DOI: 10.1594/pangaea.942789
title: Radiocarbon of ecosystem respired CO2 and DIC in Peatcosm array

description: In 2010, we harvested 24 intact 1-m-3 peat monoliths from an oligotrophic peatland in Meadowlands, MN USA (N 47.07278°, W 92.73167°) and subsequently installed them in the Mesocosm facility at the USDA Forest Service Northern Research Station Forestry Sciences Laboratory in Houghton, MI USA (N 47.11469°, W 88.54787°). Beginning in 2011, we initiated a full-factorial randomized complete block manipulation of water table levels and vegetation types. We manipulated water tables to reflect either “high” or “low” levels using a 45-year water table record from the Marcell Experimental Forest (USDA Forest Service) near the peat harvest location. The vegetation type treatments included three levels: unmanipulated (“Control”), all Ericaceae removed (“Sedge”), or all sedges removed (“Ericaceous”). To initiate treatments, we removed all aboveground and, when possible, belowground plant material. In 2014, 8 bins were selected for radiocarbon analysis of respired CO2 and dissolved inorganic carbon (DIC). Two mesocosms were chosen for each treatment combination (e.g., two bins with 'Sedge' and 'low' water table treatments) for each manipulated vegetation type. Bins were randomly selected from the stratified design across all experimental blocks. To collect respired CO2, we placed the 100cm x 100cm x 40cm silicone-sealed opaque chambers used for CH4 measurements over the mesocosm bins. We taped the chamber to create an airtight seal to the rim of the mesocosm. The chamber was in a closed loop with an IRGA (described above) to monitor headspace CO2 levels. To scrub the chamber headspace of ambient air, we employed another closed loop with a pump (3.74 m3 hr-1; 115 VAC oil-less, Cole Parmer #EW-07061-60) connected to a 5.1 diameter column containing 400g fresh Soda Lime (Indicating Type 4-8 Mesh Baker Analyzed ACS #3448-05-cs). We circulated chamber air through the Soda Lime to remove atmospheric CO2 for two hours, which brought the headspace CO2 down from ambient to approximately 130 ppm. At the end of the scrubbing period, we recorded the CO2 concentration, disconnected the pump lines, and allowed CO2 to accumulate in the chamber until a concentration of approximately eight times (&gt;1000ppm) initial scrubbed concentration was achieved in the static chamber. A purpose-built air capture apparatus (6 1 volume) consisting of an airtight stainless-steel cylinder outfitted with an internal battery-operated CPU fan was also connected in a closed loop with the flux chamber. We verified the CO2 concentration within the apparatus, and then the attached cold trap was lowered into liquid nitrogen (N) to trap CO2; the CPU fan situated above the trap ensured a heterogeneous pool of CO2 within the apparatus. After 10 minutes we sealed the trap and transferred it in a dewer filled with liquid N to the USDA Northern Research Station radiocarbon facility on site and immediately connected to a vacuum line for CO2 purification (removal of non-condensables) and transfer to evacuated glass tubes to be sealed for conversion to graphite. Purified CO2 was then graphitized with an iron powder catalyst (99.99%) in a method modified from Vogel et al. (1984). Graphite targets were sent to the Lawrence Livermore National Laboratory in Berkeley, CA USA for analysis on an accelerator mass spectrometer. The 13C was calculated on splits of the same sample based on the permil ratio of 13C/12C relative to the international standard Vienna Peedee belemnite. Radiocarbon values were normalized to a 13C of -25‰ to account for mass-dependent fractionation and 2014 was the year of measurement for decay corrections. To obtain DI14C, 60ml of porewater from 20, 40, and 70cm below peat surface from the same bins as selected for respiration 14CO2 were collected after flushing and placed in pre-evacuated 125ml glass serum bottles via septa. The collections were made through pre-installed micropiezometers made of ultra‐high‐density polyethylene casing with inner Teflon tubing, where each depth represented 10cm centered around the three depths stated above. After shaking, 10ml of headspace air was removed, purified, graphitized and analyzed for radiocarbon following the same procedure described above.