Microbial decomposition processes are typically described using first-order kinetics, and the effect of elevated temperature is modeled as an increase in the rate constant. However, there is experimental data to suggest that temperature increases the pool size of substrate C available for microbial respiration with little effect on first-order rate constants. We reasoned that changes in soil temperature alter the composition of microbial communities, wherein dominant populations at higher temperatures have the ability to metabolize substrates that are not used by members of the microbial community at lower temperatures. To gain insight into changes in microbial community composition and function following soil wanning, we used molecular techniques of phospholipid fatty acid (PLFA) and lipopolysaccharide fatty acid (LPS-OHFA) analysis and compared the kinetics of microbial respiration for soils incubated from 5 to 25°C. Substrate pools for microbial respiration and the abundance of PLFA and LPS-OHFA biomarkers for Gram-positive and Gram-negative bacteria differed significantly among temperature treatments, providing evidence for a shift in the function and composition of microbial communities related to soil warming. We suggest that shifts in microbial community composition following either large seasonal variation in soil temperature or smaller annual increases associated with global climate change have the potential to alter patterns of soil organic matter decomposition by a mechanism that is not considered by current simulation models.

Chronic N additions to forest ecosystems can enhance soil N availability, potentially leading to reduced C allocation to root systems. This in turn could decrease soil CO 2 efflux. We measured soil respiration during the first, fifth, sixth and eighth years of simulated atmospheric NO 3 À deposition (3 g N m À2 yr À1 ) to four sugar maple-dominated northern hardwood forests in Michigan to assess these possibilities. During the first year, soil respiration rates were slightly, but not significantly, higher in the NO 3 À -amended plots. In all subsequent measurement years, soil respiration rates from NO 3 À -amended soils were significantly depressed. Soil temperature and soil matric potential were measured concurrently with soil respiration and used to develop regression relationships for predicting soil respiration rates. Estimates of growing season and annual soil CO 2 efflux made using these relationships indicate that these C fluxes were depressed by 15% in the eighth year of chronic NO 3 À additions. The decrease in soil respiration was not due to reduced C allocation to roots, as root respiration rates, root biomass, and root turnover were not significantly affected by N additions. Aboveground litter also was unchanged by the 8 years of treatment. Of the remaining potential causes for the decline in soil CO 2 efflux, reduced microbial respiration appears to be the most likely possibility. Documented reductions in microbial biomass and the activities of extracellular enzymes used for litter degradation on the NO 3 À -amended plots are consistent with this explanation.

We examined fine-root (< 2.0 mm diameter) respiration throughout one growing season in four northern hardwood stands dominated by sugar maple (Acer saccharum Marsh.), located along soil temperature and nitrogen (N) availability gradients. In each stand, we fertilized three 50 x 50 m plots with 30 kg NO(3) (-)-N ha(-1) year(-1) and an additional three plots received no N and served as controls. We predicted that root respiration rates would increase with increasing soil temperature and N availability. We reasoned that respiration would be greater for trees using NO(3) (-) as an N source than for trees using NH(4) (+) as an N source because of the greater carbon (C) costs associated with NO(3) (-) versus NH(4) (+) uptake and assimilation. Within stands, seasonal patterns of fine-root respiration rates followed temporal changes in soil temperature, ranging from a low of 2.1 micro mol O(2) kg(-1) s(-1) at 6 degrees C to a high of 7.0 micro mol O(2) kg(-1) s(-1) at 18 degrees C. Differences in respiration rates among stands at a given soil temperature were related to variability in total net N mineralized (48-90 micro g N g(-1)) throughout the growing season and associated changes in mean root tissue N concentration (1.18-1.36 mol N kg(-1)). The hypothesized increases in respiration in response to NO(3) (-) fertilization were not observed. The best-fit model describing patterns within and among stands had root respiration rates increasing exponentially with soil temperature and increasing linearly with increasing tissue N concentration: R = 1.347Ne(0.072T) (r(2) = 0.63, P < 0.01), where R is root respiration rate ( micro mol O(2) kg(-1) s(-1)), N is root tissue N concentration (mol N kg(-1)), and T is soil temperature ( degrees C). We conclude that, in northern hardwood forests dominated by sugar maple, root respiration is responsive to changes in both soil temperature and N availability, and that both factors should be considered in models of forest C dynamics.

To determine the importance of microorganisms in regulating the retention of anthropogenic NO3−, we followed the belowground fate and flow of 15NO3− in a mature northern hardwood forest, dominated by Acer saccharum Marsh. Total recovery of added 15N (29.5 mg 15N/m2 as NaNO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0–10 cm depth) was 93% two hours following application of the 15NO3− but rapidly dropped to ∼29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for 15NO3− (21% in forest floor; 16% in soil microbial biomass). After one month, total 15N recovery varied little (24–18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four‐month experiment, the main fates of the 15N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%). Temporal changes in the 15N enrichment (atom % excess 15N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3− cycling in this forest. The 15N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3−. In contrast, the 15N enrichment of soil organic matter did not peak until day 1, presumably because much of the added 15N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the 15N enrichment of root biomass (<0.5‐mm diameter and 0.5–2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the 15N enrichment of soil organic matter (day 1) and, more importantly, NH4+ (day 2). From these data we conclude that microorganisms are immediate, short‐term sinks for anthropogenic NO3−. Although the long‐term fate of NO3− additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short‐term factor influencing patterns of NO3− retention in this ecosystem.

Soil moisture deficits can reduce root respiration, but the effects have yet to be quantified at the stand level or included in models of forest carbon budgets. We studied fine‐root (≤1.0 mm diameter) respiration in four sugar maple forests for three growing seasons in order to assess the combined effects of temperature, N concentration, and soil moisture on respiration rates. Fine‐root respiration at the four sites was exponentially related to soil temperature and linearly related to root N concentration and soil moisture availability. Most of the variability in respiration rates was explained by temperature. Differences in soil moisture availability explained temporal variation within sites in respiration rate at a given temperature, whereas differences among sites in respiration rates resulted from site‐specific differences in fine‐root N concentration. Periodic moisture deficits during 1995 and 1996 were sufficient to cause declines of up to 17% in total growing‐season root respiration at affected sites. Estimated reductions in respiration of up to 0.8 Mg C/ha during dry years were equivalent to a significant portion of annual aboveground woody biomass C increment, arguing for the inclusion of soil moisture availability as a predictor of root respiration when modeling C allocation in forest ecosystems.

Accurate estimates of root respiration are crucial to predicting belowground C cycling in forest ecosystems. Inhibition of respiration has been reported as a short-term response of plant tissue to elevated measurement [CO(2)]. We sought to determine if measurement [CO(2)] affected root respiration in samples from mature sugar maple (Acer saccharum Marsh.) forests and to assess possible errors associated with root respiration measurements made at [CO(2)]s lower than that typical of the soil atmosphere. Root respiration was measured as both CO(2) production and O(2) consumption on excised fine roots ( 20,000 micro l l(-1). Root respiration was significantly affected by the [CO(2)] at which measurements were made for both CO(2) production and O(2) consumption. Root respiration was most sensitive to [CO(2)] near and below normal soil concentrations (< 1500 micro l l(-1)). Respiration rates changed little at [CO(2)]s above 3000 micro l l(-1) and were essentially constant above 6000 micro l l(-1) CO(2). These findings call into question estimates of root respiration made at or near atmospheric [CO(2)], suggesting that they overestimate actual rates in the soil. Our results indicate that sugar maple root respiration at atmospheric [CO(2)] (350 micro l l(-1)) is about 139% of that at soil [CO(2)]. Although the causal mechanism remains unknown, the increase in root respiration at low measurement [CO(2)] is significant and should be accounted for when estimating or modeling root respiration. Until the direct effect of [CO(2)] on root respiration is fully understood, we recommend making measurements at a [CO(2)] representative of, or higher than, soil [CO(2)]. In all cases, the [CO(2)] at which measurements are made and the [CO(2)] typical of the soil atmosphere should be reported.

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. To determine the importance of microorganisms in regulating the retention of anthropogenic NO3-, we followed the belowground fate and flow of 5SNO3-in a mature northern hardwood forest, dominated by Acer saccharum Marsh. Total recovery of added t5N (29.5 mg t5N/m2 as NaNO3) in inorganic N, microbial immobilization in forest floor and soil microbial biomass, soil organic matter, and root biomass pools (0-10 cm depth) was 93% two hours following application of the 15NO3-but rapidly dropped to -29% within one month, presumably due to movement of the isotope into other plant tissues or deeper into soil. Microbial immobilization was initially (i.e., at 2 h) the largest sink for 15NO3-(21% in forest floor; 16% in soil microbial biomass). After one month, total t5N recovery varied little (24-18%) throughout the remainder of the growing season, suggesting that the major N transfers among pools occurred relatively rapidly. At the end of the four-month experiment, the main fates of the '5N label were in soil organic matter (7%), root biomass (6%), and N immobilized in forest floor and soil microbial biomass (6%).Temporal changes in the 15N enrichment (atom % excess 15N) of plant and soil pools during the first month of the experiment indicated the dynamic nature of NO3cycling in this forest. The 15N enrichment of soil microbial biomass and the forest floor significantly increased two hours after isotope additions, suggesting rapid microbial immobilization of NO3-. In contrast, the 15N enrichment of soil organic matter did not peak until day 1, presumably because much of the added 15N cycled through microorganisms before becoming stabilized in soil organic matter, or it directly entered soil organic matter via physical processes. Furthermore, the 15N enrichment of root biomass (<0.5-mm diameter and 0.5-2.0 mm diameter) was greatest between day 7 and day 28, following significant increases in the 15N enrichment of soil organic matter (day 1) and, more importantly, NH4+ (day 2). From these data we conclude that microorganisms are immediate, short-term sinks for anthropogenic NO3-. Although the long-term fate of NO3-additions to this forest is likely in soil organic matter and plants, the cycling of N through microorganisms appears to be the major short-term factor influencing patterns of NO3-retention in this ecosystem.Key words: belowground fate and flow of nitrate; microbial immobilization; nitrate, anthropogenic; nitrate retention and microbial pathways; nitrogen deposition; nitrogen saturation; plant and microbial competition for nitrogen; sugar maple (Acer saccharum).

Human activity has substantially increased atmospheric NO À 3 deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO À 3 deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5-7 years). Although microbial immobilization is an initial sink (i.e., within 1-2 days) for anthropogenic NO À 3 in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO À 3 additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of 15 NO À 3 in plots receiving chronic NO À 3 addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m À2 year À1) and three plots which receive ambient plus experimental N deposition (3.0 g NO 3 À-N m À2 year À1). Chronic NO À 3 deposition significantly increased the N concentration and content (g N/m 2) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO À 3 deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of 15 N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO À 3 over a 1 year duration. Our results indicate that anthropogenic NO À 3 initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short-and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.