Merging aquatic and terrestrial perspectives of nutrient biogeochemistry

Although biogeochemistry is an integrative discipline, terrestrial and aquatic subdisciplines have developed somewhat independently of each other. Physical and biological differences between aquatic and terrestrial ecosystems explain this history. In both aquatic and terrestrial biogeochemistry, key questions and concepts arise from a focus on nutrient limitation, ecosystem nutrient retention, and controls of nutrient transformations. Current understanding is captured in conceptual models for different ecosystem types, which share some features and diverge in other ways. Distinctiveness of subdisciplines has been appropriate in some respects and has fostered important advances in theory. On the other hand, lack of integration between aquatic and terrestrial biogeochemistry limits our ability to deal with biogeochemical phenomena across large landscapes in which connections between terrestrial and aquatic elements are important. Separation of the two approaches also has not served attempts to scale up or to estimate fluxes from large areas based on plot measurements. Understanding connectivity between the two system types and scaling up biogeochemical information will rely on coupled hydrologic and ecological models, and may be critical for addressing environmental problems associated with locally, regionally, and globally altered biogeochemical cycles.We dedicate this paper to the memory of Catherine Lisa Dent, a member of our working group who contributed much to the ideas presented herein, and to the joy of developing them together.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Unexpected spatial stability of water chemistry in headwater stream networks

Understanding how water and solutes enter and propagate through freshwater landscapes in the Anthropocene is critical to protecting and restoring aquatic ecosystems and ensuring human water security. However, high hydrochemical variability in headwater streams, where most carbon and nutrients enter river networks, has hindered effective modelling and management. We developed an analytical framework informed by landscape ecology and catchment hydrology to quantify spatiotemporal variability across scales, which we tested in 56 headwater catchments, sampled periodically over 12 years in western France. Unexpectedly, temporal variability in dissolved carbon, nutrients and major ions was preserved moving downstream and spatial patterns of water chemistry were stable on annual to decadal timescales, partly because of synchronous variation in solute concentrations. These findings suggest that while concentration and flux cannot be extrapolated among subcatchments, periodic sampling of headwaters provides valuable information about solute sources and subcatchment resilience to disturbance.     

Decline in Ecosystem <Emphasis Type="Italic">δ</Emphasis><Superscript>13</Superscript>C and Mid-Successional Nitrogen Loss in a Two-Century Postglacial Chronosequence

Uncertainty about controls on long-term carbon (C) and nitrogen (N) balance, turnover, and isotopic composition currently limits our ability to predict ecosystem response to disturbance and landscape change. We used a two-century, postglacial chronosequence in Glacier Bay, Alaska, to explore the influence of C and N dynamics on soil and leaf stable isotopes. C dynamics were closely linked to soil hydrology, with increasing soil water retention during ecosystem development resulting in a linear decrease in foliar and soil δ¹³C, independent of shifts in vegetation cover and despite constant precipitation across sites. N dynamics responded to interactions among soil development, vegetation type, microbial activity, and topography. Contrary to the predictions of nutrient retention theory, potential nitrification and denitrification were high, relative to inorganic N stocks, from the beginning of the chronosequence, and gaseous and hydrological N losses were highest at mid-successional sites, 140–165 years since deglaciation. Though leaching of dissolved N is considered the predominant pathway of N loss at high latitudes, we found that gaseous N loss was more tightly correlated with δ¹⁵N enrichment. These results suggest that δ¹³C in leaves and soil can depend as much on soil development and associated water availability as on climate and that N availability and export depend on interactions between physical and biological state factors.     

Nitrogen Removal by Riparian Buffers along a European Climatic Gradient: Patterns and Factors of Variation

We evaluated nitrogen (N) removal efficiency by riparian buffers at 14 sites scattered throughout seven European countries subject to a wide range of climatic conditions. The sites also had a wide range of nitrate inputs, soil characteristics, and vegetation types. Dissolved forms of N in groundwater and associated hydrological parameters were measured at all sites; these data were used to calculate nitrate removal by the riparian buffers. Nitrate removal rates (expressed as the difference between the input and output nitrate concentration in relation to the width of the riparian zone) were mainly positive, ranging from 5% m⁻¹ to 30% m⁻¹, except for a few sites where the values were close to zero. Average N removal rates were similar for herbaceous (4.43% m⁻¹) and forested (4.21% m⁻¹) sites. Nitrogen removal efficiency was not affected by climatic variation between sites, and no significant seasonal pattern was detected. When nitrate inputs were low, a very large range of nitrate removal efficiencies was found both in the forested and in the nonforested sites. However, sites receiving nitrate inputs above 5 mg N L⁻¹ showed an exponential negative decay of nitrate removal efficiency (nitrate removal efficiency = 33.6 e^{−0.11 NO3input}, r ² = 0.33, P < 0.001). Hydraulic gradient was also negatively related to nitrate removal (r = −0.27, P < 0.05) at these sites. On the basis of this intersite comparison, we conclude that the removal of nitrate by biological mechanisms (for example, denitrification, plant uptake) in the riparian areas is related more closely to nitrate load and hydraulic gradient than to climatic parameters. Received 15 August 2001; accepted 2 May 2002.     

Heat Shock Factor 1 Drives Intergenic Association of Its Target Gene Loci upon Heat Shock

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Seasonal changes in inorganic and organic phosphorus in the soil of a riparian forest

This study addresses the temporal distribution of forms of phosphorus in the soil of a temporarily flooded riparian forest of the valley of the river Garonne (Southwest of France). A sequential extraction for forms of phosphorus of increasing chemical stability was used. During the study period (13 months), the forest was flooded a few days during March and May. In winter, resin-Pi concentration was high (26 g g^–1) in comparison to spring values (<9 g g^–1). NaHCO₃-Po, NaHCO₃-Pi or NaOH-Pi concentrations increased during winter (up to 74, 124 and 78 g g^–1 respectively) and decreased significantly during spring (32, 44 and 32 g g^–1 respectively). This pattern was attributed to simultaneous mineralization and plant uptake during the growing season and to the flood events (erosional processes and P-release). During summer and fall, resin-Pi concentration increased significantly (up to 26 g g^–1 in October). NaHCO₃-Po concentrations remained low during spring and summer (<33 g g^–1), and increased significantly in fall (>45 g g^–1 NaHCO₃-Pi or NaOH-Pi increased in late spring or summer (90 g g^–1 and 68 g g^–1 respectively). Increasing concentrations of the labile forms during late spring or summer were ascribed to the warm temperature and soil dryness that limited plant growth. HCl-Pi increased regularly after the floods (174 g g^–1 before the flood events to 254 g g^–1 after the floods). Residual P presented a similar pattern i.e. 214 g g^–1 and 279 g g^–1 respectively before and after the flood events. This pattern was attributed to a progressive incorporation of flood deposits to the soil.     

The rise of research on futures in ecology: rebalancing scenarios and predictions

Concern about the ecological consequences of global change has increasingly stimulated ecologists to examine the futures of ecological systems. Studying futures is not only a crucial element of the interaction between science, management and decision making , but also a critical research challenge per se , especially because futures cannot be observed or experimented on. In addition, researchers can encounter methodological and theoretical difficulties, which make interpretations and predictions problematic. In the literature which deals with futures of ecological systems two main lines of research can be distinguished: a predictive approach, which dominates the literature, can be contrasted with a rarer number of studies that elaborate potential scenarios for ecological systems. Scenario approaches currently concern mainly contacts with stakeholders or decision makers, or the use of climate scenarios to derive projections about ecological futures. We argue that a new direction for ecological futures research could be explored by using ecological scenarios in combination with predictive models to further fundamental ecological research, in addition to enhancing its applied value.