Partitioning Of 15N Research Articles

Nonlinear models of 15N partitioning kinetics in late-lactation dairy cows from individually labeled feed ingredients

Few studies have examined the N kinetics of individual feeds with stable isotope tracing. We hypothesized that N partitioning to milk and excreta pools as well as the rates of the processes that drive this partitioning would differ for alfalfa silage, corn silage, corn grain, and soybean meal. Feed ingredients were endogenously labeled with 15N and included in 4 diets to create treatments with the same dietary composition and different labeled feed. Diets were fed to 12 late-lactation dairy cows for 4 d (96 h) and feces, urine, and milk collection proceeded during the 4 d of 15N enrichment and for 3 d (80 h) after cessation of label feeding. Nonlinear models of 15N enrichment and decay were fit to milk (MN), urine (UN), and fecal N (FN) in R with the nlme package, and feed-specific parameter estimates were compared. The estimated proportions of feed N that were excreted in feces supported our understanding that N from soybean meal and corn grain is more digestible than N from alfalfa and corn silage. Estimates for the N partitioning between milk and urine from the 2 concentrate feeds (soybean meal and corn grain) indicated that UN/MN ratios were less than or equal to 1:1, indicating either more or equal nitrogen partitioning to milk compared with urine. It is important to maintain factual accuracy in representing the results rather than implying a desired outcome unsupported by the data. In contrast, UN/MN ratios for forage feeds (corn and alfalfa silage) were >1:1, indicating more N partitioning to urine than milk. The modeled proportion of total FN that originated from feed N was 82.2%, which is in line with previous research using a similar 15N measurement timeframe. However, the proportion of urinary and MN originating from feed N was much lower (60.5% for urine, 57.9% for milk), suggesting that approximately 40% of urinary and MN directly originate from body N sources related to protein turnover.

Nitrogen redistribution and seasonal trait fluctuation facilitate plant N conservation and ecosystem N retention

Abstract Low available soil nitrogen (N) limits plant productivity in alpine regions, and alpine plants thus resorb and reallocate N from senescing tissues to conserve this limited N during the non‐growing season. However, the destination and extent of N redistribution during plant senescence among above‐ and below‐ground organs, let alone other processes of translocation outside of plants and into the soil components, remain poorly understood. Utilizing 15N stable isotope as a tracer, we quantified N redistribution among above‐ and below‐ground plant organs and different soil components during senescence in an alpine meadow ecosystem, and explored the relationship between 15N partitioning among plant–soil N pools with seasonal fluctuations of plant functional traits. We found a substantial depletion of 15N in fine roots (−40% ± 2.8%) and above‐ground tissues (−51% ± 5.1%), and an enhanced 15N retention primarily in coarse roots (+79% ± 27%) and soil organic matter (+37% ± 10%) during plant senescence, indicating a dual role of roots with coarse roots acting as an N sink and fine roots as a source of N recycling during senescence. In parallel, we observed a temporal variation in plant functional traits, representing a shift from more acquisitive to more conservative strategies as the growing season ends, such as higher coarse root N and coarse root to fine root ratio. The seasonal trait variations were highly correlated with the 15N retention in coarse roots and soil organic matter. Particularly, 15N retention in particulate and mineral‐associated organic matter increased by 30% ± 12% and 24% ± 9%, respectively, suggesting a potential pathway through which fine root and microbial mortality contribute to 15N redistribution into soil N pools during senescence. Synthesis. N redistribution and seasonal plant trait fluctuation facilitate plant N conservation and ecosystem N retention in the alpine system. This study suggests a coupled above‐ground‐below‐ground N conservation strategy that may optimize the temporal coupling between plant N demand and ecosystem N supply in N‐limited alpine ecosystems.

LDPE and biodegradable PLA-PBAT plastics differentially affect plant-soil nitrogen partitioning and dynamics in a Hordeum vulgare mesocosm

Micro and macroplastics are emerging contaminants in agricultural settings, yet their impact on nitrogen (N) cycling and partitioning in plant-soil-microbial systems is poorly understood. In this mesocosm-scale study, spring barley (Hordeum vulgare L.) was exposed to macro or microplastic produced from low density polyethylene (LDPE) or biodegradable plastic at concentrations equivalent to 1, 10 and 20 years of plastic mulch film use. Partitioning of 15N-labelled fertiliser into plant biomass, soil and leachate yielded a partial mass balance. Soil N partitioning was probed via compound-specific 15N-stable isotope analyses of soil microbial protein. Concentration-dependent decreases in plant 15N uptake occurred with increased leached nitrogen for LDPE microplastic. Assimilation into soil microbial protein was higher for biodegradable plastics, which we associate with early-stage biodegradable plastic degradation. Partitioning of 15N into inorganic soil N pools was affected by LDPE size, with lower assimilation into the microbial protein pool. While microplastics and macroplastics altered soil N cycling, the limited impacts on plant health indicated the threshold for negative effects was not reached at agriculturally relevant concentrations. This study highlights the difference between conventional and biodegradable plastics, and emphasises that the interplay of micro and macroplastics on soil N cycling must be considered in future studies.

Fire increases soil nitrogen retention and alters nitrogen uptake patterns among dominant shrub species in an Arctic dry heath tundra

Climate change increases the frequency and severity of fire in the Arctic tundra regions. We assessed effects of fire in combination with summer warming on soil biogeochemical N- and P cycles with a focus on mineral N over two years following an experimental fire in a dry heath tundra, West Greenland. We applied stable isotopes (15NH4+-N and 15NO3−-N) to trace the post-fire mineral N pools. The partitioning of 15N in the bulk soils, soil dissolved organic N (TDN), microbes and plants (roots and leaves) was established. The fire tended to increase microbial P pools by four-fold at both one and two years after the fire. Two years after the fire, the bulk soil 15N recovery has decreased to 10.4% in unburned plots while relatively high recovery was maintained (30%) in burned plots, suggesting an increase in soil N retention after the fire. The contribution of microbial 15N recovery to bulk soil 15N recovery increased from 11.2% at 21 days to 31.5% two years after the fire, suggesting that higher post-fire N retention was due largely to the increased incorporation of N into microbial biomass. Fire also increased 15N recovery in bulk roots after one and two years, but only under summer warming. This suggests that higher retention of post-fire N can strongly increase the potential for N uptake of recovering plants under a future warmer climate. There was significantly lower 15N enrichment of Betula nana leaves while higher 15N enrichment of Vaccinium uliginosum leaves (after three years) in burned than control plots. This shows that fire can alter the N uptake differently among dominant shrub species in this tundra ecosystem, and implies that wildfires may change plant species composition in the longer term.

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

The stable isotope 15N is an extremely useful tool for studying the nitrogen (N) cycle of terrestrial ecosystems. The affordability of isotope-ratio mass spectrometry has increased in the last decades and routine measurements of δ15N with an accuracy better than 1‰ are now easily achieved. Except perhaps for wood, which has a very high C/N ratio, isotope analysis of samples is, thus, no longer the main challenge in measuring the partitioning of 15N used as tracer in ecosystem studies. The central aim of such experiments is to quantitatively determine the fate of N after it enters an ecosystem, mainly as fertilizer, as atmospheric deposition or as plant litter. By measuring how much of this incoming N goes into different ecosystem pools, inferences can be made about the entire N cycle. Sample collection and preparation can be tedious work. Optimizing sampling schemes is thus an important aspect in the application of 15N in ecosystem research and can be helpful for obtaining a high precision of the results with the available manpower and budget. In this contribution, we combine statistical and practical considerations and give recommendations for the design of labeling experiments and also for assessments of natural 15N abundance. In particular, we discuss soil, vegetation and water sampling. We additionally address the most common questions arising during the calculation of tracer partitioning, and we provide some examples of the interpretation of experimental results.

ACCELERATED CELL DEATH 6 Acts on Natural Leaf Senescence and Nitrogen Fluxes in Arabidopsis.

As the last step of leaf development, senescence is a molecular process involving cell death mechanism. Leaf senescence is trigged by both internal age-dependent factors and environmental stresses. It must be tightly regulated for the plant to adopt a proper response to environmental variation and to allow the plant to recycle nutrients stored in senescing organs. However, little is known about factors that regulate both nutrients fluxes and plant senescence. Taking advantage of variation for natural leaf senescence between Arabidopsis thaliana accessions, Col-0 and Ct-1, we did a fine mapping of a quantitative trait loci for leaf senescence and identified ACCELERATED CELL DEATH 6 (ACD6) as the causal gene. Using two near-isogeneic lines, differing solely around the ACD6 locus, we showed that ACD6 regulates rosette growth, leaf chlorophyll content, as well as leaf nitrogen and carbon percentages. To unravel the role of ACD6 in N remobilization, the two isogenic lines and acd6 mutant were grown and labeled with 15N at the vegetative stage in order to determine 15N partitioning between plant organs at harvest. Results showed that N remobilization efficiency was significantly lower in all the genotypes with lower ACD6 activity irrespective of plant growth and productivity. Measurement of N uptake at vegetative and reproductive stages revealed that ACD6 did not modify N uptake efficiency but enhanced nitrogen translocation from root to silique. In this study, we have evidenced a new role of ACD6 in regulating both sequential and monocarpic senescences and disrupting the balance between N remobilization and N uptake that is required for a good seed filling.

Degradation of RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) in contrasting coastal marine habitats: Subtidal non-vegetated (sand), subtidal vegetated (silt/eel grass), and intertidal marsh

Hundreds of explosive-contaminated marine sites exist globally, many of which contain the common munitions constituent hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). Quantitative information about RDX transformation in coastal ecosystems is essential for management of many of these sites. Isotopically labelled RDX containing 15N in all 3 nitro groups was used to track the fate of RDX in three coastal ecosystem types. Flow-through mesocosms representing subtidal vegetated (silt/eel grass), subtidal non-vegetated (sand) and intertidal marsh ecosystems were continuously loaded with isotopically labelled RDX for 16–17 days. Sediment, pore-water and overlying surface water were analyzed to determine the distribution of RDX, nitroso-triazine transformation products (NXs) and nitrogen containing complete mineralization products, including ammonium, nitrate+nitrite, nitrous oxide and nitrogen gas. The marsh, silt, and sand ecotypes transformed 94%, 90% and 76% of supplied RDX, respectively. Total dissolved NXs accounted for 2%–4% of the transformed 15N-RDX. The majority of RDX transformation in the water column was by mineralization to inorganic N (dissolved and evaded; 64%–78% of transformed 15N-RDX). RDX was mineralized primarily to N2O (62–74% of transformed 15N-RDX) and secondarily to N2 (1–2% of transformed 15N-RDX) which exchanged with the atmosphere. Transformation of RDX was favored in carbon-rich lower redox potential sediments of the silt and marsh mesocosms where anaerobic processes of iron and sulfate reduction were most prevalent. RDX was most persistent in the carbon-poor sand mesocosm. Partitioning of 15N derived from RDX onto sediment and suspended particulates was negligible in the overall mass balance of RDX transformation (2%–3% of transformed 15N-RDX). The fraction of 15N derived from RDX that was sorbed or assimilated in sediment was largest in the marsh mesocosm (most organic carbon), and smallest in the sand mesocosm (largest grain size and least organic carbon). Sediment redox conditions and available organic carbon stores affect the fate of RDX in different coastal marine habitats.

Substrate availability affects the partitioning of C and N in glycine between plants and soil microorganisms

ABSTRACT Amino acids are important components of the soil organic nitrogen pool. There is, however, a current lack of knowledge concerning how exogenous C and N regulate amino acid uptake in the soil microorganism/plant system. We used a 13C,15N-glycine injection method to examine glycine-derived 13C and 15N partitioning in a planted or unplanted spring wheat pot experiment. Four treatments were used: control (CK), ammonium sulfate (N), glucose (C), and ammonium sulfate with glucose (NC). The results showed that amino acids were absorbed intact by soil microorganisms and plants at 4 h. An exogenous excess of C, rather than N, resulted in a reduced uptake of intact glycine by soil microorganisms, indicating that glycine was taken up primarily to fulfill their C requirement. Wheat was more effective in competing for glycine-C and N at 24 h. 13C and 15N partitioning in wheat was significantly influenced by exogenous C and N availability; uptake of 13C and 15N by wheat increased with increasing C and N availability. We concluded that amino acids are important not only for N but also for C to soil microorganisms and plants, which are able to respond quickly to exogenous C and N source application.

Fertilization alters nitrogen isotopes and concentrations in ectomycorrhizal fungi and soil in pine forests

Abstract To assess how nitrogen (N) availability affected ectomycorrhizal functioning and to test a theoretical model of ectomycorrhizal 15N partitioning, we measured C/N and δ15N in soils and nine fungal taxa in two Swedish N addition experiments. Sporocarp C/N and soil C/N decreased with fertilization, implying that N uptake per unit fungal growth increased. The S horizon was more responsive than the F and H horizons to changes in N addition, with N turnover for these horizons of 24, 57, and 57 y, respectively. Fungal and soil δ15N patterns identified fungal N sources, with N acquisition primarily from the S, F, or H horizon for two, five, and two taxa, respectively. With increasing N availability, sporocarp 15N enrichment increased in five taxa, in agreement with our model of fungal-plant N partitioning. However, it decreased in Lactarius rufus and Russula aeruginea, perhaps indicating shifts towards greater inorganic N uptake in these two taxa. This may relate to the generally lower sensitivity of these taxa to N deposition compared to the Cortinarius and Suillus taxa that fit our model of 15N partitioning.

A New Role for SAG12 Cysteine Protease in Roots of Arabidopsis thaliana

Senescence associated gene (SAG) 12, which encodes a cysteine protease is considered to be important in nitrogen (N) allocation to Arabidopsis thaliana seeds. A decrease in the yield and N content of the seeds was observed in the Arabidopsis SAG12 knockout mutants (sag12) relative to the wild type (Col0) under limited nitrogen nutrition. However, leaf senescence was similar in both lines. To test whether SAG12 is involved in N remobilization from organs other than the leaves, we tested whether root N could be used in N mobilization to the seeds. Root architecture, N uptake capacity and 15N partitioning were compared in the wild type and sag12 under either high nitrogen (HN) or low nitrogen (LN) conditions. No differences in root architecture or root N uptake capacity were observed between the lines under HN or LN. However, under LN conditions, there was an accumulation of 15N in the sag12 roots compared to the wild type with lower allocation of 15N to the seeds. This was accompanied by an increase in root N protein contents and a significant decrease in root cysteine protease activity. SAG12 is expressed in the root stele of the plants at the reproductive stage, particularly under conditions of LN nutrition. Taken together, these results suggest a new role for SAG12. This cysteine protease plays a crucial role in root N remobilization that ensures seed filling and sustains yields when nitrogen availability is low.

Effects of grazing on the acquisition of nitrogen by plants and microorganisms in an alpine grassland on the Tibetan plateau

Nitrogen (N) limitation leads to intense competition between plants and soil microorganisms for available N. However, it is unclear how grazing affects the acquisition of N by plants and microorganisms. We conducted short-term 15N tracer experiments during the growing season (June, early growing season; July, mid-growing season; and September, late growing season) in an alpine grassland on the Tibetan Plateau to investigate the effects of grazing on the acquisition of NO3 −-N, NH4 +-N, and glycine-N by plants and soil microorganisms. Three dominant plant species (one graminoid, Kobresia pygmaea, and two forbs, Potentilla bifurca and Potentilla multifida) were selected for the study. As these species represented >90% of the vegetation, the plant recovery of 15N reflected competition at the plant community. Grazing decreased the recovery of 15N by soil microorganisms and plants by 46 and 69%, respectively, indicating that grazing strongly reduced the uptake of 15N by plants and microorganisms and altered the partitioning of 15N between them. Significant interactions were found between grazing, season and the different forms of N. In the absence of grazing, plants acquired relatively more N than soil microorganisms for the three forms of N in July and September, whereas the microorganisms obtained relatively more 15N glycine in July and all three forms of N in September under grazing conditions. Under grazing, the plant root biomass played an important role in controlling plant–microbial N acquisition. Grazing alters the partitioning of inorganic and organic N between plants and soil microorganisms by reducing microbial 15N recovery to a lesser extent than plant 15N recovery. This indicates that heterotrophic microorganisms play an important part in N cycling in N-limited ecosystems.

Contribution of previous year's leaf N and soil N uptake to current year's leaf growth in sessile oak

Abstract. The origin of N which contributes to the synthesis of N reserves of in situ forest trees in autumn and to the growth of new organs the following spring is currently poorly documented. To characterize the metabolism of various possible N sources (plant N and soil N), six distinct 20-year-old sessile oaks were 15N labelled by spraying 15NH415NO3: (i) on leaves in May, to label the N pool remobilized in the autumn for synthesis of reserves, (ii) on soil in the autumn, to label the N pool taken up from soil and (iii) on soil at the beginning of the following spring, to label the N pool taken up from soil in the spring. The partitioning of 15N in leaves, twigs, phloem, xylem, fine roots, rhizospheric soil and microbial biomass was followed during two growing seasons. Results showed a significant incorporation of 15N into the soil–tree system; more than 30 % of the administered 15N was recovered. Analysis of the partitioning clearly revealed that in autumn, roots' N reserves were formed from foliage 15N (73 %) and to a lesser extent from soil 15N (27 %). The following spring, 15N used for the synthesis of new leaves came first from 15N stored during the previous autumn, mainly from 15N reserves formed from foliage (95 %). Thereafter, when leaves were fully expanded, 15N uptake from the soil during the previous autumn and before budburst contributed to the formation of new leaves (60 %).

Nitrogen Isotope Exchange Between NO and NO2 and Its Implication for 15N Variations in Tropospheric NOx

Nitrogen oxides (NOx = NO + NO2) are trace gases that have numerous sources that include vehicles, power plants, lightning, soil emissions, agriculture, and biomass burning. It is important to estimate the budgets of these emission sources because NOx play an important role in air quality, climate, and human health. Analysis of the nitrogen stable isotope ratio (15N/14N) has been suggested as a useful method for partitioning NOx sources, as emission sources often have characteristic 15N/14N ratios (“fi ngerprints”). However, the effects of chemical and physical processes on 15N/14N ratios of NOx have not been researched in depth, which limits this isotopic technique. It has been suggested that the nitrogen isotope exchange between NO and NO2 has an infl uence on the partitioning of 15N between these two molecules. However, previous experimental and theoretical studies disagree about the value of this isotope exchange process. We have experimentally measured this isotope exchange process in which NO and NO2 were equilibrated in a vacuum system and NO2 was selectively removed from the system and measured for its 15N/14N ratio. Our experimental values agree exceptionally well with the theoretically predicted value, using corrections for anharmonicity. Using this data, we were able to model a diurnal cycle and seasonal changes in the distribution of 15N/14N ratios between NO and NO2 resulting from nitrogen isotopic exchange. Future research will measure 15N/14N ratios of NOx in ambient air to see if our predicted diurnal and seasonal change is correct.

Allocation patterns of airborne nitrogen in mountainous heathlands – A 15N tracer study in the Cantabrian Mountains (NW Spain)

The fate of atmospheric N depositions in heathlands dominated by Calluna vulgaris (L.) Hull in the Cantabrian Mountains (NW Spain) was analyzed in this study. The aim was to identify and quantify allocation patterns of airborne nitrogen in mountain heathland ecosystems by 15N tracer experiment. Four replicated plots were established to analyze 15N partitioning among different compartments selected (Calluna biomass, soil horizons and soil microbial biomass), besides N losses by leaching, using 15N tracer pulse addition. The study was conducted over two years. The recovery of 15N tracer was significantly higher (72%) in the first year compared to the second year (5%). Most 15N was recovered in the soil compartment in both years, mainly in the O-horizon. 15N losses by leaching were negligible over two years, suggesting that the ecosystem was not N saturated. Low 15N tracer recovery was found both in the new shoots of Calluna (0.5%) and the old ones (1.3%) in the short-term. The soil microbial biomass was not an important N sink in these heathlands. This study demonstrates that Cantabrian heathlands have a capacity to immobilize nitrogen in a short time, but that N is transferred out of the ecosystem during the second year.

Compound specific amino acid δ15N in marine sediments: A new approach for studies of the marine nitrogen cycle

The nitrogen (N) isotopic composition (δ15N) of bulk sedimentary N (δ15Nbulk) is a common tool for studying past biogeochemical cycling in the paleoceanographic record. Empirical evidence suggests that natural fluctuations in the δ15N of surface nutrient N are reflected in the δ15N of exported planktonic biomass and in sedimentary δ15Nbulk. However, δ15Nbulk is an analysis of total combustible sedimentary N, and therefore also includes mixtures of N sources and/or selective removal or preservation of N-containing compounds. Compound-specific nitrogen isotope analyses of individual amino acids (δ15NAA) are novel measurements with the potential to decouple δ15N changes in nutrient N from trophic effects, two main processes that can influence δ15Nbulk records. As a proof of concept study to examine how δ15NAA can be applied in marine sedimentary systems, we compare the δ15NAA signatures of surface and sinking POM sources with shallow surface sediments from the Santa Barbara Basin, a sub-oxic depositional environmental that exhibits excellent preservation of sedimentary organic matter. Our results demonstrate that δ15NAA signatures of both planktonic biomass and sinking POM are well preserved in such surface sediments. However, we also observed an unexpected inverse correlation between δ15N value of phenylalanine (δ15NPhe; the best AA proxy for N isotopic value at the base of the food web) and calculated trophic position. We used a simple N isotope mass balance model to confirm that over long time scales, δ15NPhe values should in fact be directly dependent on shifts in ecosystem trophic position. While this result may appear incongruent with current applications of δ15NAA in food webs, it is consistent with expectations that paleoarchives will integrate N dynamics over much longer timescales. We therefore propose that for paleoceanographic applications, key δ15NAA parameters are ecosystem trophic position, which determines relative partitioning of 15N into source AA versus trophic AA pools, and the integrated δ15NAA of all common protein AA (δ15NTHAA), which serves as a proxy for the δ15N of nutrient N. Together, we suggest that these can provide a coupled picture of regime shifts in planktonic ecosystem structure, δ15N at the base of food webs, and possibly additional information about nutrient dynamics.

Prolonged summer droughts retard soil N processing and stabilization in organo-mineral fractions

Prolonged summer droughts are projected to occur as a consequence of climate change in Central Europe. The resulting reduced soil water availability may lead to alterations in rates of soil processes such as nitrogen partitioning among soil organic matter fractions and stabilization within soil. To study the effect of climate change-induced drought on (1) the distribution of nitrogen among soil organic matter fractions and (2) nitrogen stabilization, we performed a space-for-time climate change experiment. We transferred intact plant–soil–microbe mesocosms of a Rendzic Leptosol with a young beech tree from a slope with northwestern exposure in southern Germany characterized by a cool-moist microclimate across a narrow valley to a slope with southwestern exposure with a warm-dry microclimate, which reflects projected future climatic conditions. A control transfer was also done on the northwest-facing slope within the same area of origin. We combined a homogenous 15N labeling approach using ammonium nitrate with a physical fractionation procedure and chemical soil extraction protocols. Our aim was to follow the partitioning of 15N in different soil organic matter fractions, i.e. light fractions, organo-mineral fractions, and extractable soil fractions including microbial biomass, ammonium, nitrate, and dissolved organic nitrogen. Within less than one growing season, we observed a modified partitioning of recently applied inorganic 15N between different soil fractions in relation to drier summer conditions, with attenuated nitrogen turnover under drought and consequently significantly higher 15N concentrations in the relatively labile light fractions. We ascribed this effect to a decelerated mineralization immobilization turnover. We conclude that prolonged summer droughts may alter the stabilization dynamics because the induced inactivity of microorganisms may reduce the transfer of nitrogen to stabilization pathways. A retarded stabilization in organo-mineral associations enhances the risk of nitrogen losses during extreme rainfall events, which are projected to increase in the 21st century predicted by future climate change scenarios for Central Europe.

13C and 15N partitioning among shoots, roots, and soil in Brassica napus genotypes varying in seed oil content potential

Four genotypes of Brassica napus (cultivars Topas, Sentry and Polo, and the experimental line 04C204) varying in seed oil content (SOC) potential (from 42.2% to 50.0% seed DW) were used to study C and N partitioning among shoots, roots and soil, as well as nitrogen use efficiency (NUE). Topas, with lowest SOC potential, had higher biomass accumulation in shoots and roots and higher NUE than 04C204 with the highest SOC potential. Although the absolute amounts of biomass and N accumulated in Topas were greater than in 04C204, the percentage of 13C partitioned to shoots was lower, and the percentage of 15N partitioned to roots was higher. These results imply that SOC may be more associated with the proportions of C and N partitioned between shoots and roots, than with absolute amounts accumulated in these organs. This study also indicates that while Topas fixed the most CO2, it also had the greatest nitrogen fertilizer demand, while 04C204, with the highest SOC potential, had the greatest proportion of whole plant C retained within shoots of all the genotypes. These results suggest that a breeding strategy to optimize B. napus as a biodiesel feedstock may well be selection for maximum SOC at a minimal investment in vegetative biomass (and hence a lower N fertilizer requirement).

Microbial N immobilization is of great importance in acidified mountain spruce forest soils

The prevailing N saturation paradigm still considers microbial N immobilization as a less important process of ecosystem N retention. On the contrary, we hypothesize that it can even be a primary process affecting N leaching from N saturated soils. We studied N transformations in soils of acidified near-natural and primeval forests in the Bohemian Forest (watersheds of Plešné and Čertovo Lakes, Czech Republic) and Pop Ivan Massif (Ukraine). Organic soils were sampled from similar conditions (1100–1500 m a.s.l., precipitation 1400–1800 mm, acidic bedrock, forest dominated by Picea abies) and had similar chemical properties (pHKCl 2.5–3.2, BS ∼45%, Alex ∼40 meq kg−1). However, the Ukrainian soil had lower soil C/N ratio (24 vs. 30) and C availability (water soluble C and C/N ratio of 65 vs. ∼114–163 μmol g−1 and 6 vs. 21–24, respectively) than the other soils. We ran laboratory experiments in which mixtures of different N sources (N–NH4, N–NO3 and glycine) were added to the soil with only one source 15N-labelled. We followed 15N partitioning within soil N pools and analysed the composition of the microbial community (16SrDNA-DGGE fingerprint of bacteria, ergosterol analyses, qPCR of fungal 18S rDNA gene). The microbial N pool was always three to five times higher than the total soluble N pool. We found rapid (15 min) and simultaneous immobilization of all added N forms into the microbial biomass with clear preferences for organic N over inorganic sources. The total N flux to the microbial pool always exceeded N flux into mineral N pools. The pattern of N transformation in the C limited soil was different from the other soils. The microbial pool and N flux into it were smaller compared to the mineral N pools and fluxes. The contribution of N–NO3 to microbial immobilization was negligible, while nitrification was almost equal to N mineralization. Total N flux through soluble N pools was greater than total N flux to insoluble pools (residual and microbial N); this was accompanied by lower microbial N uptake efficiency and shorter residence time of N in microbial pool than in soils with higher C availability. The composition of bacterial community was related to DOC content and C and N in microbial biomass. In soils with higher fungal abundance, more glycine was immobilized regardless of soil C availability, but with higher deamination (∼50 vs. 20%) and subsequent release of N–NH4 back to the soil. Our study emphasized the role of microbial N immobilization in preventing N–NO3 loss from N saturated ecosystems as a function of C availability. Nitrification was favoured when enough N–NH4 was available in the C limited soil. The discharged N–NO3 was not immobilized by the microbes and could be, if not immobilized by plants, leached out. C limitation plays an important role in the susceptibility of ecosystems to N leaching and could partially explain the observed differences in some N-saturated ecosystems.

Impact of industrial-age climate change on the relationship between water uptake and tissue nitrogen in eucalypt seedlings.

This study explored reductions in tissue nitrogen concentration ([N]) at elevated CO2 concentrations ([CO2]), and changes in plant water and N uptake. Eucalyptus saligna Sm. seedlings were grown under three [CO2] levels (preindustrial (280μLL-1), current (400μLL-1) or projected (640μLL-1)) and two air temperatures (current, (current+4°C)). Gravimetric water use, leaf gas exchange and tissue dry mass and %N were determined. Solid-state 15N-NMR spectroscopy was used for determining the partitioning of N chemical groups in the dry matter fractions. Water use efficiency (WUE) improved with increasing [CO2] at ambient temperature, but strong leaf area and weak reductions in transpiration rates led to greater water use at elevated [CO2]. High temperature increased plant water use, such that WUE was not significantly stimulated by increasing [CO2] at high temperature. Total N uptake increased with increasing [CO2] but not temperature, less than the increase recorded for plant biomass. Tissue [N] decreased with rising [CO2] and at high temperature, but N use efficiency increased with rising [CO2]. Total N uptake was positively correlated with total water use and root biomass under all treatments. Growth [CO2] and temperature did not affect the partitioning of 15N among the N chemical groups. The reductions of tissue [N] with [CO2] and temperature were generic, not specific to particular N compounds. The results suggest that reductions in tissue [N] are caused by changes in root N uptake by mass flow due to altered transpiration rates at elevated [CO2] and temperature.

Biogeochemistry of nitrous oxide in Lake Kizaki, Japan, elucidated by nitrous oxide isotopomer analysis

[1] The biogeochemistry of nitrous oxide (N2O) was investigated in Lake Kizaki, Japan, where accumulation of N2O in the water column has been observed. The N2O concentration profile showed weak accumulation in the oxic zone, although much higher and much lower N2O concentrations were observed in the deeper oxygen-deficient zone. Intramolecular partitioning of 15N (site preference) of N2O within the oxic zone increased concomitantly with increased N2O concentration. The site preference of the newly produced N2O in the oxic zone was estimated as 33.6‰. This high site preference strongly suggests that this N2O was produced by hydroxylamine oxidation. In regions of the oxygen-deficient zone, the nitrate (NO3−) concentration decreased rapidly, concomitantly with increased nitrogen and oxygen isotope ratios, indicating denitrification. The high site preference and nitrogen isotope ratio of N2O (δ15Nbulk), combined with isotopic data of NO3−, strongly suggest denitrification as the main N2O source. Moreover, site preference and δ15Nbulk data suggest that the existing N2O in the oxygen-deficient zone was already strongly reduced (more than 75%) to N2. Results of this study demonstrate the feasibility of using isotope and isotopomer analyses of N compounds to elucidate the complex biogeochemistry of N2O in an intact ecosystem.