Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

Interactive effects of drought and N fertilization on the spatial distribution of methane assimilation in grassland soils

Soil methanotrophic bacteria constitute the only globally relevant biological sink for atmospheric methane (CH₄). Nitrogen (N) fertilizers as well as soil moisture regime affect the activity of these organisms, but the mechanisms involved are not well understood to date. In particular, virtually nothing is known about the spatial distribution of soil methanotrophs within soil structure and how this regulates CH₄ fluxes at the ecosystem scale. We studied the spatial distribution of CH₄ assimilation and its response to a factorial drought × N fertilizer treatment in a 3‐year experiment replicated in two grasslands differing in management intensity. Intact soil cores were labelled with ¹⁴CH₄ and methanotrophic activity mapped at a resolution of ～100 μm using an autoradiographic technique. Under drought, the main zone of CH₄ assimilation shifted down the soil profile. Ammonium nitrate (NH₄NO₃) and cattle urine reduced CH₄ assimilation in the top soil, but only when applied under drought, presumably because NH₄⁺ from fertilizers was not removed by plant uptake and nitrification under these conditions. Ecosystem‐level CH₄ fluxes measured in the field did show no or only very small inhibitory effects, suggesting that deeper soil layers fully compensated for the reduction in top soil CH₄ assimilation. Our results indicate that the ecosystem‐level CH₄ sink cannot be inferred from measurements of soil samples that do not reflect the spatial organization of soils (e.g. stratification of organisms, processes, and mechanisms). The autoradiographic technique we have developed is suited to study methanotrophic activity in a relevant spatial context and does not rely on the genetic identity of the soil bacterial communities involved, thus ideally complementing DNA‐based approaches.     

Inferring the history of speciation in house mice from autosomal,X‐linked,Y‐linked and mitochondrial genes

Patterns of genetic differentiation among taxa at early stages of divergence provide an opportunity to make inferences about the history of speciation. Here, we conduct a survey of DNA‐sequence polymorphism and divergence at loci on the autosomes, X chromosome, Y chromosome and mitochondrial DNA in samples of Mus domesticus, M. musculus and M. castaneus. We analyzed our data under a divergence with gene flow model and estimate that the effective population size of M. castaneus is 200 000–400 000, of M. domesticus is 100 000–200 000 and of M. musculus is 60 000–120 000. These data also suggest that these species started to diverge approximately 500 000 years ago. Consistent with this recent divergence, we observed considerable variation in the genealogical patterns among loci. For some loci, all alleles within each species formed a monophyletic group, while at other loci, species were intermingled on the phylogeny of alleles. This intermingling probably reflects both incomplete lineage sorting and gene flow after divergence. Likelihood ratio tests rejected a strict allopatric model with no gene flow in comparisons between each pair of species. Gene flow was asymmetric: no gene flow was detected into M. domesticus, while significant gene flow was detected into both M. castaneus and M. musculus. Finally, most of the gene flow occurred at autosomal loci, resulting in a significantly higher ratio of fixed differences to polymorphisms at the X and Y chromosomes relative to autosomes in some comparisons, or just the X chromosome in others, emphasizing the important role of the sex chromosomes in general and the X chromosome in particular in speciation.     

Production of juvenile salmonids in small Norwegian streams is affected by agricultural land use

1. We estimated the biomass and production of juvenile anadromous brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) (parr) in 12 streams in the Skagerrak area of Norway to identify controlling environmental factors, such as land‐use and water chemistry. 2. Production estimates correlated positively with fish density in early summer, but not with the size of the catchment. The summer biomass of age‐0 brown trout and Atlantic salmon was smaller than that of age‐1 and constituted 27.4 and 25.7%, respectively, of the total biomass of the two groups. 3. Mean production of brown trout from July to September varied between streams, but in most cases it was below 2 g 100 m^?2 day^?1. Yearly cohort production from age‐0 in July to age‐1 in July was 10 g m^?2 or less, with mean annual production of 1.32 g 100 m^?2 day^?1, equivalent to 4.8 g m^?2 year^?1. The corresponding annual cohort production of Atlantic salmon was 0.38 g 100 m^?2 day^?1 or 1.4 g m^?2 year^?1. Annual production to biomass ratio (P/B) for brown trout of the same cohort in the various streams was between 1.47 and 4.37; the overall mean (±SD) for all streams was 2.25 ± 0.94. Mean turnover rate of Atlantic salmon was 2.73 ± 0.24. 4. Production of 0+ brown trout during the summer correlated significantly with the percentage of agricultural land and forest/bogs in the catchment, with maxima at 20 and 75%, respectively. Age‐0 brown trout production also correlated with concentration of nitrogen and calcium in the water, with maxima at 2.4 and 14 mg L^?1, respectively. 5. The results support the hypothesis that brown trout parr production reflects the quality of their habitat, as indicated by the dome‐shaped relationship between percentage of agricultural land and the concentration of nitrogen and calcium in the water.     

Carbon dioxide concentrations within forest canopies—variation with time, stand structure, and vegetation type

Vertical CO₂ profiles (between 0.02 and 14.0 m) were studied in forest canopies of Pinus contorta, Populus tremuloides, and in a riparian forest with Acer negundo and Acer grandidentatum during two consecutive growing seasons. Profiles, measured continuously during 1- to 13-day periods in four to five stands differing in overstorey canopy area index (CAI < 4.5; including leaves, branches and stems), were well stratified, with highest [CO₂] just above the forest floor. Canopy [CO₂] profiles were influenced by stand structure (CAI, presence of understorey vegetation), and were highly dependent on vegetation type (deciduous and evergreen). A doubling of CAI in Acer spp. and P. tremuloides stands did not show an effect on upper canopy [CO₂], when turbulent mixing was high. However, increasing understorey biomass in Acer spp. stands had a profound effect on lower canopy [CO₂]. In open stands with a vigorous understorey layer, higher soil respiration rates were offset by increased understorey gas exchange, resulting in [CO₂] below those of the convective boundary layer (CBL). Midday depletions up to 20 ppmv below CBL values could be frequently observed in deciduous canopies. In evergreen canopies, [CO₂] stayed generally above the CBL background values, [CO₂] profiles were more uniform, and gradients were smaller than in deciduous stands with similar CAI. Seasonal changes of canopy [CO₂] reflected changes in soil respiration rates as well as plant phenology and gas exchange of both dominant tree and understorey vegetation. Seasonal patterns were less pronounced in evergreen than in deciduous forests.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Migration of anadromous brown trout Salmo trutta in a Norwegian river

1. Upstream and downstream migrating anadromous brown trout Salmo trutta were monitored daily in fish traps in the River Imsa in south-western Norway for 24 years, from 1976 to 1999. One-third of the fish descended to sea during spring (February–June) and two-thirds during autumn (September–January).
2. In spring, high water temperature appeared to influence the downstream descent. Large brown trout (> 30 cm, chiefly two or more sea sojourns) descended earlier and appeared less dependent on high water temperature than smaller and younger fish. The spring water flow was generally low and of little importance for the descent.
3. In autumn, the daily number of descending brown trout correlated positively with flow and negatively with water temperature.
4. Brown trout ascended from the sea between April and December, but more than 70% ascended between August and October. The number of ascending trout increased significantly with both decreasing temperature and flow during the autumn. This response to flow appeared to be the result of the autumn discharge which is generally high and most fish ascended at an intermediate flow of 7.5–10 m³ s⁻¹ (which is low for the season).
5. In a river like the Imsa with low spring and high autumn flows, water temperature appears to be the main environmental factor influencing the timing and rate of spring descent, while both water temperature and flow seemed to influence the timing and rate of the autumn descent and ascent. These relationships make sea trout migrations susceptible to variation in climate and human impacts of the flow regime in rivers.     

Dissolved carbon leaching from soil is a crucial component of the net ecosystem carbon balance

Estimates of carbon leaching losses from different land use systems are few and their contribution to the net ecosystem carbon balance is uncertain. We investigated leaching of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and dissolved methane (CH₄), at forests, grasslands, and croplands across Europe. Biogenic contributions to DIC were estimated by means of its δ¹³C signature. Leaching of biogenic DIC was 8.3±4.9 g m^?2 yr^?1 for forests, 24.1±7.2 g m^?2 yr^?1 for grasslands, and 14.6±4.8 g m^?2 yr^?1 for croplands. DOC leaching equalled 3.5±1.3 g m^?2 yr^?1 for forests, 5.3±2.0 g m^?2 yr^?1 for grasslands, and 4.1±1.3 g m^?2 yr^?1 for croplands. The average flux of total biogenic carbon across land use systems was 19.4±4.0 g C m^?2 yr^?1. Production of DOC in topsoils was positively related to their C/N ratio and DOC retention in subsoils was inversely related to the ratio of organic carbon to iron plus aluminium (hydr)oxides. Partial pressures of CO₂ in soil air and soil pH determined DIC concentrations and fluxes, but soil solutions were often supersaturated with DIC relative to soil air CO₂. Leaching losses of biogenic carbon (DOC plus biogenic DIC) from grasslands equalled 5–98% (median: 22%) of net ecosystem exchange (NEE) plus carbon inputs with fertilization minus carbon removal with harvest. Carbon leaching increased the net losses from cropland soils by 24–105% (median: 25%). For the majority of forest sites, leaching hardly affected actual net ecosystem carbon balances because of the small solubility of CO₂ in acidic forest soil solutions and large NEE. Leaching of CH₄ proved to be insignificant compared with other fluxes of carbon. Overall, our results show that leaching losses are particularly important for the carbon balance of agricultural systems.     

Responses of microbial phototrophs to late‐Holocene environmental forcing of lakes in south‐west Greenland

1. The biological structure of arctic lakes is changing rapidly, apparently in response to global change processes such as increasing air temperatures, although altered nutrient stoichiometry may also be an important driver. Equally important, however, are local factors (e.g. landscape setting, hydrological linkages and trophic interactions) that may mediate responses of individual lakes at the regional scale. Despite general acknowledgement of the importance of local factors, there has been little focus on among‐lake variability in the response to environmental change. 2. Sedimentary pigments, organic carbon and nitrogen, and biogenic silica (BSi) in ²¹⁰Pb and ¹⁴C‐dated sediment cores from three contrasting lakes in the Kangerlussuaq area (c. 67°N, 51°W) of south‐west Greenland were used to reconstruct algal and phototrophic bacterial ecological change during the late‐Holocene. Water chemistry for the individual lakes varies in terms of conductivity (range: 30–3000 μS cm^?1) and stratification regimes (cold monomictic, dimictic and meromictic), linked with their position along the regional climate gradient from the coast and to the present ice sheet margin. 3. Despite essentially similar regional climate forcing over the last c. 1000 years, marked differences among lake types were observed in the phototrophic communities and their temporal variability. Considerable short‐term variability occurred in an oligosaline, meromictic lake (SS1371), dominated by purple sulphur bacterial pigments, most likely due to a tight coupling between the position of the chemocline and the phototrophic community. Communities in a lake (SS86) located on a nunatak, just beyond the edge of the present ice sheet shifted in a nonlinear pattern, approximately 1000 cal. years BP, possibly due to lake‐level lowering and loss of outflow during the Medieval Climate Anomaly. This regime shift was marked by a substantial expansion of green sulphur bacteria. 4. A dilute, freshwater coastal lake (SS49) dominated by benthic algae was relatively stable until ca. 1900 AD when rates of community change began to increase. These changes in benthic algal pigments are correlated with substantial declines (1.3–0.44‰) in δ¹⁵N that are indicative of increased deposition of atmospheric inputs of industrially derived NOx into the atmosphere. 5. Climate control on lake ecosystem functioning has been assumed to be particularly important in the Arctic. This study, however, illustrates a complex spatial response to climate forcing at the regional scale and emphasises differences in the relative importance of changes in the mass (m, both precipitation and nutrients) and energy flux (E) to lakes for the phototrophic community structure of low‐arctic Greenland lakes.