Stable isotope analyses on archived fish scales reveal the long‐term effect of nitrogen loads on carbon cycling in rivers

Stable isotope analysis of organic matter in sediment records has long been used to track historical changes in productivity and carbon cycling in marine and lacustrine ecosystems. While flow dynamics preclude stratigraphic measurements of riverine sediments, such retrospective analysis is important for understanding biogeochemical cycling in running waters. Unique collections of riverine fish scales were used to analyse δ¹⁵N and δ¹³C variations in the food web of two European rivers that experience different degrees of anthropogenic pressure. Over the past four decades, dissolved inorganic N loading remained low and constant in the Teno River (70°N, Finland); in contrast, N loading increased fourfold in the Scorff River (47°N, France) over the same period. Archived scales of Atlantic salmon parr, a riverine life‐stage that feeds on aquatic invertebrates, revealed high δ¹⁵N values in the Scorff River reflecting anthropogenic N inputs to that riverine environment. A strong correlation between dissolved inorganic N loads and δ¹³C values in fish scales was observed in the Scorff River, whereas no trend was found in the Teno River. This result suggests that anthropogenic N‐nutrients enhanced atmospheric C uptake by primary producers and its transfer to fish. Our results illustrate for the first time that, as for lakes and marine ecosystems, historical changes in anthropogenic N loading can affect C cycling in riverine food webs, and confirm the long‐term interactions between N and C biogeochemical cycles in running waters.     

Ectomycorrhizal impacts on plant nitrogen nutrition: emerging isotopic patterns,latitudinal variation and hidden mechanisms

Ectomycorrhizal (EcM)‐mediated nitrogen (N) acquisition is one main strategy used by terrestrial plants to facilitate growth. Measurements of natural abundance nitrogen isotope ratios (denoted as δ¹⁵N relative to a standard) increasingly serve as integrative proxies for mycorrhiza‐mediated N acquisition due to biological fractionation processes that alter ¹⁵N:¹⁴N ratios. Current understanding of these processes is based on studies from high‐latitude ecosystems where plant productivity is largely limited by N availability. Much less is known about the cause and utility of ecosystem δ¹⁵N patterns in the tropics. Using structural equation models, model selection and isotope mass balance we assessed relationships among co‐occurring soil, mycorrhizal plants and fungal N pools measured from 40 high‐ and 9 low‐latitude ecosystems. At low latitudes ¹⁵N‐enrichment caused ecosystem components to significantly deviate from those in higher latitudes. Collectively, δ¹⁵N patterns suggested reduced N‐dependency and unique sources of EcM ¹⁵N‐enrichment under conditions of high N availability typical of the tropics. Understanding the role of mycorrhizae in global N cycles will require reevaluation of high‐latitude perspectives on fractionation sources that structure ecosystem δ¹⁵N patterns, as well as better integration of EcM function with biogeochemical theories pertaining to climate‐nutrient cycling relationships.     

Herbaria century record of increasing eutrophication in Spanish terrestrial ecosystems

Additional biological evidence is presented for the alteration of biogeochemical cycles by human activities. The leaf δ¹⁵N and the concentrations of nutrients in herbarium specimens of 24 species of vascular plants and 3 species of bryophytes collected in northern and eastern regions of Spain have substantially changed throughout the XX century. In the second half of the century, when anthropogenic nitrogen fixation and mobilization started to increase rapidly, leaf δ¹⁵N values started to decrease strongly, indicating that additional anthropogenic nitrogen is being retained in Spanish terrestrial ecosystems. The concentration of nutrients in vascular plants did not present any clear pattern, but there were increasing concentrations of N and other nutrients (P, K, and S) in the last decades in bryophytes, which are usually better biomonitors of airborne chemicals than vascular plants. Important consequences for ecosystem structure and functioning such as enhancement of the carbon sink or changes in community biodiversity and species distribution may be expected from this increase in eutrophication.     

Patterns of soil and plant nitrogen isotope composition and their relationships with climate factors in Xilingol League,Inner Mongolia,China

Background: Plant and soil nitrogen stable isotope (δ¹⁵N) can integrate several fundamental biogeochemical processes in ecosystem nitrogen dynamics, and reflect characteristics of ecosystem nitrogen cycling.

Aims: We investigated how climate change influenced plant-soil nitrogen cycling by relating soil δ¹⁵N, plant δ¹⁵N and Δδ¹⁵N (difference between soil and plant δ¹⁵N) with climatic factors.

Methods: Field investigation was conducted in temperate grasslands in Inner Mongolia during August 2015. Plant δ¹⁵N, soil δ¹⁵N and Δδ¹⁵N were determined, and their relationships with climatic factors were examined by simple regression analyses and general linear models.

Results: Soil δ¹⁵N was significantly higher than plant δ¹⁵N, and there was a positive linear correlation between them. Soil and plant δ¹⁵N were negatively related with mean annual precipitation (MAP) and positively with mean annual temperature (MAT); conversely, Δδ¹⁵N was positively related with MAP and negatively with MAT.

Conclusion: Soil δ¹⁵N was dominantly controlled by MAT, while it was MAP for plant δ¹⁵N. Climate factors influenced plant δ¹⁵N not only through their effects on soil nitrogen dynamics but also strategies of plant nitrogen acquisition. Thus, compared with plant δ¹⁵N, soil δ¹⁵N can more accurately reflect soil nitrogen dynamics, while plant δ¹⁵N may integrate soil nitrogen dynamics and plant nitrogen acquisition.     

Relationship between the natural abundance of soil nitrogen isotopes and condition in North Dakota wetlands

A statewide condition assessment of North Dakota wetlands in the summer of 2011 was conducted as part of the U.S. Environmental Protection Agency's National Wetland Condition Assessment (NWCA). Two other wetland condition assessments, the Index of Plant Community Integrity (IPCI) and North Dakota Rapid Assessment Method (NDRAM), were also completed at each wetland. Previous studies have identified how the distinct signatures of stable isotopes can be used to determine different land uses, anthropogenic impacts, nutrient cycling, and biological processes. To evaluate if these relationships existed in northern prairie wetlands, the data collected from the wetland assessments were compared with the natural abundance of soil nitrogen (δ¹⁵N) isotopes. Wetland soil δ¹⁵N was significantly higher (isotopically heavier) in wetlands surrounded by cropland compared to those surrounded by idle or grazed/hayed grasslands, possibly reflecting anthropogenic impacts and multiple nitrogen sources. Soil δ¹⁵N was significantly correlated with floristic quality, IPCI scores, NDRAM scores, and average buffer width, indicating that soil δ¹⁵N values may be representative of wetland condition. Soil δ¹⁵N exhibited significant differences among wetland types, although limited sample sizes of certain wetland types may have affected this result. Additional studies on the natural abundance of wetland soil isotopes need to be performed in northern prairie wetlands. This study is the first step in exploring the potential applications of wetland soil nitrogen isotopes regarding wetland assessment and surrounding land use and provides important insight for future studies.     

An Assessment of Contemporary and Historic Nitrogen Availability in Contrasting Coastal Douglas-Fir Forests Through δ15N of Tree Rings

Wood nitrogen isotope composition (δ¹⁵N) provides a potential retrospective evaluation of ecosystem N status but refinement of this index is needed. We calibrated current wood δ¹⁵N of Douglas-fir (Pseudotsuga menziesii), an ectomycorrhizal tree species, against a productivity gradient of contrasting coastal forests of southern Vancouver Island (Canada). We then examined historical δ¹⁵N via increment cores, and tested whether wood δ¹⁵N corresponded with climatic fluctuations. Extractable soil N ranged from 11 to 43 kg N ha^?1 along the productivity gradient, and was characterized by a progressive replacement of N forms (amino acids, NH₄ ⁺ and NO₃ ^?). Current wood δ¹⁵N was significantly less depleted (?5.0 to ?2.6 ‰) with increasing productivity, although linear correlations were stronger with Δδ¹⁵N (the difference between wood and soil δ¹⁵N) to standardize the extent of isotopic fractionation by ectomycorrhizal fungi. An overall decline in wood δ¹⁵N of 0.9 ‰ over the years 1900–2009 was detected, but trends diverged widely among plots, including positive, negative and no trend with time. We did not detect significant correlations in detrended wood δ¹⁵N with mean annual temperature or precipitation. The contemporary patterns in stand productivity, soil N supply and wood δ¹⁵N were moderately strong, but interpreting historical patterns in δ¹⁵N was challenging because of potential variations in N uptake related to stand dynamics. The lack of wood δ¹⁵N correlations with climate may be partly due to methodological limitations, but might also reflect the relative stability in N supply due to the overriding constraints of soil organic matter quantity and quality.     

Caribbean octocorals record changing carbon and nitrogen sources from 1862 to 2005

During the last century, the global biogeochemical cycles of carbon (C) and nitrogen (N) have been drastically altered by human activities. A century of land‐clearing and biomass burning, followed by fossil fuel combustion have increased the concentration of atmospheric CO₂ by approximately 20%, and since the mid‐1900s, the use of agricultural fertilizers has been the primary driver of an approximate 90% increase in bioavailable N. Geochemical records obtained through stable isotope analysis of terrestrial and marine biota effectively illustrate rising anthropogenic C inputs. However, there are fewer records of anthropogenic N, despite the enormous magnitude of change and the known negative effects of N on ecosystem health. We used stable isotope values from independent octocorals (gorgonians) sampled across the Western Atlantic over the last 143 years to document human perturbations of the marine C and N pools. Here, we demonstrate that in sea plumes δ¹³C values and in both sea plumes and sea fans δ¹⁵N values declined significantly from 1862 to 2005. Sea plume δ ¹³C values were negatively correlated with increasing atmospheric CO₂ concentrations and corroborate known rates of change resulting from global fossil fuel combustion, known as the Suess effect. We suggest that widespread input of agricultural fertilizers to near‐shore coastal waters is the dominant driver for the decreasing δ ¹⁵N trend, though multiple anthropogenic sources are likely affecting this trend. Given the interest in using δ ¹⁵N as an indicator for N pollution in aquatic systems, we highlight the risk of underestimating contributions of pollutants as a result of source mixing as demonstrated by a simple isotope‐mixing model. We conclude that signals of major human‐induced perturbations of the C and N pools are detectable in specimens collected over wide geographic scales, and that archived materials are invaluable for establishing baselines against which we can assess environmental change.     

Biogeochemical Change During Climate-Driven Afforestation: A Paleoecological Perspective from the Rocky Mountains

Shifts in an ecosystem’s state can alter biogeochemical cycling and the extent of nutrient conservation within a terrestrial landscape on multiple time scales. Transient biogeochemical changes may follow disturbance and succession, although persistent long-term differences may exist under different climates and vegetation types. We evaluate the potential for such biogeochemical changes in the context of long-term ecological history by measuring the nitrogen isotope composition of organic matter in a lake sediment core. We targeted Little Windy Hill Pond (LWH) in the Medicine Bow Mountains, Wyoming because reconstructions of the lake level, fire, and vegetation histories from the lacustrine sediments indicated a century-scale transformation from an arid, shrub-dominated landscape to a sub-alpine, tree-dominated ecosystem with extensive woody cover and large, live biomass pools. We demonstrate that the afforestation at the beginning of the Holocene transformed the Artemisia-dominated ecosystem, which had persisted for millennia during the Pleistocene. The changes affected nitrogen cycling dynamics, especially through intensified nutrient conservation when live biomass pools increased with greater woody cover. The LWH sediments record a baseline δ¹⁵N shift from 2.2–3.0 to 0.3–2.0‰ as less ¹⁵N-enriched organic matter accumulated in the lake. We also observed a transient pattern of maximum nutrient conservation and minimum δ¹⁵N values as terrestrial biomass increased during the aggradation (~175 years) and transition phases of ecological succession. Our nitrogen isotope results support theoretical expectations of long-term biogeochemical dynamics as nutrient conservation increases during afforestation.     

Iron supply prevents Cd uptake in Arabidopsis by inhibiting IRT1 expression and favoring competition between Fe and Cd uptake

Background

Anthropogenic nitrogen (N) addition has dramatically increased and significantly affected global nitrogen cycling. The natural abundance of stable N isotope ratios (δ¹⁵N) has been used as an indicator of the N status of an ecosystem. However, how plant and soil δ¹⁵N signatures would respond to N addition is still unclear.

Methods and aims

Herein, we synthesized the data of 951 observations from 48 individual studies associated with responses of plant and soil δ¹⁵N values to N addition and conducted a meta-analysis to explore a general pattern of N addition effects on δ¹⁵N values of plant and soil.

Results

Our results showed that δ¹⁵N values of plant, soil total N, and soil NO₃ ^? were significantly increased by N addition, while δ¹⁵N value of soil N₂O was significantly decreased and δ¹⁵N value of soil NH₄ ⁺ was not significantly changed. The δ¹⁵N value of soil total N of different ecosystems showed similar responses to N addition, whereas δ¹⁵N values of different plant types showed different responses. Increasing treatment duration significantly increased the effects of inorganic N addition on δ¹⁵N values of shrubs and soil NH₄ ⁺ but did not affect the responses of δ¹⁵N values of soil total N and NO₃ ^?. With increasing inorganic N addition rate, only δ¹⁵N value of plant was significantly increased, but no significant relationship was found between the effect of N addition on other components and N addition rate because of the input of isotopically depleted sources.

Conclusions

Our study revealed a comprehensive picture of the effects of N addition on δ¹⁵N signatures in terrestrial ecosystems and could help us understand how plant and soil δ¹⁵N signatures change with N addition and how these signatures can be used as an indicator of ecosystem N status under increasing N deposition or fertilization.

     

15N自然丰度法在生态系统氮素循环研究中的应用

稳定性同位素技术是现代生态学研究中的一门新兴技术,在生态学研究的诸多领域都展示了广阔的应用前景。其中,稳定性同位素     

Patterns of foliar δ15N and their control in Eastern Asian forests

Foliar δ¹⁵N has been used increasingly in research on ecosystem nitrogen (N) cycling, because it can serve as an integrator of ecosystem N cycling and thus has a potential to reveal temporal and spatial patterns of N cycling as well as how the N cycle is altered by disturbances. However, the current understanding on controls of foliar δ¹⁵N is based principally on studies from America, Europe, Australia and Africa. Here we compiled data from 65 forests at 33 sites across East Asia to explore regional patterns and what controls foliar δ¹⁵N by linking it to climate, species composition, soil depth, slope position, N deposition, and soil N availability. In East Asia, foliar δ¹⁵N ranged from ?7.1 to +2.7‰. Mean foliar δ¹⁵N values for tropical, subtropical and temperate forests were all ?3.1‰, which was unexpected. The patterns of foliar δ¹⁵N with precipitation, temperature and altitude were not clear. The variation in foliar δ¹⁵N among species and between different slope positions appeared to be small within a given forest. The δ¹⁵N for both bulk soil N and extractable inorganic N generally increased with soil depth as expected, strengthening the idea that deep-rooted trees may have access to ¹⁵N-enriched N. Different from the positive correlations reported across America and Europe, in East Asia we found that foliar δ¹⁵N decreased with increasing N deposition and did not relate to soil N availability. These discrepancies deserve more research to elucidate the mechanisms by which foliar δ¹⁵N is affected by ecosystem N availability at a regional scale.     

Strategies of carbon and nitrogen acquisition by saprotrophic and ectomycorrhizal fungi in Finnish boreal Picea abies-dominated forests

We compared the δ¹³C and δ¹⁵N of forest material with an extensive sporocarp collection to elucidate the role of litter, wood and soil as fungal carbon and nitrogen sources in Finnish boreal Picea abies-dominated forests. Ectomycorrhizal Hydnum and Cortinarius had higher δ¹⁵N than other ectomycorrhizal fungi, suggesting use of ¹⁵N-enriched, deeper nitrogen. Russula had lower δ¹⁵N than other ectomycorrhizal fungi and resembled some litter decay genera, suggesting use of litter-derived nitrogen. There was little variation in δ¹⁵N among other genera of ectomycorrhizal fungi, indicating limited functional diversity in nitrogen use. Saprotrophic Leotia, Gymnopus, Hypholoma, Pholiota, Rhodocollybia and Calocera had δ¹⁵N values similar to ectomycorrhizal fungi, indicating overlap in use of older nitrogen from soil or roots or use of newly fixed nitrogen. Genera of litter and wood decay fungi varied up to 6‰ in δ¹³C and 10‰ in δ¹⁵N, suggesting large differences in carbon and nitrogen sources and processing. Similar δ¹³C between white and brown rot wood decay fungi also suggest that white rot fungi do not use lignin-derived carbon. Together, these δ¹³C and δ¹⁵N patterns of fungi from Finnish boreal forests enhance our knowledge of fungal functional diversity and indicate broad use of litter, wood and soil resources.     

Isotope alteration caused by changes in biochemical composition of sedimentary organic matter

Stable carbon (C) and nitrogen (N) isotope ratios of sedimentary organic matter (OM) can reflect the biogeochemical history of aquatic ecosystems. However, diagenetic processes in sediments may alter isotope records of OM via microbial activity and preferential degradation of isotopically distinct organic components. This study investigated the isotope alteration caused by preferential degradation in surface sediments sampled from a eutrophic reservoir in Germany. Sediments were treated sequentially with hot water extraction, hydrochloric acid hydrolysis, hydrogen peroxide oxidation and di-sodium peroxodisulfate oxidation to chemically simulate preferential degradation pathways of sedimentary OM. Residue and extracts from each extraction step were analyzed using elemental analyzer-isotope ratio mass spectrometry and solid-state ¹³C nuclear magnetic resonance spectroscopy. Our results show that stable C and N isotope ratios reacted differently to changes in the biochemical composition of sedimentary OM. Preferential degradation of proteins and carbohydrates resulted in a 1.2‰ depletion of ¹³C, while the isotope composition of ¹⁵N remained nearly the same. Sedimentary δ¹⁵N values were notably altered when lignins and lipids were oxidized from residual sediments. Throughout the sequential fractionation procedure, δ¹³C was linearly correlated with the C:N of residual sediments. This finding demonstrates that changes in biochemical composition caused by preferential degradation altered δ¹³C values of sedimentary OM, while this trend was not observed for δ¹⁵N values. Our study identifies the influence of preferential degradation on stable C isotope ratios and provide additional insight into the isotope alteration caused by post-depositional processes.

     

Investigating patterns of symbiotic nitrogen fixation during vegetation change from grassland to woodland using fine scale δ15N measurements

Biological nitrogen fixation (BNF) in woody plants is often investigated using foliar measurements of δ¹⁵N and is of particular interest in ecosystems experiencing increases in BNF due to woody plant encroachment. We sampled δ¹⁵N along the entire N uptake pathway including soil solution, xylem sap and foliage to (1) test assumptions inherent to the use of foliar δ¹⁵N as a proxy for BNF; (2) determine whether seasonal divergences occur between δ¹⁵N_{xylem sap} and δ¹⁵N_{soil inorganic N} that could be used to infer variation in BNF; and (3) assess patterns of δ¹⁵N with tree age as indicators of shifting BNF or N cycling. Measurements of woody N‐fixing Prosopis glandulosa and paired reference non‐fixing Zanthoxylum fagara at three seasonal time points showed that δ¹⁵N_{soil inorganic N} varied temporally and spatially between species. Fractionation between xylem and foliar δ¹⁵N was consistently opposite in direction between species and varied on average by 2.4‰. Accounting for these sources of variation caused percent nitrogen derived from fixation values for Prosopis to vary by up to ～70%. Soil–xylem δ¹⁵N separation varied temporally and increased with Prosopis age, suggesting seasonal variation in N cycling and BNF and potential long‐term increases in BNF not apparent through foliar sampling alone.     

Use of stable isotope ratios of fish larvae as indicators to assess diets and patterns of anthropogenic nitrogen pollution in estuarine ecosystems

A stable isotope study was carried out to investigate the feeding ecology of the common goby Pomatoschistus microps larvae (Krøyer, 1838), and to assess differences in the response of planktonic food web to nutrient enrichment, in two ecosystems from the Southern European coast with different levels of historical pollution (estuaries of the Minho and Lima Rivers). At each estuary and time (July 2012, November 2012, February 2013, and June 2013), the fish larvae of two size classes (class 0: 0–10 mm; class 1: 10–15 mm), particulate organic matter (POM), and pelagic zooplankton were collected. The stable isotope mixing model SIAR revealed that, despite temporal differences in the relative proportion of prey items ingested, in both estuaries P. microps larvae feed on both planktonic-hyperbenthic food sources, predating mainly on copepods (from 34% to 60%), Mysidacea (from 16% to 28%), and brachyuran zoea (from 14% to 29%). Fish larvae size classes did not differ significantly for δ¹⁵N, and exhibited a very narrow range of the δ¹³C signature. Enriched δ¹⁵N values of all biota in the Lima estuary throughout the study period, with a marked nitrogen enrichment in colder months, are indicative of higher anthropogenic inputs of nitrogen (e.g. sewage and industrial discharges, agriculture) into this system. The δ¹⁵N values of fish larvae and other planktonic groups can be a sensitive bioindicator, because they are highly correlated with the nitrogen content of water (ammonium), indicating that this element has transferred through the planktonic food web. Enriched carbon isotope ratios were observed in warmer months, in both estuaries, and the heavier δ¹³C values in Lima are best explained by differences in the degree of marine influence. This research emphasises the utility of stable isotopes in trophic interactions studies, highlighting the relevance of the stable nitrogen isotope of zooplanktonic communities as a reliable bioindicator to detect patterns of anthropogenic nitrogen contamination in estuarine ecosystems.     

Evaluating the consequences of salmon nutrients for riparian organisms: Linking condition metrics to stable isotopes

Stable isotope ratios (δ¹³C and δ¹⁵N) have been used extensively to trace nutrients from Pacific salmon, but salmon transfer more than carbon and nitrogen to stream ecosystems, such as phosphorus, minerals, proteins, and lipids. To examine the importance of these nutrients, metrics other than isotopes need to be considered, particularly when so few studies have made direct links between these nutrients and how they affect riparian organisms. Our study specifically examined δ¹³C and δ¹⁵N of riparian organisms from salmon and non‐salmon streams in Idaho, USA, at different distances from the streams, and examined whether the quality of riparian plants and the body condition of invertebrates varied with access to these nutrients. Overall, quality and condition metrics did not mirror stable isotope patterns. Most notably, all riparian organisms exhibited elevated δ¹⁵N in salmon streams, but also with proximity to both stream types suggesting that both salmon and landscape factors may affect δ¹⁵N. The amount of nitrogen incorporated from Pacific salmon was low for all organisms (<20%) and did not correlate with measures of quality or condition, probably due to elevated δ¹⁵N at salmon streams reflecting historical salmon runs instead of current contributions. Salmon runs in these Idaho streams have been declining, and associated riparian ecosystems have probably seen about a 90% reduction in salmon‐derived nitrogen since the 1950s. In addition, our results support those of other studies that have cautioned that inferences from natural abundance isotope data, particularly in conjunction with mixing models for salmon‐derived nutrient percentage estimates, may be confounded by biogeochemical transformations of nitrogen, physiological processes, and even historical legacies of nitrogen sources. Critically, studies should move beyond simply describing isotopic patterns to focusing on the consequences of salmon‐derived nutrients by quantifying the condition and fitness of organisms putatively using those resources.     

Do grazers alter nitrogen dynamics on grazing lawns in a South African savannah?

The presence of grazers on grazing lawns in East Africa and North America often alters nitrogen cycling and availability. Grazing lawns can be defined as areas where grasses are kept in a short, actively growing, palatable state by the action of grazers. Our aim was to test whether lawns have enhanced leaf nitrogen (N) concentrations, total soil N and δ¹⁵N when compared to tall grass areas in a South African savannah. Previous studies have used ecosystem δ¹⁵N as a proxy of N availability, and enriched δ¹⁵N values have been suggested to indicate higher N availability or higher N transformation rates. Across all sites, foliar N concentrations (but not soil N) were higher when compared to tall grass areas, and evidence of enriched foliar and soil δ¹⁵N values was found on the lawns. These results suggest that grazers may be involved in altering the rates of N transformations directly on grazing lawns. Regardless of whether these N transformations included increased net N mineralization, higher N concentrations in above‐ground foliage attract grazers back to the lawns, encouraging their maintenance.     

Effects of Bromus tectorum invasion on microbial carbon and nitrogen cycling in two adjacent undisturbed arid grassland communities

Soil nitrogen (N) is an important component in maintaining ecosystem stability, and the introduction of non-native plants can alter N cycling by changing litter quality and quantity, nutrient uptake patterns, and soil food webs. Our goal was to determine the effects of Bromus tectorum (C₃) invasion on soil microbial N cycling in adjacent non-invaded and invaded C₃ and C₄ native arid grasslands. We monitored resin-extractable N, plant and soil δ¹³C and δ¹⁵N, gross rates of inorganic N mineralization and consumption, and the quantity and isotopic composition of microbial phospholipid biomarkers. In invaded C₃ communities, labile soil organic N and gross and net rates of soil N transformations increased, indicating an increase in overall microbial N cycling. In invaded C₄ communities labile soil N stayed constant, but gross N flux rates increased. The δ¹³C of phospholipid biomarkers in invaded C₄ communities showed that some portion of the soil bacterial population preferentially decomposed invader C₃-derived litter over that from the native C₄ species. Invasion in C₄ grasslands also significantly decreased the proportion of fungal to bacterial phospholipid biomarkers. Different processes are occurring in response to B. tectorum invasion in each of these two native grasslands that: 1) alter the size of soil N pools, and/or 2) the activity of the microbial community. Both processes provide mechanisms for altering long-term N dynamics in these ecosystems and highlight how multiple mechanisms can lead to similar effects on ecosystem function, which may be important for the construction of future biogeochemical process models.     

Possible ideas on carbon and nitrogen trophic fractionation of food chains: a new aspect of food-chain stable isotope analysis in Lake Biwa, Lake Baikal, and the Mongolian grasslands

Trophic fractionation of carbon and nitrogen isotopes (Δδ¹³C, Δδ¹⁵N) was examined using previously complied databases for food chains in Lake Biwa, Lake Baikal, and Mongolian grassland. The following two features were clarified: (1) For each ecosystem, the ratios of trophic fractionation of carbon and nitrogen isotopes (Δδ¹⁵N/Δδ¹³C) throughout food chain could be obtained as the slope of linear regression line on the δ¹⁵N–δ¹³C plot. (2) Further, analysis of covariance (ANCOVA) revealed the slopes on δ¹⁵N–δ¹³C were not significantly different among these various ecosystems and allowed us to have the regression by setting δ¹⁵N as the response variable: δ¹⁵N = 1.61 δ¹³C + [ecosystem specific constant] with standard errors of [±0.41] and [±9.7] for the slope and the intercept, respectively. It was suggested that the slope of the regression (or the ratio Δδ¹⁵N/Δδ¹³C) could be applicable to more complicated food webs in case nitrogen and carbon isotope ratios of primary producers can be assumed constant in space and time within the ecosystems. The results from simple linear regression analyses coincided well with the ANCOVA results for these ecosystems, although there was some discrepancy between the results of the two statistical analyses. Possible factors that govern the linear relationship between δ¹⁵N and δ¹³C along a food chain are discussed together with a new scope for the stable isotope food chain analyses.     

Isotopic evidence for increased carbon and nitrogen exchanges between peatland plants and their symbiotic microbes with rising atmospheric CO2 concentrations since 15,000 cal. year BP

Whether nitrogen (N) availability will limit plant growth and removal of atmospheric CO₂ by the terrestrial biosphere this century is controversial. Studies have suggested that N could progressively limit plant growth, as trees and soils accumulate N in slowly cycling biomass pools in response to increases in carbon sequestration. However, a question remains over whether longer-term (decadal to century) feedbacks between climate, CO₂ and plant N uptake could emerge to reduce ecosystem-level N limitations. The symbioses between plants and microbes can help plants to acquire N from the soil or from the atmosphere via biological N₂ fixation—the pathway through which N can be rapidly brought into ecosystems and thereby partially or completely alleviate N limitation on plant productivity. Here we present measurements of plant N isotope composition (δ¹⁵N) in a peat core that dates to 15,000 cal. year BP to ascertain ecosystem-level N cycling responses to rising atmospheric CO₂ concentrations. We find that pre-industrial increases in global atmospheric CO₂ concentrations corresponded with a decrease in the δ¹⁵N of both Sphagnum moss and Ericaceae when constrained for climatic factors. A modern experiment demonstrates that the δ¹⁵N of Sphagnum decreases with increasing N₂-fixation rates. These findings suggest that plant-microbe symbioses that facilitate N acquisition are, over the long term, enhanced under rising atmospheric CO₂ concentrations, highlighting an ecosystem-level feedback mechanism whereby N constraints on terrestrial carbon storage can be overcome.