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1. Tree symbioses sustain nitrogen fixation despite excess nitrogen supply

   https://doi.org/10.1002/ecm.1562  

   Menge, Duncan N. L.; Wolf, Amelia A.; Funk, Jennifer L.; Perakis, Steven S.; Akana, Palani R.; Arkebauer, Rachel; Bytnerowicz, Thomas A.; Carreras Pereira, K. A.; Huddell, Alexandra M.; Kou‐Giesbrecht, Sian; et al (January 2023, Ecological Monographs)

   Abstract Symbiotic nitrogen fixation (SNF) is a key ecological process whose impact depends on the strategy of SNF regulation—the degree to which rates of SNF change in response to limitation by N versus other resources. SNF that is obligate or exhibits incomplete downregulation can result in excess N fixation, whereas a facultative SNF strategy does not. We hypothesized that tree‐based SNF strategies differed by latitude (tropical vs. temperate) and symbiotic type (actinorhizal vs. rhizobial). Specifically, we expected tropical rhizobial symbioses to display strongly facultative SNF as an explanation of their success in low‐latitude forests. In this study we used¹⁵N isotope dilution field experiments in New York, Oregon, and Hawaii to determine SNF strategies in six N‐fixing tree symbioses. Nitrogen fertilization with +10 and +15 g N m⁻² year⁻¹for 4–5 years alleviated N limitation in all taxa, paving the way to determine SNF strategies. Contrary to our hypothesis, all six of the symbioses we studied sustained SNF even at high N.Robinia pseudoacacia(temperate rhizobial) fixed 91% of its N (%N_dfa) in controls, compared to 64% and 59% in the +10 and +15 g N m⁻² year⁻¹treatments. ForAlnus rubra(temperate actinorhizal), %N_dfawas 95%, 70%, and 60%. For the tropical species, %N_dfawas 86%, 80%, and 82% forGliricidia sepium(rhizobial); 79%, 69%, and 67% forCasuarina equisetifolia(actinorhizal); 91%, 42%, and 67% forAcacia koa(rhizobial); and 60%, 51%, and 19% forMorella faya(actinorhizal). Fertilization with phosphorus did not stimulate tree growth or SNF. These results suggest that the latitudinal abundance distribution of N‐fixing trees is not caused by a shift in SNF strategy. They also help explain the excess N in many forests where N fixers are common. 

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2. Nitrogen-doped CVD diamond: Nitrogen concentration, color and internal stress

   https://doi.org/10.1016/j.diamond.2020.107794  

   Zaitsev, A.M.; Kazuchits, N.M.; Kazuchits, V.N.; Moe, K.S.; Rusetsky, M.S.; Korolik, O.V.; Kitajima, Kouki; Butler, J.E.; Wang, W. (May 2020, Diamond and Related Materials)

   null (Ed.)
3. Soil nitrogen fertilization reduces relative leaf nitrogen allocation to photosynthesis

   https://doi.org/10.1093/jxb/erad195  

   Waring, Elizabeth_F; Perkowski, Evan_A; Smith, Nicholas_G; Rogers, ed., Alistair (May 2023, Journal of Experimental Botany)

   Abstract The connection between soil nitrogen availability, leaf nitrogen, and photosynthetic capacity is not perfectly understood. Because these three components tend to be positively related over large spatial scales, some posit that soil nitrogen positively drives leaf nitrogen, which positively drives photosynthetic capacity. Alternatively, others posit that photosynthetic capacity is primarily driven by above-ground conditions. Here, we examined the physiological responses of a non-nitrogen-fixing plant (Gossypium hirsutum) and a nitrogen-fixing plant (Glycine max) in a fully factorial combination of light by soil nitrogen availability to help reconcile these competing hypotheses. Soil nitrogen stimulated leaf nitrogen in both species, but the relative proportion of leaf nitrogen used for photosynthetic processes was reduced under elevated soil nitrogen in all light availability treatments due to greater increases in leaf nitrogen content than chlorophyll and leaf biochemical process rates. Leaf nitrogen content and biochemical process rates in G. hirsutum were more responsive to changes in soil nitrogen than those in G. max, probably due to strong G. max investments in root nodulation under low soil nitrogen. Nonetheless, whole-plant growth was significantly enhanced by increased soil nitrogen in both species. Light availability consistently increased relative leaf nitrogen allocation to leaf photosynthesis and whole-plant growth, a pattern that was similar between species. These results suggest that the leaf nitrogen–photosynthesis relationship varies under different soil nitrogen levels and that these species preferentially allocated more nitrogen to plant growth and non-photosynthetic leaf processes, rather than photosynthesis, as soil nitrogen increased. 

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4. Does adopting a nitrogen best management practice reduce nitrogen fertilizer rates?

   https://doi.org/10.1007/s10460-021-10227-9  

   Houser, Matthew (June 2021, Agriculture and Human Values)

   null (Ed.)

   Technical best management practices are the dominant approach promoted to mitigate agriculture’s significant contributions to environmental degradation. Yet very few social science studies have examined how farmers actually use these practices. This study focuses on the outcomes of farmers’ technical best management practice adoption related to synthetic nitrogen fertilizer management in the context of Midwestern corn agriculture in the United States. Moving beyond predicting the adoption of nitrogen best management practices, I use structural equation modeling and data from a sample of over 2500 farmers to analyze how the number of growing season applications a farmer uses influences the rate at which synthetic nitrogen is applied at the field-level. I find that each additional application of N during the growing season is associated with an average increase of 2.4 kg/ha in farmers’ average N application rate. This result counters expectation for the outcome of this practice and may suggest that structural pressures are leading farmers to use additional growing season applications to ensure sufficiently high N rates, rather than allowing them to reduce rates. I conclude by discussing the implication of this study for future research and policy. 

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5. Loss of immiscible nitrogen from metallic melt explains Earth’s missing nitrogen

   https://doi.org/10.7185/geochemlet.1919  

   Liu, J.; Dorfman, S.M.; Lv, M.; Li, J.; Zhu, F.; Kono, Y. (July 2019, Geochemical Perspectives Letters)