Dongwei Liu

Aridity threshold in controlling ecosystem nitrogen cycling in arid and semi-arid grasslands

Higher aridity and more extreme rainfall events in drylands are predicted due to climate change. Yet, it is unclear how changing precipitation regimes may affect nitrogen (N) cycling, especially in areas with extremely high aridity. Here we investigate soil N isotopic values (δ(15)N) along a 3,200 km aridity gradient and reveal a hump-shaped relationship between soil δ(15)N and aridity index (AI) with a threshold at AI=0.32. Variations of foliar δ(15)N, the abundance of nitrification and denitrification genes, and metabolic quotient along the gradient provide further evidence for the existence of this threshold. Data support the hypothesis that the increase of gaseous N loss is higher than the increase of net plant N accumulation with increasing AI below AI=0.32, while the opposite is favoured above this threshold. Our results highlight the importance of N-cycling microbes in extremely dry areas and suggest different controlling factors of N-cycling on either side of the threshold.

Microbial denitrification dominates nitrate losses from forest ecosystems

Significance Nitrogen (N) losses from terrestrial ecosystems can occur as inert forms or heat-trapping greenhouse gases, and via nitrate (NO3−) leaching to drainage waters, which can contribute to eutrophication and anoxia in downstream ecosystems. Here, we use natural isotopes to demonstrate that microbial gaseous N production via denitrification is the dominant pathway of NO3− removal from forest ecosystems, with gaseous N losses that are up to ∼60-fold higher than those based on traditional techniques. Denitrification becomes less efficient compared with NO3− leaching in more N-polluted ecosystems, which has important implications for assessing the connections between terrestrial soils and downstream ecosystems under rising anthropogenic N deposition. Denitrification removes fixed nitrogen (N) from the biosphere, thereby restricting the availability of this key limiting nutrient for terrestrial plant productivity. This microbially driven process has been exceedingly difficult to measure, however, given the large background of nitrogen gas (N2) in the atmosphere and vexing scaling issues associated with heterogeneous soil systems. Here, we use natural abundance of N and oxygen isotopes in nitrate (NO3−) to examine dentrification rates across six forest sites in southern China and central Japan, which span temperate to tropical climates, as well as various stand ages and N deposition regimes. Our multiple stable isotope approach across soil to watershed scales shows that traditional techniques underestimate terrestrial denitrification fluxes by up to 98%, with annual losses of 5.6–30.1 kg of N per hectare via this gaseous pathway. These N export fluxes are up to sixfold higher than NO3− leaching, pointing to widespread dominance of denitrification in removing NO3− from forest ecosystems across a range of conditions. Further, we report that the loss of NO3− to denitrification decreased in comparison to leaching pathways in sites with the highest rates of anthropogenic N deposition.

Reply to Comment on “Fossil Fuel Combustion-Related Emissions Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from 15N-Stable Isotope in Size-Resolved Aerosol Ammonium”

Dominate Atmospheric Ammonia Sources during Severe Haze Episodes: Evidence from N‐Stable Isotope in Size-Resolved Aerosol Ammonium” W appreciate the opportunity to respond to the comments of Chang and Ma regarding our article, and we also hope to further clarify the findings of our work. Their comments on our work focus on the source apportionment of ammonia (NH3) during haze episodes in Beijing. We do not think that their objections are well founded, and their speculations do not change our conclusions.

Chemical Method for Nitrogen Isotopic Analysis of Ammonium at Natural Abundance

We report a new chemical method to determine the (15)N natural abundance (δ(15)N) for ammonium (NH4(+)) in freshwater (e.g., precipitation) and soil KCl extract. This method is based on the isotopic analysis of nitrous oxide (N2O). Ammonium is initially oxidized to nitrite (NO2(-)) by hypobromite (BrO(-)) using previously established procedures. NO2(-) is then quantitatively converted into N2O by hydroxylamine (NH2OH) under strongly acid conditions. The produced N2O is analyzed by a commercially available purge and cryogenic trap system coupled to an isotope ratio mass spectrometer (PT-IRMS). On the basis of a typical analysis size of 4 mL, the standard deviation of δ(15)N measurements is less than 0.3‰ and often better than 0.1‰ (3 to 5 replicates). Compared to previous methods, the technique here has several advantages and the potential to be used as a routine method for (15)N/(14)N analysis of NH4(+): (1) substantially simplified preparation procedures and reduced preparation time particularly compared to the methods in which diffusion or distillation is involved since all reactions occur in the same vial and separation of NH4(+) from solution is not required; (2) more suitability for low volume samples including those with low N concentration, having a blank size of 0.6 to 2 nmol; (3) elimination of the use of extremely toxic reagents (e.g., HN3) and/or the use of specialized denitrifying bacterial cultures which may be impractical for many laboratories.

Modifications to the azide method for nitrate isotope analysis

RATIONALE The azide method for measuring the stable isotope ratios of nitrate (NO3- ) is easy to set up. However, the method requires spongy cadmium (Cd) or activated Cd powder which are not easy to prepare, and a toxic azide buffer is used. We aimed to use Cd powder directly to simplify preparation and to substantially reduce the azide dose. METHODS The reaction conditions were optimized in order to maximize the NO3- reduction yield. The original azide buffer was diluted by 10- to 10000-fold with or without addition of sodium acetate to reduce O-exchange between nitrite (NO2- ) and H2 O. The isotope ratios of the produced nitrous oxide (N2 O), used to examine the overall reaction performance, were measured using a purge and cryogenic trap system coupled to an isotope ratio mass spectrometer. RESULTS It was found that Cd powder could be directly used to reduce NO3- to NO2- . A 100-fold diluted azide buffer could be used to reduce NO2- to N2 O when only the δ15 N value was measured, and the diluted azide buffer with sodium acetate when both δ15 N and δ18 O values were measured. Using the modified method, the standard deviations of the δ15 N and δ18 O measurements of international NO3- standards were 0.1 to 1.0‰ and often better than 0.3‰ (3 replicates). CONCLUSIONS Compared with the original azide method, the techniques described here can reduce preparation time by using Cd powder without activation in the first reaction step and substantially (by >60-fold) reduce the dose of extremely toxic reagents containing azide by incorporating sodium acetate in the second reaction step. Our modified method is suitable for samples with small volume (5 mL), being different from previous methods in which 50 or 70 mL samples were used. Copyright