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Nitrogen is an essential element for all life forms and undergoes various transformation processes mostly mediated by microbes along biogeochemical gradients in the water column and in sediments. Along estuarine gradients a variety of regulating factors such as salinity or nutrient concentrations change rapidly. These processes are summarized and the interactions among processes are presented. A focus lies on dissolved organic nitrogen as an important component for the nutrition of bacteria and phytoplankton. Moreover, organic nitrogen mediates the transport of nitrogen from primary producers into the microbial food web. Most studies summarized here focus on nitrogen in temperate estuaries, but we also present a summary of nitrogen processes in tropical mangrove ecosystems.

The study of nitrogen (N) cycling requires sensitive and accurate methods to measure various N compounds. This chapter reviews existing methods to analyze dissolved and particulate fractions of N with an emphasis on those methodologies that are most commonly used or represent the state of the art, noting that N analysis does indeed seem to be an art at times. Many techniques are temperamental at best and it is not uncommon to find small fetishes devoted to keeping a given mass spectrometer running. The first main section of the review describes techniques to measure the concentrations of N pools, followed by various approaches to chemically characterize organic N fractions. The last major section reviews techniques to measure N-fluxes, including uptake, release, nitrification, denitrification, and N2 fixation. It largely provides the perspective of water column studies; most of the methods, however, could also be used more broadly to analyze sediments, particles, or pore water samples. The difference between a given method for these sample types is principally one of the detection limit required and matrix specific analytical challenges. Analyzing water column samples can be difficult because concentrations can be very low. While concentrations are generally much higher in sediment pore waters, extracting the necessary volume of pore water can be challenging. With respect to rate measurements, most of the techniques outlined could be applied in both the water column and the sediment, and several largely sedimentary rate measurements are specifically discussed below. Tracer approaches in sediments can be especially problematic because the substrate is so heterogeneous that getting uniform mixing of the tracer can be impossible.

Publisher Summary This chapter reviews the forms and sources of regenerated nitrogen (N), both inorganic and organic, to describe the various processes that result in N regeneration and to survey N regeneration rates in estuarine, coastal, and open ocean systems. It specifically focuses on water column N regeneration. Sediments are discussed from the perspective that they serve as a source of N to the water column. Nitrogen is regenerated in both inorganic and organic forms. The inorganic forms include ammonium (NH 4+) nitrate (NO3–), and nitrite (NO2– ). Organic forms include urea, dissolved free amino acids nucleic acids, and a broad suite of dissolved combined amino acids and proteins, as well as other largely unidentified components of the dissolved organic nitrogen pool including enzymes. To measure regeneration rates bioassays or 15N tracer methods are used. With the bioassay method, the accumulation of a compound is measured over time. Bioassays have the advantage that they are relatively easy to perform. However, they can only provide net release rates, will only produce a rate at all if the release rate is greater than any uptake rates, and can suffer from bottle affects if long incubations are needed to see a signal. This discussion concludes with a brief survey of methods and recommendations for future research.

14 páginas, 8 tablas, 1 figura.-- Jonathan H. Sharp ... et al., Routine determination of dissolved organic nitrogen (DON) is performed in numerous laboratories around the world using one of three families of methods: UV oxidation (UV), persulfate oxidation (PO), or high temperature combustion (HTC). Essentially all routine methods measure total dissolved nitrogen (TDN) and calculate DON by subtracting the dissolved inorganic nitrogen (DIN). While there is currently no strong suggestion that any of these methods is inadequate, there are continuing suspicions of slight inaccuracy by UV methods. This is a report of a broad community methods comparison where 29 sets (7 UV, 13 PO, and 9 HTC) of TDN analyses were performed on five samples with varying TDN and DIN concentrations. Analyses were done in a “blind” procedure with results sent to the first author. With editing out one set of extreme outliers (representing 5 out of 145 ampoules analyzed), the community comparability for analyzing the TDN samples was in the 8–28% range (coefficient of variation representing one standard deviation for the five individual samples by 28 analyses). When DIN concentrations were subtracted uniformly (single DIN value for each sample), the comparability was obviously worse (19–46% cv). This comparison represents a larger and more diverse set of analyses, but the overall comparability is only marginally better than that of the Seattle workshop of a decade ago. Grouping methods, little difference was seen other than inconclusive evidence that the UV methods gave TDN values for several of the samples higher than HTC methods. Since there was much scatter for each of the groups of methods and for all analyses when grouped, it is thought that more uniformity in procedures is probably needed. An important unplanned observation is that variability in DIN analyses (used in determining the final analyte in most UV and PO methods) is essentially as large as the variability in the TDN analyses. This exercise should not be viewed as a qualification exercise for the analysts, but should instead be considered a broad preliminary test of the comparison of the families of methods being used in various laboratories around the world. Based on many independent analyses here, none of the routinely used methods appears to be grossly inaccurate, thus, most routine TDN analyses being reported in the literature are apparently accurate. However, it is not reassuring that the ability of the international community to determine DON in deep oceanic waters continues to be poor. It is suggested that as an outgrowth of this paper, analysts using UV and PO methods experiment and look more carefully at the completeness of DIN conversion to the final analyte and also at the accuracy of their analysis of the final analyte. HTC methods appear to be relatively easy and convenient and have potential for routine adoption. Several of the authors of this paper are currently working together on an interlaboratory comparison on HTC methodology., This research was partially supported by NSF grants OCE 96-33296 and OCE 00-82238 to JHS

This chapter describes the dynamics of dissolved organic nitrogen (DON). DON is that subset of the dissolved organic matter (DOM) pool that contains nitrogen (N). From the perspective of a microorganism, this is where the action is—one-stop shopping for N, carbon (C), and energy. Research in DON, however, has lagged far behind that of the larger dissolved organic carbon (DOC) pool. This situation is primarily the result of the substantial analytical challenges inherent in DON research: (1) DON exists in substantially lower concentrations than DOC, (2) multiple chemical analyses are required for a single DON measurement, (3) inorganic N removal is a nightmarish undertaking, and (4) unless one has an easy access to a nuclear reactor manufacturing short-lived 13 N, he or she must be content with labor-intensive stable isotopes rather than the quicker and more sensitive radiotracers. Measurements of DON concentrations have become a routine component of many studies. Nonetheless, because of space limitations DON concentrations in lakes, streams, or groundwater—with some exceptions—are not included in studies.