Brian Fry

Carbon and nitrogen dynamics along the decay continuum: Plant litter to soil organic matter

Decay processes in an ecosystem can be thought of as a continuum beginning with the input of plant litter and leading to the formation of soil organic matter. As an example of this continuum, we review a 77-month study of the decay of red pine (Pinus resinosa Ait.) needle litter. We tracked the changes in C chemistry and the N pool in red pine (Pinus resinosa Ait.) needle litter during the 77-month period using standard chemical techniques and stable isotope, analyses of C and N.Mass loss is best described by a two-phase model: an initial phase of constant mass loss and a phase of very slow loss dominated by degradation of ‘lignocellulose’ (acid soluble sugars plus acid insoluble C compounds). As the decaying litter enters the second phase, the ratio of lignin to lignin and cellulose (the lignocellulose index, LCI) approaches 0.7. Thereafter, the LCI increases only slightly throughout the decay continuum indicating that acid insoluble materials (‘lignin’) dominate decay in the latter part of the continuum.Nitrogen dynamics are also best described by a two-phase model: a phase of N net immobilization followed by a phase of N net mineralization. Small changes in C and N isotopic composition were observed during litter decay. Larger changes were observed with depth in the soil profile.An understanding of factors that control ‘lignin’ degradation is key to predicting the patterns of mass loss and N dynamics late in decay. The hypothesis that labile C is needed for ‘lignin’ degradation must be evaluated and the sources of this C must be identified. Also, the hypothesis that the availability of inorganic N slows ‘lignin’ decay must be evaluated in soil systems.

Resource-based niches provide a basis for plant species diversity and dominance in arctic tundra

Ecologists have long been intrigued by the ways co-occurring species divide limiting resources. Such resource partitioning, or niche differentiation, may promote species diversity by reducing competition. Although resource partitioning is an important determinant of species diversity and composition in animal communities, its importance in structuring plant communities has been difficult to resolve. This is due mainly to difficulties in studying how plants compete for belowground resources. Here we provide evidence from a 15N-tracer field experiment showing that plant species in a nitrogen-limited, arctic tundra community were differentiated in timing, depth and chemical form of nitrogen uptake, and that species dominance was strongly correlated with uptake of the most available soil nitrogen forms. That is, the most productive species used the most abundant nitrogen forms, and less productive species used less abundant forms. To our knowledge, this is the first documentation that the composition of a plant community is related to partitioning of differentially available forms of a single limiting resource.

Compound-specific δ 13C analyses of leaf lipids from plants with differing carbon dioxide metabolisms

Abstract The stable carbon isotope (δ 13C) values of (1) total leaf tissue (2), total surface lipid extracts and (3) individual n-alkanes isolated from leaves were determined for plants utilizing the C3, C4, and Crassulacean Acid (CAM) pathways of carbon fixation. The C4 and CAM species had leaf tissue δ 13C values of −10 to −16‰; the total surface lipid extract δ 13C values were on average 8‰ more depleted. The C3 plants had leaf tissue values of −25 to −30‰; total surface lipid extracts were on average 4‰ more depleted in 13C. The average n-alkane values for all plants were around 4‰ more depleted in 13C than the total surface lipid extracts and were between 1.6 and 13.8‰ depleted in 13C relative to total tissue. This depletion relative to the total tissue was on average greater for CAM (11.0‰) and C4 plants (9.9‰) than for C3 plants (5.9‰). Variations ranging from 0.1 to 6‰ were observed between n-alkalene homologues for the plants studied, emphasizing the need for caution when interpreting small variations in the isotopic compositions of individual compounds isolated from sedimentary environments. n-Aldehydes from C3 plants had carbon-isotopic compositions similar to the n-alkanes from the same plants, suggesting that other biosynthetically related n-alkyl compounds (e.g. n-acids and n-alcohols) will have δ 13C values similar to the n-alkanes. Phytol isolated from C3 plants was, on average, 1.5‰ enriched in 13C relative to the average n-alkane δ 13C values.

Natural abundance-level measurement of the nitrogen isotopic composition of oceanic nitrate: an adaptation of the ammonia diffusion method

We have adapted the “ammonia diffusion” method of nitrate extraction for natural-abundance level nitrogen isotopic measurement of oceanic nitrate. The method involves: (1) sample concentration (by boiling or evaporation); (2) conversion of nitrate to ammonia using Devardas alloy; and (3) the gas-phase diffusion of ammonia onto an acidified glass fiber disk which is sandwiched between two porous Teflon membranes. We have investigated the conditions necessary to effect complete ammonia recovery from natural seawater samples and the use of Devardas alloy under these conditions. In addition, we have characterized the blanks in this method and designed a protocol to minimize them. Here, we report our protocol for nitrate extraction from seawater and provide an explanation of the protocol based on our method development work. To demonstrate the performance of the method, we present nitrate nitrogen isotopic data from nitrate standard additions to Sargasso Sea surface water and from several Southern Ocean depth profiles. The nitrate extraction method gives highly reproducible, complete recovery of nitrate and a standard deviation for isotopic analysis of < 0.2%c down to 5 μM nitrate (or lower). Replicate extractions of a nitrate standard added to Sargasso Sea surface water demonstrate agreement between the isotopic composition of the added and recovered N, with the extraction blank causing a ≤ 0.3%. discrepancy for 5 μM nitrate. The blanks inherent in the extraction procedure are from Devardas alloy and seawater dissolved organic nitrogen (“DON”). The N blank of the Devardas alloy reagent depends on brand and lot number. The Devardas alloy which we are currently using results in a blank of ~ 0.4 nmol N per 100 ml of seawater (effectively 0.4 μM). An isotopic correction is made for this blank. For standard incubation conditions, stored Woods Hole seawater (with ~ 10 μM DON) gives a ~ 0.6 μM DON blank, while stored Sargasso Sea (with ~ 6 μm DON) surface water gives a DON blank of 0.3–0.5 μM. The DON blank appears to cause the ≤ 0.3%. difference between the measured and actual isotopic composition of nitrate added to Sargasso Sea surface water at the 5 μM nitrate level. We discuss several ways to lower the DON blank for samples in which the DON concentration is high relative to the nitrate concentration. The nitrogen isotopic data from several Southern Ocean profiles, in conjunction with the other results presented in this paper, demonstrate the consistency of the data produced by the ammonia diffusion method. The ammonia diffusion-based protocol is more reliable and allows for better precision than the nitrate reduction/ammonia distillation method (Cline and Kaplan, 1975) in our hands. While the samples have an incubation time of 4 days or longer, we find that the diffusion method allows for higher throughput than the distillation method because samples can be run conveniently in large batches.

Stable Isotopes and Planktonic Trophic Structure in Arctic Lakes

Actual food—web structure or function is difficult to determine based on visual observation, gut analyses, or the feeding interactions expected from a given list of species. We used C and N stable—isotope distributions to define food—web structure in arctic lakes, and we compared that structure with results based on more traditional analyses. Although zooplankton species composition was similar across the eight lakes studied, the food—web structure varied greatly. In some lakes the copepod predator Heterocope fed on the herbivorous copepod Diaptomus as expected in a conventional food web. In most lakes, however, °15N data were consistent with Heterocope functioning as an herbivore rather than a predator. These inferences were supported by evidence from carbon isotopes and energy—flow data. Our study indicates that only two or three trophic levels exist in the macrozoopolankton of these lakes, in comparison to five or six trophic levels reported in temperate lakes. Isotope analyses showed that actual food—web structure is poorly predicted from simple consideration of species lists and potential trophic interactions.

Variations of marine plankton δ13C with latitude, temperature, and dissolved CO2 in the world ocean

Variations of the 13C content of marine participate organic carbon (δ13CPOC) in the modern ocean were studied using literature data to test the assumptions underlying the calculation of atmospheric pCO2 through geological time from the δ13C of sedimentary organic matter. These assumptions are that (1) concentrations of CO2 in the atmosphere and the surface ocean are at equilibrium at all times and latitudes and that (2) carbon isotopic fractionation of phytoplankton (ϵp) covaries primarily with concentrations of dissolved molecular CO2 ([CO2]aq). Previous studies and compilations have shown that the first assumption does not strictly hold, although [CO2]aq may be predicted with a reasonable degree of accuracy from sea surface temperature for specific regions of the world ocean. The second assumption is shown to be questionable due to the weak covariation of ϵp and [CO2]aq in the modern ocean. The large residual variance for regressions of ϵp against [CO2]aq suggests that factors other than [CO2]aq strongly affect carbon isotopic fractionation in phytoplankton. It is concluded that the relationship between ϵp and [CO2]aq cannot be easily calibrated using δ13CPOC data from the modern ocean.

Stable Isotope Diagrams of Freshwater Food Webs

This formula may be used to calculate 95% confidence intervals around mi values (Fig. 1), aiding the comparison of survivorship patterns in the cohorts (Lee 1980). When this procedure is applied to the data of Pyke and Thompson (1986), the analysis shows that the mortality rates differ significantly in the two cohorts during the intervals between days 0-10, 10-20, 30-40, and 50-60, confirming the conclusions of the analysis of the residuals generated during calculation of the logrank statistic.

Rapid 13C/12C turnover during growth of brown shrimp (Penaeus aztecus)

SummaryUsing natural-abundance 13C/12C ratios as tracers, carbon turnover rates were determined for postlarval brown shrimp, Penaeus aztecus, in five laboratory growth experiments. Although tissue turnover in adult animals generally occurs during maintenance metabolism and is a function of time, turnover for young postlarval shrimp was accelerated during growth, and was primarily a function of weight gained rather than time. Metabolic loss of tissue carbon during growth was usually approximated by the function, Fraction lost=1-(initial weight/final weight). For shrimp that switch diets in the sea, model calculations show that this high turnover rate coupled with a four-fold weight increase suffices for shrimp to achieve a close isotopic resemblance of 1‰ or less (δ13C units) to the new diet.In accordance with these predictive calculations, shrimp which had increased in weight by a factor of four or more in the culture experiments showed essentially constant isotopic values reflecting their new diets. For these larger animals, the average animal-diet difference varied across three diets from-0.9 to +11‰, and the δ13C range among individuals was ≦1.4‰ in each experiment.

Measuring 15N-NH4+ in marine, estuarine and fresh waters : An adaptation of the ammonia diffusion method for samples with low ammonium concentrations

Abstract We present a method for measuring 15 N –NH4+ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses large sample volumes (often 4 l) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured δ 15 N values.

THE STABLE NITROGEN ISOTOPE RATIO AS A MARKER OF FOOD‐WEB INTERACTIONS AND FISH MIGRATION

We used stable nitrogen isotopes to describe the pelagic food-web structure of three coastal Baltic Sea areas, each of which was sampled twice. Two of the areas were influenced by 15N-rich nutrient discharges from a sewage treatment plant. Analyses were made of particulate organic matter (<35 μm, mainly phytoplankton), zooplankton, mysids (Mysis mixta and M. relicta), sprat (Sprattus sprattus), smelt (Osmerus eperlanus), four size classes of herring (Clupea harengus), and pikeperch (Stizostedion lucioperca). Discharges from the sewage treatment plant significantly increased δ15N values in the whole food web, from phytoplankton to piscivorous fish. Based on nitrogen isotopic compositions, consistent trophic food-web structures were observed on both occasions and in all three areas. The results indicate that zooplankton and mysids may have more complex diets than assumed before. Apparent trophic fractionation, i.e., differences in δ15N between a consumer and its assumed food, averaged 2.4‰ with a standard e...