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.

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More than 50% of terrestrially-derived organic carbon (terrOC) flux from the continents to the ocean is remineralised in the coastal zone despite its perceived high refractivity. The efficient degradation of terrOC in the marine environment could be fuelled by labile marine-derived material, a phenomenon known as “priming effect”, but experimental data to confirm this mechanism are lacking. We tested this hypothesis by treating coastal sediments with 13C-lignocellulose, as a proxy for terrOC, with and without addition of unlabelled diatom detritus that served as the priming inducer. The occurrence of priming was assessed by the difference in lignocellulose mineralisation between diatom-amended treatments and controls in aerobic sediment slurries. Priming of lignocellulose degradation was observed only at the initial stages of the experiment (day 7) and coincided with overall high microbial activity as exemplified by total CO2 production. Lignocellulose mineralisation did not differ consistently between diatom treatments and control for the remaining experimental time (days 14–28). Based on this pattern, we hypothesize that the faster initiation of lignocellulose mineralisation in diatom-amended treatments is attributed to the decomposition of accessible polysaccharide components within the lignocellulose complex by activated diatom degraders. The fact that diatom-degraders contributed to lignocellulose degradation was also supported by the different patterns in 13C-enrichment of phospholipid fatty acids between treatments. Although we did not observe differences between treatments in the total quantity of respired lignocellulose at the end of the experiment, differences in timing could be important in natural ecosystems where the amount of time that a certain compound is subject to aerobic degradation before burial to deeper anoxic sediments may be limited.

Patterns of synthesis and breakdown of carbon (C) and nitrogen (N) stores are relatively well known. But the role of mobilized stores as substrates for growth remains less clear. In this article, a novel approach to estimate C and N import into leaf growth zones was coupled with steady-state labeling of photosynthesis ( 13 CO 2 / 12 CO 2 ) and N uptake ( 15 NO 3 2 / 14 NO 3 2 ) and compartmental modeling of tracer fluxes. The contributions of current C assimilation/N uptake and mobilization from stores to the substrate pool supplying leaf growth were then quantified in plants of a C 3 (Lolium perenne) and C 4 grass (Paspalum dilatatum Poir.) manipulated thus to have contrasting C assimilation and N uptake rates. In all cases, leaf growth relied largely on photoassimilates delivered either directly after fixation or short-term storage (turnover rate 5 1.6-3.3 d 21 ). Long-term C stores (turnover rate , 0.09 d 21 ) were generally of limited relevance. Hence, no link was found between the role of stores and C acquisition rate. Short-term (turnover rate 5 0.29-0.90 d 21 ) and long-term (turnover rate , 0.04 d 21 ) stores supplied most N used in leaf growth. Compared to dominant (well-lit) plants, subordinate (shaded) plants relied more on mobilization from long-term N stores to support leaf growth. These differences correlated well with the C-to-N ratio of growth substrates and were associated with responses in N uptake. Based on this, we argue that internal regulation of N uptake acts as a main determinant of the importance of mobilized long-term stores as a source of N for leaf growth.

We assessed the extent to which plants can acquire amino acids when supplied as single N-sources or when plants have access to a mixture of amino-and inorganic N sources. Because the uptake of different N-sources is temperaturedependent, the effects of temperature on amino-N uptake were also tested. Lolium perenne (perennial rye-grass) was grown hydroponically at 11 ∞ ∞ ∞ ∞ C or 21 ∞ ∞ ∞ ∞ C. Uptake of N was determined using 15 N tracers at the growth temperature from solutions containing either nitrate, ammonium or glycine as single N sources and from a mixture containing all three N-forms. Estimates of the relative importance of amino acids such as glycine to the total N budget of plants will have been underestimated in studies where uptake was determined in single source solutions compared with those from solutions containing a mixture of N-forms. The proportion of total N acquired from the mixed N source as ammonium increased as temperature was reduced. Regarding the uptake and initial metabolism of glycine, uptake was probably the rate limiting step at 11 ∞ ∞ ∞ ∞ C whilst it was the metabolism of glycine to serine at 21 ∞ ∞ ∞ ∞ C. Although 15 N incorporation into the plant amino-N pool was generally in proportion to the abundance of individual amino acids, its incorporation into the glycine pool was sometimes significantly less than predicted.

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Summary1. Marine copepods of the genus Calanus can reproduce prior to the spring bloom in the absence of sufficient food. Their starvation physiology, and hence the factors limiting their pre-bloom population growth (egg production), remain poorly understood. 2. Stoichiometric theory can provide insights into the factors controlling an organism's growth and the fate of elements in an ecosystem. It is underpinned by substrate utilization efficiencies that relate to key physiological processes such as absorption efficiencies (AEs) and biomass turnover. These parameters are seldom investigated, particularly in the case of essential 'micronutrients' such as the polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). 3. Calanus spp. were fed briefly and subsequently starved for 5 days to determine basal turnover rates of biomass carbon, nitrogen and essential PUFAs. The effect of short-term fasting on nitrogen isotope signatures was also examined. The elemental, fatty acid and isotopic composition of their faecal pellets were compared to that of their food, providing insights into AEs and digestive isotopic discrimination. 4. Gut AEs typically followed the sequence: PUFA > nitrogen > carbon, although low AE for DHA was a notable exception. Starvation-induced losses of carbon, nitrogen, EPA and DHA demonstrate that homeostatic organisms must ingest all of these substrates in substantial quantity to achieve positive net growth. 5. Egested material was significantly depleted in 13 C and 15 N relative to the ingested food. We attribute this to isotopic discrimination at the macromolecular level, indicating that food quality contributes to the isotopic signature of a consumer organism. Values of d 15 N in the copepods' tissues did not increase during starvation, despite significant losses of bulk nitrogen. This supports the suggestion that dissimilatory protein pathways in marine crustaceans are non-discriminating. 6. The significant basal turnover rates and variable AEs for essential PUFAs and nitrogen presented herein demonstrate that organisms cannot be assumed to utilize all nutritious substrates with the same, high efficiency, even when scarce in the diet. Our data highlight the need for a more detailed understanding of organismal physiology before isotopic and stoichiometric models can be meaningfully constructed and parameterized.