Abstract. The biogeochemical behavior of carbon in the forested watersheds of the Hubbard Brook Experimental Forest (HBEF) was analyzed in long-term studies. The largest pools of C in the reference watershed (W6) reside in mineral soil organic matter (43% of total ecosystem C) and living biomass (40.5%), with the remainder in surface detritus (14.5%). Repeated sampling indicated that none of these pools was changing significantly in the late-1990s, although high spatial variability precluded the detection of small changes in the soil organic matter pools, which are large; hence, net ecosystem productivity (NEP) in this 2nd growth forest was near zero (± about 20 g C/m 2 -yr) and probably similar in magnitude to fluvial export of organic C. Aboveground net primary productivity (ANPP) of the forest declined by 24% between the late-1950s (462 g C/m 2 -yr) and the late-1990s (354 g C/m 2 -yr), illustrating age-related decline in forest NPP, effects of multiple stresses and unusual tree mortality, or both. Application of the simulation model PnET-II predicted 14% higher ANPP than was observed for 1996-1997, probably reflecting some unknown stresses. Fine litterfall flux (171 g C/m 2 -yr) has not changed much since the late-1960s. Because of high annual variation, C flux in woody litterfall (including tree mortality) was not tightly constrained but averaged about 90 g C/m 2 -yr. Carbon flux to soil organic matter in root turnover (128 g C/m 2 -yr) was only about half as large as aboveground detritus. Balancing the soil C budget requires that large amounts of C (80 g C/m 2 -yr) were transported from roots to rhizosphere carbon flux. Total soil respiration (TSR) ranged from 540 to 800 g C/m 2 -yr across eight stands and decreased with increasing elevation within the northern hardwood forest near W6. The watershedwide TSR was estimated as 660 g C/m 2 -yr. Empirical measurements indicated that 58% of TSR occurred in the surface organic horizons and that root respiration comprised about 40% of TSR, most of the rest being microbial. Carbon flux directly associated with other heterotrophs in the HBEF was minor; for example, we estimated respiration of soil microarthropods, rodents, birds and moose at about 3, 5, 1 and 0.8 g C/m 2 -yr, respectively, or in total less than 2% of NPP. Hence, the effects of other heterotrophs on C flux were primarily indirect, with the exception of occasional 2 -yr) were small, larger quantities of C were transported within the ecosystem and a more substantial fraction of dissolved C was transported from the soil as inorganic C and evaded from the stream as CO 2 (4.0 g C/m 2 -yr). Carbon pools and fluxes change rapidly in response to catastrophic disturbances such as forest harvest or major windthrow events. These changes are dominated by living vegetation and dead wood pools, including roots. If biomass removal does not accompany large-scale disturbance, the ecosystem is a large net source of C to the atmosphere (500-1200 g C/m 2 -yr) for about a decade following disturbance and becomes a net si...

The seasonal dynamics of plant N assimilation and microbial N immobilization were studied in an alpine ecosystem to evaluate temporal patterns of plant and microbial N partitioning and the potential for plant vs. microbial competition for N. Plant N uptake was higher in the first half of the growing season than later in the season, as indicated by changes in biomass N and by 15 N uptake. Microbial N pools were low during the first half of the growing season (9.5 g N/m 2 on 1 June) and increased late in the season, from 11.4 g N/m 2 on 1 August 1991 to 38.6 g N/m 2 on 14 October 1991.Two different measures of N availability were highest in the midseason. Ion exchange resin bag N uptake was greatest in July (86.0 g N · g Ϫ1 resin · mo Ϫ1 ). Maximum N availability as indicated by net N mineralization rates occurred in August (0.54 g N · m Ϫ2 · mo Ϫ1 ). Plants took up 96.1%, and soil microorganisms took up 3.9% of the 15 N recovered from 12-d field incubations of 15 NH 4 ϩ in June; the corresponding percentages were 92.6% and 7.4% in August 1991. Thus, plants acquired N early in the season when they were actively growing, and the highest net microbial N immobilization occurred later in the season, after plant senescence. The potential for microbial competition for N may have been limited by:(1) constraints on microbial growth from the seasonal alpine freeze-thaw cycles, and (2) influences of roots on N cycling by soil microorganisms. The alternation between plant N uptake early in the season and microbial N uptake late in the season may enhance N retention in this N-limited ecosystem.

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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.Topography controls snowpack accumulation and hence growing-season length, soil water availability, and the distribution of plant communities in the Colorado Front Range alpine. Nutrient cycles in such an environment are likely to be regulated by interactions between topographically determined climate and plant species composition. We investigated variation in plant and soil components of internal N cycling across topographic gradients of dry, moist, and wet alpine tundra meadows at Niwot Ridge, Colorado. We expected that plant production and N cycling would increase from. dry to wet alpine tundra meadows, but we hypothesized that variation in N turnover would span a proportionately greater range than productivity, because of feedbacks between plants and soil microbial processes that determine N availability. Plant production of foliage and roots increased over topographic sequences from 280 g.m-2.yr--l in dry meadows to 600 gm-2 yr-I in wet meadows and was significantly correlated to soil moisture. Contrary to our expectation, plant N uptake for production increased to a lesser degree, from 3.9 g N-m-2 yr-' in dry meadows to 6.8 g N.m-2 yr-' in wet meadows. In all communities, the belowground component accounted for the majority of biomass, among communities. These results indicate that the topographic soil moisture gradient is in fundamental control of the patterns of N turnover among communities and that differences in plant species do not appear to be as important.

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Physiological and growth measurements were made on forbs and graminoids following additions of water and N+water in a graminoid-dominated dry meadow and a forb-dominated moist meadow, to determine if the community-level response was related to differential responses between the growth forms. Graminoids had higher photosynthetic rates and lower transpiration rates and foliar N concentrations than forbs, and consequently maintained higher photosynthetic N- and water-use efficiencies. Photosynthetic rates, stomatal conductance, and transpiration rates increased significantly only in response to N fertilization and only in moist meadow species. The increase in photosynthetic rates was unrelated to variation in foliar N concentration, but instead correlated with variation in stomatal conductance. Growth based N-use efficiency was higher in moist meadow graminoids than in moist meadow forbs, but did not differ between the growth forms in the dry meadow. The moist meadow community had higher biomass and N standing crops, but the relative increase in these factors in response to N fertilization was greater in the dry meadow. Graminoids had a greater relative increase in biomass and N accumulation than forbs following N fertilization, but moist meadow graminoids exhibited a greater response than dry meadow graminoids. The difference in the growth response between the dry meadow and moist meadow graminoids to N fertilization was correlated with more conservative leaf gas exchange responses in dry meadow species, presumably related to a higher frequency of soil water deficits in this community. Community-level response to the resource additions was therefore mediated by the plant growth form response, corresponding with differences between the growth forms in physiological factors related to resource acquisition and use.

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