15N Mass Balance Research Articles

15N mass balance technique for measuring ammonia losses from soil surface‐applied slurries containing various additives

AbstractBackgroundAnthropogenic ammonia emissions, primarily derived from agriculture, lead to air pollution, soil acidification, and surface water eutrophication, all of which adversely affect human health and ecosystems. Slurry treatment technologies in the form of additives represent an underutilized means of reducing gaseous emissions. Information regarding the potential of additives to reduce ammonia in soil surface‐applied slurries is scarce.AimThis study aims to develop a 15N mass balance technique to quantitatively measure ammonia losses from different slurries containing multiple additives that are applied to outdoor soil‐filled containers.MethodsThe experiments were performed under free‐air conditions. Isotopically labeled slurries from biogas, cattle, and pigs containing 18 additives were surface‐applied to soil‐filled containers and exposed for 72 or 48 h. The additives included inorganic and organic adsorbents, five amounts of sulfuric acids, molasses ± effective microorganisms, and water dilution. After termination of the ammonia loss period, a suite of soil preparation steps for the quantitative recovery of the labeled ammonium remaining in the soil was developed, and subsequently the loss of NH4‐N was determined.ResultsIn the control treatments, ammonia losses from biogas, cattle slurries, and pig slurries averaged 54.4%, 33.9%, and 11.0%, respectively. The adsorbents did not decrease or only slightly decreased ammonia emissions. Ammonia abatement by sulfuric acid was nearly complete at pH values of 5.9 and 5.8 for the biogas and pig slurry, respectively, and about 80% at pH 5.2 for the cattle slurry. In comparison, more moderately decreased pH values with sulfuric acid showed a similar reduction as molasses and a 1:1 dilution for the three slurries. Adding microorganisms to the molasses did not further decrease ammonia losses.ConclusionThe newly developed 15N mass balance technique, which allows a precise estimate of ammonia losses, can serve as a reference method to assess ammonia losses from field‐applied slurries containing various additives and as a standard comparison technique for other ammonia measurement techniques.

A new incubation system to simultaneously measure N2 as well as N2O and CO2 fluxes from plant-soil mesocosms

AbstractThis study presents a novel plant-soil mesocosm system designed for cultivating plants over periods ranging from days to weeks while continuously measuring fluxes of N2, N2O and CO2. For proof of concept, we conducted a 33-day incubation experiment using six soil mesocosms, with three containing germinated wheat plants and three left plant-free. To validate the magnitude of N2 and N2O fluxes, we used 15N-enriched fertilizer and a 15N mass balance approach. The system inherent leakage rate was about 55 µg N m− 2 h− 1 for N2, while N2O leakage rates were below the detection limit (< 1 µg N m− 2 h− 1). In our experiment, we found higher cumulative gaseous N2 + N2O losses in sown soil (0.34 ± 0.02 g N m− 2) as compared to bare soil (0.23 ± 0.01 g N m− 2). N2 fluxes accounted for approximately 94–96% of total gaseous N losses in both planted and unplanted mesocosms. N losses, as determined by the 15N mass balance approach, were found to be 1.7 ± 0.5 g N m− 2 for the sown soil and 1.7 ± 0.6 g N m− 2 for the bare soil, indicating an inconsistency between the two assessment methods. Soil respiration rates were also higher in sown mesocosms, with cumulative soil and aboveground biomass CO2 respiration reaching 4.8 ± 0.1 and 4.0 ± 0.1 g C m− 2 over the 33-day incubation period, in sown and bare soil, respectively. Overall, this study measured the effect of wheat growth on soil denitrification, highlighting the sensitivity and utility of this advanced incubation system for such studies.

Lysimeter-based full fertilizer 15N balances corroborate direct dinitrogen emission measurements using the 15N gas flow method

AbstractThe 15N gas flux (15NGF) method allows for direct in situ quantification of dinitrogen (N2) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N2 emissions of a wheat rotation using the 15NGF method, to compare these N2 emissions with those obtained from a lysimeter-based 15N fertilizer mass balance approach, and to contextualize N2 emissions with 15N enrichment of N2 in soil air. For four sampling periods, fertilizer-derived N2 losses (15NGF method) were similar to unaccounted fertilizer N fates as obtained from the 15N mass balance approach. Total N2 emissions (15NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the 15N mass balance approach overall indicated fertilizer-derived N2 emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N2O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N2:(N2O + N2) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N2 emissions was well explained by the δ15N2 in soil air down to 50 cm depth. We conclude the 15NGF method provides realistic estimates of field N2 emissions and should be more widely used to better understand soil N2 losses. Moreover, combining soil air δ15N2 measurements with diffusion modeling might be an alternative approach for constraining soil N2 emissions.

Mesopelagic particulate nitrogen dynamics in the subarctic and subtropical regions of the western North Pacific

Recently, new spatiotemporal-scale particle observations by autonomous profiling floats equipped with bio-optical sensors have revealed that, in addition to gravitational particle sinking, the downward transport of surface particles by physical mixing events, which has been overlooked, contributes to particulate organic carbon export. However, the subsequent behavior of these exported particles in the mesopelagic zone (e.g., particle fragmentation and degradation) remains unclear, although it may influence the efficiency of carbon transport to further depths. This study successfully depicted the new annual mean mesopelagic particulate nitrogen (PN) dynamics with multi-layer, steady-state suspended PN pools by reanalyzing seasonal data on the stable nitrogen isotopic compositions of both suspended and sinking particles, each with different profiles, from subarctic station K2 and subtropical station S1 in the North Pacific, which are both CO2 sinks but in different oceanic settings. As analytical conditions, we assumed that the net loss of sinking PN was entirely due to abiotic fragmentation of particle aggregates to non-sinking particles and that the apparent 15N enrichment associated with heterotrophic degradation in the suspended PN pools was vertically constant. The 15N mass balance for the PN supply to the uppermost mesopelagic pool, derived from such constraints, allowed estimating the PN export by the mixed-layer pump, which was 1.6 times greater at K2 than at S1. However, its contribution to the total export (including gravitational PN sinking) from the surface layer was approximately 20% at both stations. Moreover, the ratio of PN supplied to the uppermost pool by the mixed-layer pump and by the fragmentation of particle aggregates was also similar at both stations, approximately 1:1. Using these ratios, together with separate observations of the mixed-layer pump-driven flux, it may be possible to estimate the efficiency of the particulate organic carbon transport due to the biological gravitational pump responsible for carbon sequestration in the deep sea.

Nitrogen use efficiency and N2O emissions vary according to seasonal water supply across different cereal production systems of south eastern Australia

Application of nitrogen (N) fertiliser is vital to the productivity of grains production systems. However, losses can result in negative environmental impacts as well as having a significant impact on farmer profitability. Such losses can vary significantly, therefore it is important to benchmark nitrogen use efficiency (NUE) across a range of environments to better understand opportunities for improvement. Using a combination of 15N mass balance and monitoring for gaseous N2O flux we undertook an assessment of NUE at 29 sites spread across a range of management systems in semi-arid and temperate environments of south eastern Australia. An N rate experiment was established at each site, testing three different in-season application rates with N surface applied as urea. Timing and rates were determined in relation to farmer practice for the broader paddock. Loss of fertiliser N averaged 29% (ranging from 5 to 54%) and while daily N2O flux rates represented a fraction of this, peak flux rates ranged from 52 to 132 to 376 g N2O-N/ha/day across low/ medium rainfall, high rainfall and irrigated regions respectively. Crop recovery of applied N ranged from 3 to 65% and was positively correlated with agronomic efficiency of N application.

Nitrogen use efficiency of 15N urea applied to wheat based on fertiliser timing and use of inhibitors

Improving fertiliser nitrogen (N) use efficiency is essential to increase productivity and avoid environmental damage. Using a 15N mass balance approach, we investigated the effects of five N fertiliser management strategies to test the hypothesis that increasing uptake of applied N by wheat improves productivity and reduces loss of N in a semi-arid environment. Three experiments were conducted between 2012 and 2014. Treatments included urea application (50 kg N/ha) at sowing with and without nitrification inhibitor (3,4-dimethylpyrazole phosphate, DMPP) and surface broadcast with and without urease inhibitor (n-butyl thiophosphoric triamide, NBPT) at the end of tillering plus an unfertilised control. It was found that deferring fertiliser application until the end of tillering decreased losses of fertiliser N (35–52%) through increasing uptake by the crop and or recovery in the soil at harvest, while maintaining yield except when rainfall following application was low. In this case, deferring application reduced fertiliser uptake (− 71%) and grain yield (− 18%) and increased recovery of N in the soil (+ 121%). Use of DMPP or NBPT reduced N loss where seasonal conditions were conducive to denitrification during winter (DMPP) and volatilisation or denitrification later in the season (NBPT). Their effect on grain yield was less significant; DMPP increased yield (+ 3–31%) in all years and NBPT increased yield (+ 7–11%) in 2 of 3 years compared to unamended urea. The majority of crop N uptake was supplied from soil reserves and as a result, crop recovery of applied N was not strongly related to grain yield response.

Climatic controls on the isotopic composition and availability of soil nitrogen across mountainous tropical forest

AbstractWhile tropical forests play a critical role in global carbon (C) and nitrogen (N) cycles, how their biogeochemical dynamics will respond to changes in climate, especially warming, is uncertain. To shed light on links between climate and N cycling in tropical forests, we measured bulk surface soil C and N concentrations and isotopic content at 40 forested sites spanning an 1800 m elevation transect in Central America, possessing wide variation in mean annual temperature (MAT; range = 10°C) and precipitation (MAP; range = 1.2 m). Climate and terrain attributes were extracted from gridded data sets and regressed against soil variables, and then, empirical relationships were combined with a mass balance model to scale up to the larger landscape. Across the remote study region, elevation and soil δ15N values displayed a strong negative relationship, while elevation was positively related to percent of soil C, N, and C:N ratios. As elevation was tightly correlated with MAT and MAP, soil chemical and isotopic content varied strongly with climate. For example, for every degree increase in MAT, soil δ15N values—an indicator of relative gaseous N losses—increased by a factor of 0.4, and soil C:N ratios, which affect net N mineralization and N availability, declined by a factor of 1.1. With the 40 sites binned into bioclimatic life zones, montane, premontane, and wet‐premontane transition forests showed distinct clustering of soil chemical and isotopic properties, yet forest type alone explained less variation compared to continuous elevation–climate parameters. Results of the spatially applied 15N mass balance model implied shifts in the contribution of gaseous‐to‐total N loss, from 10% or less in cool, wet high‐elevation forests to upwards of 60% at warmer, drier, low‐elevation sites. Climate variation was thus associated with significant shifts in N dynamics across this montane tropical region, yet more work is needed to decouple direct vs. indirect climatic controls. While the mechanisms deserve further study, observed shifts in indicators of N availability and gaseous loss may be useful in managing and modeling tropical forests under climate change.

Nitrate removal processes in a constructed wetland treating drainage from dairy pasture

15N-labelled NO 3 − was used in a surface-flow constructed wetland in spring to examine the relative importance of competing NO 3 − removal processes. In situ mesocosms (0.25 m 2) were dosed with 2 l of 15NO 3 − (NaNO 3, 300 mg N l −1, 99 atom% 15N) and bromide (Br −) solution (LiBr, 4.3 g l −1, as a conservative tracer). Concentrations of NO 3 −, Br −, dissolved oxygen and 15N 2 were monitored periodically and replicate mesocosms were destructively sampled prior to and 6 days after 15N addition. Denitrification, immobilisation, plant uptake and dissimilatory NO 3 − reduction to NH 4 + (DNRA) accounted for 77, 11, 9 and 2% of 15NO 3 − transformed during the experiment. Only 6% of denitrification gases were directly measured as atmospheric or dissolved 15N 2; the remainder (71%) was determined via 15N mass balance. This indicated that a large proportion of the denitrification gases were entrapped within the soil matrix and/or plant aerenchyma. The floating plant Lemna minor exhibited a significantly higher NO 3 − uptake rate (221 mg kg −1 d −1) than Typha orientalis (10 mg kg −1 d −1), but periodic harvest of plants would remove <3% of annual NO 3 − inputs. Our results suggest that this 6-year-old constructed wetland functions effectively as a sink for NO 3 − during the growing season with less than one-quarter of the NO 3 − processed sequestered into wetland plant, algal and microbial N pools and the balance permanently removed by denitrification.

Assessing nitrogen fluxes from roots to soil associated to rhizodeposition by apple (Malus domestica) trees

The mass transfer from root to soil by means of rhizodeposition has been studied in grasses and forest trees, but its role in fruit trees is still unknown. In this study, N fluxes from roots to soil were estimated by applying a 15N mass balance technique to the soil–tree system. Apple (Malus domestica) trees were pre-labelled with 15N and then grown outdoors in 40 L pots for one vegetative season in (1) a coarse-textured, low organic matter soil, (2) a coarse-textured, high organic matter soil, and (3) a fine-textured, high organic matter soil. At tree harvest the 15N abundance of the soils was higher than at transplanting, but the total amount of 15N present in the tree–soil system was similar at transplanting and tree harvest. The soils had a strong effect on N fluxes from and to the soil. In the fine-textured soil, 11% of the total plant-derived nitrogen was transferred to the soil, compared with 2–5% in the two coarse-textured soils. Rhizodeposition was higher in the fine soil (18% of the primary production) than in the coarse-textured soils, whereas higher soil organic matter depressed rhizodeposition. Nitrogen uptake was almost double in the coarse-textured, high organic matter soil versus the other soils. Our results indicate that belowground primary productivity is significantly underestimated if based on root production data only. Rhizodeposition represents a major process, whose role should not be underestimated in carbon and nitrogen cycles in orchard ecosystems.

Foliar and fungal 15 N:14 N ratios reflect development of mycorrhizae and nitrogen supply during primary succession: testing analytical models

Nitrogen isotopes (15N/14N ratios, expressed as delta15N values) are useful markers of the mycorrhizal role in plant nitrogen supply because discrimination against 15N during creation of transfer compounds within mycorrhizal fungi decreases the 15N/14N in plants (low delta15N) and increases the 15N/14N of the fungi (high delta15N). Analytical models of 15N distribution would be helpful in interpreting delta15N patterns in fungi and plants. To compare different analytical models, we measured nitrogen isotope patterns in soils, saprotrophic fungi, ectomycorrhizal fungi, and plants with different mycorrhizal habits on a glacier foreland exposed during the last 100 years of glacial retreat and on adjacent non-glaciated terrain. Since plants during early primary succession may have only limited access to propagules of mycorrhizal fungi, we hypothesized that mycorrhizal plants would initially be similar to nonmycorrhizal plants in delta15N and then decrease, if mycorrhizal colonization were an important factor influencing plant delta15N. As hypothesized, plants with different mycorrhizal habits initially showed similar delta15N values (-4 to -6 per thousand relative to the standard of atmospheric N2 at 0 per thousand), corresponding to low mycorrhizal colonization in all plant species and an absence of ectomycorrhizal sporocarps. In later successional stages where ectomycorrhizal sporocarps were present, most ectomycorrhizal and ericoid mycorrhizal plants declined by 5-6 per thousand in delta15N, suggesting transfer of 15N-depleted N from fungi to plants. The values recorded (-8 to -11 per thousand) are among the lowest yet observed in vascular plants. In contrast, the delta15N of nonmycorrhizal plants and arbuscular mycorrhizal plants declined only slightly or not at all. On the forefront, most ectomycorrhizal and saprotrophic fungi were similar in delta15N (-1 to -3 per thousand), but the host-specific ectomycorrhizal fungus Cortinarius tenebricus had values of up to 7 per thousand. Plants, fungi and soil were at least 4 per thousand higher in delta15N from the mature site than in recently exposed sites. On both the forefront and the mature site, host-specific ectomycorrhizal fungi had higher delta15N values than ectomycorrhizal fungi with a broad host range. From these isotopic patterns, we conclude: (1) large enrichments in 15N of many ectomycorrhizal fungi relative to co-occurring ectomycorrhizal plants are best explained by treating the plant-fungal-soil system as a closed system with a discrimination against 15N of 8-10 per thousand during transfer from fungi to plants, (2) based on models of 15N mass balance, ericoid and ectomycorrhizal fungi retain up to two-thirds of the N in the plant-mycorrhizal system under the N-limited conditions at forefront sites, (3) sporocarps are probably enriched in 15N by an additional 3 per thousand relative to available nitrogen, and (4) host-specific ectomycorrhizal fungi may transfer more N to plant hosts than non-host-specific ectomycorrhizal fungi. Our study confirms that nitrogen isotopes are a powerful tool for probing nitrogen dynamics between mycorrhizal fungi and associated plants.

Effect of management history and temperature on the mineralization of methylene urea in soil

A soil incubation and a greenhouse study on processing tomato were used to test the effects of soil temperature and the size and activity of the soil microbial biomass (SMB) on the degradation (mineralization) rate of a slow-release N fertilizer, methylene urea (MU), a condensation product of urea and formaldehyde. The mineralization rates of three MUs: Short (S), Medium (M), and Long (L) with different water solubilities were measured at two temperatures in a soil with low (fallow, F) and high (cover crop, CC) microbial activity. In the greenhouse study, the fate of fertilizer N was followed using 15N-urea and 15N-MU. The fertilizer N efficiency calculated for urea using the 15N mass balance approach was 93 and 85% compared with 65 and 67% for MU-S in F and CC soils, respectively. During six months of incubation, 52 and 63% of MU-S N was mineralized at 20 and 30 °C, respectively. The accumulated N data suggested that the degradation of all three MU types followed first-order reaction kinetics. The reaction rates were similar for all three MUs and increased with increasing temperature. However, fitting discrete, non-accumulated data revealed that MU mineralization is more complex and cannot be modeled with simple exponential decay equations. The size and activity of SMB did not affect the mineralization rate of MU-N under laboratory or greenhouse conditions. Interestingly, Activity Index (AI), defined as the slowly available pool of MU-N, was not a reliable indicator for the mineralization rate and plant availability of MU-N.

GASEOUS LOSSES OF APPLIED NITROGEN FROM A CORN FIELD DETERMINED BY 15N ABUNDANCE OF N2 AND N2O

This study was carried out to investigate the magnitude of N loss through denitrification by 15N gas-flux method in a corn field where rape was incorporated with surface soils as green manure. To measure the in situ emissions of N2 and N2O, micro-plots with plastic barriers (0.70 m×0.30 m) were setup in triplicate in a corn field, (15NH4)2SO4 (70.7 atom% 15N) was mixed thoroughly with the 0–10 cm topsoil inside the plots at a rate equivalent to 150 kg N ha−1. The gas samples were taken periodically by using a static chamber method, 15N abundance in N2O and N2 was determined by mass spectrometry, and N2O concentrations were determined by gas chromatography. The 15N mass balance in the micro-plots was estimated at harvest. The total amount of gaseous N losses was about 10.8% of the applied fertilizer N, and N2O emissions accounted for only 2.9% of the gaseous N loss. The average emission rates of N2 and N2O during the corn-growing period were 13.4 and 0.041 mg N m−2 d−1, respectively. However, after 30–40 days from the sowing, both the fluxes of N2 and N2O were rather low. The 15N abundance of N2O was as high as 34.3 atom% 15N during the first week after fertilization and then decreased drastically to the background level. These temporal changes in 15N abundance of N2O coincided well with those in nitrate contained in the soil. At harvest time, the recovery of the applied fertilizer N calculated from 15N mass balance was 86%, and 55% of the applied N still remained in soil profile as nitrate.

Fate of 15N-labelled nitrate in a wet summer under different residue management regimes in young hoop pine plantations

A field study was conducted to investigate the fate of 15N-labelled nitrate applied at 20 kg N ha −1 in a wet summer to microplots installed in areas under different residue management regimes in second-rotation hoop pine ( Araucaria cunninghamii) plantations aged 1–3 years in south-east Queensland, Australia. PVC microplots of 235 mm diameter and 300 mm long were driven into 250 mm soil. There were three replications of each of eight treatments. These were areas just under and between 1-year-old windrows (ca. 2–3 m in width) of harvesting residues spaced 15 m apart, and with and without incorporated foliage residues (20 t DM ha −1); the areas just under and between 2- or 3-year-old windrows spaced 10 m apart. Only 7–29% of the added 15N was recovered from the top 750 mm of the soil profile with the leaching loss estimated to be 70–86% over the 34-day period. The 15N loss via denitrification was 3.7–6.3% by directly measuring the 15N gases emitted. The microplots with the incorporated residues at the 1-year-old site had the highest 15N loss (6.3%) as compared with the other treatments. The 15N mass balance method together with the use of bromide (Br) tracer applied at 100 kg Br ha −1 failed to obtain a reliable estimate of the denitrification loss. The microplots at the 1-year-old site had higher 15N immobilisation rate (7.5–24.7%) compared with those at 2- and 3-year-old sites (2.1–3.6%). Incorporating the residues resulted in an increase in 15N immobilisation rate (24.5–24.7%) compared with the control without the incorporated residues (8.4–14.3%). These findings suggest that climatic conditions played important roles in controlling the 15N transformations in the wet summer season and that the residue management regimes could also significantly influence the 15N transformations. Most of the 15N loss occurred through leaching, but a considerable amount of the 15N was lost through denitrification. Bromide proved to be an unsuitable tracer for monitoring the 15N leaching and movement under the wet summer conditions.

Denitrification, leaching and immobilisation of 15N-labelled nitrate in winter under windrowed harvesting residues in hoop pine plantations of 1–3 years old in subtropical Australia

A field study was carried out to investigate the impacts of windrowed harvesting residues on denitrification, immobilisation and leaching of 15N-labelled nitrate applied at 20 kg N ha −1 to microplots in second-rotation hoop pine ( Araucaria cunninghamii) plantations of 1–3 years old in southeast Queensland, Australia. The PVC microplots were 235 mm in diameter and 150 mm long, and driven into the 100 mm soil. There were three replications of such microplots for each of the six treatments which were areas just under and between 1-, 2- and 3-year-old windrows of harvesting residues. Based on gaseous N losses estimated by the difference between the recoveries of bromide (Br) applied at 100 kg Br ha −1 and 15N-labelled nitrate, denitrification was highest (23% based on 15N loss) in the areas just under the 1-year-old windrows 25 days after a simulated 75 mm rainfall and following several natural rainfall events. There was no significant difference in 15N losses (14–17%) among the other treatments. The 15N immobilisation rate was highest for microplots in the areas between the 1-year-old windrows and generally higher for microplots in the areas just under the windrows (30–39%) than that (26–30%) between the windrows. Direct measurement of 15N gas emissions ( 15 N 2+ 15 N 2 O) confirmed that the highest denitrification rate occurred in the microplots under the 1-year-old windrows although the gaseous 15N loss calculated by gas emission was only about one-quarter that estimated by the 15N mass balance method. A significant, positive linear relationship ( P<0.05) existed between the gaseous 15N losses measured by the two methods used. The research indicates that considerable mineral N could be lost via denitrification during the critical inter-rotation period and early phase of the second rotation. However, the impacts of windrowed harvesting residues on N losses via denitrification might only last for a period of about 2 years.

Transformations of inorganic-N in soil leachate under differing storage conditions

Incomplete recovery of 15N often occurs in 15N mass balance experiments, the unrecovered portion being assumed to be lost due to denitrification. Denitrification losses estimated in this way will be too high if any gaseous loss(es) of N occur during the sample collection, extraction or storage procedures. Leachate or KCl extracts of soil are mixtures of inorganic and organic substances, so changes may occur during storage. The objective of this work was to assess the potential for inorganic N transformations and potential gaseous losses when a soil leachate was treated in various ways and stored in glass bottles for 10 days at −20, 4 or 20°C. The treatments were natural pH of 6.3, acidification to pH 4.5, and adjustment of the salt strength to 2 M in KCl. The NH 4 +, NO 2 − and NO 3 − concentrations were enhanced and differentially labelled with 15N. After 10 days, headspaces were analysed to determine the fluxes of N 2O and N 2, and solutions were analysed for inorganic-N concentrations and their 15N enrichments. Concentrations changed least when the storage temperature was 4°C or when the ionic composition was primarily 2 M KCl. Ammonium concentrations changed little, a small significant ( P<0.05) increase at pH 6.3 and 20°C being due to mineralisation. Nitrate concentrations also changed little, a small significant ( P<0.05) increase at pH 4.5 and − 20°C being due to oxidation of NO 2 −. Transformations of inorganic-N were dominated by chemical reactions involving NO 2 −, particularly at pH 4.5 and −20°C. Under these conditions, the NO 2 − concentration decreased from 8.6 to 1.3 mg N l −1 partly because of oxidation to NO 3 −, reduction to N 2O, and reaction with NH 4 + to form N 2. These processes accounted for 49.0, 0.2 and 14.2% of the NO 2 − initially present, respectively. Nitrous oxide production accounted for 3.2% of the NO 2 − initially present at pH 4.5 and 20°C with headspace concentrations reaching 125 μl l −1 N 2O-N. The potential importance of the inorganic-N transformations measured in this experiment depends on the relative inorganic-N composition. Deep-freezing of acidic or acidified soil leachate which contains NO 2 − has the potential to not only increase NO 3 − concentrations but also decrease NO 2 − and NH 4 + concentrations. Storing leachates or 2 M KCl extracts from soils at 4°C without acidification is recommended for minimising N transformations of NO 2 − and NH 4 + and avoiding the potential to overestimate NO 3 − concentrations. Future work should explore these gas loss mechanisms using sterilised samples in conjunction with more interim sampling to investigate the precise timing and rates of gas production.

Denitrification, leaching and immobilisation of applied 15N following legume and grass pastures in a semi-arid climate in Australia

Four consecutive 15N mass balance experiments lasting 18 months from February 1993 to August 1994 were carried out to assess the fate of applied 15N at 3 sites after 4 years of lucerne or snail medic and 20 years of Mitchell grass/naturalised medic pastures respectively in the Roma district of Queensland, Australia. 15N loss via denitrification was estimated from the difference between the recovery of applied Br(100kg Br/ha) and that of applied 15N(40kg N/ha) in the top 250mm at the end of each mass balance experiment. From February to August 1993, denitrification losses were 12–38% of applied 15N. N losses increased to 36–51% during August to November 1993, responding to the higher rainfall during this period. With even more rainfall during the period between November 1993 and March 1994, N losses were estimated to be 16–23%while displacement of 15N below 250 mm was 74–81%. When rainfall was much less between March 1994 and August 1994, N losses of only 15–19% of the applied 15N occurred at the 3 sites. It was found that although rainfall was the dominant factor controlling denitrification of the applied 15N, soil available carbon (C) (measured as water-soluble C) and the quantity of nitrate available were also important for soils already containing a considerable quantity of organic matter and N from residues of pasture legumes.

Denitrification and nitrous oxide emissions from a Black Chernozemic soil during spring thaw in Alberta

Previous field research in Alberta has suggested that denitrification occurs mostly when soil thaws in the spring, with associated soil water saturation. Our objective was to determine if denitrification and N2O emission in fact take place in cold, thawing soil in the field. Denitrification and N2O flux were measured in two springs and the intervening summer. Cylinders were placed in soil in November, 1988, and 57 kg N ha−1 of 15Nlabeled KNO3 was added. Soil 15N mass balance technique showed 23 kg N ha−1 of added-N was lost by 15 May 1989. Gas trappings were made (28 March to 29 April) and nearly all of the N2O emission (3.5 kg N2O-N ha−1) occurred during an 11-d period of thaw. The accumulated N2O flux from 20 June to 31 August was small (0.5 kg N2O-N ha−1, or less); during that time there were no rainfall events intense enough to produce water saturated soil. In 1990, 15N-labeled KNO3 (100 kg N ha−1) was applied on 26 March (outset of the thaw) and mass balance showed 32.7 kg N ha−1of added-N was lost by 7 May. A flux of 16.3 kg N2O-N ha−1 occurred largely in a 10-d period during and immediately after soil thaw. The N2O emitted from soil left a considerable fraction of the lost N unaccounted for. This unaccounted N was most likely lost as gaseous N other than N2O (e.g., N2). We conclude that large amounts of soil nitrate may be denitrified, with smaller amounts emitted as N2O, as the soil thaws and soon thereafter. Key words: Denitrification, frozen soil, thawing soil, nitrogen, nitrous oxide

Combination Probe for Nitrogen‐15 Soil Labeling and Sampling of Soil Atmosphere to Measure Subsurface Denitrification Activity

AbstractInvestigations of subsoil denitrification have been hampered by the difficulty in adapting traditional methods for measuring denitrification in surface soils to subsoils. In this study, we present, and test, a combination probe constructed to detect denitrification in the vadose zone. The probe was designed to label a volume of the subsoil with 15NO‐3 and subsequently to collect gas samples from which the denitrifying activity in the surrounding soil can be estimated by isotopic analysis of N2. Gas samples were collected from the labeled soil volume by use of He‐flushed silicone tubing having high permeability for gases. No mass flow of gases through the soil was caused by the procedure. Nitrogen gas diffusing into the tubing can be collected after equilibration periods of 15 to 20 min. A combined tensiometer and sampler for dissolved ions was built into the probe to have direct estimates of soil water potential together with information about the 15N atom fraction of the dissolved NO‐3 in the surrounding soil. The probe was tested under laboratory conditions. The probe introduced 15NO‐3 into a roughly spherical volume of soil with a symmetric distribution of 15NO‐3. Estimates of the 15N atom fraction of the NO‐3 undergoing denitrification based on gas samples agreed favorably with destructive sampling of NO‐3. The accumulation of N2 derived from denitrification was consistent with expected trends and, along with computer simulations and 15N mass balance methods, was used to calculate denitrification rates. These different estimates of denitrification agreed well. This combination probe shows promise for measuring denitrification in subsoil.

Measurement of gaseous emissions from denitrification of applied N-15 .2. Effects of temperature and added straw

Gas emissions of applied 15N were measured beneath a soil cover daily following saturation of Vertisol and Alfisol soils repacked in pots to the original field bulk density and held at three temperatures (5, 15 or 30�C) with or without addition of wheat straw. Collective gas emissions over 57, 43 and 15 days at 5, 15 and 30 degrees C respectively were compared with the 15N loss determined by mass balance. Loss measured by gas emissions (15N2 and 15N2O) ranged from 36% to 152% of the denitrification loss as determined by 15N mass balance. In the absence of added straw, measurement by gas emissions was consistently less than loss by 15N balance. Where straw was added, 15N loss by gas emissions was overestimated, probably because of a smaller headspace (0.3 L) than considered desirable (1-1.5 L) for emission measurements. Potential denitrification rates, in the presence of added straw, were similar for the Vertisol and Alfisol. Decreasing temperature slowed potential rates of denitrification from similar to 2.5 kg ha-1 day-1 at 30 �C to 0.8 kg ha-1 day-1 at 15 �C and 0.4-0.5 kg ha-1 day-1 at 5 �C. Decreasing temperature prolonged the period of waterlogging following a saturating event. Thus, collective loss of 15N was considerable even at the lower rates of denitrification at 5 �C (52-76% over 57 days) or 15 �C (87-92% over 43 days). Straw addition (10.5 t ha-1) to the Vertisol, which contained no visible plant residues from previous crops, more than doubled the losses of applied 15N. In the absence of straw, rates of denitrification and immobilization were similar in magnitude, 0.97, 0.26 and 0.16 kg ha-1 day-1 for 30, 15 and 5 �C respectively. Very rapid loss of appliedha-1 day-1N in the presence of added straw led to decreases in immobilization of applied ha-1 day-1N, highlighting the potential effects of the much higher maximum rates for denitrification than for immobilization. The N2O emissions generally represented the smaller fraction (&lt;25%) of denitrification emissions, becoming smaller as temperature was increased. As a proportion of emissions due to denitrification, N2O emissions were very low (&lt;0.5% Vertisol, &lt;3% Alfisol) in the presence of added straw.

The fate of the missing 15N differs among marine systems

In nitrogen uptake studies with 15N tracer techniques, a common observation is that the 15N label added at the start of the experiment is not fully recovered at the end of the experiment in the particulate and dissolved N pools measured. We have made direct measurements of the 15N content of the traditionally measured NH4+ and particulate N pools, as well as the dissolved organic N (DON) and bacterial pools in studies conducted in Chesapeake Bay, the Caribbean Sea, and the Choptank River (a subestuary of Chesapeake Bay). We found that the fate of the “missing 15N” differs from eutrophic to oligotrophic waters and is dependent on the length of incubation, the dissolved inorganic N substrate used, and the length of time the sample was contained before the 15N uptake experiment. In general, it appears that it is more important to include the 15N that enters the DON and bacterial pools to achieve a 15N mass balance in more oligotrophic ecosystems and when longer (&gt;1 h) incubations are used.