Influence of sample preparation on estuarine macrofauna stable isotope signatures in the context of contaminant bioaccumulation studies.

The ratios of stable isotopes of carbon and nitrogen provide important information on food sources of aquatic organisms and trophic structure of aquatic food webs. For many studies, trophic position and food source are linked to bioaccumulation and trophic transfer of contaminants from prey to predators. In these cases, it is useful to use measurements on whole organisms to make direct comparisons of contaminant bioaccumulation and food web attributes. There is a great deal of variation in methods used for stable isotope analysis, particularly in the selection of tissue type and sample preparation prior to stable isotope analysis. While there have been aquatic studies that examined methodological differences, few have focused on estuarine organisms. In this study, the effects of depuration and tissue dissection on the stable isotope enrichment of common estuarine invertebrates and fish were examined. Homogenized tissues of non-depurated whole organisms were compared to dissected muscle tissue or depurated whole organisms. A 24 h depuration did not change the mean δ15N and δ13C values for most species examined. Additionally, as expected, significant differences in carbon and nitrogen signatures were found when muscle tissues were compared to whole organisms. However, differences were small enough that food source as inferred by δ13C or trophic level as inferred from δ15N would not be inaccurately represented (differences of <1.9‰ for δ13C and <1.2‰ for δ15N). The results of this study suggest that for these common estuarine fish and macroinvertebrates, stable isotopes ratios of samples can be analyzed without depuration in the same way as samples for contaminant analysis, but differences in tissue types must be taken into account when combining data from different sources.

Cover crop residue decomposition triggered soil oxygen depletion and promoted nitrous oxide emissions.

Cover cropping is a promising strategy to improve soil health, but it may also trigger greenhouse gas emissions, especially nitrous oxide (N2O). Beyond nitrogen (N) availability, cover crop residue decomposition may accelerate heterotrophic respiration to limit soil O2 availability, hence promote N2O emissions from denitrification under sub-optimal water-filled pore space (WFPS) conditions that are typically not conducive to large N2O production. We conducted a 21-day incubation experiment to examine the effects of contrasting cover crop residue (grass vs legume) decomposition on soil O2 and biogeochemical changes to influence N2O and CO2 emissions from 15N labeled fertilized soils under 50% and 80% WFPS levels. Irrespective of cover crop type, mixing cover crop residue with N fertilizer resulted in high cumulative N2O emissions under both WFPS conditions. In the absence of cover crop residues, the N fertilizer effect of N2O was only realized under 80% WFPS, whereas it was comparable to the control under 50% WFPS. The N2O peaks under 50% WFPS coincided with soil O2 depletion and concomitant high CO2 emissions when cover crop residues were mixed with N fertilizer. While N fertilizer largely contributed to the total N2O emissions from the cover crop treatments, soil organic matter and/or cover crop residue derived N2O had a greater contribution under 50% than 80% WFPS. Our results underscore the importance of N2O emissions from cover crop-based fertilized systems under relatively lower WFPS via a mechanism of respiration-induced anoxia and highlight potential risks of underestimating N2O emissions under sole reliance on WFPS.

Altitudinal trends of leaf delta(13)C follow different patterns across a mountainous terrain in north China characterized by a temperate semi-humid climate.

Many studies have documented that the delta(13)C values of plants increase with altitude both on a global scale and locally in humid climates, while in semi-arid areas the opposite trend has been found. The study reported herein was conducted in a mountainous area of China characterized by a temperate semi-humid climate. The delta(13)C values of C(3) species do not exhibit a consistent variation along an altitudinal gradient and the observations suggest that the pattern of increasing delta(13)C with altitude cannot be generalized. In the study area, in addition to environmental factors such as changing air pressure and light, the interaction between temperature and plant water balance determines the delta(13)C-altitude variations in C(3) plants. The delta(13)C of the leaves of C(4) plants is found to increase with altitude with a mean gradient of 0.9 per thousand/km. The altitudinal trend of C(4) plants is attributed to the combined influences of water availability and other factors rather than temperature.

Experimental measurements of leaf carbon isotope discrimination and gas exchange in the progenies of Plantago depressa and Setaria viridis collected from a wide altitudinal range.

Significant correlations between leaf carbon isotope discrimination (Delta) and altitude and between gas exchange and altitude have been reported in previous studies, raising the question of whether the altitudinal variations in discrimination and gas exchange can be attributed to genetic differences among populations from different altitudes. Studies that focus on in situ analysis cannot distinguish the effects of genetic variation from environmental variation. This article describes an experiment in which seeds of Plantago depressa (C3 species) and Setaria viridis (C4 species) collected from a wide altitudinal range were grown in the same environment. Carbon isotopic ratios (delta(13)C) and gas exchange of the seedlings were measured. The progenies of P. depressa and S. viridis no longer display any significant Delta decreases with the altitude of origin as seen in situ. Furthermore, photosynthetic rate, stomatal conductance, the ratio of intercellular to ambient CO(2) and intrinsic water use efficiency for P. depressa and S. viridis grown in the greenhouse are also not significantly related to the altitude of origin. The observations suggest that altitudinal variations in Delta and gas exchange are not because of genotypic differences, independent of photosynthetic type.