15N Tracer Method Research Articles

Innovative no-till seeding technology improves yield and nitrogen use efficiency while reducing environmental pressure in wheat after rice harvesting

Optimizing tillage and seeding strategies is important for increasing crop productivity and nitrogen use efficiency (NUE) while reducing environmental impact. A newly developed innovative no-till seeding (INtS) technology was shown to effectively enhance seeding quality and increase wheat yield in wet clay soil after puddled rice harvest in the Yangtze River Basin (YRB) of China. However, comprehensive assessments of the yield, NUE, and eco-environmental benefits of INtS technology have not been performed. The present study conducted a consecutive three-year field experiment combining field plots and microplots with the 15N tracer method using two wheat seeding technologies (INtS and typical rotary-till seeding (TRtS) technology) and three nitrogen (N) application rates (0, 90, and 180 kg ha−1). The study evaluated the yield, NUE, fate of fertilizer/straw N, N budgets, and environmental footprints of wheat production after rice harvesting. The results showed that compared with TRtS, the average yield, N uptake and NUE of wheat under INtS technology increased by 27.2%, 28.9%, and 31.9%, respectively. By tracing the multi-seasonal fate of 15N-labeled fertilizer/straw N in soil-crop systems, it was found that 43.8% and 6.0% of fertilizer N and rice straw N applied in the first wheat growing season were recovered by wheat plants in the same season under INtS technology, which significantly decreased to 29.0% and 4.1% for TRtS technology. Over the five growing seasons, the cumulative recoveries of fertilizer and straw N for INtS technology were 26.2% and 15.6% higher, while the total fertilizer and straw N losses were 20.6% and 20.0% lower than those for TRtS technology, respectively. Over 50% of straw N remained in the soil after the fifth growing season, while only 14.5–23.6% of fertilizer N remained. In addition, INtS technology significantly reduced the fertilizer/straw N losses when applied in the subsequent rice growing season. Unlike TRtS technology, INtS could maintain a low-level soil N balance (2.1 kg N ha−1 for N180) and generate less soil surface N surplus in the wheat growing season. On average, INtS significantly reduced the carbon footprint and N footprint in the wheat growing season by 26.8% and 19.1%, respectively. In conclusion, INtS technology can significantly improve crop production and agricultural sustainability while minimizing negative environmental impacts for wheat production after rice cultivation in the YRB of China and other areas of the world with similar field conditions.

Nitrogen fixation under the interaction of Kuroshio and upwelling in the northeastern South China Sea

Diazotrophic organisms are vulnerable to physical dynamic processes. However, the interactions and mechanisms of multiple dynamic processes on nitrogen fixation remain unclear. This study aimed to investigate one of these interactions; we conducted field investigations and bioassay experiments in the northeastern South China Sea, which faces the Kuroshio current and coastal upwelling. Using 15N tracer and molecular biology methods, we found that the eastern Guangdong upwelling, having high phosphorus and a lower N:P ratio, had higher nitrogen fixation rates as well as a higher abundance of diazotrophic cyanobacteria Trichodesmium spp. and UCYN-A compared with nonupwelling area. Additionally, a higher abundance of diazotrophic cyanobacteria occurred near the bottom of the euphotic zone along the shelf break that was affected by the Kuroshio current. Field incubation experiments confirmed that phosphorus increased the nitrogen fixation rates and stimulated the growth of cyanobacteria in the area affected by upwelling. Taken together, our results indicated that warmer surface water with a high N:P ratio likely mixed with upwelled phosphorus-rich water containing diazotrophic cyanobacteria, which were transported by the intensified Kuroshio current under the influence of the 2015/16 El Niño. This mixing then triggered a high abundance of diazotrophic cyanobacteria. These findings provide new insights into how the interaction of multiple dynamic processes can regulate nitrogen fixation.

Continuous remobilization from below‐ground provides more than half of all carbon and nitrogen in regrowing shoots after grassland defoliation

Abstract Remobilization of carbon (C) and nitrogen (N) from roots and crowns to regrowing shoots is an important strategy for grassland plants to tolerate herbivory. While this remobilization is thought to terminate once plants have the capacity to photosynthesize again and to take up N from the soil, this has rarely been quantified. Here we investigated the remobilization of C and N to regrowing shoots of a perennial grassland under two defoliation frequencies (every 15 and 30 days) and two N fertilization levels (0 and 2.8 g m−2) by growing intact plant–soil cores for 60 days in a growth chamber. We measured the C remobilization from roots and crowns to regrowing shoots using a novel continuous 13C labelling method, and quantified the N remobilization to regrowing shoots and plant N uptake from soil with a 15N tracer method. Regrowing shoots relied strongly on below‐ground C reserves, where up to 52% of C in regrowing shoots came from below‐ground during the first 30 days after defoliation. Below‐ground C was still remobilized when plant shoots had fully regrown and retained full capacity to photosynthesize. Plants relied more on C remobilization with shorter times between defoliation events at the expense of root biomass, particularly without N fertilization. Large amounts of C remobilization were accompanied by large amounts of N remobilization (up to 89% of total shoot N) that were positively related, suggesting a coupled C and N transfer from roots and crowns to shoots. Synthesis. The unexpectedly high dependency on C and N remobilization occurred continuously after defoliation and did not terminate once plants were fully regrown. Our results indicate that remobilization of C and N from below‐ground has profound effects on long‐term productivity and C sequestration that needs to be considered for managing similar grasslands worldwide.

Effect of Phosphorus Supply Levels on Nodule Nitrogen Fixation and Nitrogen Accumulation in Soybean (Glycine max L.)

The specific mechanism by which phosphorus affects nodule nitrogen fixation and nitrogen absorption in soybeans remains inconclusive. To further quantitatively analyze the effect of phosphorus on nodule nitrogen fixation and nitrogen accumulation in soybeans, this experiment was carried out under sand culture conditions. The experiment consisted of six phosphorus supply levels (1 mg/L, 11 mg/L, 21 mg/L, 31 mg/L, 41 mg/L, 51 mg/L). The acetylene reduction method and 15N tracer method (50 mg/L (NH4)2SO4) were used to determine and analyze the nodule growth status, nodule nitrogenase activity, nitrogen content, and nodule nitrogen fixation rate at initial flowering (R1 stage), initial pod (R3 stage), seed filling (R5 stage) and maturity stages (R8 stage). The results are described as follows: 1. The nitrogen fixation of soybean nodules at different growth stages has different requirements for phosphorus supply levels. The initial flowering stage and seed-filling stage were 31 mg/L–41 mg/L, and the initial pod stage was 51 mg/L. 2. The nitrogen source in different parts of soybean showed different trends with different growth periods and phosphorus supply concentrations. Among them, from the initial flowering stage to the seed filling stage, the main body of the nitrogen supply of soybean shoots in the low phosphorus treatment (1 mg/L–31 mg/L) gradually changed from fertilizer nitrogen to nodule nitrogen fixation, while the main body of the nitrogen supply of soybean shoots in the high phosphorus treatment (41 mg/L–51 mg/L) always showed nodule nitrogen fixation and was transformed into fertilizer nitrogen at the mature stage. The main nitrogen supply to the roots of soybean at different levels of phosphorus supply from the initial flowering to the initial pods and maturity stage was fertilizer nitrogen, and the main nitrogen supply at the seed filling stage was nodule nitrogen fixation. The nitrogen supply to the main body of soybean nodules was constantly nodule nitrogen fixation. 3. Different phosphorus supply levels significantly affected the nitrogen fixation of soybean nodules (R2 ≥ 0.803), and both the acetylene reduction method and the 15N tracer method could be used to determine the nitrogen fixation capacity of soybean nodules. This study indicated the optimal phosphorus supply level of nodules in different growth stages of soybean and clarified the main body of phosphorus supply in different parts of soybean at different growth stages, which pointed out the direction for further improving the utilization efficiency of soybean nitrogen and phosphorus fertilizer.

Evaluating the impact of biochar on biomass and nitrogen use efficiency of sugarcane using 15N tracer method

N is an essential nutrient for sugarcane (Saccharum spp. Hybribds) growth. Excess chemical nitrogen fertilizer applied still a serious problem of China sugarcane plant. Biochar has shown promise in improving crop yield and N use efficiency (NEU).However its impact on sugarcane is not well-studied. To investigate how biochar impacts on sugarcane growth and nitrogen N use efficiency (NUE), a glasshouse pot experiment was conducted using the 15N tracer method. Two cultivars, GT11 and B8, were chosen as test objects and were planted under low N(120 kg N hm-2) and high N(600 kg N hm-2)condition, respectively. The effects of low and high biochar application rates (10 t hm-2 and 20 t hm-2) on growth, nitrogen uptake, accumulation and distribution as well as NUE in GT11 and B8 were studied. Results showed that sugarcane biomass was not significantly affected by biochar application. N uptake by GT11 was significantly increased 23.91% - 45.42% by C20 and N120 condition at tillering stage and elongation stage. While N uptake by B8 showed a significant response to B10 and B20 with an increase of 27.27% and 30.40% at tillering stage,respectively. Biochar application led to 0.28% - 23.75% and 1.08% - 30.07% increase in NUE of GT11 and B8,respectively. The effect of biochar application of N from fertilizer(FF) was significant,however only C20 treatment shown remarkable response when under low N treatment. Our study suggest that the effects of biochar on sugarcane depend on varieties and the applied rate of biochar and N fertilizer.Biochar application with inorganic N could improve N uptake and N use of sugarcane.

Exploring the Functions of Efficient Canonical Denitrifying Bacteria as N2O Sinks: Implications from 15N Tracer and Transcriptome Analyses.

In denitrifying reactors, canonical complete denitrifying bacteria reduce nitrate (NO3-) to nitrogen via N2O. However, they can also produce N2O under certain conditions. We used a 15N tracer method, in which 15N-labeled NO3-/nitrite (NO2-) and nonlabeled N2O were simultaneously supplied with organic electron donors to five canonical complete denitrifying bacteria affiliated with either Clade I or Clade II nosZ. We calculated their NO3-, NO2-, and N2O consumption rates. The Clade II nosZ bacterium Azospira sp. strain I13 had the highest N2O consumption rate (3.47 ± 0.07 fmol/cell/h) and the second lowest NO3- consumption rate (0.20 ± 0.03 fmol/cell/h); hence, it is a N2O sink. A change from peptone- to acetate/citrate-based organic electron donors increased the NO3- consumption rate by 4.8 fold but barely affected the N2O consumption rate. Electron flow was directed to N2O rather than NO3- in Azospira sp. strain I13 and Az. oryzae strain PS only exerting a N2O sink but to NO3- in the Clade I nosZ N2O-reducing bacteria Pseudomonas stutzeri strain JCM 5965 and Alicycliphilus denitrificans strain I51. Transcriptome analyses revealed that the genotype could not fully describe the phenotype. The results show that N2O production and consumption differ among canonical denitrifying bacteria and will be useful for developing N2O mitigation strategies.

Dense Planting with Reducing Nitrogen Rate Increased Nitrogen Use Efficiency and Translocated Nitrogen in Grains in Double-Cropped Rice

Nitrogen fertilization and planting density are two key factors that influence the yield of rice. Reducing nitrogen fertilizer input and increasing planting density will help to improve nitrogen use efficiency and stabilize yield. Field and 15N tracer method in plot experiments were conducted to study the trends of yield, nitrogen use efficiency (NUE) and nitrogen transfer of hybrid rice and conventional rice under dense planting with a reduced nitrogen rate (DPRN) and sparse planting with a high nitrogen rate (SPHN). Among the nitrogen in rice plants, the proportion of nitrogen from fertilizer under the DPRN was reduced by 1.8–13%. The late-season rice (LSR) had a higher rate of decrease compared with the early-season rice (ESR). The uptake efficiency of nitrogen fertilizer was significantly higher under the DPRN than that under the SPHN, with an increase of 7.7–21.9%. The accumulated nitrogen and translocated ratio under the DPRN before the heading stage were 6.1–10.8% and 2.0–9.6% higher than those under the SPHN, respectively. The yield did not change under different treatments. Those findings suggest that the DPRN could guarantee a stabilized yield while increasing the NUE and the amount of translocated nitrogen in the double-cropped rice system.

Nitrous oxide-sink capability of denitrifying bacteria impacted by nitrite and pH

Nitrous oxide (N2O) is a highly potent greenhouse gas and ozone-depleting substance, produced and consumed during denitrification. Evaluation of the N2O production and consumption activities of complete denitrifying bacteria is essential for understanding their capacity to act as N2O sinks in engineered systems for cost-effective nitrogen removal via nitrite (NO2−). However, the physiologies of these N2O-reducing bacteria (N2ORB) are poorly understood. This study aimed to evaluate the physiologies of two N2ORB, Azospira sp. strain I13 and Alicycliphilus denitrificans strain I51. A 15N tracer method was applied to determine N2O production and consumption activities in the co-presence of NO2− and N2O. Both N2ORB displayed a higher N2O consumption rate (RN2O by Azospira sp. strain I13 and Alicycliphilus denitrificans strain I51, 23.85 and 7.60 μmol-N mg-biomass−1h−1, respectively) than N2O production rate (PN2O, 5.88 and 1.32 μmol-N mg-biomass−1h−1, respectively) at an initial NO2− concentration of 2.14 mmol-N L−1 with exogenous addition of N2O, indicating that these N2ORB acted as N2O sinks. On increasing the NO2− concentration from 0.36 to 7.14 mmol-N L−1, the net N2O consumption rate RO_N2O (=RN2O – PN2O) decreased for both N2ORB; the magnitude of the decrease was greater for Azospira sp. strain I13 than for Alicycliphilus denitrificans strain I51. The formation of free nitrous acid (FNA) from NO2− in acidic conditions noticeably affected the N2O sink activities of the N2ORB. A higher FNA concentration decreased RO_N2O for both N2ORB, creating the risk of N2O emission at pH 6 and high NO2− concentration. Our results show the ranges of pH and NO2− concentration where Azospira sp. strain I13 is promising for use as an N2O sink in shortcut nitrogen removal via NO2−.

Combination of 15N Tracer and Microbial Analyses Discloses N2O Sink Potential of the Anammox Community.

Although nitrogen removal by partial nitritation and anammox is more cost-effective than conventional nitrification and denitrification, one downside is the production and accumulation of nitrous oxide (N2O). The potential exploitation of N2O-reducing bacteria, which are resident members of anammox microbial communities, for N2O mitigation would require more knowledge of their ecophysiology. This study investigated the phylogeny of resident N2O-reducing bacteria in an anammox microbial community and quantified individually the processes of N2O production and N2O consumption. An up-flow column-bed anammox reactor, fed with NH4+ and NO2- and devoid of oxygen, emitted N2O at an average conversion ratio (produced N2O: influent nitrogen) of 0.284%. Transcriptionally active and highly abundant nosZ genes in the reactor biomass belonged to the Burkholderiaceae (clade I type) and Chloroflexus genera (clade II type). Meanwhile, less abundant but actively transcribing nosZ strains were detected in the genera Rhodoferax, Azospirillum, Lautropia, and Bdellovibrio and likely act as an N2O sink. A novel 15N tracer method was adapted to individually quantify N2O production and N2O consumption rates. The estimated true N2O production rate and true N2O consumption rate were 3.98 ± 0.15 and 3.03 ± 0.18 mgN·gVSS-1·day-1, respectively. The N2O consumption rate could be increased by 51% (4.57 ± 0.51 mgN·gVSS-1·day-1) with elevated N2O concentrations but kept comparable irrespective of the presence or absence of NO2-. Collectively, the approach allowed the quantification of N2O-reducing activity and the identification of transcriptionally active N2O reducers that may constitute as an N2O sink in anammox-based processes.

Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

AbstractExamination of dinitrogen (N2) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N2 fixation rates were measured using a robust 15N tracer method (bubble removal) that accounts for the slow dissolution of N2 gas and calculated using a conservative approach. N2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20 μmol O2 kg−1) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non‐cyanobacterial diazotrophs. Nevertheless, this study indicates that N2 fixation is neither widespread nor quantitatively important throughout this region.

Dissimilatory nitrate reduction to ammonium increased with rising temperature

Although recent studies have explored dissimilatory nitrate reduction to ammonium (DNRA), little is known about the dependence of this process on temperature. A laboratory experiment was conducted to investigate if temperature regulates nitrate (NO3ˉ) reduction by DNRA and denitrification. Soil samples were incubated at 10 to 40 °C for 28 days, and 15N tracer methods were applied to estimate the reduction of 15NO3ˉ to 15NH4+ by DNRA organisms and to 15N2O or 15N2 by denitrifiers. Raising the temperature from 10 to 40 °C increased DNRA, which contributed to enrichment of the NH4+ pool, particularly at 40 °C. The percentage of 15NO3ˉ reduced by DNRA was from 4 to 30% of that reduced by denitrification, and decreased with increasing temperature. DNRA and denitrification were positively correlated with the abundance of nrfA and nosZ genes, respectively. The accumulation of 15NH4+ slowed in the final 12 h of sampling times, which may be due to a re-conversion of 15NH4+ to 15NO3ˉ by nitrification. High temperatures (30 to 40 °C) enhanced 15N2 production, which suggests that temperature plays an important role in regulating complete denitrification from NO3ˉ to N2.

Assessing the Systematic Effects of the Concentration of Nitrogen Supplied to Dual-Root Systems of Soybean Plants on Nodulation and Nitrogen Fixation

The specific mechanisms by which nitrogen affects the nodulation and nitrogen fixation of soybean plants are unclear. Investigating the relationship between nitrogen, nodulation and nitrogen fixation can provide new insights for the rational and proper use of nitrogen fertilizer in soybean plants. In this study, we grafted soybean roots to construct a dual-root system with a single nodulated side. Experiment I was performed at the third trifoliate leaf to initial seed filling (V3-R3) growth stages (for 30 days) for long-term nitrogen supply, and Experiment II was performed at the third trifoliate leaf to fourth trifoliate leaf (V3-V4) growth stages (for 5 days) for short-term nitrogen supply. For the two experiments, a nutrient solution providing 15NH415NO3 or NH4NO3 as the nitrogen source was added to the non-nodulated side, while a nitrogen-free nutrient solution was added to the nodulated side. The concentrations of nitrogen supplied were 0 mg/L, 25 mg/L, 50 mg/L, 75 mg/L, and 100 mg/L. The results showed the following: (1) Short-term nitrogen supply systematically regulated the specific nitrogenase activity (SNA), thereby inhibiting the acetylene reduction activity (ARA). Under long-term nitrogen supply, the recovery of SNA was generally consistent across treatments, and the concentration of nitrogen supplied systematically regulated the growth of root nodules, thereby inhibiting the ARA. (2) Using the 15N tracer method, the concentration of fertilizer nitrogen was positively correlated with the amount of nitrogen redistributed to other organs. Although the percentage of nitrogen derived from the atmosphere (Ndfa%) decreased significantly with increasing concentrations of nitrogen supplied, the effect on the accumulation of nitrogen fixed by nodules (Naccumulation of nodules) was not significant. By establishing the relationships between the ARA (measured by the acetylene reduction method), Ndfa% (based on 15N calculations), and Naccumulation nodules (based on 15N calculations), it was found that the ARA reliably reflected the Ndfa% but not the Naccumulation of nodules.

Testing deep placement of an 15N tracer as a method for in situ deep root phenotyping of wheat, barley and ryegrass

BackgroundDeep rooting is one of the most promising plant traits for improving crop yield under water-limited conditions. Most root phenotyping methods are designed for laboratory-grown plants, typically measuring very young plants not grown in soil and not allowing full development of the root system.ResultsThis study introduced the 15N tracer method to detect genotypic variations of deep rooting and N uptake, and to support the minirhizotron method. The method was tested in a new semifield phenotyping facility on two genotypes of winter wheat, seven genotypes of spring barley and four genotypes of ryegrass grown along a drought stress gradient in four individual experiments. The 15N labeled fertilizer was applied at increasing soil depths from 0.4 to 1.8 m or from 0.7 to 2.8 m through a subsurface tracer supply system, and sampling of aboveground biomass was conducted to measure the 15N uptake. The results confirm that the 15N labeling system could identify the approximate extension of the root system. The results of 15N labeling as well as root measurements made by minirhizotrons showed rather high variation. However, in the spring barley experiment, we did find correlations between root observations and 15N uptake from the deepest part of the root zone. The labeled crop rows mostly had significantly higher 15N enrichment than their neighbor rows.ConclusionWe concluded that the 15N tracer method is promising as a future method for deep root phenotyping because the method will be used for phenotyping for deep root function rather than deep root growth. With some modifications to the injection principle and sampling process to reduce measurement variability, we suggest that the 15N tracer method may be a useful tool for deep root phenotyping. The results demonstrated that the minirhizotrons observed roots of the tested rows rather than their neighboring rows.

Contrasting impact of elevated atmospheric CO2 on nitrogen cycle in eutrophic water with or without Eichhornia crassipes (Mart.) Solms

The elevation of atmospheric CO2 is an inevitable trend that would lead to significant impact on the interrelated carbon and nitrogen cycles through microbial activities in the aquatic ecosystem. Eutrophication has become a common trophic state of inland waters throughout the world, but how the elevated CO2 affects N cycles in such eutrophic water with algal bloom, and how vegetative restoration helps to mitigate N2O emission remains unknown. We conducted the experiments to investigate the effects of ambient and elevated atmospheric CO2 (a[CO2], e[CO2]; 400, 800 μmol﹒mol−1) with and without the floating aquatic plant, Eichhornia crassipes (Mart.) Solms, on N-transformation in eutrophic water using the 15N tracer method. The nitrification could be slightly inhibited by e[CO2], due mainly to the competition for dissolved inorganic carbon between algae and nitrifiers. The e[CO2] promoted denitrification and N2O emissions from eutrophic water without growth of plants, leading to aggravation of greenhouse effect and forming a vicious cycle. However, growth of the aquatic plant, Eichhornia crassipes, slightly promoted nitrification, but reduced N2O emissions from eutrophic water under e[CO2] conditions, thereby attenuating the negative effect of e[CO2] on N2O emissions. In the experiment, the N transformation was influenced by many factors such as pH, DO and algae density, except e[CO2] and plant presence. The pH could be regulated through diurnal photosynthesis and respiration of algae and mitigated the acidification of water caused by e[CO2], leading to an appropriate pH range for both nitrifying and denitrifying microbes. Algal respiration at night could consume DO and enhance abundance of denitrifying functional genes (nirK, nosZ) in water, which was also supposed to be a critical factor affecting denitrification and N2O emissions. This study clarifies how the greenhouse effect caused by e[CO2] mediates N biogeochemical cycle in the aquatic ecosystem, and how vegetative restoration mitigates greenhouse gas emission.

Role of organic carbon, nitrate and ferrous iron on the partitioning between denitrification and DNRA in constructed stormwater urban wetlands

Denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) are two competing nitrate reduction pathways that remove or recycle nitrogen, respectively. However, factors controlling the partitioning between these two pathways are manifold and our understanding of these factors is critical for the management of N loads in constructed wetlands. An important factor that controls DNRA in an aquatic ecosystem is the electron donor, commonly organic carbon (OC) or alternatively ferrous iron and sulfide. In this study, we investigated the role of natural organic carbon (NOC) and acetate at different OC/NO3− ratios and ferrous iron on the partitioning between DNF and DNRA using the 15N-tracer method in slurries from four constructed stormwater urban wetlands in Melbourne, Australia. The carbon and nitrate experiments revealed that DNF dominated at all OC/NO3− ratios. The higher DNF and DNRA rates observed after the addition of NOC indicates that nitrate reduction was enhanced more by NOC than acetate. Moreover, addition of NOC in slurries stimulated DNRA more than DNF. Interestingly, slurries amended with Fe2+ showed that Fe2+ had significant control on the balance between DNF and DNRA. From two out of four wetlands, a significant increase in DNRA rates (p < .05) at the cost of DNF in the presence of available Fe2+ suggests DNRA is coupled to Fe2+ oxidation. Rates of DNRA increased 1.5–3.5 times in the Fe2+ treatment compared to the control. Overall, our study provides direct evidence that DNRA is linked to Fe2+ oxidation in some wetland sediments and highlights the role of Fe2+ in controlling the partitioning between removal (DNF) and recycling (DNRA) of bioavailable N in stormwater urban constructed wetlands. In our study we also measured anammox and found that it was always <0.05% of total nitrate reduction in these sediments.

A Comparison of Nitrogen Transfer and Transformation in Traditional Farming and the Rice–Duck Farming System by 15N Tracer Method

A field experiment was conducted in Ninghe, Tianjin, China, using the 15N isotope method to determine the fate of N sources, application effect of organic fertilizer on the growth of rice plant organs, N uptake by rice, and N use efficiency. The experiment included eight treatments: CK-N (control + no-duck), CK-D (control + ducks), CF-N (chemical fertilizer + no-ducks), CF-D (chemical fertilizer + ducks), CM-N (chemical fertilizer + organic fertilizer + no-ducks), CM-D (chemical fertilizer + organic fertilizer + ducks), CD-N (chemical fertilizer 30% off + organic fertilizer + no-ducks), and CD-D (chemical fertilizer 30% off + organic fertilizer + ducks). The results showed that the application of organic fertilizer whether CM or CD in grain and leaf significantly increased N concentration; leaf and root P concentrations over control (CK) and chemical fertilizer (CF). In contrast, straw and root N concentrations, including grain and straw P concentrations did not show any difference between duck and no-duck treatment. Moreover, non-significant differences were found in 15N fresh grain and husk concentration. Both organs ranged from 14.2–14.4 g·kg−1 and 6.2–6.3 g·kg−1, respectively. Likewise, N uptake and N use efficiency in fresh grain and husk were not significantly differed within duck and without duck treatment. However, N uptake in fresh grain and husk ranged at the rates of 54.90–93.69 and 6.43–11.04 kg ha−1 with duck and without duck treatment. N use efficiency in fresh grain and husk ranged from 21.55%–34.61% and 2.61%–4.24%, respectively. Overall organic fertilizer has a significant influence on rice growth and promotes crop productivity.

Correction: Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

[This corrects the article DOI: 10.1371/journal.pone.0189831.].

Fates of atmospheric deposited nitrogen in an Asian tropical primary forest

The impacts of increasing nitrogen (N) deposition on forest ecosystems, including on carbon (C) sequestration, largely depend on the extent to which forests are N-limited and so whether and where deposited N is retained within the ecosystem. The 15N tracer method can provide excellent insight into the ecosystem fates of N, but while it has been extensively used in temperate forests it has yet to be sufficiently employed in tropical forests, which are often thought not to be N-limited. Here, we used stable isotope 15NH4+ and 15NO3− tracers applied as solutions to the forest floor to examine the fates of different forms of N in a tropical montane primary forest with low background atmospheric N deposition (6 kg N ha−1 yr−1) in China. We found that a substantial amount of 15N was assimilated by plants over time and significantly more 15N was recovered following 15NO3− addition than following 15NH4+ addition: 7% and 16% of 15N were recovered three months after the respective 15NH4+ and 15NO3− tracer additions and 11% and 29% respectively after one year. In contrast to plants, the organic layer was only an important short-term sink for deposited N: while 21% and 12% of the 15N from 15NH4+ and 15NO3− additions were accumulated in the organic layer after three months, more than half of the retained 15N was lost after one year. Mineral soil was the largest sink for deposited N, and the 15N retained in soil was relatively stable over time for both N forms, with 39% and 32% of the initial 15N input recovered after one year for 15NH4+ and 15NO3− tracer additions, respectively. Overall, the total ecosystem 15N recovery one year after the 15NH4+ and 15NO3− tracer additions was large (60% and 66% respectively), and not significantly different from total recovery after three months, suggesting that a large proportion of deposited N could be retained in the longer term. Based on the measured fate of 15N one year after labeling and the C:N ratios of different plant components, this tropical forest’s carbon sequestration efficiency is calculated to be 17 kg C per kg N added, comparable to the values reported for temperate and boreal forests in Europe and North America and indicating substantial N limitation of this tropical forest. Our results suggest that anthropogenic N input in moderate levels may contribute to enhance C sequestration in some tropical forests, without significant long-term loss of N to the environment.

Five-year study of the effects of simulated nitrogen deposition levels and forms on soil nitrous oxide emissions from a temperate forest in northern China.

Few studies have quantified the effects of different levels and forms of nitrogen (N) deposition on soil nitrous oxide (N2O) emissions from temperate forest soils. A 5-year field experiment was conducted to investigate the effects of multiple forms and levels of N additions on soil N2O emissions, by using the static closed chamber method at Xi Mountain Experimental Forest Station in northern China. The experiment included a control (no N added), and additions of NH4NO3, NaNO3, and (NH4)2SO4 that each had two levels: 50 kg N ha−1 yr−1 and 150 kg N ha−1 yr−1. All plots were treated to simulate increased N deposition on a monthly schedule during the annual growing season (March to October) and soil N2O emissions were measured monthly from March 2011 to February 2016. Simultaneously, the temperature, moisture, and inorganic N contents of soil were also measured to explore how the main factors may have affected soil N2O emission. The results showed that the types and levels of N addition significantly increased soil inorganic N contents, and the accumulation of soil NO3–-N was significantly higher than that of soil NH4+–N due to N addition. The three N forms significantly increased the average N2O emissions (P < 0.05) in the order of NH4NO3 > (NH4)2SO4 > NaNO3 by 355.95%, 266.35%, and 187.71%, respectively, compared with control. The promotion of N2O emission via the NH4+–N addition was significantly more than that via the NO3––N addition, while N addition at a high level exerted a stronger effect than at the low-level. N addition exerted significantly stronger effects on cumulative N2O emissions in the initial years, especially the third year when the increased cumulative N2O emission reached their maximum. In the later years, the increases persisted but were weakened. Increasing inorganic N concentration could change soil from being N-limited to N-rich, and then N-saturated, and so the promotion on soil available N effect increased and then decreased. Moreover, the soil NH4+–N, NO3–-N, temperature, and water-filled pore space were all positively correlated with soil N2O emissions. These findings suggest that atmospheric N deposition can significantly promote soil N2O emission, and that exogenous NH4+–N and NO3–-N inputs into temperate forests can have synergic effects on soil N2O emission. In future research, both aspects should be better distinguished in the N cycle and balance of terrestrial ecosystems by using 15N tracer methods.

High-rate nitrogen removal from waste brine by marine anammox bacteria in a pilot-scale UASB reactor.

The goal of this study was to develop a startup strategy for a high-rate anaerobic ammonium oxidation (anammox) reactor to treat waste brine with high concentrations of ammonium from a natural gas plant. An upflow anaerobic sludge blanket (UASB) anammox reactor with an effective volume of 294L was fed continuously with waste brine with a salinity of 3% and a NH4+ concentration of 180mg-N/L, as well as a NaNO2 solution. By inoculating a methanogenic granular biomass as a biomass carrier, the reactor attained the maximum volumetric nitrogen removal rate (NRR) of 10.7kg-N/m3/day on day 209, which was 1.7 times higher than the highest reported NRR for wastewater of comparable salinity. High-throughput sequencing of 16S rRNA gene amplicons revealed that Candidatus Scalindua wagneri was enriched successfully in granules in the UASB, and it replaced Methanosaeta and became dominant in the granule. The inhibitory effect of NO2- on the anammox reaction in the granules was assessed by a 15N tracer method, and the results showed that anammox activity was maintained at 60% after exposure to 300mg-N/L of NO2- for 24h. Compared with previous studies of the susceptibilities of Candidatus Brocadia and Candidatus Kuenenia to NO2-, the enriched marine anammox bacteria were proven to have comparable or even higher tolerances for high NO2- concentrations after a long exposure.