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PubMed2004

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams.

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH4+ uptake rates were measured in eight streams using short-term 15N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.

Carbon↗
PubMed2004

Successional changes in soil nitrogen availability, non-symbiotic nitrogen fixation and carbon/nitrogen ratios in southern Chilean forest ecosystems.

Vast areas of southern Chile are now covered by second-growth forests because of fire and logging. To study successional patterns after moderate-intensity, anthropogenic fire disturbance, we assessed differences in soil properties and N fluxes across a chronosequence of seven successional stands (2-130 years old). We examined current predictions of successional theory concerning changes in the N cycle in forest ecosystems. Seasonal fluctuations of net N mineralization (N(min)) in surface soil and N availability (N(a); N(a)=NH4+-N+NO3--N) in upper and deep soil horizons were positively correlated with monthly precipitation. In accordance with theoretical predictions, stand age was positively, but weakly related to both N(a) ( r(2)=0.282, P<0.001) and total N (N(tot); r(2)=0.192, P<0.01), and negatively related to soil C/N ratios ( r(2)=0.187, P<0.01) in surface soils. A weak linear increase in soil N(min) (upper plus deep soil horizons) was found across the chronosequence ( r(2)=0.124, P<0.022). N(min) occurred at modest rates in early successional stands, suggesting that soil disturbance did not impair microbial processes. The relationship between N fixation (N(fix)) in the litter layer and stand age best fitted a quadratic model ( r(2)=0.228, P<0.01). In contrast to documented successional trends for most temperate, tropical and Mediterranean forests, non-symbiotic N(fix) in the litter layer is a steady N input to unpolluted southern temperate forests during mid and late succession, which may compensate for hydrological losses of organic N from old-growth ecosystems.

Age Factors↗
PubMed2003

Boreal bog plants: nitrogen sources and uptake of recently deposited nitrogen.

The main goals of this study were to determine the delta15N signature of quantitatively important boreal bog plants as basis for discussing their N sources, and to assess the effects of five different 3 year N treatments (i.e. 0, 5, 10, 20 and 40 kg N ha(-1) year(-1)) on the bog plants and surface peat at different depths (i.e. 0, 5, 10, 20 and 40 cm) by using 15N as tracer. Plants and peat were analyzed for N concentration, 15N natural abundance and 15N at.%. From the results we draw three main conclusions: First, the relative importance of different N sources is species-specific among bog plants. Second, an annual addition of 5 kg N ha(-1) year(-1) was sufficient to significantly increase the N concentration in Sphagnum mosses, liverworts and shallow rooted vascular plants, and an annual addition of 40 kg N ha(-1) year(-1) during 3 years was not sufficient to increase the N concentration in deep rooted plants, although the 15N content increased continuously, indicating a possible longer term effect. Third, an annual addition of 40 kg N ha(-1) year(-1) during 3 years increased the N content in surface peat at depths of 5 and 10 cm, but not at depths of 20 and 40 cm, indicating the capacity of the living Sphagnum mosses and the surface peat to take up deposited N, and thereby function as a filter.

Bryophyta↗
PubMed1983

Collisions between nitrogen-14 and nitrogen-15 spin-labels. 1. Lipid-lipid interactions in model membranes.

A new method is described to evaluate collision frequencies between spin-labeled molecules in membranes. This method utilizes a mixture of 14N and 15N nitroxides. The extent of spin-spin interaction between the two different isotopes can be determined by comparing experimental ESR spectra with simulated spectra. The latter are calculated by using modified Bloch equations and by assuming that the spin exchange process is the dominant broadening mechanism. A more approximate analysis is also described; it is based on the evaluation of the line broadening from the measurement of peak heights. Both analyses show that relatively low concentrations of label can be utilized: 1-5% labeled lipids in a membrane give a broadening sufficient to allow quantitation. Collision frequencies equal to or larger than 0.1 MHz can be evaluated by this method. Because of the limited overlap between 15N and 14N spectra, this method allows the investigation of the collisions between unlike molecules of very different concentration. From the collision frequencies, the topological domain explored by a probe can be inferred as well as its lateral diffusion constant. In this paper, it is shown that in spite of the low order of the chain residues near their methyl terminal, a probe linked at the omega - 2 position of an ionized stearic acid does not interact with a probe positioned near the polar region. Nevertheless the former probe, deeply embedded in the membrane, explores a larger volume. It is also shown that if the nitroxide is attached to a fatty ester, the probe "sees" evenly the whole hydrophobic region of the membrane.

Chemical Phenomena↗
PubMed2006

Sedimented organic nitrogen isotopes in freshwater wetlands record long-term changes in watershed nitrogen source and land use.

Although historic land use is widely recognized as an important determinant of watershed N cycling, efforts to examine land use legacy effects are limited by incomplete historical data. This research evaluates N isotopes of sedimented organic matter (delta15N(org)), in a palynological context, as a long-term proxy of changes in N source to wetland biota. N and S isotope measurements of organic sediments, fossil plant fragments, and living plants are used to explore isotope stratigraphies of wetland sediment cores. Processes potentially contributing to isotope stratigraphies are investigated including the following: a change in N source, diagenesis, and denitrification. We document the delta15N(org) stratigraphy of a core from the Smithsonian Environmental Research Center, MD, U.S.A. spans approximately 350 years, during which time delta15N(org) increases from +2 per thousand to +7 per thousand. Reconstructed population density and wastewater inputs to the watershed suggest that the increase in delta15N reflects changing land use from forested conditions to increasing nutrient inputs from human waste. Our results illustrate the importance of hydrologic connectivity in delivering waste-derived N in a watershed characterized by relatively low human population density. These results also demonstrate how this approach can expand the temporal horizon over which we can assess human impacts to watershed N dynamics.

Agriculture↗