Nitrogen deposition alters soil chemical properties and bacterial communities in the Inner Mongolia grassland

Nitrogen deposition has dramatically altered biodiversity and ecosystem functioning on the earth; however, its effects on soil bacterial community and the underlying mechanisms of these effects have not been thoroughly examined. Changes in ecosystems caused by nitrogen deposition have traditionally been attributed to increased nitrogen content. In fact, nitrogen deposition not only leads to increased soil total N content, but also changes in the NIL^-N content, NO3--N content and pH, as well as changes in the heterogeneity of the four indexes. The soil indexes for these four factors, their heterogeneity and even the plant community might be routes through which nitrogen deposition alters the bacterial community. Here, we describe a 6-year nitrogen addition experiment conducted in a typical steppe ecosystem to investigate the ecological mechanism by which nitrogen deposition alters bacterial abundance, diversity and composition. We found that various characteristics of the bacterial community were explained by different environmental factors. Nitrogen deposition decreased bacterial abundance that is positively related to soil pH value. In addition, nitrogen addition decreased bacterial diversity, which is negatively related to soil total N content and positively related to soil NOa--N heterogeneity. Finally, nitrogen.addition altered bacterial composition that is significantly related to soil NH4＋-N content. Although nitrogen deposition significantly altered plant biomass, diversity and composition, these characteristics of plant community did not have a significant impact on processes of nitrogen deposition that led to alterations in bacterial abundance, diversity and composition. Therefore, more sensitive molecular technologies should be adopted to detect the subtle shifts of microbial community structure induced by the changes of plant community upon nitrogen deposition.

Soil Microbial Responses to Experimental Warming and Nitrogen Addition in a Temperate Steppe of Northern China

The responses of soil microbes to global warming and nitrogen enrichment can profoundly affect terrestrial ecosystem functions and the ecosystem feedbacks to climate change.However,the interactive effect of warming and nitrogen enrichment on soil microbial community is unclear.In this study,individual and interactive effects of experimental warming and nitrogen addition on the soil microbial community were investigated in a long-term field experiment in a temperate steppe of northern China.The field experiment started in 2006 and soils were sampled in 2010 and analyzed for phospholipid fatty acids to characterize the soil microbial communities.Some soil chemical properties were also determined.Five-year experimental warming significantly increased soil total microbial biomass and the proportion of Gram-negative bacteria in the soils.Long-term nitrogen addition decreased soil microbial biomass at the 0-10 cm soil depth and the relative abundance of arbuscular mycorrhizal fungi in the soils.Little interactive effect on soil microbes was detected when experimental warming and nitrogen addition were combined.Soil microbial biomass positively correlated with soil total C and N,but basically did not relate to the soil C/N ratio and pH.Our results suggest that future global warming or nitrogen enrichment may significantly change the soil microbial communities in the temperate steppes in northern China.

Mowing weakens the positive effects of nitrogen deposition on fundamental ecosystem service of grassland

Forage yield is the fundamental ecosystem service of grasslands.While the quantitative responses of forage yield to nitrogen(N)enrichment are well known,its qualitative responses remain unclear.Even less known is the relative contribution of changes in community composition to the quality of the yield at the community level.We examined the quantitative and qualitative responses of forage yield at both plant functional group and community levels with factorial treatments of N addition and mowing in a temperate steppe.Nitrogen addition significantly enhanced the community-level yield by favoring the growth of rhizomatous grass.Mowing tended to mediate the impacts of N addition on the yield.Nitrogen addition increased the concentrations of crude protein and crude fat in forage at the community level.Neither the main effects of mowing nor its interactive effects with N addition affected forage quality.The N-induced shifts in plant species composition significantly contributed to the effects of N addition on forage quality at the community level.Our results suggest that mowing wound weaken the positive effects of N deposition on the quantity but not the quality of forage yield.Changes in plant community composition are important in driving the qualitative responses of yield to N deposition.

Resolving the influence of nitrogen abundances on sediment organic matter in macrophyte-dominated lakes, using fluorescence spectroscopy

A controlled experiment was designed to resolve the influence of nitrogen abundance on sediment organic matters in macrophyte-dominated lakes using fluorescence analysis.Macrophyte biomass showed coincident growth trends with time, but different variation rates with nitrogen treatment. All plant growth indexes with nitrogen addition（N, NH4Cl100, 200, 400 mg/kg, respectively） were lower than those of the control group. Four humiclike components, two autochthonous tryptophan-like components, and one autochthonous tyrosine-like component were identified using the parallel factor analysis model. The results suggested that the relative component changes of fluorescence in the colonized sediments were in direct relation to the change of root biomass with time. In the experiment, the root formation parameters of the plants studied were significantly affected by adding N in sediments, which may be related to the reason that the root growth was affected by N addition.Adding a low concentration of N to sediments can play a part in supplying nutrients to the plants. However, the intensive uptake of NH4^＋may result in an increase in the intracellular concentration of ammonia, which is highly toxic to the plant cells. Hence, our experiment results manifested that organic matter cycling in the macrophyte-dominated sediment was influenced by nitrogen enrichment through influencing vegetation and relevant microbial activity.

Mowing did not mitigate the negative effects of nitrogen deposition on soil nematode community in a temperate steppe

Soil nematodes are the most numerous components of the soil fauna in terrestrial ecosystems.The occurrence and abundance of nematode trophic groups determine the structure and function of soil food webs.However,little is known about how nitrogen deposition and land-use practice(e.g.mowing)affect soil nematode communities.We investigated the main and interactive effects of nitrogen addition and mowing on soil nematode diversity and biomass carbon in nematode trophic groups in a temperate steppe in northern China.Nitrogen addition and mowing significantly decreased the abundance of soil nematodes and trophic diversity but had no effects on nematode richness and the Shannon-Wiener diversity.Nitrogen addition influenced soil nematode communities through decreasing soil pH.Mowing influenced soil nematode communities through decreasing soil moisture.Nitrogen addition enhanced the bacterial energy channel but mowing promoted fungal energy channel in the soil micro-food web.Our study emphasizes that ecosystem function supported by soil organisms can be greatly influenced by nitrogen deposition,and mowing cannot mitigate the negative effects of nitrogen deposition on soil food webs.

Soil burial has a greater effect on litter decomposition rate than nitrogen enrichment in alpine grasslands

Aims Litter is frequently buried in the soil in alpine grasslands due to grassland degradation,serious rodent infestation and frequent strong winds.However,the effects of various litter positions on litter decomposition rates and nutrient dynamics under nitrogen(N)enrichment in such areas remain unknown.Methods A field experiment was performed in the alpine grasslands of northwest China to investigate the influence of litter position(surface,buried in the soil and standing)and N enrichment on litter decomposition,using data from two dominant grass species(Festuca ovina and Leymus tianschanicus)in control and N-enriched plots.Important Findings Litter decomposition rates were much faster in buried litter and slower in standing litter than in surface litter.N enrichment significantly affected litter quality and then influenced decomposition.But no significant differences in litter mass remaining were observed between control and N-enriched soil burial.These results indicated that N enrichment significantly affected litter decomposition by changes in litter quality.In addition,all litter exhibited net carbon(C)and phosphorus(P)release regardless of treatments.Litter exhibited net N accumulation for litter from the control plots but showed N release for litter from N enrichment plots.These suggested that litter decomposition can be limited by N and N enrichment influenced N cycling of litter.Current study presented direct evidence that soil buried litter exhibited faster mass loss and C release,and that soil burial can be a candidate explanation why litter decomposes faster than expected in dryland.

Nitrogen-rich carbon nitrogen polymers for enhancing the sorption of uranyl

Nitrogen enrichment and increased nitrogen content is an effective strategy for enhancing adsorption of uranium by carbon nitride polymers.Herein,we reported the uranium absorption by using a structurally well-defined and nitrogen-rich carbon nitride polymer with C_(3)N_(5) stoichiometry for the first time.In comparison with the adsorption performance of g-C_(3)N_(4) for U(Ⅵ),the conjugation system of the material was increased by connecting the heptazine unit through the azo bridge in the structure of C_(3)N_(5),so that C_(3)N_(5) exhibited several times higher adsorption performance than that of g-C_(3)N_(4).The C_(3)N_(5) has high kinetics for uranyl ions,which can adsorb100 mg/g U(Ⅵ)in only 10 min and reach complete adsorption equilibrium in 60 min;the theoretical maximum adsorption capacity is 207 mg/g,meanwhile,the material exhibits high selectivity.The results of spectral analysis and theoretical calculations indicate that the process of uranyl ion capture by C3N5is a combination of physical and chemical adsorption,and its higher density of electronic states makes the electrostatic binding ability enhanced,which is favorable to the adsorption of uranyl ions by C_(3)N_(5).This work indicates that C_(3)N_(5) has great promise and application in the separation and enrichment of uranyl ions,and also provides a reference for the systematic investigation of the adsorption ability of nitrogenrich carbon nitrogen polymers on uranyl ions.

Elevated CO_2 facilitates C and N accumulation in a rice paddy ecosystem

Elevated CO2 can stimulate wetland carbon （C） and nitrogen （N） exports through gaseous and dissolved pathways, however, the consequent influences on the C and N pools are still not fully known. Therefore, we set up a free-air CO2 enrichment experiment in a paddy field in Eastern China. After five year fumigation, we studied C and N in the plant-water-soil system. The results showed： （1） elevated CO2 stimulated rice aboveground biomass and N accumulations by 19.1% and 12.5%, respectively. （2） Elevated CO2 significantly increased paddy soil TOC and TN contents by 12.5% and 15.5%, respectively in the 0-15 crn layer, and 22.7% and 26.0% in the 15-30 cm soil layer. （3） Averaged across the rice growing period, elevated CO2 greatly increased TOC and TN contents in the surface water by 7.6% and 11.4%, respectively. （4） The TOC/TN ratio and natural 5ISN value in the surface soil showed a decreasing trend under elevated CO2. The above results indicate that elevated CO2 can benefit C and N accumulation in paddy fields. Given the similarity between the paddies and natural wetlands, our results also suggest a great potential for long-term C and N accumulation in natural wetlands under future climate patterns.

Theoretical study of ullage washing with mixed inert gas in a non-equilibrium state

The Mixed Inert Gas(MIG)produced by the novel Green On Board Inerting Gas Generation System(GOBIGGS)mainly consists of carbon dioxide,nitrogen and oxygen.Because of the large solubility of carbon dioxide in jet fuel compared with nitrogen,the no gas release or equilibrium model could not be employed any more.In this paper,first,a mathematical model of the ullage washing was set up to predict the variation of the oxygen concentration on ullage and in the fuel,and the gas evolution and dissolution rate were calculated by Fick's second law.Then,an experimental apparatus was constructed to verify the accuracy of the model.Finally,the numerical comparisons of ullage washing using Nitrogen Enriched Air(NEA)and MIG are presented under various flow rates and fuel loads,and the result reveals that the variation of the oxygen concentration on ullage is nearly identical whatever the inert gas is NEA or MIG.However,the variation of the oxygen concentration in the fuel is disparate,and the oxygen concentration decreases rapidly if the inert gas is MIG,especially when the fuel load is low or the flow rate of the inert gas is high.Besides,MIG could suppress the rising trend of the oxygen concentration on ullage when the aircraft ascends if the fuel tank is fully washed into an equilibrium state on ground.