Soil Organic Carbon:
Quantity and Distribution in Arctic Tundra Soils
Associated with the ATLAS Winter C-Flux Study
G.J. Michaelson1, and C.L. Ping1
1University of Alaska Fairbanks, Agric. and Forestry Exp. Stn.
Palmer Res. Center, Palmer, Alaska, 533 E. Fireweed, Palmer, AK 99645.
Introduction
Research is underway to develop and improve models that describe the exchange of carbon from arctic ecosystems. Recent measurements of carbon flux have indicated that wintertime flux of CO2 from the arctic tundra region can be a very significant part of the seasonal fluxes, up to 60% (Oechel et al., 1997). Models often use total OC stocks of the surface soil layers, mostly surface organic layers, to represent the pool available for biological activity. This is to represent the system as it functions in the warm season. However during the cold-season, soil OC stocks down to the permafrost are often at nearly the same temperatures (0 to –10oC) for an extended period of time up to 8 months. This seasonal temperature shift presents a challenge with respect to assessing the quantity and quality of soil OC that may be important to soil respiration at low temperature. It is likely that there is a very substantial change in both the quantity and quality of SOC available to microbes during this cold period. Soil OC stocks down to 50 or even 100 cm depth could play a more significant role in total soil respiration. Due to the subzero temperatures, soil OC dissolved or with the potential to be dissolved in the soils unfrozen water could become the most significant substrate for microbes. The result of this could be that the total OC stocks of the surface organic layer may not be the best indicators of OC available for wintertime respiration or for indicators in predictive models that assess year round activity.
Objective
Determine the quantity of soil OC and soil water-soluble OC (OCws) and their distribution in soil profiles at the ATLAS Winter C-Flux Study sites along the western Alaska transect.
Materials and Methods
Sampling
The soils of each study site (Fig. 1) were examined and described in the field according to Soil Survey Manual (Soil Survey Division Staff, 1993). Soil samples were taken from the major soil genetic horizons that were exposed in the walls of pits excavated on each ATLAS Study site (approx. 1 m3). Samples were taken to the Palmer Soils for analysis.
Laboratory
Soil bulk density was determined on dimensional samples taken in the field-weighed in the lab. Total OC was determined by LECO CHN analyzer, water soluble OC (OCws) by equilibration of soil with water (1.45:10 v/v) for 12 hrs at 25oC and C-stores for site profiles calculated by the method of Michaelson et al., (2001).
Results
Quantity and distribution of soil OC
Soil profile distribution of OC for each site is presented in the top row of Figure 2. Carbon stores were generally within the range of stores found for the Kuparuk River Basin soils (Michaelson et al., 1996). The Ivotuk tussock tundra tower site had the highest stores at 104 kgOC m-3 followed by the more northern Oumalik acidic tundra site, the nonacidic tundra site at Ivotuk and the Council tundra site at 71, 62 and 58 kgOC m-3 respectively. Soil profiles at these sites contained relatively high amounts of OC compared with other study sites, largely due to the effects of cryoturbation. This was evident from the substantial portions of the OC stores found at depth in the combination (O/B or B/O) and Cf horizons. Similarly at the Quartz Creek sites the tussock tundra contained large proportions of its stocks in the Bw/O horizon with total stocks similar to those found at the other tundra sites. Soils under the lichen stripes at Quartz Creek contain OC stocks similar to the alpine soils of the Kuparuk Basin and only slightly lower than shrub sites at Council. Study sites at Council contained varying amounts of OC ranging from high to low in the order of tundra (w/permafrost), forest, open shrub, open woodland, and shrub sites at 58, 38, 28, 19 and 15 kgOC m-3 respectively. High tundra stocks were as mentioned above due to cryoturbation. Forest stocks were largely in the Oa (well decomposed organic) and Bsh (containing illuvial organic matter). Stocks of the open shrub site were a large part due to the mixing/burying effects of slope movement processes as evidenced by the substantial stocks found at depth in the profile. The open woodland profile mirrors that of the forest but with smaller amounts of OC. The shrub site also had a distribution pattern similar to the open shrub and forest but with shallower depth and lower stock levels.
Quantity and distribution of water soluble OC
It can be presumed that at low temperatures, biological activity and therefore respiration should be highly dependent on liquid water and the organic substrates that may be available in it. Therefore soil profile stocks of water-soluble organic carbon (OCws) were determined for each site and the data are presented in the lower part of Figure 2. Two points are most apparent in these data: (1) there is a quantity variation among profiles, and a range in distribution of OCws relative to the amount of total OC in the profiles, and (2) the ratio for stocks of OCws:OC is not the same between study areas.
(1) In general, the largest soil profile OCws stocks were found in the Council tundra and the Ivotuk tussock (tower) sites. However these sites were highest due to the large amounts of OCws stocks in the upper permafrost horizons. Vegetation types such as mature forest, lichens and shrubs apparently serve to elevate the OCws stocks found in a profile when compared to tussock tundra sites. The quantity of OCws was not necessarily related to the amount of total OC in a profile.
(2) There is quite a wide range of ratios for profile OCws: OC (gOCws m-3: kgOC m-3) stocks among soils at different locations. Sites with a significant presence of shrubs had the highest proportions of OCws . The highest ratios or proportions of OCws were in the Quartz Creek inter stripe (high in shrubs at 5.5), Council shrub (3.9) and open woodland (2.7), Council forest and tundra (2.2), Quartz Creek lichen (2.2), Council open shrub (1.7), and Ivotuk shrub (1.5). Tussock tundra sites all had ratios between 0.6 and 1.0.
Conclusions
1. Organic C stocks of ATLAS study sites were similar in quantity and distribution to comparable sites in the Kuparuk River Basin of the central Alaska Arctic. The presence of mixed mineral/organic horizons at depth due to cryoturbation, significantly elevate OC stocks.
2. Water-soluble OC stocks are elevated in soils under forest, lichen and shrub vegetation relative to tussock tundra. The quantity of OCws was not necessarily related to the amount of total OC in a soil profile.
References
Michaelson, G.J., C.L. Ping, and J.M. Kimble, Carbon storage and distribution in tundra soils of Arctic Alaska, U.S.A., Arctic and Alpine Res. 28(4), 414-424, 1996.
Michaelson, G.J., C.L. Ping, and J.M. Kimble, Effects of soil morphological and physical properties on estimation of carbon storage in arctic soils, in Lal et al., (eds) Assessment Methods For Soil Carbon, Ch. 23, p339-348, CRC Press, 2001.
Oechel, W.C., G. Vourlitis , and S.J. Hastings, Cold-season CO2 emissions from arctic soils. Global Biogeoch. Cycles 11:163-172, 1997.
Soil Survey Division Staff, Soil Survey Manual, USDA Handbook No. 18. U.S. Government Printing Office, Washington, D.C., 1993.