J. A. Laundre

EFFECTS OF TEMPERATURE AND SUBSTRATE QUALITY ON ELEMENT MINERALIZATION IN SIX ARCTIC SOILS

We compared the effects of temperature on rates of microbial respiration, N mineralization, nitrification, and P mineralization in soils from six arctic ecosystems located along a toposequence on Alaskas North Slope. Soils from these ecosystems were incubated aerobically in the laboratory for 13 wk and at temperatures representative of field values during a typical growing season. Rates of C and N mineralization were insen- sitive to temperature between 30 and 90C but increased by factors of 2 or more between 90 and 15?. For both C and N, differences in mineralization rates among soils were greater than differences due to incubation temperature within single soils. This suggests that the quality of soil organic matter varies widely among these ecosystems and is more important than soil temperature differences in controlling rates of these processes in the field. Nitri- fication occurred in all soils, even at 30, but there were large differences among soils in nitrification potentials. Overall differences in P mineralization between soils were small. Rates of P mineralization, however, decreased with increasing temperature in soils from some sites and increased with temperature in others. Carbon respired during the 1 3-wk incubations ranged between 1.5 and 8% of total soil organic C across soil types and incubation temperatures. In contrast to the relatively high C mineralization rates in these soils, net N and P mineralization rates were very low and were likely due to high microbial demands for these nutrients. High microbial demand for mineral nutrients can severely limit plant N and P availability in arctic soils.

Biogeochemical Diversity Along a Riverside Toposequence in Arctic Alaska

Nitrogen and phosphorus pool sizes, distribution, and cycling rates were described and compared for six different ecosystem types occurring along a single topose- quence in northern Alaska. The toposequence was located on a series of old floodplains of the Sagavanirktok River, in the northern foothills of the Brooks Range. From tussock tundra in the uplands, the toposequence passed through a relatively dry hilltop heath zone, a hillslope shrub/lupine/Cassiope zone, a footslope Equisetum zone, a wet sedge tundra, and a riparian shrub zone. A late-melting snowbank covered the hillslope site in early June of each year, and the sites consistently varied in soil temperature, soil moisture, thaw depth, and the seasonal pattern of soil thaw. The standing stocks of N, P, and C in soils of these six ecosystem types varied dra- matically but not monotonically along the toposequence, as did the turnover rates of these elements. Several measures were used in comparisons of N and P availability, including soil solution concentrations, in situ accumulation on ion-exchange resins, and levels of KCl-extractable N and P. Annual rates of net N mineralization were assayed using a buried bag method, and ecosystem respiration was measured by trapping CO2 in soda lime (NaOH + Ca (OH)2). Soil P pools were characterized by sequential extraction methods into four major pools, including loosely bound P, Al- and Fe-bound P, primary mineral P, and organic P. Both N and P availability were low in all six ecosystems when compared with temperate forests or wetlands. Among ecosystems, however, there was considerable variation in the relative availability of N vs. P, and in the apparent relative importance of nitrate as a nitrogen source.

BIOMASS AND CO2 FLUX IN WET SEDGE TUNDRAS: RESPONSES TO NUTRIENTS, TEMPERATURE, AND LIGHT

The aim of this research was to analyze the effects of increased N or P availability, increased air temperature, and decreased light intensity on wet sedge tundra in northern Alaska. Nutrient availability was increased for 6–9 growing seasons, using N and P fertilizers in factorial experiments at three separate field sites. Air temperature was increased for six growing seasons, using plastic greenhouses at two sites, both with and without N + P fertilizer. Light intensity (photosynthetically active photon flux) was reduced by 50% for six growing seasons at the same two sites, using optically neutral shade cloth. Responses of wet sedge tundra to these treatments were documented as changes in vegetation biomass, N mass, and P mass, changes in whole-system CO2 fluxes, and changes in species composition and leaf-level photosynthesis. n nBiomass, N mass, and P mass accumulation were all strongly P limited, and biomass and N mass accumulation also responded significantly to N addition with a small N × P interaction. Greenhouse warming alone had no significant effect on biomass, N mass, or P mass, although there was a consistent trend toward increased mass in the greenhouse treatments. There was a significant negative interaction between the greenhouse treatment and the N + P fertilizer treatment, i.e., the effect of the two treatments combined was to reduce biomass and N mass significantly below that of the fertilizer treatment only. Six years of shading had no significant effect on biomass, N mass, or P mass. n nEcosystem CO2 fluxes included net ecosystem production (NEP; net CO2 flux), ecosystem respiration (RE, including both plant and soil respiration), and gross ecosystem production (GEP; gross ecosystem photosynthesis). All three fluxes responded to the fertilizer treatments in a pattern similar to the responses of biomass, N mass, and P mass, i.e., with a strong P response and a small, but significant, N response and N × P interaction. The greenhouse treatment also increased all three fluxes, but the greenhouse plus N + P treatment caused a significant decrease in NEP because RE increased more than GEP in this treatment. The shade treatment increased both GEP and RE, but had no effect on NEP. Most of the changes in CO2 fluxes per unit area of ground were due to changes in plant biomass, although there were additional, smaller treatment effects on CO2 fluxes per unit biomass, per unit N mass, and per unit P mass. n nThe vegetation was composed mainly of rhizomatous sedges and rushes, but changes in species composition may have contributed to the changes in vegetation nutrient content and ecosystem-level CO2 fluxes. Carex cordorrhiza, the species with the highest nutrient concentrations in its tissues in control plots, was also the species with the greatest increase in abundance in the fertilized plots. In comparison with Eriophorum angustifolium, another species that was abundant in control plots, C. cordorrhiza had higher photosynthetic rates per unit leaf mass. Leaf photosynthesis and respiration of C. cordorrhiza also increased with fertilizer treatment, whereas they decreased or remained constant in E. angustifolium. n nThe responses of these wet sedge tundras were similar to those of a nearby moist tussock tundra site that received an identical series of experiments. The main difference was the dominant P limitation in wet sedge tundra vs. N limitation in moist tussock tundra. Both tundras were relatively unresponsive to the increased air temperatures in the greenhouses but showed a strong negative interaction between the greenhouse and fertilizer treatments. New data from this study suggest that the negative interaction may be driven by a large increase in respiration in warmed fertilized plots, perhaps in relation to large increases in P concentration.

Effects of drainage and temperature on carbon balance of tussock tundra micrososms

We examined the importance of temperature (7°C or 15°C) and soil moisture regime (saturated or field capacity) on the carbon (C) balance of arctic tussock tundra microcosms (intact blocks of soil and vegetation) in growth chambers over an 81-day simulated growing season. We measured gaseous CO2 exchanges, methane (CH4) emissions, and dissolved C losses on intact blocks of tussock (Eriophorum vaginatum) and intertussock (moss-dominated). We hypothesized that under increased temperature and/or enhanced drainage, C losses from ecosystem respiration (CO2 respired by plants and heterotrophs) would exceed gains from gross photosynthesis causing tussock tundra to become a net source of C to the atmosphere. The field capacity moisture regime caused a decrease in net CO2 storage (NEP) in tussock tundra micrososms. This resulted from a stimulation of ecosystem respiration (probably mostly microbial) with enhanced drainage, rather than a decrease in gross photosynthesis. Elevated temperature alone had no effect on NEP because CO2 losses from increased ecosystem respiration at elevated temperature were compensated by increased CO2 uptake (gross photosynthesis). Although CO2 losses from ecosystem respiration were primarily limited by drainage, CH4 emissions, in contrast, were dependent on temperature. Furthermore, substantial dissolved C losses, especially organic C, and important microhabitat differences must be considered in estimating C balance for the tussock tundra system. As much as ∼ 20% of total C fixed in photosynthesis was lost as dissolved organic C. Tussocks stored ∼ 2x more C and emitted 5x more methane than intertussocks. In spite of the limitations of this microcosm experiment, this study has further elucidated the critical role of soil moisture regime and dissolved C losses in regulating net C balance of arctic tussock tundra.

Plant carbon-nutrient interactions control CO2 exchange in Alaskan wet sedge tundra ecosystems.

We explored the long-term (8-yr) effects of separate field manipulations of temperature and nutrient availability on carbon balance in wet sedge tundra near the Arctic Long Term Ecological Research (LTER) site at Toolik Lake, Alaska. Our goals were (1) to assess the relative importance of chronic warming (with field greenhouses) and increased N and P availability (by fertilization) in controlling gross ecosystem photosynthesis, ecosystem respiration (plant plus heterotrophic respiration), and ultimately ecosystem C balance; and (2) to attempt to partition ecosystem responses to these treatments between plant and soil contributions. We present results of the effects of these manipulations on whole-system CO2 exchange over seasonal and diel cycles, and on nonrhizosphere soil microbial respiration using in situ soil incubations. n nWet sedge control plots were, at best, a weak sink for carbon even during the peak growing season. Chronic nutrient additions of N + P shifted wet sedge carbon balance to a strong sink throughout the growing season; nutrient availability regulated seasonal and diel CO2 exchanges in these two wet sedge ecosystems. The N + P plots had significantly higher photosynthesis and ecosystem respiration in spite of the unanticipated effect of ∼30% reduction in thaw depth in these plots, apparently due to a twofold increase in litter accumulation insulating the soil surface and/or possible shading from greater plant cover in these plots. These results highlighted the prevailing importance of nutrient–carbon interactions in controlling ecosystem processes and ecosystem C balance in arctic tundra. n nIn contrast, warming had only subtle effects on CO2 exchanges. Increased temperatures in the warmed plots had little effect on instantaneous rates of photosynthesis or respiration. After eight years of chronic warming with an average 5.6°C higher air temperature over the growing season and a 40–200% increase in net N mineralization rate, it was surprising that warming did not have more profound effects on CO2 exchange and plant cover. If there were an effect of warming, increased temperatures might cause early canopy development and lengthen the growing season, rather than directly affect instantaneous rates of photosynthesis. Based on photosynthesis–light response curves developed from the early- and late-season diel measurements, we demonstrated that the main effect of warming was to accelerate the development of the canopy early in the season. By midseason, however, there were no significant differences in C exchange between warmed and control plots. n nPerhaps the most important and novel result emerging from this study is the prevailing importance of plant C exchange, not soil processes, in driving ecosystem C fluxes. First, nonrhizosphere soil microbial respiration as estimated CO2 flux from in situ soil incubations was a small fraction of whole-system respiration and did not vary among treatments. This suggests that anaerobic conditions or some other factor may limit soil microbial respiration more than do temperature or nutrients. Second, plant respiration contributed most (90%) of the ecosystem respiration in fertilized plots. This unanticipated and large contribution from plant respiration highlights the critical importance of understanding the response of plant respiration to global environmental change in these wet sedge ecosystems.

Changes in Live Plant Biomass, Primary Production, and Species Composition along a Riverside Toposequence in Arctic Alaska, U.S.A.

In the arctic landscape, vegetation composition and structure are strongly affected by topographic position and associated variation in microclimate. Along a single riverside toposequence in northern Alaska, six distinct plant communities were studied including a riparian shrub community, a wet sedge tundra, a footslope Equisetum community, a hillslope shrub/lupine community, a hilltop birch-heath community, and a moist tussock tundra. Total live plant biomass varied threefold along the toposequence (450-1400 g m-2) while live vascular plant biomass (including belowground stems and rhizomes but not roots) varied sevenfold (160970 g m-2). Aboveground vascular plant production varied less than fourfold (80265 g m-2). Although all six communities showed some signs of nutrient limitation, measures of soil nutrient availability were highly variable among communities. Contrary to expectations, the most productive community along the toposequence was the hillslope shrub/lupine community, where a late-lying snowbank delayed the start of the growing season by 2 wk. The second most productive community was the hilltop birch-heath, which was exposed to winter winds and where snow melted earliest; most of the production in this community occurred in relatively protected depressions where there were dense accumulations of plant mass. A conclusion is that soil fertility, soil thaw, and protection from wind are more important than length of the snow-free season in regulating productivity along the toposequence. Also contrary to expectations, overall production:live biomass ratios of the six communities varied little despite large differences in growth form composition among communities and in biomass turnover among growth forms. High-biomass, highly productive communities had the lowest production:live biomass ratios, and thus the lowest biomass turnover. Because production and live biomass were linearly correlated over the entire range sampled, it may be possible to use live biomass and/or leaf area as a reasonably accurate predictor of productivity at the landscape or regional level in the Arctic.

Fine root production and nutrient content in wet and moist arctic tundras as influenced by chronic fertilization

We used ingrowth cores to estimate fine root production in organic soils of wet sedge and moist tundra ecosystems near Toolik Lake on Alaskas North Slope. Root-free soil cores contained in nylon mesh tubes (5 cm diameter, 20–30 cm long) were placed in control and chronically fertilized (N plus P) plots in mid-August 1994 and were retrieved 1 year later. Estimated fine root production in control plots was 75 g m−2 year−1 in wet sedge and 56 g m−2 year−1 in moist tussock tundra. Fine root production in fertilized plots was 85 g m−2 year−1 in wet sedge and 67 g m−2 year−1 in moist tussock tundra. Although our estimates of fine root production were higher on fertilized than control plots, differences were not statistically significant within either tundra type. Comparisons between our estimates of fine root production and other estimates of aboveground (plus rhizome) production on the same (wet sedge tundra) or similar (moist tussock tundra) plots suggest that fine root production was about one-third of total net primary production (NPP) under non-fertilized conditions and about one-fifth of total NPP under chronic fertilization. Fine root N and P concentrations increased with fertilization in both tundra types, but P concentrations increased more than N concentrations in wet sedge tundra, whereas relative increases in N and P concentrations in moist tundra roots were similar. These data are consistent with other studies suggesting that NPP in wet sedge tundra is often P limited and that co-limitation by N and P is more important in moist tussock tundra.