Summary• The flux of carbon from tree photosynthesis through roots to ectomycorrhizal (ECM) fungi and other soil organisms is assumed to vary with season and with edaphic factors such as nitrogen availability, but these effects have not been quantified directly in the field.• To address this deficiency, we conducted high temporal-resolution tracing of 13 C from canopy photosynthesis to different groups of soil organisms in a young boreal Pinus sylvestris forest.• There was a 500% higher below-ground allocation of plant C in the late (August) season compared with the early season (June). Labelled C was primarily found in fungal fatty acid biomarkers (and rarely in bacterial biomarkers), and in Collembola, but not in Acari and Enchytraeidae. The production of sporocarps of ECM fungi was totally dependent on allocation of recent photosynthate in the late season. There was no short-term (2 wk) effect of additions of N to the soil, but after 1 yr, there was a 60% reduction of below-ground C allocation to soil biota.• Thus, organisms in forest soils, and their roles in ecosystem functions, appear highly sensitive to plant physiological responses to two major aspects of global change: changes in seasonal weather patterns and N eutrophication.

Summary• Half of the biological activity in forest soils is supported by recent tree photosynthate, but no study has traced in detail this flux of carbon from the canopy to soil microorganisms in the field.• Using 13 CO 2 , we pulse-labelled over 1.5 h a 50-m 2 patch of 4-m-tall boreal Pinus sylvestris forest in a 200-m 3 chamber.• Tracer levels peaked after 24 h in soluble carbohydrates in the phloem at a height of 0.3 m, after 2-4 d in soil respiratory efflux, after 4-7 d in ectomycorrhizal roots, and after 2-4 d in soil microbial cytoplasm. Carbon in the active pool in needles, in soluble carbohydrates in phloem and in soil respiratory efflux had half-lives of 22, 17 and 35 h, respectively. Carbon in soil microbial cytoplasm had a half-life of 280 h, while the carbon in ectomycorrhizal root tips turned over much more slowly. Simultaneous labelling of the soil with showed that the ectomycorrhizal roots, which were the strongest sinks for photosynthate, were also the most active sinks for soil nitrogen.• These observations highlight the close temporal coupling between tree canopy photosynthesis and a significant fraction of soil activity in forests.

Summary• Gas exchange, fluorescence, western blot and chemical composition analyses were combined to assess if three functional groups (forbs, grasses and evergreen trees/ shrubs) differed in acclimation of leaf respiration ( R ) and photosynthesis ( A ) to a range of growth temperatures (7, 14, 21 and 28 ° C).• When measured at a common temperature, acclimation was greater for R than for A and differed between leaves experiencing a 10-d change in growth temperature (PE) and leaves newly developed at each temperature (ND). As a result, the R : A ratio was temperature dependent, increasing in cold-acclimated plants. The balance was largely restored in ND leaves. Acclimation responses were similar among functional groups.• Across the functional groups, cold acclimation was associated with increases in nonstructural carbohydrates and nitrogen. Cold acclimation of R was associated with an increase in abundance of alternative and/or cytochrome oxidases in a speciesdependent manner. Cold acclimation of A was consistent with an initial decrease and subsequent recovery of thylakoid membrane proteins and increased abundance of proteins involved in the Calvin cycle.• Overall, the results point to striking similarities in the extent and the biochemical underpinning of acclimation of R and A among contrasting functional groups differing in overall rates of metabolism, chemical composition and leaf structure.

Summary Symbioses between plant roots and mycorrhizal fungi are thought to enhance plant uptake of nutrients through a favourable exchange for photosynthates. Ectomycorrhizal fungi are considered to play this vital role for trees in nitrogen (N)‐limited boreal forests. We followed symbiotic carbon (C)–N exchange in a large‐scale boreal pine forest experiment by tracing 13CO2 absorbed through tree photosynthesis and 15N injected into a soil layer in which ectomycorrhizal fungi dominate the microbial community. We detected little 15N in tree canopies, but high levels in soil microbes and in mycorrhizal root tips, illustrating effective soil N immobilization, especially in late summer, when tree belowground C allocation was high. Additions of N fertilizer to the soil before labelling shifted the incorporation of 15N from soil microbes and root tips to tree foliage. These results were tested in a model for C–N exchange between trees and mycorrhizal fungi, suggesting that ectomycorrhizal fungi transfer small fractions of absorbed N to trees under N‐limited conditions, but larger fractions if more N is available. We suggest that greater allocation of C from trees to ectomycorrhizal fungi increases N retention in soil mycelium, driving boreal forests towards more severe N limitation at low N supply.

The response of plant respiration (R) to temperature is an important component of the biosphere's response to climate change. At present, most global models assume that R increases exponentially with temperature and does not thermally acclimate. Although we now know that acclimation does occur, quantitative incorporation of acclimation into models has been lacking. Using a dataset for 19 species grown at four temperatures (7, 14, 21, and 28 1C), we have assessed whether sustained differences in growth temperature systematically alter the slope and/or intercepts of the generalized log-log plots of leaf R vs. leaf mass per unit leaf area (LMA) and vs. leaf nitrogen (N) concentration. The extent to which variations in growth temperature account for the scatter observed in log-log R-LMA-N scaling relationships was also assessed. We show that thermal history accounts for up to 20% of the scatter in scaling relationships used to predict R, with the impact of thermal history on R-LMA-N generalized scaling relationships being highly predictable. This finding enabled us to quantitatively incorporate acclimation of R into a coupled global climate-vegetation model. We show that accounting for acclimation of R has negligible impact on predicted annual rates of global R, net primary productivity (NPP) or future atmospheric CO 2 concentrations. However, our analysis suggests that accounting for acclimation is important when considering carbon fluxes among thermally contrasting biomes (e.g. accounting for acclimation decreases predicted rates of R by up to 20% in high-temperature biomes). We conclude that acclimation of R needs to be accounted for when predicting potential responses of terrestrial carbon exchange to climatic change at a regional level.

Phosphorus (P) is a major factor limiting the response of carbon acquisition of plants and ecosystems to increasing atmospheric CO 2 content. An important consideration, however, is the effect of P deficiency at the low atmospheric CO 2 content common in recent geological history, because plants adapted to these conditions may also be limited in their ability to respond to further increases in CO 2 content. To ascertain the effects of low P on various components of photosynthesis, white lupin ( Lupinus albus L.) was grown hydroponically at 200, 400 and 750 m mol mol -1 CO 2 , under sufficient and deficient P supply (250 and 0.69 m M P, respectively). Increasing growth CO 2 content increased photosynthesis only under sufficient growth P. Ribulose 1,5-biphosphate carboxylase/oxygenase (Rubisco) content and activation state were not reduced to the same degree as the net CO 2 assimilation rate ( A ), and the in vivo rate of electron transport was sufficient to support photosynthesis in all cases. The rate of triose phosphate use did not appear limiting either, because all the treatments continued to respond positively to a drop in oxygen levels. We conclude that, at ambient and elevated CO 2 content, photosynthesis in low-P plants appears limited by the rate of ribulose biphosphate (RuBP) regeneration, probably through inhibition of the Calvin cycle. This failure of P-deficient plants to respond to rising CO 2 content above 200 m mol mol -1 indicates that P status already imposes a widespread restriction in plant responses to increases in CO 2 content from the pre-industrial level to current values. Key-words :phosphorus deficiency; photosynthetic limitations; triose phosphate use.Abbreviations : A , net CO 2 assimilation rate; A / C i curve, the response of net CO 2 assimilation rate versus the intercellular content of CO 2 ; A 1200 , net CO 2 assimilation rate at a C i of ≈ 1200 µ mol mol − 1 CO 2 ; Abs , absorbance, percentage of incident light absorbed by a leaf; A ETR , electron-transport limited net CO 2 assimilation rate; C a , ambient content of CO 2 in the air; C i , intercellular content of CO 2 ; DAP, days after planting; E c , the number of electrons required per CO 2 reduced in photosynthesis; E o , the number of electrons required per O 2 reduced in photosynthesis; ETR , electron transport rate; Fv ′ / Fm ′ , ratio of variable to maximal fluorescence under growth light; in vivo kcat , in vivo catalytic turnover rate of Rubisco; K c , Michaelis constant of Rubisco with respect to CO 2 ; kcat , the maximum catalytic turnover rate of Rubisco; K o , Michaelis constant of Rubisco with respect to O 2 ; O i , intercellular O 2 content; PPFD , photosynthetic photon flux density, ( µ mol photons m − 2 s − 1 ); TPU limitation, triose phosphate use limitation of photosynthesis; V c , calculated rate of carboxylation under measurement conditions, using the model of Wullschleger, Hanson & Sage (1992); V cmax , maximum carboxylation velocity of fully activated Rubisco; V omax , maximum oxygenation velocity o...

The goal of criterion development in Project A was to construct multiple measures of the major components of job performance such that the total performance domain for a representative sample of the population of entry‐level enlisted positions in the U.S. Army was covered. These measures were to be used as criteria against which to validate both experimental and existing predictors of job performance. The initial model specified that performance is multidimensional within two major categories of dimensions designated as organization‐wide and job specific. The development strategy involved describing the total domain of job content via extensive task analyses and critical incident analyses, generating the critical performance dimensions that constitute it, constructing measures for each dimension, and evaluating each measure using expert judgment and field test data. The specific measures developed consisted of rating scales, tests of job knowledge, hands‐on job samples, and archival records. The major steps in the job analyses, content sampling, instrument construction, and instrument evaluation are described, and the final array of criterion measures is presented.

Summary1. The Amazon region may experience increasing moisture limitation over this century. Leaf dark respiration (R) is a key component of the Amazon rain forest carbon (C) cycle, but relatively little is known about its sensitivity to drought. 2. Here, we present measurements of R standardized to 25°C and leaf morphology from different canopy heights over 5 years at a rain forest subject to a large-scale through-fall reduction (TFR) experiment, and nearby, unmodified Control forest, at the Caxiuana˜reserve in the eastern Amazon. 3. In all five post-treatment measurement campaigns, mean R at 25°C was elevated in the TFR forest compared to the Control forest experiencing normal rainfall. After 5 years of the TFR treatment, R per unit leaf area and mass had increased by 65% and 42%, respectively, relative to pre-treatment means. In contrast, leaf area index (L) in the TFR forest was consistently lower than the Control, falling by 23% compared to the pre-treatment mean, largely because of a decline in specific leaf area (S). 4. The consistent and significant effects of the TFR treatment on R, L and S suggest that severe drought events in the Amazon, of the kind that may occur more frequently in future, could cause a substantial increase in canopy carbon dioxide emissions from this ecosystem to the atmosphere.