TITLE: bilbrough.soil.readme.txt AUTHORS: PI: Jeffrey M. Welker co-PI/data questions: Carol J. Bilbrough Department of Renewable Resources University of Wyoming Laramie, WY 82071 USA phone: JMW: 307 766-2172 phone: CJB: 307 766-5472 facsimile: 307 766- web page: email JMW: jeff@uwyo.edu email CJB: carolb@uwyo.edu ABSTRACT FROM SUBMITTED MANUSCRIPT: Nongrowing season nitrogen (N) dynamics in arctic tundra ecosystems are poorly understood, constraining our understanding of the annual cycle of soil and plant N processes. Because the nongrowing season (fall, winter, and early spring) is so long, understanding N processes during these seasons is critical to a holistic perspective of arctic biogeochemical cycles. As increases in winter temperatures and associated increases in snow fall are a consistent prediction of climate change models, quantifying N processes throughout the year under experimentally increased snow depth is an important component to predicting how arctic N dynamics may function in the future. We measured extractable soil N and net N mineralization at time scales reflecting seasonal changes throughout an annual cycle under ambient snow cover and increased snow cover in moist tussock and dry heath tundra. In moist tussock tundra, we sampled tussocks dominated by Eriophorum vaginatum and intertussocks dominated by sphagnum mosses. In dry heath, we sampled soils dominated by Dryas octopetela or Arctostaphylos alpina. Our objectives were to: 1) compare nongrowing season N dynamics under ambient and increased snow cover conditions in moist tussock and dry heath tundra systems, 2) examine how different winter snow cover regimes affect growing season N dynamics, and 3) evaluate how these responses are affected by dominant species present in each system. We used intact core incubations sampled in September, November, March, May, June, and August to measure soil inorganic N, mineralization, and nitrification. In moist tussock tundra, tussock and intertussock soils with increased winter snow cover had high net N mineralization rates during the fall and winter, followed by immobilization during snowmelt. In contrast, N mineralization only occured during the autumn in soils with ambient snow cover. During the growing season when N immobilization dominated in areas with ambient snow cover, soils with increased winter snow cover had positive net mineralization and nitrification rates, indicating that plant N availability increased during the growing season as a consequence of changing winter conditions. In dry heath tundra, soils with increased snow depth also had high late winter net N mineralization rates, but these rates were: a) comparable to early winter rates in soils under Arctostaphylos plants with ambient snow cover, b) greater in soils under Arctostaphylos plants than in soils under Dryas plants, and c) less than the rates found in tussock tundra. Our findings suggest that soil microbes are carbon limited during the nongrowing season when positive mineralization rates occur, and switch to N limitation during immobilization phases. Thus, changes in climate that result in deeper snow and warmer winter soil temperatures will dramatically alter soil N dynamics by increasing the quantity of inorganic N present in the soil, resulting in greater plant N availability and fundamental shifts in substrate limitation for soil microbes. TIME PERIOD: August 30, 1998 through August 30, 1999 PHYSICAL LOCATION: Toolik Lake Field Station, dry heath tundra and moist tussock tundra snow fence sites (68o38'N, 149o38'W, elevation 760 m) DATA COLLECTION AND PROCESSING: Site description. This research was conducted in dry heath and moist tussock tundra near the Toolik Field Station (6838'N, 14938'W, elevation 760 m asl), located in the northern foothills of the Brooks Range, Alaska, USA. The dry site is sparsely covered with prostrate plants dominated by Dryas octopetela, Arctostaphylos alpina, Loiseleuria procumbens, and many lichens. Dryas and Arctostaphylos grow in near monotypic stands 0.25 to several meters across. Vegetation cover is usually about 50%, with open spaces occupied by some lichens. The well-drained mineral soils are overlain by a thin organic horizon up to 5 cm deep. The moist tussock site is acidic tundra dominated by the tussock-forming sedge Eriophorum vaginatum intermixed with mosses, deciduous shrub, and evergreen shrub species. The soil substrate of Eriophorum tussocks is composed of tightly woven dead Eriophorum roots, and may be 10 to 30 cm taller than the intertussock moss mats. The saturated intertussock soils are covered with an organic mat composed primarily of mosses up to 20 cm thick. Snow fences were erected perpendicular to prevailing winds in the dry heath and moist tussock sites in July of 1994 to increase the depth and duration of snow cover, simulating one possible climate change scenario. Snow drifts behind the fences typically reach a maximum depth of 3 m, and extend out for about 35 m, diminishing to ambient snow depths 50 to 60 m from the fence (Jones et al. 1999). Snow accumulates behind the snowfence earlier than adjacent sites, and remains at least three weeks later in the deepest area of the drift. Thus, this treatment extends snow cover in the fall and spring, resulting in a shortened growing season. Producing a 3 m deep drift may be an extreme treatment, but it was necessary to produce a large enough study zone and it should adequately represent the effects of increased winter insulation. Methods. Net N mineralization in the fall, winter, and growing season was measured using an in situ soil incubation method. Because of the difficulty in collecting soil samples from frozen soil, cores for all incubations were installed the first week of September 1998. Replicate cores for each incubation period were installed in either ambient or increased snow depth treatments in dry heath and moist tussock tundra. Increased snow cover cores were located at maximum snow depth (3 m) behind the snow fence. Ambient snow cover cores were located adjacent to the snowfence area where vegetation and soils were similar to the snow fence site. In the dry heath, cores were inserted into spaces between plants in monotypic clusters of Arctostaphylos or Dryas. In moist tussock tundra, cores were placed in tussocks dominated by Eriophorum and in adjacent intertussock spaces dominated by Sphagnum mosses. For the tussock cores, aboveground tissue was removed, the core extruded and live roots removed from th For soil incubations, PVC tubes 4 cm in diameter were inserted into the ground to 10 cm, removed, and a mixed-bed ion exchange resin bag placed at the base of the core as a leachate trap. Cores were returned to the soil for incubation. At each harvest, cores were returned to the lab and processed within 6 hours. Frozen soils (November, March, and May) were hammered into small pieces, and all soils were homogenized, and analyzed for gravimetric water content and inorganic soil N (NH4+ and NO3-). Within 30 minutes of extrusion from the core, soil subsamples were extracted for one hour with 0.5 mol/L K2SO4, filtered through prewashed filters, and the filtrate frozen until analyzed. Soil temperatures on frozen soil samples ranged from -5 to -1.5 when placed in extractant solution. Resins were rinsed free of soil with deionized water, extracted with 0.5 mol/L K2SO4 for 30 minutes, filtered and frozen in the same manner as the soils. Extract solutions were analyzed for NH4+ and NO3- by flow injection analysis In summary, cores were installed in two sites, dry heath and moist tussock tundra. Within each site, there were two snow treatments (ambient and increased snow depth) and two subplot types based on the dominant species present for a total of four treatment combinations per tundra type. The subplot types in the dry heath were separate clusters of Arctostaphylos or Dryas. Moist tussock subplot types were tussock, dominated by Eriophorum, and intertussock, dominated by Sphagnum mosses. Subplots will be referred to as the species level treatment. Three replicate cores were installed for each of the 5 harvest dates (November, March, May, June, August), plus initial measurements in September. DATA FORMAT: -file structure: ASCII, tab delimited -data format and layout: The data base includes header with information on PI, location, and dates. The data consist of 8 columns and 112 rows of data and are both numeric and alphanumeric. - list of parameters 1. date: year, month, day 2. location: longitude and latitude 3. site: moist tussock tundra or dry heath tundra 4. plot type: the plant species or community that is dominant where the soil was sampled. In dry heath tundra, this was soil under Dryas octopetela or soil under Arcostaphylos alpina. In moist tussock tundra, this was tussock soils dominated by Eriophorum vaginatum and intertussock soils dominated by spaghnum mosses. 5. snow treatment: The winter snow cover treatments. Soils were sampled under ambient snow cover or in the lee of the snow fences where the snow drifted to a depth of approximately 3 meters. 6. NH4-N, soil NH4 contents, measured with 0.5 Mol K2SO4 extracts. 7. NO3-N, soil NO3 contents, measured with 0.5 Mol K2SO4 extracts. 8. total N, soil NH4 plus soil NO3 contents - data version: preliminary version, September 25, 2000. DATA REMARKS The soil N data are presented on a per unit area basis, and were calculated based on soil sampled to 10 cm.