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 Alaska's 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.     

Studies on the molecular ecology of Blastomyces dermatitidis

The microecology of Blastomyces dermatitidis, the dimorphic etiologic agentof the potentially fatal systemic fungal infection, blastomycosis, is not well defined.Blastomyces dermatitidis may occur periodically at natural sites, perhaps aided by rotting organic material, animal droppings and physical changes. Semi-quantitative growth studies of B. dermatitidis on 2% agar plates determined the ability toutilize or tolerate a variety of substrates including simple and complex molecules as carbon source, and organic and inorganic nitrogen sources. Allantoin, creatinine, quanidoacetic acid, guanidine and cysteine may be used as sole nitrogen source. Allantoin in combination with dextrose, glycerol, lichenen, celloboise and other wood by-products support growth of B. dermatitidis at room temperature. The nutritional conversion of the fungus to the yeast form at room temperature, well demonstrated on allantoin/glycerol/yeast extract media, appears to be affected by certain inorganic compounds. The organism tolerates low to moderate levels of alpha-pinene, tannic acid, shikimic acid, veratryl alcohol, vanillic acid, and polyethyleneglycol-200. There are significant differences among isolates regarding growth on various substances at 20° and 37° centigrade. It appears that a variety of wood by-products and animal waste substrates, in combination, support the growth of B. dermatitidis. Their role in the ecological niche of B. dermatitidis, and the importance of nutritional dimorphism in the natural environment warrants further investigation. This revised version was published online in June 2006 with corrections to the Cover Date.     

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.     

Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes

Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.     

Desaturation of trans-octadecenoyl-acyl carrier protein by stearoyl-acyl carrier protein delta9 desaturase

Positional isomers of mono-unsaturated 18:1-ACP have been used as substrates for stearoyl-acyl carrier protein delta9 desaturase to test whether a C-H bond abstraction from either the C-9 or C-10 position could lead to rearranged products diagnostic for the production of an allylic radical intermediate. The reconstituted enzyme complex was able to desaturate trans-delta11-18:1-ACP and trans-delta7-18:1-ACP, but not trans-delta9-18:1-ACP, or any of the corresponding cis-isomers. Enzymatic desaturation of trans-delta11-18:1-ACP gave a single product, cis-delta9,trans-delta11-18:2-ACP, as characterized by gas chromatography-electron ionization mass spectrometry of the molecular ions, the fragmentation products of pyrrolidide and 4,4-dimethyloxazoline derivatives, and by comparison of chromatographic retention times with authentic standards. Reaction of trans-delta7-18:1-ACP gave two enzymic products, trans-delta7,cis-delta9-18:2 (approximately 80%) and trans-delta7,cis-delta11-18:2 (approximately 20%). The major product was likely formed in a reaction identical to that of 18:0-ACP desaturation, while the minor product was likely formed by alternative placement of the C-10 and C-11 positions of the substrate analog in a cis configuration relative to the diiron oxidant. Since none of the products observed are indicative of rearrangements originating with an allylic radical, a discussion of the origins and possible implications of these results is presented.     

Interannual variability of plant phenology in tussock tundra: modelling interactions of plant productivity, plant phenology, snowmelt and soil thaw

We present a linked model of plant productivity, plant phenology, snowmelt and soil thaw in order to estimate interannual variability of arctic plant phenology and its effects on plant productivity. The model is tested using 8 years of soil temperature data, and three years of bud break data of Betula nana. Because the factors that trigger the end of the growing season of arctic vegetation are less well known than those of the start of the growing season, three hypotheses were formulated and tested for their effects on productivity and its sensitivity to climate change; the hypothesised factors determining the end of the growing season were frost, photoperiod and periodic constraints. The performance of the soil thermal model was good; both the onset of soil thaw in spring and the initiation of freezing in autumn were predicted correctly in most cases. The phenology model predicted the bud break date of Betula nana closely for the three different years. The soil thaw model predicted similar growing season start dates under current climate as the models based on sum of temperatures, but it made significantly different predictions under climate change scenarios, probably because of the non‐linear interactions between snowmelt and soil thaw. The uncertainty about the driving factors for the end of the growing season, in turn, resulted in uncertainty in the interannual variability of the simulated annual gross primary productivity (GPP). The interannual variability ranged from ? 25 to + 26% of the mean annual GPP for the frost hypothesis, from ? 20 to + 20% for the photoperiod hypothesis and only from ? 7 to + 7% for the periodic hypothesis. The different hypotheses also resulted in different sensitivity to climate change, with the frost hypothesis resulting in 30% higher annual GPP values than the periodic hypothesis when air temperatures were increased by 3 °C.     

Nitrogen dynamics in arctic tundra soils of varying age: differential responses to fertilization and warming

In the foothills of the Brooks Range, Alaska, different glaciation histories have created landscapes with varying soil age. Productivity of most of these landscapes is generally N limited, but varies widely, as do plant species composition and soil properties (e.g., pH). We hypothesized that the projected changes in productivity and vegetation composition under a warmer climate might be mediated through differential changes in N availability across soil age. We compared readily available [water-soluble NH₄ ⁺, NO₃ ^?, and amino acids (AA)], moderately available (soluble proteins), hydrolyzable, and total N pools across three tussock-tundra landscapes with soil ages ranging from 11.5k to 300k years. The effects of fertilization and warming on these N pools were also compared for the two younger sites. Readily available N was highest at the oldest site, and AA accounted for 80–89 % of this N. At the youngest site, inorganic N constituted the majority (80–97 %) of total readily available N. This variation reflected the large differences in plant functional group composition and soil chemical properties. Long-term (8–16 years) fertilization increased the soluble inorganic N by 20- to 100-fold at the intermediate-age site, but only by twofold to threefold at the youngest site. Warming caused small and inconsistent changes in the soil C:N ratio and AA, but only in soils beneath Eriophorum vaginatum, the dominant tussock-forming sedge. These differential responses suggest that the ecological consequences of warmer climates on these tundra ecosystems are more complex than simply elevated N-mineralization rates, and that the responses of landscapes might be impacted by soil age, or time since deglaciation.