ALBERT

Description: Nitrogen (N) mineralization and nitrification were compared among ecosystems representing a primary successional sequence on the Tanana River floodplain of interior Alaska. These processes displayed marked seasonality, were closely related to substrate chemistry, and reflected the impact of vegetation clearing. The highest rates of N mineralization were encountered in the June to July incubation periods, and rates generally declined during the remainder of the summer. The early season period (June to July) was the interval of most favorable litter and mineral soil temperature and most available energy supply for microbial mineralization of detrital materials. Minimal rates were encountered during the winter. Litter layer N mineralization rates were highest in the early-successional poplar–alder (Populusbalsamifera–Alnustenuifolia (Nutt.) stage and declined with advancing succession in poplar (Populusbalsamifera) and mature white spruce (Piceaglauca) (Moench) Voss) stands. The poplar–alder stage displayed the highest rate of nitrification. Nitrate constituted 98% of the mineralized N in early-successional poplar–alder forest floors but fell to 4 and 0% in poplar and white spruce forest floors, respectively. Nitrogen mineralization was closely related to significant increases in the lignin/N ratio across the sequence of vegetation types. The rate of surface mineral soil net N mineralization increased with succession in response to higher soil organic matter content. The range of average total seasonal net N mineralization (260–1600 mg N•m−2) for litter layer plus mineral soil among successional stages in this study was generally lower than the 1200–8400 mg N•m−2 reported by investigators for other studies in temperate latitudes. Vegetation clearing increased the magnitude of temporal fluxes as well as total annual mineral N production. The most consistent increases were encountered in the poplar–alder vegetation type. The average seasonal total net N mineralization for forest floor plus mineral soil in this vegetation type increased from 1500 to 3264 mg N•m−2 as a result of clearing. Soil temperature declined with advancing succession and generally increased as a consequence of clearing. However, these changes were not as closely correlated with N mineralization as were the changes in substrate chemistry encountered across this successional sequence.

Description: Fine root production and turnover were studied in hardwood and coniferous taiga forests using three methods. (1) Using soil cores, fine root production ranged from 1574 ± 76 kg•ha−1•year−1 in the upland white spruce (Piceaglauca (Moench) Voss) stand to 4386 ± 322 kg•ha−1•year−1 in the floodplain balsam poplar (Populusbalsamifera L.) stand, accounting for 49% of total production for coniferous stands and 32% of total production for deciduous stands. Fine root turnover rates were higher in floodplain (0.90 ± 0.06 year−1) stands than in upland (0.42 ± 0.10 year−1) stands. Across all sites, the ratio of fine root turnover to litter fall averaged 2.2 for biomass and 2.8 for N. Both values were higher in floodplain stands than in upland stands, and in coniferous stands than in deciduous stands. (2) The C budget method showed that C allocation to fine roots varied from 150 to 425 g C•m−2•year−1 and suggested that soil respiration was more dependent on C derived from roots than from aboveground inputs. The C allocation ratio (C to roots: C to litter fall) was inversely correlated with litter-fall C and varied from 0.3 to 69.5; there was a tendency for higher proportional belowground allocation in coniferous stands than in deciduous stands and the highest levels were at the earliest successional sites. (3) Estimates of apparent N uptake (Nu), N allocation to fine roots, and fine root production based on N budget calculations showed that annual aboveground N increments exceeded Nu estimates at half the sites, indicating that the method failed to account for large amounts of N acquired by plants. This suggests that plant and (or) mycorrhizal uptake of soil organic N may be more significant to ecosystem N cycling than mineral N turnover by the soil microbial biomass.

Description: Forest floors and mineral soils from early (open willow), middle (poplar–alder), and late (white spruce) floodplain primary successional stages were examined for nitrogen fixation and denitrification. The acetylene-reduction and acetylene-inhibition techniques were used separately and in combination to measure nitrogenase and denitrification activities, both in laboratory and field studies. In situ N2O production was undetectable at all sites and during all sampling periods. Denitrifying activity measured in the field with acetylene amendments was low to undetectable, except after a brief flood in the open willow stand when N2O production ranged from undetectable to 34 ng N•cm−2•h−1 within the newly deposited alluvium–old mineral soil interface. Intact core assays also had low to undetectable denitrification activities; the highest activities (259 ng N•g−1 h−1) were measured in the poplar–alder forest floor in the fall. Laboratory studies showed that potential denitrification enzyme activity (DEA) was also greatest in the poplar–alder forest floor (4332 ng N•g−1•h−1), once again occurring in the fall. In early and midsuccessional stages, the interactive effects of temperature, carbon, and NO3− limited denitrification, yet even with the addition of the limiting amendments, low to undetectable DEA was observed in mineral soils. The later white spruce successional stage also had low to undetectable DEA, increasing only with the addition of the full DEA media and independent of temperature changes. Nonsymbiotic nitrogenase activities were highly variable, ranging from undetectable to 30 ng N•cm−2•h−1. Highest activities were seen in the open willow, newly deposited alluvium–old mineral soil interface immediately after a flood and approximately 1 month after the flood on the newly deposited silt surface. Only the white spruce forest floor had measurable nonsymbiotic nitrogenase activity at all sampling times. Alder root nodule nitrogenase activity showed no significant differences between sampling periods. The estimated annual nitrogen fixation rate of 164 kg N•ha−1 for alder root nodules is a substantial N contribution to the alder stand and to the floodplain ecosystem in general.

Description: Low addition levels of high enrichment isotope (〉1% of the total nitrogen pool with 99 at.% excess 15N) were used to follow nitrogen movement through selected forest floor components of permafrost-free and permafrost-dominated black spruce ecosystems in subarctic Alaska. The nitrogen pool examined in this study was the total nitrogen pool. 15N was retained most effectively by the feather moss layer (Pleuroziumschreberi (BSG.) Mitt. and Hylocomiumsplendens (Hedw.) BSG.) on both black spruce sites. Twenty-eight months after isotope application the feather moss layer still contained over 90% of the 15N that could be recovered. The limited movement of 15N between feather moss layers and underlying forest floor horizons appeared to be slightly affected by climatological events. Differences in 15N movement patterns between permafrost-free and permafrost-dominated black spruce sites are discussed in terms of precipitation, soil temperature, and biological controls.

Description: Soil chemical properties were studied after a wildfire in stands of white spruce (Piceaglauca (Moench) Voss), black spruce (Piceamariana (Mill.) B.S.P.), paper birch (Betulapapyrifera Marsh.), and quaking aspen (Populustremuloides Michx.). Samples of the forest floor and surface 5 cm of mineral soil were collected from burned sites and unburned controls and analyzed soon after the fire. With the exception of soil pH, effects of the fire on soil chemistry differed among the four forest types. Generally, amounts of exchangeable K, Ca, and Mg did not appreciably increase in the forest floor and surface mineral soil except in heavily burned areas in white spruce and black spruce. Fire reduced amounts of N by about 50% in white spruce, aspen, and birch forest floors. In black spruce, quantities of N were slightly higher in heavily burned locations. Forest floor C:N ratios were substantially lower in heavily burned locations in white spruce and black spruce than in unburned controls. Burning did not have a marked influence on supplies of available P in the forest floor, except in heavily burned black spruce, where average amounts were 12.50 g/m2 versus only 0.46 g/m2 in the control. Burning caused more moderate gains in available P in surface mineral soils under aspen and white spruce. We concluded that fire caused marked short-term changes in soil chemistry in the four forest types. How long these changes will persist is unknown.

Description: A variety of evergreen and deciduous forests in the taiga of interior Alaska were studied over a 5-year period to examine how the chemical quality of forest-floor organic matter affected its rate of decomposition and mineral cycling within and outside the tree vegetation. Litterbag and respiration studies were used to monitor decomposition. Natural forest-floor substrates and others altered by addition of N, P, and K fertilizer and glucose as a carbon source were studied in the laboratory and field for rates of weight loss and O2 consumption. Forest floors differing in C/N ratios, including those deficient in N, were used to measure substrate quality influences on seedling growth, nutrient content, and tannin content. Microbial (bacteria and fungi) biomass was measured across a range of forest types along with pH, base saturation total pool sizes of N and P, and annual mineralization of organic matter per square metre. Under identical moisture and temperature conditions average respiration rates in evergreen forest-floor L, F, and H substrates were 1.8, 2.8, and 2.0 times less than in the corresponding deciduous forest horizons, respectively. Birch L and F horizons had respiration rates 11.5 times higher than the corresponding black spruce layers. Weight losses in birch L, F, and H horizons were 6, 3, and 2 times higher, respectively, than in the corresponding black spruce substrates. Substrates had a quality-dependent decay rate which did not change when they were relocated within or between sites indicating that measured field climatic differences were not as influential on decay rates as substrate quality components. Fungal biomass was significantly correlated with the quantity of organic matter in all sites (n = 15, r = 0.62) but correlations were better for deciduous (n = 9, r = 0.89), and evergreen (n = 6, r = 0.82) forests separately. Strong correlations exist also between grams of organic matter decayed per square metre per year and fungal biomass (n = 13, r = 0.86), and fungal biomass and grams of N and P mineralized per square metre per year (n = 14, r = 0.95) and (n = 11, r = 0.94, respectively). Seedlings on mineral-deficient substrates produced more tannins than the controls, and seedlings on substrates with widening C/N ratios had successively less tissue with lower N content, and proportionally more roots. Nitrogen content of litter fall in increasingly nitrogen-poor forest floors was correspondingly lower. Nitrogen content of litter fall on N rich forest floors and N fertilized forest floors was proportionately higher. Nitrogen withdrawal in leaves at senescence was inversely correlated with grams N mineralized per square metre per year in forest floors. Fertilization did not influence microbial processes in the field, though lab studies indicated a negative influence of NH4, P, and K on microbial respiration. Glucose added in the laboratory and field markedly increased forest-floor microbial respiration. In vitro glucose-induced increases in respiration were not influenced by addition of ammonium nitrate and were significantly depressed by addition of P and K. In the field, fertilization had no effect on either glucose-induced respiration or microbial biomass.

Description: Alkaline soils exist under forest vegetation on the Tanana River floodplain of interior Alaska. The objectives of this study were to describe the soil chemical properties and to examine controls on these chemical properties along a forest primary-successional sequence. Soil saturation pastes were prepared from duplicate sites along the successional sequence representing bare alluvium (stage I), open willow (stage III), poplar–alder (stage V), and white spruce (stage VIII). Calcium, Mg, SO4, and ions responsible for alkalinity were the dominant solutes in the saturation extracts. Soil horizons were generally calcareous (CaCO3) and therefore alkaline (pH 〉 7.0) across the successional sequence. The CaCO3-containing soil horizons were saturated to supersaturated with respect to calcite. Many soil horizons in the plots of stages III and V from one site were saturated with respect to gypsum (CaSO4•2H2O), whereas other stages and sites were generally undersaturated. Phosphate availability in CaCO3-containing soil horizons was apparently constrained by CaCO3 solubility and the solubility products of β-tricalcium phosphate and hydroxyapatite. There was a strong positive relationship between soluble salt content and silt concentration in the early stages, suggesting that initial salt content is controlled by the texture of the alluvial material; the variation in initial texture largely accounts for the site to site variation in salts. Evaporation apparently plays a role in concentrating soluble salts at the surface during the early stages (III and V), but by the late stages (VIII), the biotic and topographic factors apparently reduce surficial evaporation. This reduction in evaporation, coupled with the production of organic acids, leads to a 42% loss of soluble salts from the soils. This study supports our original hypotheses that physical factors (texture, evaporation) are most important early in the successional sequence and that biotic factors (transpiration, acid leaching) are most important late in the successional sequence in controlling salt distribution in soils on the Tanana River floodplain.

Description: Cold climatic conditions govern the productivity of taiga forests, yet within a successional sequence the microclimatic and biogeochemical variations also have a major effect on soil microbial activities, thus affecting plant productivity through nutrient availability. Nitrogen mineralization and nitrification were measured in primary-successional floodplain forests of interior Alaska. Forest floor and mineral soils from an early (open willow), middle (poplar–alder), and late (white spruce) successional stage were used. The effects of temperature, moisture, and NH4+ were tested in the laboratory for each of the successional stages. Potential nitrification was estimated using the chlorate-inhibition technique. Surface mineral soils and white spruce forest floor had low to undetectable rates of nitrogen mineralization and nitrification (

Description: Alluvial soils on the Tanana River floodplain near Fairbanks, Alaska, were examined for development of physical and chemical properties in relation to soil depth and across a 200-year vegetation development sequence. Development was mediated by ecosystem controls including successional time, vegetation, terrace height, soil physical and chemical properties, and microclimate. These controls interact and are conditioned by the state factors time, flora, topography, parent material, and climate, respectively. On early-successional (