coupled, with sea lochs storing significant amounts of terrestrially derived C [3]. Land use change and management decisions therefore not only impact the terrestrial C cycle but also the coastal C cycle [4], an aspect largely overlooked. Recent work has shown that over Holocene timescales the removal/loss of forests approximately 5000 years ago triggered a substantial change in the quantity of C sequestration in the coastal ocean [5]. We hypothesize that more recent afforestation (last 100 years) had a similar if not greater effect on the coastal ocean because the rate of recent forest cover change exceeds that seen through the Holocene. In this project we will investigate the effect of conifer afforestation on carbon transport to, and accumulation in, the coastal ocean.

To fully understand the current impact of forestry, we will characterise the source C material in detail. We will investigate forest plots (both growing and clear-cut, e.g. Fig. 1) for isotopic, elemental and mineralogical signatures of the soil, living vegetation and dead vegetation, particle size and density, and magnetic susceptibility. Suitable experimental locations will be identified following consultation with Forest Research and the Forestry Commission. We will determine carbon transport pathways by sampling the rivers draining these plots at different points and at different times of the year, measuring δ13C and δ15N and elemental concentrations of both Particulate Organic Carbon (POC) and Dissolved Organic Carbon (DOC). Further we will sample the mouths of these rivers (water and sediment) to build a time series. Iron minerals which play an important role in storage and transport of carbon [6-8] will be characterised in soil, water and sediment using novel Mössbauer spectroscopy methods developed through projects funded by the Scottish Alliance for Geoscience, Environment and Society (SAGES) and the Marine Alliance for Science and Technology for Scotland (MASTS). The results from the isotopic analysis will be combined with Bayesian mixing models to fingerprint the source of the POC and DOC in the river and coastal water and sediments [3]. The past role of forestry will be examined by using existing sediment cores with precise chronologies covering the modern period of afforestation which began in the early 20th Century. Through the application of the above-mentioned techniques the terrestrial C input associated with forestry activities to the coastal ocean will be quantified. This project will provide insight into the role that forestry plays in coastal C dynamics.

References:
[1] Smeaton et al. (2016): Substantial stores of sedimentary carbon held in mid-latitude fjords. Biogeosciences 13, 5771–5787.
[2] Smeaton et al. (2017): Scotland’s forgotten carbon: a national assessment of mid-latitude fjord sedimentary carbon stocks. Biogeosciences 14, 5663-5674.
[3] Smeaton & Austin (2017): Sources, Sinks, and Subsidies: Terrestrial Carbon Storage in Mid‐latitude Fjords. Journal of Geophysical Research: Biogeosciences, 122, 2754-2768.
[4] Bauer et al. (2013): The changing carbon cycle
of the coastal ocean. Nature 504, 61-70.
[5] Smeaton et al. (2017), Earth and Planetary Science Letters, in review.
[6] Kramer et al. (2017): Depth trends of soil organic matter C:N and 15N natural abundance controlled by association with minerals. Biogeochemistry 136, 237–248.
[7] Fritzsche et al. (2015): Structure and composition of Fe-OM co-precipitates that form in soil-derived solutions. Geochimica et Cosmochimica Acta 169, 167–183.
[8] Jilbert et al. (2017): Impacts of flocculation on the distribution and diagenesis of iron in boreal estuarine sediments. Biogeosciences 15, 1243–1271.