We studied the nitrogen biogeochemistry of the ice‐covered eastern Bering Sea shelf using the isotope ratios (15N/14N and 18O/16O) of NO3− and other N species. The 15N/14N of late winter NO3− on the shelf decreases inshore and is inversely correlated with bottom water [NH4+], consistent with an input of low‐15N/14N NO3− from partial nitrification of NH4+ remineralized from the sediments. An inshore 15N/14N increase in total dissolved N (TDN) suggests that (1) the sediment‐derived NH4+ is elevated in 15N due to the same partial nitrification that yields the low‐15N/14N NO3−, and (2) 15N‐deplete NO3− from partial nitrification within the sediments is denitrified to N2. The proportion of newly nitrified NO3− on the shelf, evidenced by an inshore decrease in NO3− 18O/16O, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO3− into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the 15N/14N of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO3− isotope data indicating that the fixed N transported through Bering Strait has a 15N/14N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6–8 ‰, is at odds with previous assumptions regarding the global ocean's N isotope budget.

Within the invasion ecology literature, it is often noted that abiotically stressful environments are typically less invaded by non-native plants than nearby less-stressful environments. However, until now no one had collected and summarized examples of this pattern. This paper first compiles evidence that plant communities in many harsh habitats are less invaded, and then synthesizes possible explanations for this pattern. We discuss that harsh sites may be less invaded because, compared to moderate sites, they may receive lower propagule pressure, particularly from well-suited plants, and because their abiotic and biotic characteristics may make them inherently more resistant to invasion.

No abstract

R������. Los bosques de Polylepis, endémicos de las montañas de Sudamérica, suelen estar asociados con sitios relativamente inaccesibles como quebradas, roquedales o laderas empinadas. Las hipótesis más discutidas que explican esta asociación son: 1) factores abióticos como la humedad, la temperatura o el viento, ó 2) factores antropogénicos como la protección del ramoneo por ganado doméstico, la tala o los fuegos iniciados por el ser humano. Para resumir el estado de avance en el conocimiento, realizamos una revisión cuantitativa sobre los estudios de ecología y conservación de bosques de Polylepis a lo largo de toda su distribución. Identificamos 139 publicaciones relevantes, de las que el 36% estuvo concentrado en la Argentina. Las publicaciones incluyen 20 de las ~27 especies existentes, de las cuales las más estudiadas son P. australis y P. tarapacana, con 45 y 21 publicaciones, respectivamente. En el otro extremo, hay 7 especies no representadas en publicaciones. Las disciplinas más populares fueron la ecología de poblaciones (35%) y los estudios sobre biodiversidad y comunidades (18%). Muy pocos estudios se repiten de una región a la otra. Los estudios que intentan comprender la contribución relativa de factores abióticos vs. antropogénicos para explicar la asociación de los bosques con lugares relativamente inaccesibles señalan una contribución mayor de los factores antrópicos, aunque todos están concentrados en el extremo sur de la distribución de Polylepis en la Argentina. Sugerimos áreas de vacancia en estudios y posibles mejoras en la producción de nuevas publicaciones y en la red de interacciones entre investigadores interesados en los ecosistemas de Polylepis.

Ground-truthing the stable isotope ratio of diatom frustule-bound organic nitrogen (N) as a paleoceanographic proxy of phytoplankton nutrient consumption calls for studies of modern diatoms from cultures and the field. This work has been hindered by the lack of a method to prepare fresh diatom material, which has significant geochemical differences from sedimentary fossil diatom, for analysis. We tested and compared the performance of five oxidative treatment protocols in cleaning fresh diatom frustules of external organic material for isotopic analysis of diatom frustule-bound N. We applied each treatment to diatom material from a T. weissflogii culture and measured the diatom-bound N content and isotopic composition (d 15 N) of samples. A favored treatment is described that includes a multi-step, progressive, oxidative cleaning. The results suggest that the application of H 2 O 2 to fresh biogenic opal for more than 20 min increased the accessibility of the N within the opal matrix to subsequent chemical treatment, an effect not observed for diatom frustules from deep ocean sediments. We attribute this effect to structural alteration and dissolution of the diatom opal by the H 2 O 2 , consistent with previous observations regarding the high reactivity of fresh diatom opal and its subsequent chemical hardening in the sedimentary environment. In addition, the presence of high levels of organic matter upon treatment with H 2 O 2 may exacerbate the chemical alteration of frustules. We find that the d 15 N of diatom-bound N is ~3 ‰ higher than that of the bulk culture biomass, consistent with expectations from surface sediment and down-core measurements.