Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity— have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of ¹⁵N to ¹⁴N, termed δ¹⁵N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ¹⁵N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ¹⁵N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ¹⁵N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ¹⁵N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions

Foliar δ15N values characterize soil N cycling and reflect nitrate or ammonium preference of plants along a temperate grassland gradient

The natural abundance of stable 15N isotopes in soils and plants is potentially a simple tool to assess ecosystem N dynamics. Several open questions remain, however, in particular regarding the mechanisms driving the variability of foliar δ15N values of non-N2 fixing plants within and across ecosystems. The goal of the work presented here was therefore to: (1) characterize the relationship between soil net mineralization and variability of foliar Δδ15N (δ15Nleaf − δ15Nsoil) values from 20 different plant species within and across 18 grassland sites; (2) to determine in situ if a plant’s preference for NO3− or NH4+ uptake explains variability in foliar Δδ15N among different plant species within an ecosystem; and (3) test if variability in foliar Δδ15N among species or functional group is consistent across 18 grassland sites. Δδ15N values of the 20 different plant species were positively related to soil net mineralization rates across the 18 sites. We found that within a site, foliar Δδ15N values increased with the species’ NO3− to NH4+ uptake ratios. Interestingly, the slope of this relationship differed in direction from previously published studies. Finally, the variability in foliar Δδ15N values among species was not consistent across 18 grassland sites but was significantly influenced by N mineralization rates and the abundance of a particular species in a site. Our findings improve the mechanistic understanding of the commonly observed variability in foliar Δδ15N among different plant species. In particular we were able to show that within a site, foliar δ15N values nicely reflect a plant’s N source but that the direction of the relationship between NO3− to NH4+ uptake and foliar Δδ15N values is not universal. Using a large set of data, our study highlights that foliar Δδ15N values are valuable tools to assess plant N uptake patterns and to characterize the soil N cycle across different ecosystems

Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

Background and aims The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δ15N). Methods Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δ15N. Results All foliar δ15N values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δ15N of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δ15N values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δ15N in the two forest types suggesting a changing uptake ratio of different N forms between forest types. Conclusions These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δ15N values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δ15N between the two forest types

The Causal Cascade to Multiple Sclerosis: A Model for MS Pathogenesis

BACKGROUND: MS pathogenesis seems to involve both genetic susceptibility and environmental risk factors. Three sequential factors are implicated in the environmental risk. The first acts near birth, the second acts during childhood, and the third acts long thereafter. Two candidate factors (vitamin D deficiency and Epstein-Barr viral infection) seem well suited to the first two environmental events. METHODOLOGY/PRINCIPAL FINDINGS: A mathematical Model for MS pathogenesis is developed, incorporating these environmental and genetic factors into a causal scheme that can explain some of the recent changes in MS-epidemiology (e.g., increasing disease prevalence, a changing sex-ratio, and regional variations in monozygotic twin concordance rates). CONCLUSIONS/SIGNIFICANCE: This Model suggests that genetic susceptibility is overwhelmingly the most important determinant of MS pathogenesis. Indeed, over 99% of individuals seem genetically incapable of developing MS, regardless of what environmental exposures they experience. Nevertheless, the contribution of specific genes to MS-susceptibility seems only modest. Thus, despite HLA DRB1*1501 being the most consistently identified genetic marker of MS-susceptibility (being present in over 50% of northern MS patient populations), only about 1% of individuals with this allele are even genetically susceptible to getting MS. Moreover, because genetic susceptibility seems so similar throughout North America and Europe, environmental differences principally determine the regional variations in disease characteristics. Additionally, despite 75% of MS-patients being women, men are 60% more likely to be genetically-susceptible than women. Also, men develop MS at lower levels of environmental exposure than women. Nevertheless, women are more responsive to the recent changes in environmental-exposure (whatever these have been). This explains both the changing sex-ratio and the increasing disease prevalence (which has increased by a minimum of 32% in Canada over the past 35 years). As noted, environmental risk seems to result from three sequential components of environmental exposure. The potential importance of this Model for MS pathogenesis is that, if correct, a therapeutic strategy, designed to interrupt one or more of these sequential factors, has the potential to markedly reduce or eliminate disease prevalence in the future

Customer orientation: The interactive effect of role clarity and learning goal orientation

Purpose Customer orientation of service employees relates to customer satisfaction and loyalty, sales growth and business performance. Drawing from conservation of resources (COR) theory, the aim of this study was to test the interactive effects of service employees' role clarity and learning goal orientation on customer orientation. Specifically, it was hypothesized that even under conditions of low role clarity, service employees with high learning goal orientation would maintain a high level of customer orientation. Design/methodology/approach Participants were 323 employees of 4- and 5-star hotels in Singapore. Using questionnaires, they reported their role clarity, learning goal orientation and customer orientation. For hypothesis testing, moderated regression analysis was performed. Findings Role clarity and learning goal orientation were significantly related to customer orientation, and in support of the hypothesis, the interaction effect of role clarity and learning goal orientation was also significant. With high role clarity, all employees showed high customer orientation. But with low role clarity, only employees with high learning goal orientation demonstrated high customer orientation. Practical implications The recommendations from this study are to include learning goal orientation as a selection criterion for service employees and to clearly define the roles of existing service employees, especially for those with low learning goal orientation. Originality/value The originality and value of this study lies in highlighting the importance of learning goal orientation especially under conditions of low role clarity.falsePurpose Customer orientation of service employees relates to customer satisfaction and loyalty, sales growth and business performance. Drawing from conservation of resources (COR) theory, the aim of this study was to test the interactive effects of service employees' role clarity and learning goal orientation on customer orientation. Specifically, it was hypothesized that even under conditions of low role clarity, service employees with high learning goal orientation would maintain a high level of customer orientation. Design/methodology/approach Participants were 323 employees of 4- and 5-star hotels in Singapore. Using questionnaires, they reported their role clarity, learning goal orientation and customer orientation. For hypothesis testing, moderated regression analysis was performed. Findings Role clarity and learning goal orientation were significantly related to customer orientation, and in support of the hypothesis, the interaction effect of role clarity and learning goal orientation was also significant. With high role clarity, all employees showed high customer orientation. But with low role clarity, only employees with high learning goal orientation demonstrated high customer orientation. Practical implications The recommendations from this study are to include learning goal orientation as a selection criterion for service employees and to clearly define the roles of existing service employees, especially for those with low learning goal orientation. Originality/value The originality and value of this study lies in highlighting the importance of learning goal orientation especially under conditions of low role clarity