Description:
This data set reports surface and porewater chemistry from the bog and lagg of the S2 peatland at the Marcell Experimental Forest (MEF) in Itasca County, Minnesota. Lagg porewaters have been collected weekly from shallow depth samplers (0-10 cm) at two sites, 2009 to ongoing. Bog porewaters have been collected weekly or biweekly from shallow depth samplers (0-10 cm) at three sites, 2010 to ongoing. Bog waters have been collected monthly at multiple depths (0, 30, 50, 100, and approximately 200 cm) at three piezometers nests, 2014 to ongoing. Lagg and bog porewaters were synoptically collected from up to 40 and 13 sites (0-10 cm depth), respectively, multiple times from 2009 to 2014. Samples have never been collected when samplers were frozen (typically November to May).
Samples are measured for pH, specific conductivity, anions (chloride, sulfate), cations (calcium, magnesium, potassium, sodium, aluminum, iron, manganese, strontium), silicon, nutrients (ammonium, nitrate, soluble reactive phosphorus, total nitrogen, total phosphorus), total organic carbon, and ferrous and ferric iron. Occasionally, water isotopes have been measured.
The MEF is operated and maintained by the USDA Forest Service, Northern Research Station.
SITE DESCRIPTION:
The S2 catchment has a 6.5-ha deciduous upland forest and a natural, undrained 3.2 ha peatland (raised-dome bog with a surrounding lagg). The peatland includes a 3 ha bog and a 0.2 ha lagg. A stream forms in the lagg and flow is intermittent throughout the year (Verry et al. 2011). Surface elevation ranges from 420 m a.s.l at the outlet to 430 m a.s.l. in the uplands.
The upland forest is dominated by aspen (Populus tremuloides), white birch (Betula papyrifera), red maple (Acer rubrum), and balsam fir (Abies balsamea, with some red oak (Quercus rubra), basswood (Tilia americana), and jack pine (Pinus banksiana). The upland forest was last harvested during the 1910s. In the uplands, a Warba sandy clay loam developed in glacial till atop deep (50 m) outwash sand deposits. The Warba soil series is a fine-loamy, mixed, superactive, frigid Haplic Glossudalfs; Alfisol (Nyberg 1987).
The peatland has a black spruce (Picea mariana)-Sphagnum community with some tamarack (Larix laricinia). Below the overstory tree canopy, there is nearly complete coverage of ericaceous shrubs (primarily Rhododendron groenlandicum and Chamaedaphne calyculata with some Vaccinium angustifolium). Pitcher plant (Sarracenia purpurea) and three-leaved false Solomon’s seal (Maianthemum trifolium) also grow throughout the bog. The lagg has most of the same species but is richer in species than the bog (Verry and Janssens 2011). The more noticeable additional species include speckled alder (Alnus incana), paper birch (Betula papyrifera), various Carex species, cotton grass (Eriophorum spissum), and and haircap moss (Polytrichum sp.). Alder covers about 50% of the lagg (Hill et al. 2016).
Peat depth has been surveyed across the bog (Verry and Janssens 2011). Peat is less than 1 m deep around the perimeter of the bog to about 7-m deep at the deepest location. The Loxley peat (Dysic, frigid Typic Haplosaprists; Nyberg 1987) has accumulated in the last 10,000 yr since Wisconsin glaciation (Verry and Janssens 2011). The peat has hummock and hollow microtropography. Hummocks are uneven, elevated areas that rise various heights, up to about 50 cm, above the adjacent hollows. Hollows have a relatively uniform elevation within a localized area, with an overall raised-dome profile to the entire bog surface. The highest elevation hollows in the bog are about 15-20 cm higher relative to the lagg (Richardson et al. 2009). The peatland water table fluctuates from about 0.20 m above the surface to as much as 0.30 m below during a typical year.
The climate is continental with warm summers, cold winters, and an average air temperature since 1961 of 3.5 deg C (1961 to 2019, Sebestyen et al. 2021). Mean precipitation since 1961 is 787 mm. Most precipitation occurs as rainfall during summer and a winter snowpack starts to accumulate around December and fully melts in March or April.
The catchment has been used as a reference basin since 1960 for catchment and paired-catchment studies (Verry et al. 2011; Sebestyen et al. 2011). At the S2 catchment, streamflow, air temperature, precipitation, snow depth, snow water equivalent, ground frost, and water levels have been monitored since the 1960s. The 10,000 yr history of the peatland has been reconstructed from peat cores and a paleoecological study (Verry and Janssens 2011). Some chemistry (mostly unpublished) was measured as early as 1966. Solute chemistry and element budgets have been presented for major elements and some trace metals (Bay 1967; Jeremiason et al. 2018; Kolka et al. 2011a; Urban et al. 1990, 2011). Carbon dioxide and methane emissions have been measured occasionally since the 1980s (Dise et al. 2011; Harris et al. 1985). Peat temperature (0-2 m) has been measured since 1989. Many other studies have occurred in the S2 peatland. It is likely one of the longest-running peatland research programs and is among the most studied peatlands on the planet (Kolka et al. 2011b)
LOCATIONS OF WATER SAMPLING:
Lagg waters have been collected as surface water and porewater since 2009. Sampling of bog waters was added during 2010. Surface water or porewater have been collected every week at four sites, every other week at one site, monthly at three nests of piezometers, and occasionally during synoptic surveys at up to 40 sites.
During 2009, the first lagg samples were synoptically collected by dipping standing water from around the entire perimeter of the peatland. During a one-time synoptic sampling during 2009, the samples were labeled as S2 lagg followed by a value from 1 to 13. We do not include S2 lagg 1 or S2 lagg 13 in this data publication since we do not have geographic coordinates of those sites. During March and April 2010, two more synoptic surveys were completed. There was some overlap, but most site locations differed from the 2009 sampling. The samples were labeled S2 lagg MJ followed by a value from 1 to 10. We do not include S2 lagg MJ1 since we do not have the geographic coordinate of that location. All samples were dipped with a plastic kitchen ladle and poured into bottles (details follow). The ladle was rinsed with 18 megaohm deionized water before the start of sampling. The ladle was rinsed with lagg water from near each sampling location. The rinse water was discarded away from the point of sampling. Though the sites were accessed by walking in the lagg, the walking corridors were avoided when rinsing the ladle and sampling.
During 2010, 40 piezometers were placed around the perimeter of the peatland and along three transects in two different areas. The piezometers along transects extended from the toe-slope in upland mineral soils (not included here) through the lagg and into the bog with 7 to 9 piezometers along each transect. All of the piezometers were placed in hollows to sample near surface waters, with a 10 cm screen at about 5-10 cm below hollow surfaces. All of the piezometers were synoptically sampled during 2010 (many times) and occasionally during 2011, 2012, and 2014. The sampling was intended to quantify the chemistry of near-surface water/porewater: 1) around the perimeter of the peatland, and 2) across the lagg zone through the transition to the bog.
Piezometers were made from 5-cm (2 inch) internal-diameter (ID) PVC pipe. A screened section (a 0.3 mm slot every 1 cm over a 10-cm interval cut from a 1.5-m long Diedrich Drill, Inc, Laporte, Indiana, V-2042 well screen well screen) of a piezometer was glued to a 30-cm unscreened section of PVC on each end. A cap was glued to the bottom of each piezometer. The top of the screened interval was placed at about 5 to 10 cm below a hollow surface. Piezometers were pushed or hammered into peat. Piezometers placed in peat need to be deeply anchored to prevent toppling in the unconsolidated and saturated surficial peat, and to prevent frost heaving during winter. That is why a section of unscreened PVC was placed below the screened interval. Accordingly, each piezometer had at about 0.45-m of pipe below the peat surface (about 5 cm of the top unscreened PVC, the 10 cm PVC screen and the full 30 cm of the bottom unscreened interval). The additional belowground pipe stabilized the piezometer and served as a reservoir to accumulate and hold water. The tops of piezometers were loosely capped when not being sampled and the piezometers were vented with an ~1-mm (1/8 inch) hole, immediately below the cap.
Piezometers were developed by scrubbing with a bottle brush, then pumped of all water over successive days until no particles were observed in the evacuated water. In subsequent years, if particular piezometers yielded peat particles when pumped, that piezometer would have been similarly re-developed.
A piece of 0.6-cm (1/4 inch) internal-diameter (ID) PVC tubing, reaching to the bottom, was placed inside each piezometer. That 0.6-cm PVC remained inside a particular sampler and was attached via a flexible silicone hose to a portable pump for purging and sampling.
The piezometers were accessed via boardwalks (laid on the lagg surface). Elevated boardwalks were built along the transects and to several perimeter samplers. The samples are designated as KF followed by a value of 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 18A, 18B, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 31, 32, 34, 35, 36, 37, 38, 40, 41, 42, 43, 44, 45, 45A, 46, 47, 48, 49, 50, 51, 52, 53, 54, or 55. Missing numbers in the range of 1-55 represent identical samplers that were placed in the toeslope of the uplands to collect near-surface water in the mineral soil at the end of each transect. Samplers KF12, KF18, KF18A, KF18B, KF24, KF32, KF37, KF38, KF43, KF44 KF45, KF45A, and KF45B are used to collect bog water. Samplers KF1, KF2, KF3, KF4, KF5, KF6, KF8, KF9, KF10, KF11, KF14, KF15, KF16, KF17, KF20, KF21, KF22, KF23, KF25, KF26, KF28, KF29, KF30, KF31, KF34, KF35, KF36, KF40, KF41, KF42, KF46, KF47, KF48, KF49, KF50, KF51, KF52, KF53, and KF54 are used to collect lagg water.
During 2009, a hole was excavated in the lagg along the north edge of the peatland. The hole (the S2N lagg site) is about 20 cm deep by 20 cm wide. The excavation was occasionally cleared of ingrowing plant material and infilling peat. A boardwalk was built during September 2010 to access the S2N site, which is also the location of the KF26 piezometer (about 0.5 m of separation). Before that, the excavation was approached by walking through the lagg. During May 2015, we switched to collecting the weekly sample from the KF26 piezometer. S2N lagg and KF26 are functionally equivalent means of getting near-surface porewater at the location. After the change to sampling KF26, we labeled samples as KF26/S2N lagg. While basic chemistry is run from the KF26 sampler, a paired sample from the S2N lagg is collected for mercury and methylmercury analysis. Mercury concentration values may be included in a future version of this data product.
Beyond the weekly S2N lagg sample, we started to sample several of the piezometers (KF5, KF45, and KF26) every week during 2011. At the same time, 1) we added a KF5A bog sampler and started weekly sampling, and 2) started to collect water from KF45A about every other week.
Three nests of piezometers were installed during 2014 (probably during May). The samplers were first sampled during June 2014. The piezometer nests roughly bisect the oval-shaped bog from north to south. Three nests are named KF45 (northmost), BW (middle), and S2S (southmost). The KF45 nest is about 1 m north of the KF45 sampler that is used for weekly collections. The BW (Bog Well) nest is about 5 m from the well where water level has been measured since the 1960s. The S2S nest is located about 1 m north of the KF5A sample that is used for weekly collections.
Piezometers were installed along boardwalks (untreated lumber before summer 2019 and treated lumber thereafter) for access, to prevent peat compaction, and to eliminate trampling of the peat near and around the piezometers during installation and sampling. These piezometers were made from 5-cm (2 inch) internal-diameter (ID) PVC pipe. Piezometers were screened (machine-cut 0.025 mm slot every 1 cm) over 10-cm intervals. Sectional pipe with polytetrafluoroethylene (PTFE) o-rings on male threads at connection points were then taped with PTFE before threading pieces together to the required length of pipe for each piezometer. About 1 m of each piezometer rises above the surface. A well point was screwed/threaded on (joint was PTFE taped) to the bottom of each piezometer. In each piezometer nest, the top of the screened interval was placed at 0, 30, 50, 100 cm below the hollow surface. One deeper piezometer was installed to either 200 cm (KF45 and S2S) or 175 cm (BW). Piezometers were pushed or hammered into peat. Each piezometer had at least 1.5-m of pipe below the peat surface, such that the 0 cm depth piezometers, for example, had about 140 cm of pipe beneath the screen that served as a reservoir to accumulate and hold water. Piezometers were separated by no more than 10 to 15 cm between any adjacent pair of piezometers within a nest.
WATER SAMPLING: Before May 2010, unfiltered surface water was ladled into a new, 250-mL low density polyethylene (LDPE) bottle for pH, specific conductivity, ion, and nutrient analyses. Samples were not collected during periods when water was frozen or the water table was too low for sampling. A dipper (CXBA00, Global Water Instrumentation, Phoenix, Arizona) with an approximately 1 m handle was used after about April 2010 when sampling the S2N lagg site. The dipper was occasionally used for weekly sampling during periods when the samplers were frozen during snowmelt and there was standing water at the sampler. At those times, surface water was dipped adjacent to a sampler.
All water in a piezometer was evacuated immediately before to a day prior to sampling. Zero to 50 cm depth samplers usually refilled within minutes. Deeper samplers usually required one day to refill with enough water for sampling. A manual bellows pump (various Guzzler 400 series pumps, The Bosworth Co., East Providence, Rhode Island) was used to purge piezometers.
A peristaltic pump (Cole Parmer, Vernon Hills, Illinois, Masterflex PSF/CRS easy-load pump head mounted to a Dewalt, Townson, Maryland, portable drill) was used to sample piezometers. At least 3 volumes of the tubing volume (or 5-20 s of pumping) were purged prior to filling bottles to clean tubing between samples. Flexible pump tubing was rinsed before and after sampling with 18.0 megaohm deionized water.
Unfiltered pore water was ladled, dipped, or pumped in to a 250-ml LDPE bottle. An aliquot of most samples was collected in a 16-mL scintillation vial with a Polyseal cap for liquid water isotope analysis (stored at room temperature). Scintillation vials for water isotope samples were completely filled, with no headspace or bubbles. For all samples collected during and after 2019, a separate 60-mL aliquot was saved in a new, HDPE bottle solely for nutrient (nitrate, ammonium, total nitrogen, and total phosphorus analyses) analyses. From September 12, 2016 to October 26, 2020, a 30-mL or 60-mL HDPE bottle was filled with water from KF5, KF5A, KF45, KF45A, and S2N Lagg/KF26 for ferrous and ferric iron concentration measurement.
For all analyses except ferrous and ferric iron determination, sample bottles and vials were triple rinsed with sample water before filling. Samples bottles for ferrous and ferric iron analysis were not rinsed because each bottle was received 0.5 mL (30-mL bottle) or 1 mL (60-mL) of 12 mol/L high purity trace-metal grade hydrochloric acid in advance of sampling to preserve the samples.
When collected, date/time of retrieval, sample location, and associated notes were recorded on field data sheets. A unique serial ID number was assigned to all aliquots of the same sample for tracking purposes in the laboratory and data reporting. The serial ID, date/time, and sample location were also written on label tape on each sample bottle or vial. Sample ID numbers are 5 or 6 digit integers. Samples IDs are not necessarily consecutive because water from other sites at the MEF are interspersed in the numbering series. Surface water samples were labelled with the name of each site. For porewaters from the nests of piezometers, samples were labelled by piezometer nest (KF45, BW, or S2S) and the corresponding depth as a three character integer for a depth in cm (i.e., 000, 030, 050, 100, and 185 or 200). For example, the sample from the 0 cm depth at KF45 was labeled KF45-000.
Samples were transported on ice in a cooler to the Grand Rapids chemistry laboratory, and then were refrigerated (250-mL bottles), frozen (60-mL bottles for nutrient analysis), or stored at room temp (isotope vials) until analyzed.
Bottles for ferric/ferrous iron analysis were either refrigerated or stored at room temperature. They were later shipped to Iowa State University for analysis.
ANALYTICAL METHODS:
Water samples were analyzed for pH, specific conductivity, and concentrations of cations, silicon, anions, nutrients, and total organic carbon at the Forestry Science Laboratory in Grand Rapids, Minnesota. Samples were analyzed for ferrous and ferric iron concentrations at Iowa State University.
Unfiltered water was used for all laboratory analyses. It is important to keep in mind that porewater and surface waters in peatlands are free of inorganic particulates due to flowpaths through peat and slow transit times due to low hydraulic gradients that allow for deposition of particulates. For that reason, we have considered our unfiltered water samples of surface and peatland porewaters to be dissolved. The samples are likely to include colloids, but no inorganic particulates and rarely peat particles. Attempts are made to avoid or eliminate aquatic organisms (mostly mosquito larvae during late spring when abundant) or plant leaves and needles (after senescence) when water was ladled or dipped from standing water.
Forestry Science Laboratory in Grand Rapids, Minnesota:
For each type of laboratory measurement, every tenth sample was run in duplicate followed by two reference standards. Analytical duplicates and the 10 preceding samples were acceptable for reporting when the relative error was less than 10 percent between duplicates. When check standards differed by more than 5 percent from actual values, a batch of samples was reanalyzed. When a particular sample was higher in concentration than the highest calibration standard, that sample was diluted and re-run until within the range of the calibration standards.
Unless otherwise noted, autosamplers were used with instruments for analysis.
For each instrument and sample, we record the data and time of analysis and that information is stored in our unpublished records.
For anion, cation, silicon, nutrient, and TOC analyses, check and reference solutions were made in volumetric flasks with deionized water (18.0 megaohm/cm).
pH: A Mettler Toledo (Columbus, OH) DL53 Autotitrator was used to measure pH according to Standard Method 4500-H+ B (APHA 2017). A four-point calibration was performed before each batch of 15 samples. A sodium carbonate reference was run after every ten samples. Samples were only analyzed if the reference values were accurate to within 10 percent and pH is reported to the nearest tenth of a decimal place. Samples for pH analysis typically were analyzed within days of collection. Although rare, samples sometimes were held for weeks to several months while awaiting maintenance on the meter or for a replacement pH probe.
Specific conductivity: Conductivity was measured on a Yellow Springs Instruments (YSI; Yellow Spring, Ohio) Model 3100 meter. A YSI 3403 probe (cell constant = 1.0/cm) was used until March 2017 and a YSI 3253 probe (cell constant = 1.0/cm) thereafter. The meters were calibrated with a 46.7 microSiemen/cm standard, and periodically checked with 23.8, 84.0, or 150 microSiemen/cm references. The manually loaded cell of the conductivity probe was twice rinsed and then conductivity was measured on the third poured aliquot. Samples, the standard, and reference standards were measured at room temperature and in a laboratory maintained at 21 degree C. Conductivity values were recorded on paper. Specific conductivity (conductivity at 25 degree C) was calculated from conductivity (at 21 degree C) when values were transferred to spreadsheets. Samples for conductivity measurement typically were analyzed within days of collection. Although rare, samples sometimes were held for weeks to several months while awaiting maintenance on the analyzers.
Anions: Anion (chloride and sulfate) concentrations were measured using suppressed conductivity and conductimetric detection on two different ion chromatographs: on a Dionex (Sunnyvale, CA) DX-500 for samples collected through 2012 and a Thermo Scientific Dionex ICS-2100 thereafter. Samples were injected through 20 micrometer filter caps and through an IonPac AG14 pre-column and AS14 column (DX500) and AG22 pre-column and AS22 column (ICS-2100). Standard Method 4110-C was used (APHA 2017). The ion chromatograph was typically operated every business day. Daily throughput of samples is lower than the rate at which samples are sometimes collected. For that reason, samples for anion measurement were sometimes analyzed within several days of collection, but oftentimes held for months to a year before analysis.
Cations and silicon: Cation (calcium, magnesium, potassium, sodium, aluminum, iron, manganese, and strontium) and silicon concentrations were analyzed by inductively coupled optical emission spectroscopy (ICP-OES). A Thermo Electron Corporation (Waltham, Massachusetts) Iris Intrepid ICP-OES was used for all samples collected through 2015, and a Thermo Scientific (Waltham, Massachusetts) ICAP 7600 Duo for samples collected during and after 2016. Standard method 3120 was used (APHA 2017). Samples for cation analyses were typically run one to four times a year in large batches of samples; analysis occurred within several days of collection for some samples to a year from collection for those that were held longest.
Nutrients:
Ammonium was measured according to the Lachat (Milwaukee, Wisconsin) QuikChem 10-107-06-1-F method on a Lachat (Hach Company, Loveland, Colorado) QuickChem 8500 starting with samples collected during 2019. Ammonium is reported as the amount of nitrogen in ammonium. The Lachat methods are equivalent to the flow injection analysis method to form indephenol blue for colorimetric analysis (Standard Method 4500-NH3 H; APHA 2017).
Nitrate+nitrate was measured according to Lachat QuikChem 10-107-04-1-B on a QuickChem 8500 starting with samples collected during 2019. Nitrate was reduced to nitrite using the flow injection analysis (cadmium reduction) method and concentration was colorimetrically determined as the amount of nitrogen in the resulting nitrite (Standard Method 4500-NO3- I; APHA 2017). Though nitrite was not separately measured using this method, it has been rare in our experience to observe nitrite in MEF water samples. All samples were also analyzed by ion chromatography and nitrite peaks were not visible in sample chromatograms.
Total nitrogen (TN) concentrations were measured colorimetrically after in-line automated persulfate-ultraviolet oxidation to nitrate (Standard Method 4500-N B; APHA 2017). Concentrations were measured according to the Lachat QuikChem 10-107-04-1-P method on a Lachat QuickChem for samples collected before 2016 and Lachat QuikChem E10-107-04-3-D method on a Lachat QuickChem 8500 thereafter.
Soluble reactive phosphorus was measured according to the Lachat QuikChem 10-115-01-1-B method on a Lachat QuikChem 8000 for samples collected through 2012 and Lachat QuikChem 8500 thereafter. The Lachat methods are equivalent to the flow injection analysis method with ascorbic acid reduction (Standard Method 4500-P F; APHA 2017).
Total phosphorus (TP) concentrations were measured colorimetrically using automated persulfate-UV digestion and flow injection analysis with ascorbic acid reduction for colorimetric detection (Standard Method 4500-PI; APHA 2017). Concentrations were measured according to the Lachat QuikChem 10-115-01-3-A method on a Lachat QuickChem 8000 for samples collected before 2016 and Lachat QuikChem E10-115-01-3-A method on a Lachat QuickChem 8500 thereafter.
Samples for nitrogen and phosphorus chemistry were typically run once or twice a year in large batches of samples. Analysis occurred within several days of collection for some samples to a year from collection for those that were held longest.
Total organic carbon (TOC): Concentrations of TOC were measured by high-temperature combustion with infrared detection (Standard Method 5310-C, APHA 1995) on a Shimadzu TOC-V CPH with External Sparge Kit for samples collected through 2012. A Shimadzu (Columbia, Maryland) TOC-VCP was used for samples collected from 2013 onward. Concentrations were measured as total carbon minus inorganic carbon (TC-IC) before June 2010 and as non-purgeable organic carbon (NPOC) after that. Potassium hydrogen phthalate (KHP) was used for reference and check standards. Sebestyen et al. (2020) show that the TC-IC and NPOC are comparable to within 10% relative error as a measure of TOC concentration. We consider the two instruments and methods to be equivalent for our sites. Samples were typically measured within days of sample collection. Rarely, samples were held for weeks or months when the instrument needed maintenance or laboratory personnel were awaiting consumables such as compressed gas.
Liquid water isotopes: The glass scintillation vials for water isotope measurement are stored in a sample archive and are available for eventual analysis. Some samples have been analyzed on a Los Gatos Research (Mountain View, California) DLT-100 Water Isotope Analyzers at Plymouth State University (Center for the Environment Analytical Laboratory), University of California (Stable Isotope Facility, Davis), University of Minnesota (Biometeorology Laboratory, St. Paul), or the University of Toronto (Integrated Watershed Hydrology and Biogeochemistry Research Facility, Scarborough, Ontario, Canada); or a Los Gatos Research T-LWIA-45-EP liquid water isotope analyzer at the Grand Rapids chemistry laboratory. Further information can be found in the data entity lab_info.csv.
We report the relative abundance of deuterium (D) and oxygen-18 (O-18). The natural abundances of stable isotopes were measured using laser absorption spectroscopy (Lis et al. 2008). All laboratories used similar procedures; 6-7 injections of 0.5-1.2 uL sample were analyzed. Isotopic values were scaled relative to the Vienna Standard Mean Ocean Water (VSMOW)-Standard Light Antarctic Precipitation (SLAP) scale. A series of secondary standards were calibrated to VSMOW and SLAP Machine raw data was then post-processed to account for machine drift and between-sample memory (Wassenaar et al., 2014). Values for D and O-18 are reported in delta-notation (permil relative to VSMOW; Craig 1961).
Ferrous and ferric iron: Total iron and ferrous iron concentrations were colorimetrically measured in the Hall Laboratory at Iowa State University using a ferrozine method (Huang and Hall 2017) on a microplate spectrophotometer (Biotek Synergy HT, Winooski VT). Ferric iron concentration was calculated as the difference between total iron and ferrous iron concentrations (Huang and Hall 2017). The calculation of ferric iron concentration resulted in small negative values for samples with extremely low ferric iron concentrations. We defined the median absolute value of these negative numbers as the detection limit for ferric iron concentration (0.07 mg/L) and set positive ferric iron concentration values within this range to zero. Acidified samples were held for weeks or months to a year before analysis.
When a particular sample was higher in concentration than the highest calibration standard, that sample was diluted and re-run until within the range of the calibration standards.
The sum of ferrous and ferric iron concentrations from colorimetric analysis occasionally exceeded the total iron values measured by ICP-OES at the Grand Rapids Forestry Sciences Laboratory. This discrepancy is likely a consequence of greater release or dissolution of particulate iron from occasional peat fragments in the acidified (pH > 2) samples used for ferrous and ferric iron analysis. Samples for colorimetric analysis were collected in pre-acidified bottles to inhibit oxidation of ferrous iron to ferric iron during sample storage. Samples for ICP-OES analysis were not similarly acidified for storage and analysis.
REPORTED VALUES:
To document when a sample was collected, we include a laboratory ID, sample name, and date/time of collection. Sometimes chemistry values are assigned -9999 for individual solutes or for all analytes (i.e., pH, specific conductivity, and solute concentrations), which may have resulted from insufficient sample volume to complete all analyses, contamination that affected individual solutes or suites of analytes that were simultaneously measured on a single instrument for a particular sample, or contamination that affected all solutes for a particular sample. Samples pending analysis are also assigned -9999.
Concentrations of nitrate and ammonium are only reported onward from 2019 when aliquots were frozen for analysis. Values are reported as -9999 prior to preservation with freezing.
Concentrations of ferrous and ferric iron are only reported between September 12, 2016 to October 26, 2020. Not all samples were analyzed, and some values are pending completion of analysis.
Data values below the detection limit are reported in the data file and are not flagged. Detection limits, as listed above, must be considered when using these data.
The method detection limits were:
- 0.01 mg chlorine/L,
- 0.02 mg sulfate/L,
- 0.05 mg calcium/L,
- 0.05 mg magnesium/L,
- 0.5 mg potassium/L,
- 0.1 mg sodium/L,
- 0.01 mg aluminum/L,
- 0.05 mg iron/L,
- 0.01 mg manganese/L,
- 0.05 mg silicon/L,
- 0.01 mg strontium/L,
- 0.01 mg nitrogen/L for ammonium,
- 0.002 mg nitrogen/L for nitrate+nitrite,
- 0.001 mg phosphorus/L for soluble reactive phosphorus,
- 0.05 mg nitrogen/L for total nitrogen,
- 0.05 mg phosphorus/L for total phosphorus,
- 1 mg carbon/L for the TC-IC method, and 0.5 mg carbon/L for the NPOC method of TOC analysis,
- 0.04 mg iron/L for ferrous iron,
- 0.07 mg iron/L for ferric iron.
The analytical precision for water isotopes was:
- 0.8 permil for D and 0.1 permil for O-18 at Plymouth State University,
- 2 permil for D and 0.25 permil for O-18 at the University of California,
- 1 permil for D and 0.25 permil for O-18 at the University of Minnesota,
- 0.8 permil for D and 0.25 permil for O-18 at the University of Toronto,
- 0.5 permil for D and 0.1 permil for O-18 at the Grand Rapids Forestry Sciences Laboratory.
MARCELL EXPERIMENTAL FOREST sites and data collection are described in further detail in:
Sebestyen, S.D., C. Dorrance, D.M. Olson, E.S. Verry, R.K. Kolka, A.E. Elling, and R. Kyllander (2011). Chapter 2: Long-Term Monitoring Sites and Trends at the Marcell Experimental Forest. In Randall K. Kolka, Stephen D. Sebestyen, Elon S. Verry, and Kenneth N. Brooks (Ed.). Peatland Biogeochemistry and Watershed Hydrology at the Marcell Experimental Forest (pp 15-71). CRC Press, Boca Raton, FL. https://www.fs.usda.gov/treesearch/pubs/37979.
RELEVANT PUBLICATIONS
Griffiths, N. A., Sebestyen, S. D., and Oleheiser, K. C. (2019). Variation in peatland porewater chemistry over time and space along a bog to fen gradient. Science of The Total Environment, 697, 134152. https://doi.org/10.1016/j.scitotenv.2019.134152
Sebestyen, S. D., Funke, M. M., and Cotner, J. B. (2021). Sources and biodegradability of dissolved organic matter in two peatland catchments with different upland forest types, northern Minnesota, USA. Hydrological Processes. https://doi.org/10.1002/hyp.14049
REFERENCES
APHA. (2017). Standard methods for the examination of water and wastewater (23rd ed.). Washington, DC: American Public Health Association.
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