Decreased Soil Nitrification Rate with Addition of Biochar to Acid Soils

This study was conducted to investigate the effects of mixed biochar on the nitrification rate in acidic soils. A15 N tracer experiment with(15 NH4)2 SO4 was conducted to determine the nitrification rates of 4 acidic agricultural soils with p H 4.03-6.02 in Yunnan Province, Southern China. The accumulation of15 N-NO3-and nitrification rates decreased with the addition of biochar at the end of incubation, suggesting that biochar could be a nitrification inhibitor in acidic fertilized soil. Nitrification rates in soil with p H 4.03 were evidently lower than those in soil with p H 4.81-6.02 with or without biochar. Decreased nitrification rates were detected in the acidic soils with biochar. Soil p H controlled nitrification more than biochar in certain strongly acidic soils.

Investigating drivers of active nitrification in organic horizons of tropical forest soils

High nitrate leaching has been observed from the O horizons of some tropical forests;however,the drivers of high nitrate production(active nitrification)in these O horizons have not yet been identified.This study investigated the drivers of active nitrification in the O horizon of tropical forest soils by focusing on two of the most widely recognized controlling factors of nitrification,total N,and pH.We collected mineral and O horizons from eight tropical forests in Cameroon,Indonesia,and Malaysia and measured gross nitrification rates.Some O horizons showed significantly higher gross nitrification rates than mineral horizons,indicating that these O horizons have a high potential for nitrification.Gross nitrification rates in the O horizons were positively correlated with both total N and pH,and the chemical properties(e.g.,total content of N,P,and base cations)were intercorrelated.These correlations suggested that the underlying driver of nitrification in the O horizon was nutrient richness in the litter.Results also indicated a threshold of gross nitrification rates around pH values of 5.5–6.0.We elucidate that active nitrification and subsequent high nitrate leaching from the O horizon could be driven by nutrient-rich litter,possibly derived from soil fertility and tree species.

Spatiotemporal evolution and assembly processes of ammonia-oxidizing prokaryotic communities in 1 000-year-old coastal reclaimed soils

Coastal marshes are transitional areas between terrestrial and aquatic ecosystems.They are sensitive to climate change and anthropogenic activities.In recent decades,the reclamation of coastal marshes has greatly increased,and its effects on microbial communities in coastal marshes have been studied with great interest.Most of these studies have explained the short-term spatiotemporal variation in soil microbial community dynamics.However,the impact of reclamation on the community composition and assembly processes of functional microbes(e.g.,ammonia-oxidizing prokaryotes)is often ignored.In this study,using quantitative polymerase chain reaction and the Ion S5™XL sequencing platform,we investigated the spatiotemporal dynamics,assembly processes,and diversity patterns of ammonia-oxidizing prokaryotes in 1000-year-old reclaimed coastal salt marshes.The taxonomic and phylogenetic diversity and composition of ammonia oxidizers showed apparent spatiotemporal variations with soil reclamation.Phylogenetic null modelling-based analysis showed that across all sites,the archaeal ammonia-oxidizing community was assembled by a deterministic process(84.71%),and deterministic processes were also dominant(55.2%)for ammonia-oxidizing bacterial communities except for communities at 60 years of reclamation.The assembly process and nitrification activity in reclaimed soils were positively correlated.The abundance of the amoA gene and changes in ammonia-oxidizing archaeal and bacterial diversities significantly affected the nitrification activity in reclaimed soils.These findings suggest that long-term coastal salt marsh reclamation affects nitrification by modulating the activities of ammonia-oxidizing microorganisms and regulating their community structures and assembly processes.These results provide a better understanding of the effects of long-term land reclamation on soil nitrogen-cycling microbial communities.

Nitrification characteristics of nitrobacteria immobilized in waterborne polyurethane in wastewater of corn-based ethanol fuel production

A technology to achieve stable and high ammonia nitrogen removal rates for corn distillery wastewater （ethanol fuel production） treatment has been designed.The characteristics of nitrifying bacteria entrapped in a waterborne polyurethane （WPU） gel carrier were evaluated after acclimation.In the acclimation period,nitrification rates of WPU-immobilized nitrobacteria were monitored and polymerase chain reaction （PCR） was also carried out to investigate the change in ammonium-oxidizing bacteria.The results showed that the pellet nitrification rates increased from 21 to 228 mg-N/（L-pellet·hr） and the quantity of the ammonia oxidation bacteria increased substantially during the acclimation.A continuous ammonia removal experiment with the anaerobic pond effluent of a distillery wastewater system was conducted with immobilized nitrifying bacteria for 30 days using an 80 L airlift reactor with pellets at a fill ratio of 15% （V/V）.Under the conditions of 75 mg/L influent ammonia,hydraulic retention time （HRT） of 3.7-5.6 hr,and dissolved oxygen （DO） of 4 mg/L,the effluent ammonia concentration was lower than 10 mg/L and the ammonia removal efficiency was 90%.While the highest ammonia removal rate,162 mg-N/（L-pellet·hr）,was observed when the HRT was 1.3 hr.

REGIONAL ASSESSMENT OF SOIL NITROGEN MINERALIZATION IN DIVERSE CROPLAND OF A REPRESENTATIVE INTENSIVE AGRICULTURAL AREA

Soil nitrogen mineralization(Nmin)is a key process that converts organic N into mineral N that controls soil N availability to plants.However,regional assessments of soil Nmin in cropland and its affecting factors are lacking,especially in relation to variation in elevation.In this study,a 4-week incubation experiment was implemented to measure net soil Nmin rate,gross nitrification(Nit)rate and corresponding soil abiotic properties in five field soils(A-C,maize;D,flue-cured tobacco;and E,vegetables;with elevation decreasing from A to E)from different altitudes in a typical intensive agricultural area in Dali City,Yunnan Province,China.The results showed that soil Nmin rate ranged from 0.10 to 0.17 mg·kg^(-1)·d^(-1)N,with the highest value observed in field E,followed by fields D,C,B,and A,which indicated that soil Nmin and Nit rates varied between fields,decreasing with elevation.The soil Nit rate ranged from 434.2 to 827.1μg·kg^(-1)·h^(-1)N,with the highest value determined in field D,followed by those in fields E,C,B,and A.The rates of soil Nmin and Nit were positively correlated with several key soil parameters,including total soil N,dissolved organic carbon and dissolved inorganic N across all fields,which indicated that soil variables regulated soil Nmin and Nit in cropland fields.In addition,a strong positive relationship was observed between soil Nmin and Nit.These findings provide a greater understanding of the response of soil Nmin among cropland fields related to spatial variation.It is suggested that the soil Nmin from cropland should be considered in the evaluation of the N transformations at the regional scale.

Autotrophic and Heterotrophic Nitri?cation in a Highly Acidic Subtropical Pine Forest Soil

The occurrence of nitri?cation in some acidic forest soils is still a subject of debate.Identi?cation of main nitri?cation pathways in acidic forest soils is still largely unknown.Acidic yellow soil(Oxisol) samples were selected to test whether nitri?cation can occur or not in acidic subtropical pine forest ecosystems.Relative contributions of autotrophs and heterotrophs to nitri?cation were studied by adding selective nitri?cation inhibitor nitrapyrin.Soil NH^+_4-N concentrations decreased,but NO^-_3-N concentrations increased signi?cantly for the no-nitrapyrin control during the ?rst week of incubation,indicating that nitri?cation did occur in the acidic subtropical soil.The calculated net nitri?cation rate was 0.49 mg N kg^(-1)d^(-1)for the no-nitrapyrin control during the ?rst week of incubation.Nitrapyrin amendment resulted in a signi?cant reduction of NO^-_3-N concentration.Autotrophic nitri?cation rate averaged0.28 mg N kg^(-1)d^(-1)and the heterotrophic nitri?cation rate was 0.21 mg N kg^(-1)d^(-1)in the ?rst week.Ammonia-oxidizing bacteria(AOB) abundance increased slightly during incubation,but nitrapyrin amendment signi?cantly decreased AOB amo A gene copy numbers by about 80%.However,the ammonia-oxidizing archaea(AOA) abundance showed signi?cant increases only in the last 2weeks of incubation and it was also decreased by nitrapyrin amendment.Our results indicated that nitri?cation did occur in the present acidic subtropical pine forest soil,and autotrophic nitri?cation was the main nitri?cation pathway.Both AOA and AOB were the active biotic agents responsible for autotrophic nitri?cation in the acidic subtropical pine forest soil.

Applicability of the Arrhenius model for Ammonia Oxidizing Bacteria subjected to temperature time gradients

The aim of this work is to identify the range of applicability of Arrhenius type temperature dependence for Ammonia Oxidizing Bacteria （AOB） subjected to tem- perature time gradients through continuous titrimetric tests. An innovative online differential titrimetric technique was used to continuously monitor the maximum biologic ammonia oxidation rate of the biomass selected in a pilot scale membrane bioreactor, as a function of temperature time gradients. The monitoring technique is based on the measurement of alkalinity and hydrogen peroxide con- sumption rates in two parallel reactors operated in non- limiting substrate conditions for AOB; both reactors were continuously fed with mixed liquor and in one of them AOB were inhibited with allylthiourea. The effects of temperature decrease rates in the range 1 to 4℃h^-1 were evaluated by controlling the titrimetric reactor in the temperature range 10℃-20℃. The dependence of growth kinetics on temperature time gradients and the range of applicability of Arrhenius model for temperature depen- dency of AOB growth kinetics were assessed. The Arrhenius model was found to be accurate only with temperature gradients lower than 2℃·h^-1. The estimated Arrhenius coefficients （θ） were shown to increase from 1.07 to 1.6 when the temperature decrease rate reached 4℃.h^-1.

Responses of soil ammonia oxidizers to a short-term severe mercury stress

The responses of soil ammonia-oxidizing archaea（AOA） and ammonia-oxidizing bacteria（AOB） to mercury（Hg） stress were investigated through a short-term incubation experiment.Treated with four different concentrations of Hg（CK,Hg25,Hg50,and Hg100,denoting 0,25,50,and 100 mg Hg/kg dry soil,respectively）,samples were harvested after 3,7,and 28 day incubation.Results showed that the soil potential nitrification rate（PNR） was significantly inhibited by Hg stress during the incubation.However,lower abundances of AOA（the highest in CK： 9.20 × 10~7 copies/g dry soil; the lowest in Hg50： 2.68 × 10~7 copies/g dry soil） and AOB（the highest in CK： 2.68 × 10~7 copies/g dry soil; the lowest in Hg50：7.49 × 10~6 copies/g dry soil） were observed only at day 28 of incubation（P 〈 0.05）.Moreover,only the community structure of soil AOB obviously shifted under Hg stress as seen through DGGE profiles,which revealed that 2-3 distinct AOB bands emerged in the Hg treatments at day 28.In summary,soil PNR might be a very useful parameter to assess acute Hg stress on soil ecosystems,and the community structure of soil AOB might be a realistic biological indicator for the assessment of heavy metal stress on soil ecosystems in the future.

Aeration optimization through operation at low dissolved oxygen concentrations:Evaluation of oxygen mass transfer dynamics in different activated sludge systems

In wastewater treatment plants（WWTPs）using the activated sludge process,two methods are widely used to improve aeration efficiency — use of high-efficiency aeration devices and optimizing the aeration control strategy. Aeration efficiency is closely linked to sludge characteristics（such as concentrations of mixed liquor suspended solids（MLSS）and microbial communities）and operating conditions（such as air flow rate and operational dissolved oxygen（DO）concentrations）. Moreover,operational DO is closely linked to effluent quality. This study,which is in reference to WWTP discharge class A Chinese standard effluent criteria,determined the growth kinetics parameters of nitrifiers at different DO levels in small-scale tests. Results showed that the activated sludge system could meet effluent criteria when DO was as low as 0.3 mg/L,and that nitrifier communities cultivated under low DO conditions had higher oxygen affinity than those cultivated under high DO conditions,as indicated by the oxygen half-saturation constant and nitrification ability. Based on nitrifier growth kinetics and on the oxygen mass transfer dynamic model（determined using different air flow rate（Q′air）and mixed liquor volatile suspended solids（MLVSS）values）,theoretical analysis indicated limited potential for energy saving by improving aeration diffuser performance when the activated sludge system had low oxygen consumption; however,operating at low DO and low MLVSS could significantly reduce energy consumption. Finally,a control strategy coupling sludge retention time and MLVSS to minimize the DO level was discussed,which is critical to appropriate setting of the oxygen point and to the operation of low DO treatment technology.