Soil Nitrification Potential Research Articles

Microbial N-cycling gene abundance is affected by cover crop specie and development stage in an integrated cropping system.

Grasses of the Urochloa genus have been widely used in crop-livestock integration systems or as cover crops in no-till systems such as in rotation with maize. Some species of Urochloa have mechanisms to reduce nitrification. However, the responses of microbial functions in crop-rotation systems with grasses and its consequence on soil N dynamics are not well-understood. In this study, the soil nitrification potential and the abundance of ammonifying microorganisms, total bacteria and total archaea (16S rRNA gene), nitrogen-fixing bacteria (NFB, nifH), ammonia-oxidizing bacteria (AOB, amoA) and archaea (AOA, amoA) were assessed in soil cultivated with ruzigrass (Urochloa ruziziensis), palisade grass (Urochloa brizantha) and Guinea grass (Panicum maximum). The abundance of ammonifying microorganisms was not affected by ruzigrass. Ruzigrass increased the soil nitrification potential compared with palisade and Guinea grass. Ruzigrass increased the abundance of N-fixing microorganisms at the middle and late growth stages.The abundances of nitrifying microorganisms and N-fixers in soil were positively correlated with the soil N-NH4+ content. Thus, biological nitrogen fixation might be an important input of N in systems of rotational production of maize with forage grasses. The abundance of microorganisms related to ammonification, nitrification and nitrogen fixing and ammonia-oxidizing archea was related to the development stage of the forage grass.

The Structure and Diversity of Nitrogen Functional Groups from Different Cropping Systems in Yellow River Delta.

The Yellow River Delta (YRD) region is an important production base in Shandong Province. It encompasses an array of diversified crop systems, including the corn–wheat rotation system (Wheat–Corn), soybean–corn rotation system (Soybean–Corn), fruits or vegetables system (Fruit), cotton system (Cotton) and rice system (Rice). In this study, the communities of ammonia oxidizer–, denitrifier– and nitrogen (N)–fixing bacteria in those cropping systems were investigated by Illumina Miseq sequencing. We found that Rice soil exhibited significantly higher diversity indices of investigated N–cycling microbial communities than other crop soils, possibly due to its high soil water content. Wheat–Corn soils had higher abundances of nitrification gene amoA and denitrification genes nirK and nirS, and exhibited higher soil potential nitrification rate (PNR), compared with Soybean–Corn, Cotton and Fruit soils. Consistently, redundancy analysis (RDA) showed that soil water content (SWC), electrical conductivity (EC), and total nitrogen (TN) were the most important influencing factors of the diversity and structure of the investigated N–cycling microbial.

Decreased N2O and NO emissions associated with stimulated denitrification following biochar amendment in subtropical tea plantations

Acidic soils are hotspots of nitrous oxide (N2O) and nitric oxide (NO) and biochar is documented to have the potential for mitigating N2O and NO. The N2O and NO emissions associated with soil functional genes and physicochemical properties under biochar amendment remains unclear in acidic soils. Here, we carried out a two-year field study to examine the responses of soil N2O and NO emissions to biochar amendment in a subtropical tea plantation in China. Measurements of N2O and NO fluxes were taken from inter-row soils using the static chamber method. We also measured the seasonal changes in soil key nitrogen (N)-cycling functional genes and physicochemical properties. Annual N2O and NO emissions averaged 27.31 kg N2O-N ha−1 yr−1 and 8.75 kg NO-N ha−1 yr−1 for the N fertilizer applied plots, which were decreased by 24% and 16% due to biochar application, respectively. In addition, both potential nitrification (PNR) and denitrification (PDR) rates were stimulated by biochar amendment, which significantly increased the abundances of bacterial amoA (AOB), nirK and nosZ genes. Changes in the composition of the N2O-related microbial functional community were closely associated with soil PNR, pH, DOC, and NO3−-N contents. The ratios of NO/N2O were mainly lower than 1, suggesting that N2O was produced mostly through denitrification rather than nitrification. There were negative correlations between soil N2O and NO emissions and soil PDR and pH, and soil N2O emissions were negatively correlated with nosZ gene abundances. Together, the decrease in N2O and NO emissions following biochar application could be largely attributed to the enhanced denitrification process, in which biochar enriched the nirK and nosZ genes abundance, resulting from the enhancement of soil DOC and pH in acidic soils.

Relative Abundance of Ammonia Oxidizing Archaea and Bacteria Influences Soil Nitrification Responses to Temperature.

Ammonia oxidizing archaea (AOA) and bacteria (AOB) are thought to contribute differently to soil nitrification, yet the extent to which their relative abundances influence the temperature response of nitrification is poorly understood. Here, we investigated the impact of different AOA to AOB ratios on soil nitrification potential (NP) across a temperature gradient from 4 °C to 40 °C in twenty different organic and inorganic fertilized soils. The temperature responses of different relative abundance of ammonia oxidizers for nitrification were modeled using square rate theory (SQRT) and macromolecular rate theory (MMRT) models. We found that the proportional nitrification rates at different temperatures varied among AOA to AOB ratios. Predicted by both models, an optimum temperature (Topt) for nitrification in AOA dominated soils was significantly higher than for soils where AOA and AOB abundances are within the same order of magnitude. Moreover, the change in heat capacity () associated with the temperature dependence of nitrification was positively correlated with Topt and significantly varied among the AOA to AOB ratios. The temperature ranges for NP decreased with increasing AOA abundance for both organic and inorganic fertilized soils. These results challenge the widely accepted approach of comparing NP rates in different soils at a fixed temperature. We conclude that a shift in AOA to AOB ratio in soils exhibits distinguished temperature-dependent characteristics that have an important impact on nitrification responses across the temperature gradient. The proposed approach benefits the accurate discernment of the true contribution of fertilized soils to nitrification for improvement of nitrogen management.

Soil Nitrification Potential Influences the Performance of Nitrification Inhibitors DCD and DMPP in Cropped and Non-Cropped Soils

The application of nitrification inhibitors (NIs) shows promise in prolonging the ammonium presence in soil with beneficial effects for agriculture ecosystems and climate change mitigation. Although the inhibitory effect (IE) of NIs has been studied in the presence of various environmental and edaphic conditions, little is known about the effect of soil nitrification potential (NP) on the effectiveness of NIs. Here, laboratory-scale experiments were conducted to investigate the effect of the variation in soil NP rates, among land-use type and temperature, on the performance of two nitrification inhibitors, dicyandiamide (DCD) and 3,4-dimethypyrazole phosphate (DMPP), at four NI application rates imposed upon eight cropland and non-cropland soils. We found that the IE of DCD and DMPP were organized according to soil NP rates. Nevertheless, NP was lower in non-cropped soil than in cropped systems, and DMPP-based inhibition was higher than DCD. The IE of both NIs decreased with NP and the amount of NI required to achieve an IE ≈ 50%, was significantly reduced for soils that exhibited the lowest NP rates, especially for DMPP. However, the temperature did not appear to have a major influence on IE of both DCD and DMPP, demonstrating the potential of NIs to inhibit nitrification for a wider temperature range, dependent on the NI application rate. Our findings provide evidence that change in soil NP rate has important influences on the efficacy of NI which required great consideration for N-fertilizer optimization with the application of nitrification inhibitors.

Inhibitory Effects of Different Types and Doses of Herbicides on Soil Nitrification Potentials

To elucidate the inhibitory effects of different herbicides on soil nitrification, eight widely used herbicides, i.e., acetochlor, atrazine, dicamba, isoproturon, paraquat, puma, tribenuron-methyl, and 2,4-dichlorophenoxyacetic acid butyl ester (2,4-Dbe), which represent different chemical taxonomy were selected. Our results indicated that herbicide 2,4-Dbe displayed the best inhibitory effect on nitrification, followed by puma and tribenuron-methyl, whereas the remaining five herbicides exhibited less effect when 10 mg of active ingredient (A.I.) of every herbicide per kg of soil was applied in vegetable-planting soil. The inhibition appeared when 5–100 mg of A.I. 2,4-Dbe was employed, which was enhanced with an increment in its dose in both vegetable-planting and fluvo-aquic soils. However, the inhibitory effect of 10 mg of A. I. 2,4-Dbe exhibited obvious differences in these two types of soils, where the duration of inhibition was shorter as it only continued about a week in fluvo-aquic and calcic cambisols soils with strong nitrification activity but poorer effect as compared to 10 mg of dicyandiamide (DCD). In contrast, the duration of inhibition exceeded 2 months in dryland red and shajiang black soils with a weak nitrification activity which was equivalent to DCD. In addition, comparing with five nitrification inhibitors, 10 mg of 2,4-Dbe had better inhibition than the substituted pyrimidine (AM) and sulfocarbamide (SU), but was equivalent to DCD, nitrapirin, and 3,4-dimethylpyrazole phosphate (DMPP) at their recommended application rates in dryland red soil. These obtained data clearly indicated that 2,4-Dbe could play a stronger role as a nitrification inhibitor in soils.

Genetic variation in tree leaf chemistry predicts the abundance and activity of autotrophic soil microorganisms

AbstractGenetic variation in the chemistry of plant leaves can have ecosystem‐level consequences. Here, we address the hypothesis that genetic variation in foliar condensed tannins along a Populus hybridization gradient influences soil ammonia oxidizers, a group of autotrophic microorganisms that perform the first step of nitrification and are not dependent on carbon derived from plant photosynthesis. Evidence that genetically based plant traits influence the abundance and activity of autotrophic soil microbes would greatly expand the concept of extended plant phenotypes. We found that increasing foliar condensed tannin concentration reduced rates of soil nitrification potential by ~75%, reduced the abundance of ammonia‐oxidizing archaea by ~66%, but had no effect on ammonia‐oxidizing bacteria. Other indices that often drive nitrification rates, including soil total nitrogen, foliar nitrogen, and soil pH, were not significant predictors of either the activity or the abundance of ammonia oxidizers, suggesting genetic variation in foliar condensed tannins may be the dominant regulating factor. These results demonstrate the condensed tannin phenotypes of two different tree species and their naturally occurring hybrids have extended effects on a key ecosystem process and provide evidence for indirect genetic linkages among autotrophs across at least two domains of life.

Effect of Straw and Straw Biochar on the Community Structure and Diversity of Ammonia-oxidizing Bacteria and Archaea in Rice-wheat Rotation Ecosystems

Ammonia oxidation is the first and rate-limiting step of nitrification, driven by ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA). Straw and straw biochar retention are the popular ways to utilize the agricultural by-products in China, but their long-term effects on AOB and AOA still remain poorly understood. Based on a 7-year plot experiment, which had 4 fertilization regimes: no fertilizer (CK), regular fertilization (RT), straw retention (SR) and straw biochar retention (SB), the abundance and the composition of AOB and AOA was investigated before both the harvest of rice and wheat season by quantitative PCR and 454 high-throughput pyrosequencing, respectively. (1) Compared to RT, straw and straw biochar increased AOB abundance and diversity significantly only in wheat season (P < 0.05), and they both ranked as SB > SR > RT. Among fertilized treatments, a significant difference between SR and RT was found in AOB community composition of the winter season (R value = 0.58, P value = 0.02); (2) In contrast, AOA was almost not responsive to organic addition, except the significant enhancement of abundance by biochar in wheat season; (3) After straw and straw biochar addition, soil potential nitrification rates (PNR) was positive correlated to AOB abundance in both rice and wheat season (P < 0.01), not to AOA abundance (P = 0.211 and 0.068, respectively). This study provides scientific support for the potential of straw utilization to improve nitrification in rice-wheat rotation system with respect to soil ammonia oxidation microorganism.

Hierarchical drivers of soil microbial community structure variability in \u201cMonte Perdido\u201d Massif (Central Pyrenees)

Microbial activity is highly dependent on climatic factors (moisture and temperature) and edaphic characteristics in temperate ecosystems. Moreover, soil microbial community composition in high mountain areas is less known when compared to plant communities. In this study we investigated the soil microbial community from a functional perspective using PLFA (phospholipid fatty acid) methods in the four aspects of four summits (2,242 – 3,012 m above sea level) in the Spanish Central Pyrenees. Soil organic carbon (C), microbial biomass and nutrient dynamics (N{H}_{4}^{+} + N{O}_{3}^{-}, N mineralization and nitrification potential) were also determined. Microbial biomass C was highest in the lowermost summit and decreased by approximately 50, 14 and 12% with increasing altitude. In each summit soil N{H}_{4}^{+} and N{O}_{3}^{-} concentrations differed significantly among summits and aspects. Soil nitrification potential varied significantly between the factors summit and aspects, e.g., southerly vs. northerly, easterly vs. westerly aspects. Gram negative bacteria and Actinobacteria functional groups dominated the microbial community, with almost 40% of the total PLFA. Non-metric multidimensional scale (NMS) analysis showed that most of the PLFA functional groups were present in all summits and aspects, although with specific biomarkers. A high abundance of biomarkers 16:1ω9c and 16:0 2OH (gram negative bacteria) were obtained in the lowermost summit, while the biomarkers 16.1ω7cDMA (anaerobes) and 19:3ω6c (Eukaryote) were only found in the uppermost summit. Linear mixed model (lmm) analysis was used with summit as fixed effect and aspect as random effect. In general, our results demonstrate a fundamental role for environment, principally moisture, temperature and organic matter in explaining the pattern observed for soil PLFA biomarkers. Under a global change scenario, we need to shed light on the relationships between soil microbial functional groups and soil nutrient-related variables in order to identify the associated patterns of decomposition rates and soil processes driven by microbial communities in mountain areas. The results could thus be used in global predictive models on climate change impact on C or N cycles in these environments.

Responses of nitrification and denitrification to nitrogen and phosphorus fertilization: does the intrinsic soil fertility matter?

Nitrogen (N) is not only a major regulator of productivity in terrestrial systems but can also be a pollutant. While the effects of fertilizer addition to soil N cycling processes and the associated soil microbial groups have been extensively studied, little attention has been paid to the interaction of fertilizer amendment and the intrinsic fertility of the soil. We conducted a filed study on the Chinese Loess Plateau with soils of two fertility levels (low fertility and high fertility), amended with or without synthetic fertilizer (N and phosphorus, NP). Soil samples were collected three times in a continuous winter wheat cropping system and analyzed for soil potential nitrification (PNA) and denitrification (DEA) rates, nitrifiers and denitrifiers. Populations of ammonia-oxidizing archaea (AOA) and bacteria (AOB) and the resulting PNA were increased by the NP application regardless of intrinsic soil fertility. The NP application increased DEA by 15–228% in the low fertility soil, but decreased DEA by 18–46% in the high fertility soil. Soil denitrifiers also showed divergent responses to NP application in low and high fertility soils, i.e., the abundance of nirS-nitrite reducers increased in the low fertility soil, while nirS- and nirK-nitrite reducers were unchanged in the high fertility soil. The narG-nitrate reducers, AOB and AOA were more responsive to the NP application in the tillering stage (BBCH 25–26) than in other periods, whereas PNA, DEA and denitrifier gene abundance (nirS and nirK) varied more in the flowering stage (BBCH 65). Our results suggest that the responses of soil nitrification/nitrifiers and denitrification /denitrifiers to NP application varied with intrinsic soil fertility levels. The contrasting responses of nitrification and denitrification to NP application in high fertility soil indicate a decoupling of both processes.

Long-term fertilization regimes change soil nitrification potential by impacting active autotrophic ammonia oxidizers and nitrite oxidizers as assessed by DNA stable isotope probing.

Chemoautotrophic ammonia-oxidizers and nitrite-oxidizers are responsible for a significant amount of soil nitrate production. The identity and composition of these active nitrifiers in soils under different long-term fertilization regimes remain largely under-investigated. Based on that soil nitrification potential significantly decreased in soils with chemical fertilization (CF) and increased in soils with organic fertilization (OF), a microcosm experiment with DNA stable isotope probing was further conducted to clarify the active nitrifiers. Both ammonia-oxidizing archaea (AOA) and bacteria (AOB) were found to actively respond to urea addition in soils with OF and no fertilizer (CK), whereas only AOB were detected in soils with CF. Around 98% of active AOB were Nitrosospira cluster 3a.1 in all tested soils, and more than 90% of active AOA were Nitrososphaera subcluster 1.1 in unfertilized and organically fertilized soils. Nitrite oxidation was performed only by Nitrospira-like bacteria in all soils. The relative abundances of Nitrospira lineage I and VI were 32% and 61%, respectively, in unfertilized soils, and that of Nitrospira lineage II was 97% in fertilized soils, indicating long-term fertilization shifted the composition of active Nitrospira-like bacteria in response to urea. This finding indicates that different fertilizer regimes impact the composition of active nitrifiers, thus, impacting soil nitrification potential.

Chronic Nitrogen Fertilization Modulates Competitive Interactions Among Microbial Ammonia Oxidizers in a Loess Soil

Nitrogen (N) application may lead to niche segregation of soil ammonia-oxidizing archaea (AOA) and bacteria (AOB), thereby reducing the competitive interactions between AOA and AOB due to higher ammonium substrate availability. However, the adaptive mechanisms of AOA and AOB under N enrichment remain poorly understood. Stable isotope probing (SIP) microcosm incubation was employed to reveal community changes of active AOA and AOB in a loess soil from a field experiment growing potatoes that received no N (control, CK), low N (LN, 75 kg N ha−1), and high N (HN, 375 kg N ha−1). The results showed that the soil potential nitrification rate (PNR) was measured by culturing of the soil samples from the field experiment. Soil PNR was significantly increased in HN by 87.5% and 67.5% compared with CK and LN, respectively. Compared with CK, the 13C-amoA genes of soil AOA and AOB in HN had 2.58 × 104 and 1.55 × 106 copies, representing 1.6- and 16.2-fold increase respectively. It was indicated that AOB dominated soil ammonia oxidation. A phylogenetic analysis of the 13C-amoA gene showed that N application significantly increased the proportion of 54d9-like AOA up to 90% in HN, while the Nitrososphaera gargensis-like and Nitrososphaera viennensis-like AOA were inhibited and completely disappeared. Nitrogen application also resulted in the community shift of active AOB-dominant group from Nitrosospira briensis-like to Nitrosospira sp. TCH711-like. Our study provides compelling evidence for the emergence and maintenance of active nitrifying communities under the intensified N input to an agricultural ecosystem.

Impacts of Cu and sulfadiazine on soil potential nitrification and diversity of ammonia-oxidizing archaea and bacteria

ABSTRACTSoil potential nitrification rate (PNR) and diversity of ammonia-oxidizing microbes were investigated in spiked soils with Cu and Sulfadiazine (SDZ). An obvious decrease of PNR was observed with the increase of Cu and SDZ concentrations in the soil. Real-time fluorescence quota PCR result showed that AOA and AOB were slightly stimulated at the gene level in both contaminated soils. Sequential analysis indicated that 200 mg kg−1 Cu could improve AOA diversity but reduce AOB diversity, but 5 mg kg−1 SDZ caused a decrease of both AOA and AOB diversity. Microbial community’s analysis also found that 200 mg kg−1 Cu and 5 mg kg−1 SDZ had different influence on the populations of AOA and AOB. It could be concluded that Cu and SDZ might have a different ecological effect mechanism on soil potential nitrification and ammonia-oxidizing microbial communities.

Responses difference analysis of soil potential nitrification rates to Cu and sulfadiazine

ABSTRACTTo investigate responses difference of soil potential nitrification rates (PNR) to heavy metals and antibiotics, batch toxicity experiments of Cu and sulfadiazine (SDZ) were conducted in four typical soils which were collected from different locations of China. Toxic differences of Cu and SDZ to soil PNR were evaluated based on their values of EC20 and EC50 calculated with logistic model. It was found that soil nitrification was a good indicator of Cu contamination in soils except for paddy soil. For SDZ, their ECX values were significantly higher than its concentration level (0.002 ~ 0.760 mg·kg-1) in soils. Thus, it was concluded that soil nitrification might be more sensitive to Cu than SDZ in field soil. The further Pearson correlation analysis indicated that the response of soil PNR to Cu and SDZ contamination were significantly related with soil PNR0 and pH.

Assessing thermodynamic parameter sensitivity for simulating temperature responses of soil nitrification.

Soil nitrification responses to temperature have major implications for the global nitrogen cycle. Temperature sensitivity of soil nitrification has been modeled using several mathematical models, yet the extent to which model-generated thermodynamic parameters are accurate and sensitive in describing temperature sensitivity is unclear. In this study, we performed global sensitivity analysis to identify the key thermodynamic parameters that are most influential when simulating the temperature response of the soil nitrification potential (NP) across two different temperature gradients (4-40 °C and 20-45 °C) which are imposed upon sixteen different soils with square root growth (SQRT) and macromolecular rate theory (MMRT) models. We found that two thermodynamic parameters stand out as moderately to highly sensitive, and are uniquely identifiable in each model, regardless of the temperature range. The minimum and maximum measured temperatures seem to have no impact on the list of sensitive parameters but do influence the parameter ranges, especially for the SQRT model. However, parameters that control the minimum temperature and curvature of the NP response curve (Tmin and ΔC‡P) were found to have little to no sensitivity to SQRT and MMRT model outputs, respectively. We show that the parameter sensitivity and range of measured temperatures influence the complementary model's ability to describe the temperature sensitivity of soil nitrification. Our proposed framework enhances the accurate interpretation of existing thermodynamic parameters that explain the temperature sensitivity of soil biochemical processes, and provides methodological recommendations for future temperature sensitivity studies.

Long-term greenhouse vegetable cultivation alters the community structures of soil ammonia oxidizers

Continuous cropping obstacles derived from long-term intensive cultivation has severely affected the healthy development of greenhouse vegetable industry in China. The alterations in soil microbial communities are commonly thought to be the important reasons behind continuous cropping obstacles. However, little attention is focused on soil functional microorganisms, such as soil ammonia oxidizers, which convert NH3 to NO2− in soils and play an important role in terrestrial ecosystem N cycling. With the help of quantitative real-time PCR and high-throughput sequencing methods, this study investigated the effects of consecutive greenhouse vegetable cultivation on the abundances and community structures of ammonia-oxidizing archaea (AOA) and bacteria (AOB). Continuous greenhouse production significantly affected soil physicochemical properties but slightly influenced on soil biochemical properties. Long-term greenhouse production significantly decreased soil potential nitrification rates. The abundances of AOA and AOB were insensitive to continuous greenhouse cultivation, whereas their community structures were significantly altered. A significant increase in average relative abundance of Nitrososphaerales Clade B4 in AOA community was observed after long-term greenhouse cultivation but just the opposite for Nitrososphaerales Clade Nitrososphaera. Increasing greenhouse cultivation years decreased the average relative abundance of Nitrosospira in AOB community, and their ecological niches were gradually occupied by Nitrosomonas; consequently, Nitrosomonas dominated AOB community under long-term greenhouse vegetable cultivation. Soil physicochemical properties, but not biological activity, were directly linked to the shifts of community structures of ammonia oxidizers. Moisture and total nitrogen significantly regulated AOA community structure, whereas organic matter and salinity mediated that of AOB. Our results demonstrated that consecutive greenhouse production obviously affected soil N turnover through altering community structures of nitrifiers, and soil functional microorganisms should be taken more attention when exploring the mechanisms for continuous cropping obstacles of greenhouse vegetable. The current study also highlights the fast adaptation of both AOA and AOB communities to complexly varied soil physicochemical environments.

Impact of Heavy Metal Pollution on Ammonia Oxidizers in Soils in the Vicinity of a Tailings Dam, Baotou, China.

Soil heavy metal pollution has received increasing attention due to their toxicity to soil microorganisms. We have analyzed the effects of heavy metal pollution on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) in soils in the vicinity of a tailings dam of Baotou region, China. Results showed that AOB were dominated with Nitrosomonas-like clusters, while AOA was dominated by group1.1b (Nitrososphaera cluster). Single Cd and Cr contents, as well as compound heavy metal pollution levels, had a significant negative impact on soil potential nitrification rate and both diversities of AOA and AOB. No clear relationship was found between any single heavy metal and abundance of AOA or AOB. But compound pollution could significantly decrease AOA abundance. The results indicated that heavy metal pollution had an obviously deleterious effect on the abundance, diversity, activity and composition of ammonia oxidizers in natural soils.

Effects of Rhizophora mangle leaf litter and seedlings on carbon and nitrogen cycling in salt marshes \u2013 potential consequences of climate-induced mangrove migration

Background and aimsDue to the production of large amounts of tannins and phenolics by Rhizophora mangle, it was hypothesized that the invasion of this mangrove species in salt marshes due to global warming will result in changes in the cycling of carbon and nitrogen.MethodsLeaf litter and/or seedlings of R. mangle were placed into 1-m2 experimental plots in a Distichlis spicata-dominated salt marsh on the Atlantic Coast of central Florida (USA). An additional litter decomposition experiment was conducted in all plots by adding litter bags containing 10 g of dried D. spicata shoot litter. Seedling growth was measured yearly. One and four years after the start of the experiment, soil samples were collected to determine physical and chemical soil conditions, potential nitrification and denitrification activities and abundances of genes that are related to microbial processes in the nitrogen cycle.ResultsGrowth of R. mangle seedlings was stimulated in the presence of R. mangle litter, while decomposition rates of D. spicata litter were lower in plots with R. mangle litter and seedlings. The presence of R. mangle litter and/or seedlings had no significant effect on potential nitrification and denitrification activities and on the abundances of genes.ConclusionThe colonization of R. mangle into D. spicata-dominated salt marshes will affect the carbon cycle, but not necessarily the nitrogen cycle, which is likely due to the pre-existing nitrogen-limited conditions in the salt marsh.

Grassland plant species and cultivar effects on nitrous oxide emissions after urine application

We tested whether plant species identity influenced emissions of nitrous oxide (N2O) in response to the addition of urine at a rate of 530 kg nitrogen (N) ha−1. The plants tested are all used in temperate pastoral agriculture and comprised 11 C3 grasses, 3 forbs and 2 legumes with controls of bare ground and a standard ryegrass/white clover mixture used in the region. Total N2O emitted over 6 weeks after the application of cattle urine was highest in the bare ground. Plant identity was important with emissions ranging from an average of 0.67 kg N2O-N ha−1 for the Italian ryegrass Grasslands Moata (Lolium multiflorum) to 3.20 kg N2O-N ha−1 for the upland brome Grasslands Gala (Bromus stamineus). The 4 perennial ryegrass (Lolium perenne) cultivars had generally low emissions (average of 1.02 kg N2O-N ha−1) and the white clover (Trifolium repens) cultivars high emissions (average of 2.86 kg N2O-N ha−1). Plants with high N uptake rates had low N2O emissions. However, this was not always due to a high N uptake rate per se as during the first 2 weeks after urine was applied, when N2O emissions were at their highest, soil mineral N was in excess of plant N demand. It seems that some traits related to a high plant N uptake, such as low soil nitrification potential, might be important in driving differences in emissions associated with different plant species.

Influence of Cu application on ammonia oxidizers in fluvo-aquic soil

Nitrification driving by archaeal and bacterial ammonia oxidizers is an essential step of the global nitrogen cycle. This laboratory study screened toxicity of copper (Cu) stress in the fluvo-aquic system, investigated the ecological niche of ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities and analyzed the response of AOA and AOB communities to Cu stress in the system. We added Cu as CuCl2 solution at 0 mg, 100 mg, 300 mg, 600 mg, and 1200 mg Cu per kg soil in the fresh fluvo-aquic soil. Soil samples were incubated for 6 h, 12 h, 1 d, 3 d, 7 d, 14 d, 28 d, and 56 d in lab condition. The soil potential nitrification rates (PNR) were measured at each stage. Denaturing gradient gel electrophoresis (DGGE) analysis of amoA of AOA and AOB were performed at 56 d of Cu incubation. Soil RNA was extracted and the abundance of amoA transcripts was analyzed by real time polymerase chain reaction (real-time PCR) at 1, 3, and 7 d. The amoA gene abundance analysis and DGGE profile analysis showed that the abundance and diversity of AOA were higher than that of AOB in fluvo-aquic soil. However, the correlations between the soil PNR and the number of amoA transcript indicated that the AOB community, rather than AOA community, dominates ammonia oxidation in Cu-polluted fluvo-aquic soil ecosystems. Combining the results of the soil PNR and the abundance and diversity of ammonia oxidizers community, Cu hormetic concentration was identified as a range from 100 to 600 mg kg−1, while the toxic concentration was higher than 600 mg kg−1 in fluvo-aquic soil system. Furthermore, the results of amoA transcript numbers showed that AOA may be resistant to Cu stress and AOB may be more sensitive to Cu stress under our experiment conditions.