DOI: 10.5061/dryad.6wwpzgn6m
title: Moss species and precipitation mediate experimental warming stimulation of growing season N2 fixation in subarctic tundra

description: Climate change in high latitude regions leads to both higher temperatures
 and more precipitation but their combined effects on terrestrial ecosystem
 processes are poorly understood. In nitrogen (N) limited and often
 moss-dominated tundra and boreal ecosystems, moss-associated N2 fixation
 is an important process that provides new N. We tested if high mean annual
 precipitation enhanced experimental warming effects on growing season N2
 fixation in three common arctic-boreal moss species adapted to different
 moisture conditions and evaluated their N contribution to the landscape
 level. We measured in situ N2 fixation rates in
 Hylocomium splendens, Pleurozium schreberi and Sphagnum spp. from June to
 September in subarctic tundra in Sweden. We exposed mosses occurring along
 a natural precipitation gradient (mean annual precipitation: 571-1155 mm)
 to eight years of experimental summer warming using open-top chambers
 before our measurements. We modelled species-specific seasonal N input to
 the ecosystem at the colony and landscape level. Higher mean
 annual precipitation increased N2 fixation, especially during peak growing
 seasons and in feather mosses. For Sphagnum-associated N2 fixation, high
 mean annual precipitation reversed a small negative warming response. By
 contrast, in the dry-adapted feather moss species higher mean annual
 precipitation led to negative warming effects. Modelled total growing
 season N inputs for Sphagnum spp. colonies were 2-3 times that of feather
 mosses on an area basis. However, at the landscape level where feather
 mosses were more abundant, they contributed 50% more N than
 Sphagnum. The discrepancy between modelled estimates of
 species-specific N input via N2 fixation at the moss core versus ecosystem
 scale exemplifies how moss cover is essential for evaluating the impact of
 altered N2 fixation. Importantly, combined effects of warming and higher
 mean annual precipitation may not lead to similar responses across moss
 species, which could affect moss fitness and their abilities to buffer
 environmental
 changes.         
description: The study took place above the natural subarctic-alpine tree line
 in Northern Sweden. The study included eight sites distributed along a
 precipitation gradient where mean annual precipitation varied from 571 mm
 y<sup>-1 </sup>at the driest site to 1155 mm y<sup>-1
 </sup>at the wettest site. Patches dominated by one of three moss
 species were passively warmed through hexagonal Perspex open-top chambers,
 OTCs during summer for 8 years, or left as controls. Moss species were
 <em>Hylocomium splendens </em>(Hedw.) Schimp.,
 <em>Pleurozium schreberi</em> (Brid.) Mitt. and
 <em>Sphagnum</em> spp. (including <em>S.
 capillifolium</em> (Ehrh.) Hedw.<em> </em>and
 <em>S. fuscum</em> (Schimp.). To determine
 N<sub>2</sub> fixation, we used acetylene reduction assays
 (ARA). At all sites, <em>in situ</em> ARAs were performed five
 times during the growing season of 2018 (14 June – 21 September). ARA
 results were converted to actual N<sub>2</sub> fixation for
 our three moss species through <sup>5</sup>N<sub>2
 </sub>labelling
 <sup>15</sup>N<sub>2</sub>.<em>
 </em>For full method details, see the published article in Global
 Change Biology.
description: # Moss species and precipitation mediate experimental warming stimulation
 of growing season N2 fixation in subarctic tundra
 [https://doi.org/10.5061/dryad.6wwpzgn6m](https://doi.org/10.5061/dryad.6wwpzgn6m) ## Description of the data and file structure R scripts and data were used to investigate the effect of experimental field warming and mean annual precipitation on N fixation in 3 moss species in subarctic Sweden. Various supporting data include daily values of soil temperature and moisture in the experimental plots, measured rainfall at site level along precipitation gradient, moss shoot d13C, and density. Also data for converting acetylene reduction to N2 fixation.  Data and units are in .rds format, which is readily read by the two provided R scripts. Across data sets variables are: site: 8 sites in the Tornetrask area, Northern Sweden prec_int: Site precipitation, mm y-1. Gridded data of climate normals 1961-1991. otc: warming treatment with open-top chamber (W) or controls (C) moss: moss species, *Hylocomium splendens* (H), *Pleurozium schreberi* (P), *Sphagnum* ssp. (S) serie: measurement round (A-E) during growing season 2018 serie_date: date interval for each measurement round doy: day of the year of measurement date: date of measurement ### **List of files and additional descriptions of variables:**  **ethylene_chamber_field_long.rds** – ethylene production after Acetylene Reduction Assays on 48 moss cores during 5 field campaigns and under standardized conditions at 7 C. serie: date interval for each measurement round, 'clim' refers to climate chamber measurements at 7C **ARA_15N_conversion.rds** – climate chamber ethylene production and 15N uptake in thr 48 moss cores. etylen_korr_um_pr_hr_m2: ethylene production, microgram per hour per square meter top_N_uptake_um_m2_hr: calculated N fixation in 0-2 cm moss + brown moss, microgram N per hour per square meter top_15N_uptake_um_m2_hr: 15N uptake in 0-2 cm moss + brown moss, microgram 15N per hour per square meter soil_15N_uptake_um_m2_hr: 15N uptake in the soil layer, microgram per hour 15N per square meter **moss_15Nuptake.rds** – the uptake of 15N in each fraction of moss cores which were incubated in 15N2 gas after the field season.  fraction moss: top 2 cm (top_0_2), brown layer under 2 cm (brown_u2 soil), non-target moss and lichens (other), top 1 cm(top_0_1), 1-2 cm (top_1_2) ug15N_pr_gDW: 15N content of dry weight biomass, mikro g g-1 DW_g: Dry weight of fraction, g **N2_ARA_d13C_density_incubation_T_moist.rds** – data for N2 fixation, incubation conditions during field sampling, and final d13C and biomass in green parts of moss shoots in the 48 moss cores. Tmean: incubation temperature, C  watercontent_g_pr_g_bm: moss core moisture content during incubation etylen_korr_um_pr_hr_m2: ethylene production, microgram per hour per square meter  N_um_hr_m2: calculated N fixation, microgram per hour per square meter d13C_toplayer: delta 13 C in moss shoots (upper 2 cm for H and P and upper 1 cm for *Sphagnum*) density_toplayer: density, g cm-3 in moss shoots (upper 2 cm for H and P and upper 1 cm for *Sphagnum*)  **HOBO_raingauges.rds** – Data summarizing total precipitation and number of days with at least one rain event in the period 25 June – 20 September 2014 at the eight sites along the mean annual precipitation gradient. event_days: No of days with precipitation MAP: Mean annual precipitation, Site precipitation, mm y-1. Gridded data of climate normals 1961-1991.   **VWC_temp_wm_daily.rds** – Data summarizing daily mean, max, and min of hourly logged soil temperature and volumetric moisture content in all 48 experimental plots during the study period (2018). sensor placed in - 5cm of moss mat. Missing values occur. place: 4 places including two sites each; jiepren = jiepren1 + jiperen2, palno = palno1 + palno2, paddus = paddus1 + paddus2, katter = katter + vassi VWC_day_mean: daily mean of hourly measurements, Volumetric water content, cm^3/cm^3 VWC_day_min: daily minimum of hourly measurements, Volumetric water content, cm^3/cm^3 VWC_day_max: daily maximum of hourly measurements, Volumetric water content, cm^3/cm^3 Temp_day_mean: daily mean of hourly measurements, temperature, C Temp_day_min: daily minimum of hourly measurements, temperature, C Temp_day_max: daily maximum of hourly measurements, temperature, C Moss_moisture_g_g_mean: daily mean of hourly measurements, gravimetric moisture content, cm^3/cm^3 Moss_moisture_g_g_min: daily min of hourly measurements, moisture content, cm^3/cm^3 Moss_moisture_g_g_max: daily max of hourly measurements, moisture content, cm^3/cm^3 n: number of observations summarised, max is 24 **Gapfilled_VWC_temp_wm_daily_place.rds** - same logger data as in **VWC_temp_wm_daily.rds** but averaged across “twin sites”  to overcome missing data so that instead of 8 sites there are 4 places. Due to persistent missing data, one treatment and moss combination (warming and Sphagnum) in one place (Palno) data set has been gap-filled for half the season by assuming a linear relationship between warming and control plots. ## Code/Software R version 4.3.3 (2024-02-29) was used. Following files call necessary libraries and load uploaded data to reproduce graphs and tables for associated paper, incl. supporting information. **Stats_Figs.R** – calls the .rds files to reproduce all figures and statistics in the main paper and supplementary files (except Figure S1 (Map with site positions) and S2 (uses publically available data from the Swedish Meteorological and Hydrological Institute). **GAMM_by_moss_precip_otc.R** – Runs generalized additive models and predicts moss species-specific N2 fixation based on model parameters and microclimate data measured in plots.