Dissolved Organic Nitrogen Concentrations Research Articles

Fate and Transport of Organic Nitrogen in Minimally Disturbed Montane Streams of Colorado, USA

In two montane watersheds that receive minimal deposition of atmospheric nitrogen, 15–71% of dissolved organic nitrogen (DON) was bioavailable in stream water over a 2-year period. Discharge-weighted concentrations of bulk DON were between 102 and 135 μg/l, and the C:N ratio differed substantially between humic and non-humic fractions of DON. Approximately 70% of DON export occurred during snowmelt, and 40% of that DON was biologically available to microbes in stream sediments. Concentrations of bioavailable DON in stream water were 2–16 times greater than dissolved inorganic nitrogen (DIN) during the growing season, and bioavailable DON was depleted within 2–14 days during experimental incubations. Uptake of DON was influenced by the concentration of inorganic N in stream water, the concentration of non-humic DON in stream water, and the C:N ratio of the non-humic fraction of dissolved organic matter (DOM). Uptake of DON declined logarithmically as the concentration of inorganic N in stream water increased. Experimental additions of inorganic N also caused a decline in uptake of DON and net production of DON when the C:N ratio of non-humic DOM was high. This study indicates that the relative and absolute amount of bioavailable DON can vary greatly within and across years due to interactions between the availability of inorganic nutrients and composition of DOM. DOM has the potential to be used biotically at a high rate in nitrogen-poor streams, and it may be generated by heterotrophic microbes when DIN and labile DOM with low relative nitrogen content become abundant.

Dissolved organic carbon and nitrogen in precipitation, throughfall and stemflow fromSchima superba andCunninghamia lanceolata plantations in subtropical China

Despite growing attention to the role of dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) in forest nutrient cycling, their monthly concentration dynamics in forest ecosystems, especially in subtropical forests only were little known. The goal of this study is to measure the concentrations and monthly dynamics of DOC and DON in precipitation, throughfall and stemflow for two plantations ofSchima superba (SS) and Chinese fir (Cunninghamia lanceolata, CF) in Jianou, Fujian, China. Samples of precipitation, throughfall and stemflow were collected on a rain event base from January 2002 to December 2002. Upon collection, all water samples were analyzed for DOC, NO3 −−N, NH4 +−N and total dissolved N (TDN). DON was calculated by subtracting NO3 −−N and NH4 +−N from TDN. The results showed that the precipitation had a mean DOC concentration of 1.7 mg·L−1 and DON concentration of 0.13 mg·L−1. The mean DOC and DON concentrations in throughfall were 11.2 and 0.24 mg·L−1 in the SS and 10.3 and 0.19 mg·L−1 in the CF respectively. Stemflow DOC and DON concentrations in the CF (19.1 and 0.66 mg·L−1 respectively) were significantly higher than those in the SS (17.6 and 0.48 mg·L−1 respectively). No clear monthly variation in precipitation DOC concentration was found in our study, while DON concentration in precipitation tended to be higher in summer or autumn. The monthly variations of DON concentrations were very similar in throughfall and stemflow at both forests, showing an increase at the beginning of the rainy season in March. In contrast, monthly changes of the DOC concentrations in throughfall of the SS and CF were different to those in stemflow. Throughfall DOC concentrations were higher from February to April, while relatively higher DOC concentrations in stemflow were found during September–November period.

Spring distribution of dissolved organic matter in a system encompassing the Northeast Water Polynya: Implications for early-season sources and sinks

This study addresses sources and diagenetic state of early-season dissolved organic matter (DOM) in the Northeast Water Polynya (NEWP) area northeast of Greenland from distributions of humic substance fluorescence (HSfl), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) in the water column inside and outside the NEWP area. The water masses of the polynya area had acquired their spring/summer temperature–salinity characteristics at the time of sampling, and also had individual, different DOM signatures. DOC concentrations were variable within and among water masses in the polynya area, indicating patchy local sources and sinks of DOC. PySW and polynya intermediate water (PyIW) had higher average DON concentrations and average lower C:N ratios than polynya bottom water (PyBW), indicating a larger fraction of fresh DOM in PySW and PyIW than in PyBW. Ice-covered, polynya area surface waters (PySW) had higher DOC concentrations (113±14 μM, n=68) than surface water (SW) outside the polynya area (96±18 μM, n=6). The DOM C:N ratios in a low-salinity, ice-melt subgroup of PySW samples indicate labile material, and these low-salinity surface waters appeared to have a local DOC and DON source. In contrast, HSfl was significantly lower inside than outside the NEWP area. Despite the lower HSfl values within the NEWP area, the PySW values were high when compared to open-ocean water. There were no local terrestrial sources for HSfl to the NEWP area and the East Greenland Current is therefore proposed as a likely source of allochtonous HSfl. When HSfl was used as a conservative tracer, up to ∼70% of the water in PySW and PyIW was found to be derived from SW, which contains a high fraction of water from the East Greenland Current. Similarly, a mixing model based on HSfl indicated that ∼80% of early-season DOC and 90–100% of early-season DON in PySW and PyIW were derived from SW, indicating a potentially high fraction of terrestrially-derived, relatively refractory DOM in the early-season NEWP area.

Effect of ectomycorrhizal fungi on nitrogen mineralisation and the growth of Scots pine seedlings in natural peat

The aim of this study was to investigate the potential of different isolates of ectomycorrhizal (EM) fungi to enhance the growth of Pinus sylvestris seedlings in five natural peat substrates with different nitrogen concentrations, and the effect of the Scots pine seedlings and fungal inoculum on the formation of dissolved inorganic and organic nitrogen in peat. Utilization of different organic nitrogen compounds by microbial community in the peat was also investigated using Biolog MT MicroPlates. Inoculation of the seedlings with EM fungi enhanced seedling growth. Piloderma croceum increased root growth especially, whereas Lactarius rufus increased needle growth and Suillus variegatus I, II and III improved both root and needle growth. All the EM fungi also significantly affected stem growth. Nitrogen concentration of the peat did not affect seedling growth as much as the EM fungi. At the lowest peat N concentration (1.17%) NH 4 + mineralisation was lower and DON (dissolved organic nitrogen) accumulation higher than at higher peat N concentrations. The EM fungal isolates had different effects on NH 4 + and DON accumulation/degradation in peat. The EM fungal isolates significantly increased NH 4 + formation in the peat, whereas L. rufus and P. croceum had an opposite effect on DON accumulation. S. variegatus I significantly decreased the DON concentrations during peat incubation. The N concentration of the peat slightly affected the utilization of amino acids and polyamines by the microbial community, whereas inoculation with S. variegatus I, II or III had no effect.

Dissolved Organic Nitrogen Measurement Using Dialysis Pretreatment

Dissolved organic nitrogen (DON) is important for ecological and engineering researches. Quantification of low DON concentrations in waters with elevated dissolved inorganic nitrogen (DIN) using existing methods is inaccurate. In this study, a dialysis-based pretreatment technique was optimized and adopted to reduce the interference from DIN to the quantification of DON in natural water. A cellulose ester dialysis tube (nominal molecular weight cutoff = 100 Da) was used in batch and continuous-flow dialysis steps with model compounds, natural organic matter isolates, and bulk waters to develop a dialysis pretreatment approach that selectively reduces DIN from solutions containing DON. By reducing DIN concentrations, propagation of analytical variance in total dissolved nitrogen (TDN) and DIN species concentrations allows more accurate determination of DON (DON = TDN - NO3 - NO2- - NH3/NH4+). Dialysis for 24 h against continuously flowing distilled water reduced DIN species by 70%. With dialysis pretreatment, DON recoveries of more than 95% were obtained for surface water and finished drinking water, but wastewater experienced a slight loss (approximately 10%) of DON possibly due to the adsorption of organics onto the dialysis membrane, permeation of low molecular weight fractions, or biodegradation. Dialysis experiments using surface water spiked with different DIN/TDN ratios concluded that dialysis pretreatment leads to more accurate DON determination than no dialysis when DIN/TDN ratios exceed 0.6 mg of N/mg of N.

Soil acidification used as a management strategy to reduce nitrate losses from agricultural land

pH is known to be a primary regulator of nutrient cycling in soil. Increasing soil acidity in agricultural systems has the potential to slow down N cycling and reduce N losses from leaching thereby enhancing sustainability and reducing pollution. We conducted a field experiment to investigate the impact of acidity on N leaching in arable and grassland agricultural systems. The results showed that nitrate (NO 3 −) concentrations in soil water were greater under arable than under grassland. Soil acidification significantly lowered NO 3 − concentrations in soil water over winter and spring under grassland, whilst in cereal plots a similar effect was only observed in spring. Our results suggest that soil acidification decreased nitrification causing an accumulation of NH 4 + which was not subject to leaching. Dissolved organic nitrogen (DON) concentrations in soil water were significantly greater under arable than grassland. Soil acidification lowered concentrations of DON in soil water, usually to a greater extent in grassland than in arable plots. It was concluded that it may be possible to use careful soil pH management as a tool to control NO 3 − leaching without compromising the quality of drainage water, and that this may be more effective on grassland than on arable crops.

Dissolved organic nitrogen dominates in European bogs under increasing atmospheric N deposition

To assess the effects of increased atmospheric N input on N availability in ombrotrophic peatlands, the relative concentrations of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) were measured in bog waters along a natural gradient of atmospheric N deposition. Six European bogs were selected, spanning a range of chronic atmospheric N inputs from 0.2 to 2.0 g m−2 yr−1. DIN as well as DON concentrations increased with N deposition, the latter increasing at a sharper incline. The increase in DIN concentrations was related to the reduced capacity of the moss layer to trap atmospheric N, which in turn was a result of N saturation of the moss layer. The enhanced DON concentrations appear to be a consequence of increased leaching of organic N compounds by Sphagnum. The importance of DON on N biogeochemistry in bogs opens new perspectives in relation to nutrient limitation and organic matter turnover.

Dissolved organic carbon, nitrogen and phosphorus in the N-E Atlantic and the N-W Mediterranean with particular reference to non-refractory fractions and degradation

Dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) measured in deep profiles in the N-E Atlantic and in the N-W Mediterranean in the period 1984–2002 are described. After accurate validation, they show close agreement with those previously published. Classic profiles were obtained, with concentrations decreasing in deep waters. In the Mediterranean and in the Atlantic comparable concentrations were found in the 1500–2000 m waters, 44–46 μmol l −1 DOC, 2.6–2.8 μmol l −1 DON and 0.02–0.03 μmol l −1 DOP. In the surface layers, DOC concentrations were higher, but DON and DOP concentrations lower, in the Mediterranean than in the Atlantic, leading to higher element ratios in the Mediterranean. In autumn, values were, respectively, DOC:DON ∼17 vs. ∼14, DOC:DOP ∼950 vs. 500 and DON:DOP ∼55 vs. 35. The data suggest an increase in DOC and DON in the North Atlantic Central Water over 15 years, which may be linked to the North Atlantic climatic oscillations. Refractory DOM found in the 1500–2000 m layer exhibited C:N:P ratios of 1570:100:1. The labile+semi-labile (=non-refractory) DOM (nrDOM) pool was computed as DOM in excess of the refractory pool. Its contribution to total DOM above the thermocline in the open sea amounted to 25–35% of DOC, 30–35% of DON, and 60–80% of DOP. Element ratios of the nrDOM varied among stations and were lower than those of refractory DOM, except for C:N in the Mediterranean: nrDOC:nrDON ∼10–19, nrDOC:nrDOP ∼160–530 and nrDON:nrDOP ∼15–38. The specific stoichiometry of DOM in the Mediterranean led us to postulate that overconsumption of carbon is probably a main process in that oligotrophic sea. By coupling non-refractory DOM stoichiometry and relationships between the main DOM elements in the water column, the relative mineralization of C, N and P from DOM was studied. Below the thermocline, the preferential removal of phosphorus with regard to carbon from the semi-labile DOM can be confirmed, but not the preferential removal of nitrogen. In the ocean surface layers, processes depend on the oceanic area and can differ from deep waters, so preferential carbon removal seems more frequent. Bacterial growth efficiency data indicate that bacteria are directly responsible for mineralization of a high proportion of DON and DOP in the deep water.

Distributions, speciation and stable isotope composition of organic matter in the southeastern Bering Sea

Seawater samples were collected from the southeastern Bering Sea for determinations of dissolved (DOC, <0.7 μm), colloidal (COC, 1 kDa–0.7 μm) and particulate organic carbon (POC, >0.7 μm), dissolved organic nitrogen (DON), particulate nitrogen (PN), and dissolved carbohydrate (DCHO) species, as well as stable isotopic composition ( δ 13C and δ 15N) of suspended particulate organic matter (POM). DOC concentrations ranged from 61 to 92 μM (average 76±9 μM) with increasing concentrations from outer shelf to inner shelf along 166°W. DOC concentrations were significantly inversely correlated with salinity, indicating that the DOC distribution was predominantly controlled by water mixing although primary production is the ultimate source of organic matter. DON concentrations averaged 6.4±0.9 μM, ranging from 4.3 to 8.4 μM, resulting in an average DOC/DON ratio of 12±2 (ranging from 8 to 15). Total dissolved carbohydrates (TDCHO) comprised 13±3% of bulk DOC, with 35±15% of TDCHO in the polysaccharide and 65±15% in the monosaccharide fractions. Suspended POM had an average C/N ratio of 7.4±1.1, significantly lower than the average DOC/DON ratio (12±2). There was a significant correlation between chlorophyll- a and POC concentrations. However, Chl- a concentration was not closely coupled to DOC abundance. On average, POC comprised 14±5% of the total organic carbon (TOC) pool in the water column, and COC comprised 34±8% of the TOC, leaving 52±5% of the TOC in the <1 kDa low-molecular-weight dissolved phase. Values of δ 13C and δ 15N in the suspended POM ranged from −27.1‰ to −23.4‰ (average −24.5±0.9‰) and from 2.8‰ to 9.9‰ (average 6.0±1.9‰), respectively. Interestingly, particulate δ 15N values were negatively correlated with nitrate (NO 3) concentrations at low NO 3 concentrations (≤5 μM) but positively correlated with NO 3 concentrations when NO 3 concentrations were >5 μM, indicating the effects of nutrient drawdown in surface waters and diagenesis below the euphotic zone on the isotope fractionation. POM containing lighter C and N isotopes appeared to be decomposed preferentially during their transport from upper water column to bottom sediments.

Spatial and temporal trends in nutrient concentrations in the Belgian Continental area of the North Sea during the period 1993–2000

Statistical analysis of nutrient data obtained during 8 years of monitoring indicated strong seasonal and spatial variability, with highest concentrations in winter and significantly higher concentrations (on average >2 times higher) in the salinity zone S < 33 than in the salinity zone S ≥ 33. In the North Sea ammonium concentrations significantly decreased from 1995 on (47% in zone S < 33 and 64% in zone S ≥ 33), while for nitrate no significant decrease was observed in the zone S ≥ 33. Despite a drastic reduction (50%) in the riverine inputs of phosphorus to the North Sea during the last decade, phosphate concentrations decreased only slightly in the zone S < 33 (<20%). This observation was somewhat similar to what happened with nitrate. Phosphate concentrations were generally low (mean: 0.75 μM; maximum: 2.4 μM) compared to the other dissolved nutrients. Mean nitrate concentrations were about 18 μM but some extreme concentrations, up to 140 μM were observed. Finally, the dissolved organic nitrogen (DON) content represented a major fraction of the total dissolved nitrogen (TDN) (median of 72% for all data), especially in the zone S ≥ 33, but no significant spatial differences in the DON concentrations were found in the Belgian area of the North Sea.

Role of wetlands and developed land use on dissolved organic nitrogen concentrations and DON/TDN in northeastern U.S. rivers and streams

Previous studies have shown that watersheds with significant human development (i.e., urban and agricultural land use) generally have higher concentrations and fluxes of dissolved inorganic nitrogen (DIN) in comparison to less‐developed or forested watersheds. However, the impact of watershed development on dissolved organic nitrogen (DON) concentrations in drainage waters has received little attention. We present data from 39 watersheds in Massachusetts (Ipswich River watershed) encompassing a gradient of developed land use (0%–92% urban plus agriculture) and wetland abundance (0%–32%) to assess controls on mean annual DON concentrations and DON/total dissolved nitrogen (TDN) in drainage waters. In addition, we compiled published data from 119 northeastern U.S. watersheds to evaluate broader‐scale relationships between DON, developed land use, and wetlands. The percentage of developed land is a poor predictor of DON concentrations in the Ipswich watersheds (r2 = 0.09) and the compiled dataset (r2 = 0.27). In contrast, wetland percentage explains 56% of the variability in DON concentrations in the Ipswich watersheds, and 60% when all literature data are included. Excluding watersheds with direct wastewater inputs to surface waters improves the regional relationship significantly (r2 = 0.79). The DON:TDN ratio is best explained by a multiple regression of wetland percentage and developed land use percentage for both the Ipswich watersheds (r2 = 0.73) and the compiled dataset (r2 = 0.50). Watersheds with abundant wetlands may therefore have high DON concentrations and DON:TDN ratios despite elevated anthropogenic nitrogen inputs associated with human development.

Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution

Increased atmospheric deposition of N to forests is an issue of global concern, with largely undocumented long-term effects on soil solution chemistry. In contrast to bulk soil properties, which are typically slow to respond to a chronic stress, soil solution chemistry may provide an early indication of the long-term changes in soils associated with a chronic stress. At the Harvard Forest, soil solution was collected beneath the forest floor in zero tension lysimeters for 10 years (1993–2002) as part of an N saturation experiment. The experiment was begun in 1988 with 5 or 15 g N m −2 per year added to hardwood and pine forest plots, and our samples thus characterize the long-term response to N fertilization. Samples were routinely analyzed for inorganic nitrogen, dissolved organic nitrogen (DON), and dissolved organic carbon (DOC); selected samples were also analyzed to determine qualitative changes in the composition of dissolved organic matter. Fluxes of DOC, DON, and inorganic N were calculated based on modeled water loss from the forest floor and observed concentrations in lysimeter samples. The concentration and flux of inorganic N lost from the forest floor in percolating soil solution are strongly affected by N fertilization and have not shown any consistent trends over time. On average, inorganic N fluxes have reached or exceeded the level of fertilizer application in most plots. Concentrations of DOC were unchanged by N fertilization in both the hardwood and pine stands, with long-term seasonal averages ranging from 31–57 mg l −1 (hardwood) and 36–93 mg l −1 (pine). Annual fluxes of DOC ranged from 30–50 g m −2 per year. DON concentrations more than doubled, resulting in a shift toward N-rich organic matter in soil solution percolating from the plots, and DON fluxes of 1–3 g m −2 per year. The DOC:DON ratio of soil solution under high N application (10–20) was about half that of controls. The organic chemistry of soil solution undergoes large qualitative changes in response to N addition. With N saturation, there is proportionally more hydrophilic material in the total DON pool, and a lower C:N ratio in the hydrophobic fraction of the total DOM pool. Overall, our data show that fundamental changes in the chemistry of forest floor solution have occurred in response to N fertilization prior to initiation of our sampling. During the decade of this study (years 5–14 of N application) both inorganic N and dissolved organic matter concentrations have changed little despite the significant biotic changes that have accompanied N saturation.

Ammonium, nitrate and dissolved organic nitrogen in seepage water as affected by compost amendment to European beech, Norway spruce, and Scots pine forests

Fertilization of nutrient-depleted and degraded forest soils may be required to sustain utilization of forests. In some European countries, the application of composts may now be an alternative to the application of inorganic fertilizers because commercial compost production has increased and compost quality has been improved. There is, however, concern that compost amendments may cause increased leaching of nitrogen, trace metals and toxic organic compounds to groundwater. The objective of this study was to assess the risk of ammonium (NH + ), nitrate (NO − ) and dissolved organic nitrogen (DON) leaching following a single compost application to silty and sandy soils in mature beech (Fagus sylvatica L.), pine (Pinus silvestris L.) and spruce (Picea abies Karst.) forests at Solling and Unterlus in Lower Saxony, Germany. Mature compost from separately collected organic household waste was applied to the soil surface at a rate of 6.3 kg m −2 in the summer of 1997 and changes in NH + ,N O − and DON concentrations in throughfall and soil water at 10 and 100 cm soil depths were determined for 32 months. The spruce forests had the highest N inputs by throughfall water and the highest N outputs in both the control and compost plots compared with the pine and beech forests. Overall, the differences in total N outputs at 100 cm soil depth between the control and compost plots ranged between 0.3 and 11.2 g N m −2 for the entire 32-month period. The major leaching of these amounts occurred during the first 17 months after compost amendments, but there was no significant difference in total N outputs (− 0.2 to 1.8 gNm −2 ) between the control and compost plots during the remaining 15 months. Most of the mineral soils acted as a significant sink for NO − and DON as shown by a reduction of their outputs from 10 to 100 cm depth. Based on these results, we conclude that application of mature compost with high inorganic N contents could diminish the groundwater quality in the first months after the amendments. A partial, moderate application of mature compost with low inorganic N content to nutrient depleted forest soils can minimize the risk of NO − leaching.

Evidence for dissolved organic nitrogen and phosphorus uptake during a cyanobacterial bloom in Florida Bay

Florida Bay, a shallow, seagrass-dominated bay in southern Florida, USA, receives significant nutrient inputs and has experienced seagrass losses and microalgal blooms within the last several decades. Inorganic nutrient inputs have been well characterized, but the role of organic nutri- ents, specifically of dissolved organic nitrogen (DON) and organic phosphorus (DOP), in supporting microbial processes in the bay is unknown. In this study various techniques were used to assess the importance of these nutrients along a transect in Florida Bay when a cyanobacterial bloom occurred in the central region in November 2002. These techniques included measurements of ambient particu- late and dissolved nutrients, enzyme (urease and alkaline phosphatase) activities, and experiments to determine rates of 15 N uptake (nitrate, ammonium, urea, and amino acids over a period of 0.5 h) and long-term (48 h) changes in microbial biomass and 15 N natural abundance in enrichment bioassays. The cyanobacterial bloom in central Florida Bay was associated with the highest concentrations of DON and DOP, whereas the microflagellate- and diatom-dominated eastern bay region was associ- ated with the highest concentrations of inorganic nutrients. The zeaxanthin:chlorophyll a ratio (an indicator of the relative contribution of cyanobacteria to phytoplankton biomass) was positively corre- lated with the rate of uptake of urea, and negatively correlated with the rate of uptake of inorganic nitrogen. The opposite pattern was observed for the fucoxanthin:chlorophyll a ratio (indicative of relative diatom biomass) and the peridinin:chlorophyll a ratio (indicative of relative photosynthetic dinoflagellate biomass), suggesting that different algal groups were using different N substrates. Bio- mass responses in the bioassay experiments showed that phytoplankton (as chlorophyll a) responded to DON additions in the western region and to DOP additions in the eastern region, but heterotrophic bacteria, in contrast, responded to DOP additions in the west and DON additions in the east. These findings thus demonstrate the potential for different sources of N, including DON, to stimulate differ- ent components of the algal community, and for the phytoplankton and bacteria to respond differently to N and P.

Dissolved Organic Nitrogen Regulation in Freshwaters

Dissolved organic nitrogen (DON) has been hypothesized to play a major role in N cycling in a variety of ecosystems. Our aim was to assess the seasonal and concentration relationships between dissolved organic carbon (DOC), DON, and NO3- within 102 streams and 16 lakes within catchments of differing complexity situated in Wales. Further, we aimed to assess whether patterns of land use, soil type, and vegetation gave consistent trends in DON and dissolved inorganic nitrogen (DIN) relationships over a diverse range of catchments. Our results reinforce that DON constitutes a significant component of the total dissolved N pool typically representing 40 to 50% of the total N in streams and lakes but sometimes representing greater than 85% of the total dissolved N. Generally, the levels of DON were inversely correlated with the concentration of DIN. In contrast to DIN concentrations, which showed distinct seasonality, DON showed no consistent seasonal trend. We hypothesize that this reflects differences in the bioavailability of these two N types. The amount of DON, DOC, and DIN was significantly related to soil type with higher DON export from Histosol-dominated catchments in comparison with Spodosol-dominated watersheds. Vegetation cover also had a significant effect on DON concentrations independent of soil type with a nearly twofold decrease in DON export from forested catchments in comparison with nonforested watersheds. Due to the diversity in catchment DON behavior, we speculate that this will limit the adoption of DON as a broad-scale indicator of catchment condition for use in monitoring and assessment programs.

Nitrogen Export from Forested Watersheds in the Oregon Coast Range: The Role of N 2 -fixing Red Alder

Variations in plant community composition across the landscape can influence nutrient retention and loss at the watershed scale. A striking example of plant species importance is the influence of N2-fixing red alder (Alnus rubra) on nutrient cycling in the forests of the Pacific Northwest. To understand the influence of red alder on watershed nutrient export, we studied the chemistry of 26 small watershed streams within the Salmon River basin of the Oregon Coast Range. Nitrate and dissolved organic nitrogen (DON) concentrations were positively related to broadleaf cover (dominated by red alder: 94% of basal area), particularly when near-coastal sites were excluded (r 2 = 0.65 and 0.68 for nitrate-N and DON, respectively). Nitrate and DON concentrations were more strongly related to broadleaf cover within entire watersheds than broadleaf cover within the riparian area alone, which indicates that leaching from upland alder stands plays an important role in watershed nitrogen (N) export. Nitrate dominated over DON in hydrologic export (92% of total dissolved N), and nitrate and DON concentrations were strongly correlated. Annual N export was highly variable among watersheds (2.4–30.8 kg N ha−1 y−1), described by a multiple linear regression combining broadleaf and mixed broadleaf–conifer cover (r 2 = 0.74). Base cation concentrations were positively related to nitrate concentrations, which suggests that nitrate leaching increases cation losses. Our findings provide evidence for strong control of ecosystem function by a single plant species, where leaching from N saturated red alder stands is a major control on N export from these coastal watersheds.

Earthworm additions affect leachate production and nitrogen losses in typical midwestern agroecosystems.

Earthworms affect soil structure and the movement of agrochemicals. Yet, there have been few field-scale studies that quantify the effect of earthworms on dissolved nitrogen fluxes in agroecosystems. We investigated the influence of semi-annual earthworm additions on leachate production and quality in different row crop agroecosystems. Chisel-till corn (Zea mays L.)-soybean [Glycine max (L.) Merr.] rotation (CT) and ridge-till corn-soybean-wheat (Triticum aestivum L.) rotation (RT) plots were arranged in a complete randomized block design (n = 3) with earthworm treatments (addition and ambient) as subplots where zero-tension lysimeters were placed 45 cm below ground. We assessed earthworm populations semi-annually and collected leachate biweekly over a three-year period and determined leachate volume and concentrations of total inorganic nitrogen (TIN) and dissolved organic nitrogen (DON). Abundance of deep-burrowing earthworms was increased in addition treatments over ambient and for both agroecosystems. Leachate loss was similar among agroecosystems, but earthworm additions increased leachate production in the range of 4.5 to 45.2% above ambient in CT cropping. Although leachate TIN and DON concentrations were generally similar between agroecosystems or earthworm treatments, transport of TIN was significantly increased in addition treatments over ambient in CT cropping due to increased leachate volume. Losses of total nitrogen in leachate loadings were up to approximately 10% of agroecosystem N inputs. The coincidence of (i) soluble N production and availability and (ii) preferential leaching pathways formed by deep-burrowing earthworms thereby increased N losses from the CT agroecosystem at the 45-cm depth. Processing of N compounds and transport in soil water from RT cropping were more affected by management phase and largely independent of earthworm activity.

Seasonal variability and export of dissolved organic nitrogen in the northwestern Mediterranean Sea

Hydrodynamic and biogeochemical monthly time series were carried out in the Gulf of Lions (northwestern Mediterranean) between 1993 and 1994. Detailed analysis of vertical profiles of dissolved organic nitrogen (DON) revealed an accumulation of nitrogen in the Upper Mixed Layer (UML) above the seasonal thermocline. DON was not corrected for ammonia concentration because of negligible concentrations of this element during the whole year in the studied area. In midwinter (January–February), DON concentrations were quasi‐homogeneous throughout the water column (about 3.5 μM). Subsequently, surface DON concentrations increased up to autumn, with a subsurface maximum greater than 6.0 μM observed in September. During this time, inorganic nutrients became virtually undetectable in the UML. The total DON content (0–1000 m) increased by about 784 mmol m−2 from winter through to the end of summer. At this time, labile and semilabile DON represented a maximum of 20% of the bulk DON. Assuming that DON was essentially removed from surface water by diffusive process during the stratified period and by vertical mixing during the rest of the year, 210 to 723 mmol m−2 a−1 was exported as DON, which represented 40–90% of the total new production. Following this DON export enabled us to balance the nitrogen budget of the area studied over the duration of the experiment.

Dissolved organic carbon (doc) and dissolved organic nitrogen (don) content of an arenosol as affected by liming in a pot experiment

The effect of liming materials was investigated on the dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) content of the soil in a pot experiment on an acidic soil using oat (Avena sativa L. ) as an indicator plant. Soil samples were taken three times during the growing season. The lime rates applied were 0, 1, 2, 3 g calcite/pot and 0.92, 1.84, 2.76 g dolomite/pot, respectively. Due to an increase in soil pH and microbial activity the DOC concentration significantly increased with increasing lime doses at all three samplings. An exponential relationship was found between soil pH and DOC concentration: y v = v 0.3733e 0.7893x , r v = v 0.903***. Lime had a significant effect on DON concentration at the first sampling, while for the remainder of the growing period no further significant increases were found. This could be explained by the fact that the biodegradability of DOC and DON differs under conditions of the experiment, resulting in a decrease in the N content of the dissolved organic matter, while the amount of DOM and DOC increased with increasing pH. Because of the above mentioned facts the DOC/DON ratio increased significantly with liming. There were no significant changes in the dissolved organic carbon content of the soil during the 15-week growing period, while DON concentration decreased significantly. It can be explained by the initial addition of N fertilizer, which increased the DON quantity at the first sampling in the soil.

Dissolved organic nitrogen: a dynamic participant in aquatic ecosystems

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 31:279-305 (2003) - doi:10.3354/ame031279 Dissolved organic nitrogen: a dynamic participant in aquatic ecosystems Tom Berman1,* , Deborah A. Bronk2 1Yigal Alon Kinneret Limnological Laboratory, Israel Oceanographic and Limnological Research, PO Box 447, Migdal 14950, Israel 2College of William and Mary/Virginia Institute of Marine Science, Route 1208, Greate Road, Gloucester Point, Virginia 23062, USA *Email: tberman@ocean.org.il ABSTRACT: In both marine and freshwaters, the concentration of dissolved organic nitrogen (DON) frequently exceeds that of dissolved inorganic nitrogen (DIN), including ammonium, nitrate, and nitrite. Recent evidence indicates that many organic N compounds are released into the DON pool and taken up from this pool by planktonic microbiota on timescales of hours to days. This observation suggests that many components of the DON pool can play an active role in supplying N nutrition directly or indirectly to phytoplankton and bacteria and, in so doing, may affect the species composition of the ambient microbial assemblage. Here we present an overview of the state of knowledge of DON pools in aquatic environments, focused mainly on data gathered in the last decade. We review information on DON concentrations in freshwater and marine systems, analytical methods for the determination of DON, and the biotic and abiotic sources and sinks of DON. More detailed discussion addresses specific components of the DON pool: urea, dissolved combined and free amino acids, proteins, nucleic acids, amino sugars, and humic substances. The DON pool in natural waters is not inert and can be an important sink and source for N. There is a need for greater appreciation and understanding of the potential role of DON as a dynamic participant in the nitrogen cycle within aquatic ecosystems, particularly in freshwater environments. KEY WORDS: Dissolved organic nitrogen · Phytoplankton · Bacteria · N nutrition · N cycling · Marine/freshwater ecosystems Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 31, No. 3. Online publication date: April 03, 2003 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 2003 Inter-Research.