The productivity of ecosystems and their capacity to support life depends on access to reactive nitrogen (N). Over the past century, humans have more than doubled the global supply of reactive N through industrial and agricultural activities. However, long-term records demonstrate that N availability is declining in many regions of the world. Reactive N inputs are not evenly distributed, and global changes—including elevated atmospheric carbon dioxide (CO ₂ ) levels and rising temperatures—are affecting ecosystem N supply relative to demand. Declining N availability is constraining primary productivity, contributing to lower leaf N concentrations, and reducing the quality of herbivore diets in many ecosystems. We outline the current state of knowledge about declining N availability and propose actions aimed at characterizing and responding to this emerging challenge.

The unified model of active galactic nuclei (AGNs) predicts silicate emission features at 10 and 18 μm in type 1 AGNs, and such features have now been observed in objects ranging from distant quasi-stellar objects to nearby LINERs. More surprising, however, is the detection of silicate emission in a few type 2 AGNs. By combining Gemini and Spitzer mid-infrared imaging and spectroscopy of NGC 2110, the closest known Seyfert 2 galaxy with silicate emission features, we can constrain the location of the silicate-emitting region to within 32 pc of the nucleus. This is the strongest constraint yet on the size of the silicate-emitting region in a Seyfert galaxy of any type. While this result is consistent with a narrow-line region origin for the emission, comparison with clumpy torus models demonstrates that emission from an edge-on torus can also explain the silicate emission features and 2-20 μm spectral energy distribution of this object. In many of the best-fitting models the torus has only a small number of clouds along the line of sight, and does not extend far above the equatorial plane. Extended silicate-emitting regions may well be present in AGNs, but this work establishes that emission from the torus itself is also a viable option for the origin of silicate emission features in active galaxies of both type 1 and type 2.

It has been widely argued that agroecological science, which originally developed as the application of ecological principles to agricultural systems, should engage with the social and political issues that affect production agriculture, and incorporate knowledge from a variety of sources. In this paper, we use techniques from network science and bibliometrics to evaluate the degree to which this transformation has taken place. By creating networks based on over 3,000 agroecology papers and the roughly 160,000 references they cite, we distinguish the sub-fields (“research fronts”) that made up agroecology in three time intervals: 1982–2004, 2005–2013, and 2014–2018. We also identify the main disciplines from which the research fronts in 2014–2018 drew their supporting knowledge. We suggest that, very broadly, themes in agroecological research include: Ecosystem services; (agro)biodiversity; approaches to agricultural intensification; tropical agroecosystems (particularly coffee); pest and weed management; organic agriculture; cropping systems; system transitions, modeling, and design; climate change adaptation; food sovereignty; education; and the nature and purpose of agroecology itself. Some research fronts mainly cite papers in natural science fields such as ecology, environmental science, agriculture, and entomology. However, others draw upon work in social science areas including development studies, environmental studies, and anthropology. The analysis presented in this paper demonstrates that agroecology has indeed evolved to possess many of the characteristics of an “ecology of [the entire] food system.” We anticipate that this work will also be of use to those wishing to gain an overview of the field or identify key papers, knowledge gaps, and potential collaborations.

In order to test the silicate-core/organic-mantle model of galactic interstellar dust, we have performed spectropolarimetry of the 3.4 μm C–H bond stretch that is characteristic of aliphatic hydrocarbons, using the nucleus of the Seyfert 2 galaxy, NGC 1068, as a bright, dusty background source. Polarization calculations show that if the grains in NGC 1068 had the properties assigned by the core-mantle model to dust in the Galactic diffuse interstellar medium (ISM), they would cause a detectable rise in polarization over the 3.4 μm feature. No such increase is observed. We discuss modifications to the basic core-mantle model, such as changes in grain size or the existence of additional nonhydrocarbon aligned grain populations, that could better fit the observational evidence. However, we emphasize that the absence of polarization over the 3.4 μm band in NGC 1068—and, indeed, in every line of sight examined to date—can be readily explained by a population of small, unaligned carbonaceous grains with no physical connection to the silicates.