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Dissolved organic nitrogen (DON) is actively involved in N cycling and transformation processes in the ocean, yet how this large N pool is formed remains elusive. Here we incubated¹⁵N‐labeled individual amino acids (Ala, Val, and Phe) in seawater and monitored their fates over 21 days. About 25%–45% of Phe‐N and Val‐N were transformed to “uncharacterizable DON,” as compared to only about 6% for Ala‐N, indicating the formation of refractory DON is related to specific amino acids. Through a stable isotope probing approach, 5 Phe‐derived DON molecules from the incubations were found to be present in natural waters, and their possible structures were proposed. These results shed new lights on the formation mechanisms of refractory DON, including the roles of specific amino acids and particular chemical structures that may resist decomposition in millennial time scales.

Quantifying the fate of organic nitrogen in aquatic systems is important to improve understanding of its recycling efficiency and long‐term preservation. The fate of organic nitrogen can be investigated with¹⁵N labeling techniques, but relative amounts of¹⁵N in different chemical forms are difficult to quantify. We present a “streamlined” method by combining Ammonium Retention Time Shift‐High Performance Liquid Chromatography with zinc reduction, and UV oxidation. This method does not require a pre‐isolation step of different forms of nitrogen from the sample. At a sample volume of 50 mL, and a total N concentration in the range of 0.5–40 μmol N L⁻¹, and an¹⁵N atom% of 20–80%,¹⁵N concentrations for all N forms can be measured with this streamlined method, with a precision of within ±7%, and an accuracy of over 97%. We applied the method to investigating the short‐term fates of¹⁵N during the degradation of¹⁵N‐labeled amino acid and peptide. Recovery rates ranged from 93% to 110%, with an average of 102 ± 1.94%. As spiked¹⁵N labeled alanine and/or peptide (Ala‐Val‐Phe‐Val) disappeared during sample incubations, a large fraction (ca. 13–66%) of the¹⁵N was progressively transformed to non‐amino acid or non‐peptide dissolved organic nitrogen. This streamlined method offers quantitative estimates of potential fates of labile organic N compounds added to water samples containing in situ microbial consortia, and helps fulfill knowledge gaps in building the budget of N transformations of labile amino acids and peptides in aquatic systems.

In light of rapid shifts in biodiversity associated with human impacts, there is an urgent need to understand how changing patterns in biodiversity impact ecosystem function. Functional redundancy is hypothesized to promote ecological resilience and stability, as ecosystem function of communities with more redundant species (those that perform similar functions) should be buffered against the loss of individual species. While functional redundancy is being increasingly quantified, few studies have linked differences in redundancy across communities to ecological outcomes. We conducted a review and meta‐analysis to determine whether empirical evidence supports the asserted link between functional redundancy and ecosystem stability and resilience. We reviewed 423 research articles and assembled a data set of 32 studies from 15 articles across aquatic and terrestrial ecosystems. Overall, the mean correlation between functional redundancy and ecological stability/resilience was positive. The mean positive effect of functional redundancy was greater for studies in which redundancy was measured as species richness within functional groups (vs. metrics independent of species richness), but species richness itself was not correlated with effect size. The results of this meta‐analysis indicate that functional redundancy may positively affect community stability and resilience to disturbance, but more empirical work is needed including more experimental studies, partitioning of richness and redundancy effects, and links to ecosystem functions.