Abstract

Authors

---more---

Dataset Citation

Verity Salmon, Colleen Iversen. 2019. NGEE Arctic Plant Traits: Nitrogen fixation, Kougarok Road Mile Marker 64, Seward Peninsula, Alaska, 2017-2018. Next Generation Ecosystem Experiments Arctic Data Collection, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Tennessee, USA. Dataset accessed on [INSERT_DATE] at https://doi.org/10.5440/1493688.

Dates

2017-07-29 - 2018-07-17

Geographic Location

Place Keywords:

GCMD Keywords

N/A

Methodology

1. Nodule Collection & Incubation Alder is found within two ecotypes at Kougarok, Alder shrublands and Tussock tundra willow birch tundra. Within Alder shrublands, alder is the dominant species and forms a relatively closed canopy but in Tussock tundra willow birch tundra, alder grows with a mixture of other deciduous shrubs (Salix, Betula, and Vaccinium) and a graminoids. Measurements were made July 29-30th, 2017 and July 13-17th, 2018. Rates of N fixation were determined for 21 alder shrubs growing in Alder Shrublands and 12 alder shrubs growing in Tussock tundra willow birch tundra. When an individual shrub was selected for measurement, the basal area of all ramets making up the individual shrub were measured at the soil surface using digital calipers. The maximum height of alder in the plot was measured to the nearest centimeter. GPS coordinates of the shrub were marked with a handheld GPS unit (Garmin Montana 680t) or an iphone equipped with Avenza mapping application and a site-specific geoPDF. Sun and shade leaves were collected also (see Supplemental Files). Nodules were found by tracing roots out from the base of the alder. Once nodules were found, they were removed from the roots and were brushed free of loose soil. If sufficient (2.5 g) nodules could not be found within ten minutes of excising the first root nodule, sampling started over at a new shrub. Nodules excised for over 20 minutes have reduced rates of N fixation (Ruess personal communication) and longer search times could bias results. If 2.5 g of nodules were found, they were placed in a 60 ml syringe with a rubber plunger. The plunger was pulled to 50 ml volume and then 10 ml of 98 atm% 15N2 gas was drawn into the syringe, resulting in a 20 atm% mixture in the headspace of the syringe. Immediately following the introduction of the 15N2 gas, a ten-minute timer was started. To determine the exact signature, a 15 ml sample of headspace gas was injected into 12ml evacuated cylinder glass container (839W, Labco, UK) for analysis. The syringe was then gently buried in the surface soil near the alder. The temperature of the soil at this location was recorded in 2018, in 2017 the temperature could not be recorded due to a broken thermometer. When the 15N2 gas had been in the syringe for ten minutes, the incubation was halted by removing the nodules from the syringe, placing them in a 15 ml centrifuge tube, and dropping the centrifuge tube into a bucket of liquid N. Another 2.5 g of fresh nodules were then collected from the same alder individual for determination of the initial 15N signature of the incubated nodule. Within twelve hours, all nodules were transferred to coin envelopes and placed in a drying oven at 70 degrees Celsius to dry. 2. Sample processing When dried, all nodules were weighed and ground to a fine powder. Nodule 15N and %N by weight was determined by isotope ratio mass spectrometry (IRMS, Integra CN, SerCon, Crewe, UK) at Oak Ridge National Laboratory. Gas samples collected from the headspace of the syringe were sent to UC Davis Mass spectrometry facilities for determination of atm% 15N2 (15Nheadspace). 3. N fixation rate Atom percent excess of the nodule (APE nodule) was calculated as the difference between atm% 15N of incubated and non-incubated nodules. The rate of N fixation (Nfix) in micromole N/g nodule/hr was then calculated as: Nfix=(APEnodule x N_nodule))/(incubation time x ^15 Nheadspace) Where Nnodule is the micromole N present per g dry nodule. In 2017, the average temperature of surface soils under alders was 11 degrees Celsius (HOBO station dataloggers installed in Alder shrublands). Temperatures recorded during the 2018 incubations and a Q10 of 2 were used to normalize 2018 rates to 11 degrees Celsius. References Anderson, M. D., Ruess, R. W., Uliassi, D. D., & Mitchell, J. S. (2004). Estimating N-2 fixation in two species of Alnus in interior Alaska using acetylene reduction and 15N2 uptake. Ecoscience, 11(1), 102-112.

Related References

N/A

Related Identifiers

N/A

Metadata Contact

Point of Contact