DOI: 10.5061/dryad.95x69p8vm
title: Data from: Plant cover changes drive plant and soil carbon pool responses in High Arctic dry heath exposed to decades of experimentally increased summer rain and nutrient addition

description: With accelerating climate change, higher summer rainfall and warmer soils
 are expected for high Arctic ecosystems. Yet, how increased inputs of
 moisture and nutrients to soils will affect plant composition and
 ecosystem C storage in these arid, low-productivity ecosystems remains
 unclear. We utilised a long-term experiment in a dry shrub heath tundra in
 Zackenberg, NE Greenland, in which N and P availability was increased and
 precipitation doubled experimentally every summer for 25 years. We
 determined soil and vegetation C pools, plant cover and leaf chemistry,
 and ecosystem CO2 fluxes were measured over three weeks in peak growing
 season. Watering increased the cover of graminoids, which likely drove a
 moderate increase in upper soil carbon stocks. Soil respiration was
 consistently stimulated in watered plots, confirming the high sensitivity
 of soil microbes to moisture in dry tundra environments. As aboveground
 biomass and plant C uptake were not equivalently enhanced by watering, we
 suggest that belowground processes linked to root growth and/or microbial
 turnover are important in driving the C pool changes observed. Our results
 show evidence that increased summer rainfall can lead to greening and
 enhanced soil C storage in high arctic dry heaths, potentially providing
 moderate negative feedback to climate change.
description: Fieldwork was facilitated by the Zackenberg Ecological Research
 Operations station in July 2022. Plants and organic materials were
 collected and analysed in accordance with the non-exclusive licence
 G22-053 for utilization of Greenland genetic resources.
 The experimental work was conducted in 2022 in the Zackenberg
 valley, NE Greenland (74°30’N, 21°00’ W). The
 experiment consists of a fully factorial setup, with 48 plots of 0.5m ×
 0.5m dimensions in 6 blocks. To study the effects of increased summer
 precipitation and moderately increased nutrient availability, the full
 factorial block design includes 3 factors: Water (W), nitrogen (N) and
 phosphorus (P) additions, applied<strong> </strong>in all
 combinations within each block, hence with eight treatment combinations
 including controls in each block. To analyse soil C
 pool changes &gt;25 years after initiation of the experiment, four 10
 cm-deep soil cores were taken from each of the 48 plots under the main
 vegetation types/species which were: <em>Dryas</em>,
 <em>Salix</em>, <em>Kobresia</em> and cryptogamic
 crust. The cores were separated into two depths:  0-5 (topsoil) and 5-10cm
 (mineral subsoil). Aboveground vegetation cover in
 plots was estimated in late July 2022 using the
 point-intercept/point-framing method with 50 pin hits per plot. To
 determine plant %C, %N, <sup>13</sup>C and
 <sup>15</sup>N natural abundance, ten mature leaves of the
 three dominant plant species (<em>Dryas</em>,
 <em>Salix</em> and <em>Kobresia</em>) were
 collected in each plot. All CO<sub>2
 </sub>flux data was collected in summer 2022 using an EGM-4 portable
 Infrared Gas Analyser (PP Systems Unit 2, Hitchin, Hertfordshire, U.K). To
 measure the net ecosystem production (NEP), a transparent 8.43L plexiglass
 chamber was placed onto a 20 × 20 cm metal frame located in the centre of
 the plots (in place since 2004). To estimate Ecosystem Respiration
 (R<sub>eco</sub>), the chamber was covered with a black
 plastic cover and CO<sub>2</sub> concentrations were measured
 as for NEP. To isolate the respiration component stemming from soil
 microbial sources (as well as a minor contribution from root respiration
 and faunal heterotrophs), also known as
 R<sub>soil</sub>,<sub>, </sub>we used a
 cylindrical closed system soil respiration chamber (SRC-1 – Probe type
 8) We measured CO<sub>2</sub> flux in all
 48 plots in 3 separate rounds within a two-week timespan (July
 22<sup>nd</sup> -August 3<sup>rd</sup>, 2022)
 coinciding with peak growing season. In parallel to chamber flux
 measurements, we measured photosynthetic active radiation (PAR) using a
 PAR Meter (Sun System, U.S) and plot surface temperature with a Proscan
 530 Dual focus infrared hand thermometer (Dostmann Electronic, Germany) at
 each plot prior to flux measurement. Soil volumetric water content and
 temperature were measured at 0-5 cm depth (TEROS-11 sensors, Meter Group
 AG, München, Germay) and logged every one hour (ZL6 Cellular Data Loggers,
 Meter Group AG, Müncen Germany) in all plots.
description: # Data from: Plant cover changes drive plant and soil carbon pool
 responses in High Arctic dry heath exposed to decades of experimentally
 increased summer rain and nutrient addition
 [https://doi.org/10.5061/dryad.95x69p8vm](https://doi.org/10.5061/dryad.95x69p8vm) ## Description of the data and file structure Fieldwork was facilitated by the Zackenberg Ecological Research Operations station in July 2022. Plants and organic materials were collected and analysed in accordance with the non-exclusive licence G22-053 for utilization of Greenland genetic resources. The experimental work was conducted in 2022 in the Zackenberg valley, NE Greenland (74°30’N, 21°00’ W). The experiment consists of a fully factorial setup, with 48 plots of 0.5m × 0.5m dimensions in 6 blocks. To study the effects of increased summer precipitation and moderately increased nutrient availability, the full factorial block design includes 3 factors: Water (W), nitrogen (N) and phosphorus (P) additions, applied** **in all combinations within each block, hence with eight treatment combinations including controls in each block. To analyse soil C pool changes &gt;25 years after initiation of the experiment, four 10 cm-deep soil cores were taken from each of the 48 plots under the main vegetation types/species which were: *Dryas*, *Salix*, *Kobresia* and cryptogamic crust. The cores were separated into two depths:  0-5 (topsoil) and 5-10cm (mineral subsoil). Aboveground vegetation cover in plots was estimated in late July 2022 using the point-intercept/point-framing method with 50 pin hits per plot. To determine plant %C, %N, 13C and 15N natural abundance, ten mature leaves of the three dominant plant species (*Dryas*, *Salix* and *Kobresia*) were collected in each plot. All CO2 flux data was collected in summer 2022 using an EGM-4 portable Infrared Gas Analyser (PP Systems Unit 2, Hitchin, Hertfordshire, U.K). To measure the net ecosystem production (NEP), a transparent 8.43L plexiglass chamber was placed onto a 20 × 20 cm metal frame located in the centre of the plots (in place since 2004). To estimate Ecosystem Respiration (Reco), the chamber was covered with a black plastic cover and CO2 concentrations were measured as for NEP. To isolate the respiration component stemming from soil microbial sources (as well as a minor contribution from root respiration and faunal heterotrophs), also known as Rsoil,, we used a cylindrical closed system soil respiration chamber (SRC-1 – Probe type 8) We measured CO2 flux in all 48 plots in 3 separate rounds within a two-week timespan (July 22nd -August 3rd, 2022) coinciding with peak growing season. In parallel to chamber flux measurements, we measured photosynthetic active radiation (PAR) using a PAR Meter (Sun System, U.S) and plot surface temperature with a Proscan 530 Dual focus infrared hand thermometer (Dostmann Electronic, Germany) at each plot prior to flux measurement. Soil volumetric water content and temperature were measured at 0-5 cm depth (TEROS-11 sensors, Meter Group AG, München, Germay) and logged every one hour (ZL6 Cellular Data Loggers, Meter Group AG, Müncen Germany) in all plots. Any missing values in the data set are due to absence of a plant species in a particular plot (for leaf C and N concentration and isotope data), and variable not measured at the specific time point (for CO2 fluxes). "NA" in soil moisture content and soil temperature data is due to data not available, due to logger not in operation.     ### Files and variables #### File: ALL **Description:**  ##### Variables * Plot_ID: individual number tags attributed to each specific plot * Treat: experimental manipulations applied : C = Control, N = Nitrogen, P = Phosphorus, NP = Nitrogen + Phosphorus, W = Watered, WN = Water + Nitrogen, WP = Water + Phosphorus , WNP = Water + Nitrogen + Phosphorus; * Block: block randomly assigned as part of replication in experimental setup * Component: ecosystem component split per functional type, see main Oldcorn et al. paper for more detail * Species: genus name of main plant species present at site: Dryas = Dryas octopetala, Salix= Salix arctica, Kobresia = Kobresia myosuroides * Watered?: yes/no refers to whether the plots had received added water, or not (plots that received water are those with treatments W, WN, WP, WNP as described above)     #### File: Oldcorn_et_al_fig1_soil_CNP.csv **Description:**  ##### Variables * Layer: soil depth slices measured,with topsoil representing the upper 5cm and mineral subsoil from 5 down to 10cm * Percent_N:  nitrogen concentration in homogeneized soil samples (dimensionless ratio) , using an Eurovector elemental analyser (EA) coupled to an Isoprime isotope ratio mass spectrometer (IRMS) * Percent_P: phosphorus concentration in homogeneized soil samples (dimensionless ratio), estimated using the flow injection analyser molybdenum blue method * SOC_kg_m2: plot-based SOC stocks per soil layer in kg/m2; derived from the EA-IRMS Carbon concentration data in samples multiplied by sample mass #### File: Oldcorn_et_al_fig5_swc.csv **Description:**  ##### Variables * Doy: day of year * SWC_m3m3:  Soil volumetric water content in m3 water/ m3 available air volume; measured down to 5cm depth and logged every hour * "NA" in soil moisture content data is due to data not available, due to logger not in operation.  #### File: Oldcorn_et_al_fig2_plant_cover.csv **Description:**  ##### Variables * Cover: Pinpointed hits per component during grid cover analysis #### File: Oldcorn_et_al_figS11_soil_temp.csv **Description:**  ##### Variables * Temp_DegC: Temperature in degrees celsius; measured down to 5cm depth and logged every hour * "NA" in soil temperature data is due to data not available, due to logger not in operation.  #### File: Oldcorn_et_al_fig3_plant_C.csv **Description:**  ##### Variables * Plant_C_g_m2: Plot-level carbon stocks in total aboveground vegetation in kg /m2; derived by multiplying the biomass of each species or type with the corresponding EA-IRMS C concentrations * Missing values in the data set are due to absence of a plant species in a particular plot. #### File: Oldcorn_et_al_fig4_leaf_15N.csv **Description:**  ##### Variables * Leaf_15N: 15-N isotope concentration in plant leaves, in permil ; determined through EA-IRMS * Missing values in the data set are due to absence of a plant species in a particular plot. #### File: Oldcorn_et_al_fig6_co2_fluxes.csv **Description:**  ##### Variables * Msrmt_Round: Round of ecosystem fluxes measurements within two-week field campaign (July 22nd -August 3rd, 2022), see main Oldcorn et al. paper for more detail * NEP_gCO2_m2_h: Net ecosystem production in gCO2/m2/h; measured using a portable infrared gas analyser * ER_gCO2_m2_h: Ecosystem respiration in gCO2/m2/h; measured using a portable infrared gas analyser * GEP_gCO2_m2_h: Gross ecosystem production in gCO2/m2/h; measured using a portable infrared gas analyser * Rsoil_gCO2_m2_h: Soil respiration in gCO2/m2/h; measured using a portable infrared gas analyser * Missing values in the data set are due to variable not measured at the specific time point. #### **File: Oldcorn_et_al_figS4_EC_soil.csv** **Description:**  ##### Variables EC_soil: Electrical conductivity in µS/ cm, measured in 1:5 slurries of sample soil in distilled water #### File: Oldcorn_et_al_figS3_pH_soil.csv **Description:**  ##### Variables * pH_soil: pH (dimensionless log units) measured in 1:5 slurries of sample soil in distilled water #### File: Oldcorn_et_al_figS5_soil_NP.csv **Description:**  ##### Variables * Soil_N_kg_m2:  Plot-based nitrogen stocks per soil layer in kg/m2; derived from the EA-IRMS N concentration data in samples multiplied by sample mass * Soil_P_kg_m2:  Plot-based phosphorus stocks per soil layer in kg/m2; derived from the flow injection P concentration data in samples multiplied by sample mass #### File: Oldcorn_et_al_figS7_leaf_percent_C.csv **Description:**  ##### Variables * Percent_C:  Carbon concentration in the leaf of main plant species at our site (dimensionless ratio) , using an Eurovector elemental analyser (EA) coupled to an Isoprime isotope ratio mass spectrometer (IRMS) * Missing values in the data set are due to absence of a plant species in a particular plot. #### File: Oldcorn_et_al_figS8_leaf_percent_N.csv **Description:**  ##### Variables * Percent_N:  Nitrogen concentration in the leaf of main plant species at our site (dimensionless ratio) , using an Eurovector elemental analyser (EA) coupled to an Isoprime isotope ratio mass spectrometer (IRMS) * Missing values in the data set are due to absence of a plant species in a particular plot. #### File: Oldcorn_et_al_figS9_leaf_13C.csv **Description:**  ##### Variables * Leaf_13C: 13-C isotope concentration in plant leaves, in permil ; determined through EA-IRMS * Missing values in the data set are due to absence of a plant species in a particular plot. #### File: Oldcorn_et_al_tblS7_plant_C.csv **Description:**  ##### Variables * C_shrubs_biomass_g_m2: Plot-level carbon stocks in aboveground shrub parts, in g /m2; derived by multiplying estimated shrub biomass with the corresponding EA-IRMS C concentrations in leaf tissue * C_grams_biomass_g_m2: Plot-level carbon stocks in aboveground graminoid parts, in g /m2; derived by multiplying estimated shrub biomass with the corresponding EA-IRMS C concentrations in leaf tissue * C_crypto_biomass_g_m2:  Plot-level carbon stocks in aboveground cryptogamic parts, in g /m2; derived by multiplying estimated shrub biomass with the corresponding EA-IRMS C concentrations in leaf tissue #### File: Oldcorn_et_al_tblS8_litter_C.csv **Description:**  ##### Variables * Tot_litter_C: Plot-level C stocks in total aboveground plant litter, in g/m2. Values are higher than the sum of species litter components as also include a background litter constant used to account for the undistinguishable litter fractions and from other minor species for which correlation not found * C_shrubs_litter: Plot-level C stocks in the aboveground litter of shrubs, in g/m2. Values are the sum of litter carbon stocks for the evergreen shrub dryas and the deciduous shrub salix in each plot * C_litter_dryas: Plot-level C stocks in the evergreen shrub dryas litter, in g/m2. * C_litter_salix: Plot-level C stocks in deciduous shrub salix litter, in g/m2. * C_litter_grams: Plot-level C stocks in the aboveground litter of graminoids, in g/m2. ## Code/software All data can be read with R statistical software (version 4.1.2). To ascertain treatment effects and potential interactions on CO2 fluxes, as well as soil and vegetation C pools, a repeated measures linear mixed effects modelling approach (RM LMM) can be employed using the package *nlme (Pinheiro and Bates 2023)* .