Patterns of free amino acids in tundra soils reflect mycorrhizal type, shrubification, and warming.

The soil nitrogen (N) cycle in cold terrestrial ecosystems is slow and organically bound N is an important source of N for plants in these ecosystems. Many plant species can take up free amino acids from these infertile soils, either directly or indirectly via their mycorrhizal fungi. We hypothesized that plant community changes and local plant community differences will alter the soil free amino acid pool and composition; and that long-term warming could enhance this effect. To test this, we studied the composition of extractable free amino acids at five separate heath, meadow, and bog locations in subarctic and alpine Scandinavia, with long-term (13 to 24 years) warming manipulations. The plant communities all included a mixture of ecto-, ericoid-, and arbuscular mycorrhizal plant species. Vegetation dominated by grasses and forbs with arbuscular and non-mycorrhizal associations showed highest soil free amino acid content, distinguishing them from the sites dominated by shrubs with ecto- and ericoid-mycorrhizal associations. Warming increased shrub and decreased moss cover at two sites, and by using redundancy analysis, we found that altered soil free amino acid composition was related to this plant cover change. From this, we conclude that the mycorrhizal type is important in controlling soil N cycling and that expansion of shrubs with ectomycorrhiza (and to some extent ericoid mycorrhiza) can help retain N within the ecosystems by tightening the N cycle.

Evolution of nitrogen cycling in regrowing Amazonian rainforest.

Extensive regions of tropical forests are subjected to high rates of deforestation and forest regrowth and both are strongly affect soil nutrient cycling. Nitrogen (N) dynamics changes during forest regrowth and the recovery of forests and functioning similar to pristine conditions depends on sufficient N availability. We show that, in a chronosequence of Amazonian forests, gross nitrification and, as a result, nitrate-to-ammonium (NO3-: NH4+) ratio were lower in all stages of regrowing forests (10 to 40 years) compared to pristine forest. This indicates the evolution of a more conservative and closed N cycle with reduced risk for N leaking out of the ecosystem in regrowing forests. Furthermore, our results indicate that mineralization and nitrification are decoupled in young regrowing forests (10 years), such as that high gross mineralization is accompanied by low gross nitrification, demonstrating a closed N cycle that at the same time maintains N supply for forest regrowth. We conclude that the status of gross nitrification in disturbed soil is a key process to understand the mechanisms of and time needed for tropical forest recovery.

Distinguishing vegan-, vegetarian-, and omnivorous diets by hair isotopic analysis.

BACKGROUND: Dietary risks contribute heavily to disability-adjusted life years (DALYs), being more important than hypertension, obesity, or smoking. To measure dietary exposure remains a challenge in nutrition research. AIMS: The aim of the present study was to test the hypothesis that isotope ratios of 15N and 13C in human hair could distinguish between subjects adhering to different habitual diets. METHODS: 20 male and 29 female subjects average 31 years old (range 19-53), with stable dietary habits volunteered. Diets were vegan, vegetarian and omnivorous. Hair samples were processed on an elemental analyser coupled to isotope-ratio mass spectrometry. RESULTS: δ15N differed between vegan, vegetarian and omnivorous diets, p < 0.05 for all. δ13C differed between vegan and omnivorous diets, p < 0.05, but neither of these diets were separated from the vegetarian diet. CONCLUSION: Elemental Analysis of δ13C and especially δ15N with isotope ratio mass spectrometry seems to be a promising, non-invasive and objective way to distinguish groups of subjects on different habitual diets, at least if n=>10.

Full 15N tracer accounting to revisit major assumptions of 15N isotope pool dilution approaches for gross nitrogen mineralization.

The 15N isotope pool dilution (IPD) technique is the only available method for measuring gross ammonium (NH4 +) production and consumption rates. Rapid consumption of the added 15N-NH4 + tracer is commonly observed, but the processes responsible for this consumption are not well understood. The primary objectives of this study were to determine the relative roles of biotic and abiotic processes in 15N-NH4 + sconsumption and to investigate the validity of one of the main assumptions of IPD experiments, i.e., that no reflux of the consumed 15N tracer occurs during the course of the experiments. We added a 15N-NH4 + tracer to live and sterile (autoclaved) soil using mineral topsoil from a beech forest and a grassland in Austria that differed in NH4 + concentrations and NH4 + consumption kinetics. We quantified both biotic tracer consumption (i.e. changes in the concentrations and 15N enrichments of NH4 +, dissolved organic N (DON), NO3 - and the microbial N pool) and abiotic tracer consumption (i.e., fixation by clay and/or humic substances). We achieved full recovery of the 15N tracer in both soils over the course of the 48 h incubation. For the forest soil, we found no rapid consumption of the 15N tracer, and the majority of tracer (78%) remained unconsumed at the end of the incubation period. In contrast, the grassland soil showed rapid 15N-NH4 + consumption immediately after tracer addition, which was largely due to both abiotic fixation (24%) and biotic processes, largely uptake by soil microbes (10%) and nitrification (13%). We found no evidence for reflux of 15N-NH4 + over the 48 h incubation period in either soil. Our study therefore shows that 15N tracer reflux during IPD experiments is negligible for incubation times of up to 48 h, even when rapid NH4 + consumption occurs. Such experiments are thus robust to the assumption that immobilized labeled N is not re-mobilized during the experimental period and does not impact calculations of gross N mineralization.

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2.

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO2 (eCO2 ), and whether or not this constraint will become stronger over time. We explored the ecosystem-scale relationship between responses of plant productivity and N acquisition to eCO2 in free-air CO2 enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r(2) = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO2 caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO2 likely acquired less N than ambient CO2 -grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO2 , and this decrease was independent of the presence or magnitude of eCO2 -induced productivity enhancement, refuting the long-held hypothesis that this effect results from growth dilution. (iii) Effects of eCO2 on productivity and N acquisition did not diminish over time, while the typical eCO2 -induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO2 -induced terrestrial productivity enhancement is associated with negative effects of eCO2 on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.

Advances in 15N-tracing experiments: new labelling and data analysis approaches.

To obtain an in-depth understanding of soil nitrogen dynamics, it is necessary to quantify a variety of simultaneously occurring gross nitrogen transformation processes. In order to do so, most studies apply 15N in a disturbed soil-microbial-root system and quantify gross rates based on the principles of 15N isotope dilution. However, this approach has several shortcomings. First, studying disturbed soil provides only limited information on in situ soil nitrogen dynamics. Secondly, the analytical data analysis allows the quantification of total production and consumption rates of the labelled pool, but does not provide information on process-specific transformation rates. Combining in situ 15N isotope labelling over 1-2 weeks with numerical data analysis allows determining process-specific gross nitrogen transformations in undisturbed soils under field conditions in the presence of live roots and their associated microbial communities. This has the potential to increase our understanding of nitrogen dynamics in the soil environment.

Contrasting effects of wood ash application on microbial community structure, biomass and processes in drained forested peatlands.

The effects of wood ash application on soil microbial processes were investigated in three drained forested peatlands, which differed in nutrient status and time since application. Measured variables included the concentrations of soil elements and phospholipid fatty acids (PLFAs), net nitrogen (N) mineralization, nitrification and denitrification enzyme activity, potential methane (CH(4)) oxidation, CH(4) production and microbial respiration kinetics. Wood ash application had a considerable influence on soil element concentrations. This mirrored a decrease in the majority of the microbial biomarkers by more than one-third in the two oligotrophic peatlands, although the microbial community composition was not altered. The decreases in PLFAs coincided with reduced net ammonification and net N mineralization. Other measured variables did not change systematically as a result of wood ash application. No significant changes in microbial biomass or processes were found in the mesotrophic peatland, possibly because too little time (1 year) had elapsed since the wood ash application. This study suggests that oligotrophic peatlands can be substantially affected by wood ash for a period of at least 4 years after application. However, within 25 years of the wood ash application, the microbial biomass seemed to have recovered or adapted to enhanced element concentrations in the soil.