Nitrogen Cycling In Ecosystems Research Articles

Hydrochar and microplastics disturb soil dissolved organic matter and prominently mitigate ammonia volatilization from wheat growing soil

The impacts of microplastics (MPs) pollution on the nitrogen (N) cycle in soil ecosystems have attracted worldwide attention. Meanwhile, dissolved organic matter (DOM)-rich hydrochar (HBC) is being tried as an emerging soil conditioner for gaseous N reduction load to the atmosphere. However, the influence of MPs and HBC on soil DOM and ammonia (NH3) volatilization in wheat soils remains poorly understood. In this study, a soil column experiment was established in wheat growing soils to determine the impacts of HBC, MPs (i.e., polyethylene and polyacrylonitrile), and the co-occurrence of HBC and MPs (HBC + MPs) on soil NH3 volatilization, physicochemical properties and wheat growth. In comparison with the control (CK, without MPs or HBC), HBC, MPs, and HBC + MPs reduced cumulative NH3 volatilization by 45.4%, 66.7–67.4% and 29.8–58.7%, respectively. Soil DOM fulvic acid-like and humic aid substance abundances of the other treatments were increased by 18.5–56.2% and 12.7–39.3%, respectively, compared to CK. Besides, soil NH3 fluxes were positively correlated to soil NH4+-N concentration at basal fertilization and soil NO3−-N concentration at first supplementary fertilization. HBC, MPs and HBC + MPs treatments promoted soil urease activity and plant N uptake by 6.5–24.2% and 31.9–74.3%, respectively, compared to CK. This study provides an insight into the variation of NH3 volatilization caused by anthropogenic carbon in wheat soil ecosystems.

Main environmental drivers of abundance, diversity and community structure of comammox Nitrospira in paddy soils

The recently discovered complete ammonia oxidizers comammox Nitrospira contain clades A and B that can establish an independent one-step nitrification process; however, little is known about their environmental drivers or habitat distributions in agricultural soils. Previous studies on comammox Nitrospira in paddy soils have mainly focused on small-scale samples, and there is a lack of multisite research on comammox Nitrospira in paddy soils. In this study, we conducted a survey of 36 paddy soils to understand the community structure, abundance, and diversity of comammox Nitrospira and the degree to which they are affected by environmental factors at a large scale. Comammox Nitrospira were found to be widely distributed among the paddy soils. The abundance of comammox Nitrospira clade A was mostly lower than that of clade B, whereas its diversity was mostly higher than that of clade B. Correlation analysis showed that multiple factors affected (P < 0.05) the abundance of comammox Nitrospira, including soil pH, organic matter, total carbon, and total nitrogen, latitude, mean annual temperature, and mean annual precipitation. Moreover, there was a clear relationship between the comammox Nitrospira community and habitat, indicating that some amplicon sequence variants (ASVs) had a unique dominant position in specific habitats. Phylogenetic analysis showed that the ASVs of comammox Nitrospira clade A clustered with the known sequences in the paddy soils and were significantly different from the known sequences in other habitats, which may be related to the unique paddy field habitat. In contrast, comammox Nitrospira clade B showed no clear habitat dependence. These results support the wide distribution and high abundance of comammox Nitrospira in paddy soils and provide novel insights into nitrogen cycling and nutrient management in agricultural ecosystems.

Belowground responses to altered precipitation regimes in two semi-arid grasslands

Predicted climate change extremes, such as severe or prolonged drought, may considerably impact carbon (C) and nitrogen (N) cycling in water-limited ecosystems. However, we lack a clear and mechanistic understanding of how extreme climate change events impact ecosystem processes belowground. This study investigates the effects of five years of reoccurring extreme growing season drought (66% reduction, extreme drought treatment) and two-month delay in monsoon precipitation (delayed monsoon treatment) on belowground productivity and biogeochemistry in two geographically adjacent semi-arid grasslands: Chihuahuan Desert grassland dominated by Bouteloua eriopoda and Great Plains grassland dominated by B. gracilis. After five years, extreme drought reduced belowground net primary productivity (BNPP) in the Chihuahuan Desert grassland but not in the Great Plains grassland. Across both grasslands, extreme drought increased soil pH and available soil nutrients nitrate and phosphate. The delayed monsoon treatment reduced BNPP in both grasslands. However, while available soil nitrate decreased in the Chihuahuan Desert grassland, the delayed monsoon treatment overall had little effect on soil ecosystem properties. Extreme drought and delayed monsoon treatments did not significantly impact soil microbial biomass, exoenzyme potentials, or soil C stocks relative to ambient conditions. Our study demonstrates that soil microbial biomass and exoenzyme activity in semi-arid grasslands are resistant to five years of extreme and prolonged growing season drought despite changes to soil moisture, belowground productivity, soil pH, and nutrient availability.

Nitrite and nitrate reduction drive sediment microbial nitrogen cycling in a eutrophic lake

The anaerobic microbial nitrogen (N) removal in lake sediments is one of the most important processes driving the nitrogen cycling in lake ecosystems. However, the N removal and its underlying mechanisms regulated by denitrifying and anaerobic ammonia oxidation (anammox) bacteria in lake sediments remain poorly understood. With the field sediments collected from different areas of Lake Donghu (a shallow eutrophic lake), we examined the denitrifying and anammox bacterial communities by sequencing the nirS/K and hzsB genes, respectively. The results indicated that denitrifiers in sediments were affiliated to nine clusters, which are involved in both heterotrophic and autotrophic denitrification. However, anammox bacteria were only dominated by Candidatus Brocadia. We found that NO3− and NO2− concentrations, as well as Nar enzyme activity were the key factors affecting denitrifying and anammox communities in this eutrophic lake. The enrichment experiments in bioreactors confirmed the divergence of denitrification and anammox rates with an additional complement of NO2−, especially under a condition low nitrate reductase activity. The coupled denitrification and anammox may play significant roles in N removal, and the availability of electronic acceptors (i.e., NO2− and NO3−) strongly influenced the N loss in lake sediments. Further path analysis indicated that NO2−, NO3− and some N-related enzymes were the key factors affecting microbial N removal in lake sediments. This study advances our understanding of the mechanisms driving the of denitrification and anammox in lake sediments, which also provides new insights into coupled denitrification-anammox N removal in eutrophic lake ecosystems.

Indices enhance biological soil crust mapping in sandy and desert lands

Biological soil crusts (BSCs) covering drylands worldwide are an important functional vegetation unit and play an important role in the carbon and nitrogen cycling of dryland ecosystems. However, there is a lack of accurate data on the spatial distribution and change of BSCs. Here, two indices were developed to enhance BSC mapping, including the sandy land ratio crust index (SRCI) for sandy land with moss-dominated BSCs and the desert ratio crust index (DRCI) for desert with lichen-dominated BSCs. The SRCI combines the near infrared band in which moss-dominated BSCs and other surface components have significant spectral differences, and red edge bands in which such differences can be enhanced. Another index, the DRCI, highlights different spectral characteristics of lichen-dominated BSCs, gravel and sand in the shortwave infrared and red-edge bands. A random forest (RF) machine learning framework was developed to map the large-scale BSC distribution with 10 m resolution using Sentinel 2 satellite data. The results show that the BSC area in the Mu Us Sandy Land and Gurbantunggut Desert of China covers 10,647 km2 and 14,036 km2, accounting for 5.7% and 29.4% of the total area, respectively, indicating that BSC is one of the dominant land cover types in the Gurbantunggut Desert. Validations and comparisons demonstrate that the overall accuracy of BSC detection is 90% and 94% with kappa coefficients of 0.80 and 0.86 in sandy land and desert, respectively, and the proposed SRCI and DRCI increase the overall mapping accuracy by 6% for sandy land and desert compared to the non-index scheme. Therefore, indices can enhance the BSC mapping accuracy for sandy land and desert. Moreover, this work provides basic data for exploring the ecological effects of BSCs in arid areas.

A comprehensive dataset of microbial abundance, dissolved organic carbon, and nitrogen in Tibetan Plateau glaciers

Abstract. Glaciers are recognized as a biome dominated by microorganisms and a reservoir of organic carbon and nutrients. Global warming remarkably increases glacier melting rate and runoff volume, which have significant impacts on the carbon and nitrogen cycles in downstream ecosystems. The Tibetan Plateau (TP), dubbed “the water tower of Asia”, owns the largest mountain glacial area at mid- and low-latitudes. However, limited data on the microbial abundance, organic carbon, and nitrogen in TP glaciers are available in the literature, which severely hinders our understanding of the regional carbon and nitrogen cycles. This work presents a new dataset on microbial abundance, dissolved organic carbon (DOC), and total nitrogen (TN) for TP glaciers. In this dataset, there are 5409 records from 12 glaciers for microbial abundance in ice cores and snow pits, and 2532 records from 38 glaciers for DOC and TN in the ice core, snow pit, surface ice, surface snow, and proglacial runoff. These glaciers are located across diverse geographic and climatic regions, where the multiyear average air temperature ranges from −13.4 to 2.9 ∘C and the multiyear average precipitation ranges from 76.9 to 927.8 mm. This makes the constructed dataset qualified for large-scale studies across the TP. To the best of our knowledge, this is the first dataset of microbial abundance and TN in TP glaciers and also the first dataset of DOC in ice cores of the TP. This new dataset provides important information for studies on carbon and nitrogen cycles in glacial ecosystems, and is especially valuable for the assessment of potential impacts of glacier retreat on downstream ecosystems under global warming. The dataset is available from the National Tibetan Plateau/Third Pole Environment Data Center (https://doi.org/10.11888/Cryos.tpdc.271841; Liu, 2021).

Archaeal Communities of South China Mangroves and Their Potential Roles in the Nitrogen Cycle

Mangroves are unique ecosystems that harbor a wide variety of microbial communities including Archaea. Although Archaea have widely been reported to play important roles in biogeochemical cycles, the involvement of archaeal communities in mangrove ecosystems and nitrogen cycling remains largely underexplored. To bridge this gap, we investigated the distribution of archaeal communities and their functional genes involved in the nitrogen cycle in South China mangroves covering seven different geographical areas. Our results indicated that mangroves are an important reservoir of unclassified Archaea, and therefore these unique ecosystems could fuel important discoveries in the future. Moreover, total nitrogen, total carbon and nitrate concentrations were found to be critical drivers of archaeal community composition in these mangroves. Selective Archaea, particularly Crenarchaeota and Euryarchaeota, are the major contributors to nitrogen cycling in the mangroves, especially involved in nitrification. Taken together, our findings advance our understanding of Archaea-driven nitrogen cycling in mangrove ecosystems.

The amounts and ratio of nitrogen and phosphorus addition drive the rate of litter decomposition in a subtropical forest

Nitrogen (N) and phosphorus (P) control biogeochemical cycling in terrestrial ecosystems. However, N and P addition effects on litter decomposition, especially biological pathways in subtropical forests, remain unclear. Here, a two-year field litterbag experiment was employed in a subtropical forest in southwestern China to examine N and P addition effects on litter biological decomposition with nine treatments: low and high N- and P-only addition (LN, HN, LP, and HP), NP coaddition (LNLP, LNHP, HNLP, and HNHP), and a control (CK). The results showed that the decomposition coefficient (k) was higher in NP coaddition treatments (P < 0.05), and lower in N- and P-only addition treatments than in CK (P < 0.05). The highest k was observed with LNLP (P < 0.05). The N- and P-only addition treatments decreased the losses of litter mass, lignin, cellulose, and condensed tannins, litter microbial biomass carbon (MBC), litter cellulase, and soil pH (P < 0.05). The NP coaddition treatments increased the losses of litter mass, lignin, and cellulose, MBC concentration, litter invertase, urease, cellulase, and catalase activities, soil arthropod diversity (S) in litterbags, and soil pH (P < 0.05). Litter acid phosphatase activity and N:P ratio were lower in N-only addition treatments but higher in P-only addition and NP coaddition treatments than in CK (P < 0.05). Structural equation model showed that litter MBC, S, cellulase, acid phosphatase, and polyphenol oxidase contributed to the loss of litter mass (P < 0.05). The litter N:P ratio was negatively logarithmically correlated with mass loss (P < 0.01). In conclusion, the negative effect of N addition on litter decomposition was reversed when P was added by increasing decomposed litter soil arthropod diversity, MBC concentration, and invertase and cellulase activities. Finally, the results highlighted the important role of the N:P ratio in litter decomposition.

The first complete genome sequence of Microbulbifer celer KCTC12973T, a type strain with multiple polysaccharide degradation genes.

Genus Microbulbifer plays important roles in element cycling process in marine environments, and the first type strain KCTC 12973T (=ISL-39T=CCUG 54356T) of M. celer was isolated and identified in 2007. However, the genome sequence of M. celer KCTC 12973T is still unclear, which complicates the functional exploration and new species identification of other species belonged to this genus. This study reported the complete genome sequence of M. celer KCTC 12973T with a genome size of 4,346,001bp. A total of 3601 protein-coding genes were annotated in the genome. The potential genes involved in the polysaccharide degradation, including cellulose, chitin, xylan, and pectate, were found in the protein-coding genes. Besides, the reductase genes of nitrate and nitrite were also annotated in the genome. These findings indicated the potential crucial ecological functions of M. celer KCTC 12973T for carbon and nitrogen cycles in marine ecosystems.

Biochar application affects Nitrobacter rather than Nitrospira in plastic greenhouse vegetable soil

Nitrite oxidation, driven by nitrite-oxidizing bacteria, is an important step of nitrification and thus plays a vital role in biogeochemical nitrogen cycling in agricultural ecosystems. Although biochar has been widely recognized as a promising material for use in vegetable fields to slowly release nutrients, the current understanding of how nitrite oxidizers respond to the application of biochar in a plastic greenhouse vegetable field is very limited. A soil incubation experiment showed that soil nitrite oxidation was increased by 13.0%, 35.0% and 64.2% (P < 0.05) with the application of 0.5% (C0.5), 1.5% (C1.5), and 4.0% (C4.0) biochar (mass ratio), respectively. Methods including qPCR and Illumina MiSeq sequencing were also used to explore the impact of biochar on functional communities involved in nitrite oxidation. Nitrobacter and Nitrospira were the main genera of the nitrite-oxidizing bacteria and were assumed to be very important in the soil environment. Moreover, the increases in soil nitrite oxidation were positively correlated with the abundance of Nitrobacter-like NOB (P < 0.05) but not the Nitrospira-like NOB gene abundance. Furthermore, biochar application combined with nitrogen fertilizer had a significant effect on the Nitrobacter community composition, with lineages such as Nitrobacter alkalicus and Nitrobacter vulgaris being significantly enriched. Redundancy analysis indicated that the observed variations in the Nitrobacter community structure were associated with changes in soil microbial biomass nitrogen and nitrate nitrogen induced by biochar. These results implied that Nitrobacter rather than Nitrospira was significantly improved by biochar added to the soil for vegetable production.

Patterns of free amino acids in tundra soils reflect mycorrhizal type, shrubification, and warming

The soil nitrogen (N) cycle in cold terrestrial ecosystems is slow and organically bound N is an important source of N for plants in these ecosystems. Many plant species can take up free amino acids from these infertile soils, either directly or indirectly via their mycorrhizal fungi. We hypothesized that plant community changes and local plant community differences will alter the soil free amino acid pool and composition; and that long-term warming could enhance this effect. To test this, we studied the composition of extractable free amino acids at five separate heath, meadow, and bog locations in subarctic and alpine Scandinavia, with long-term (13 to 24 years) warming manipulations. The plant communities all included a mixture of ecto-, ericoid-, and arbuscular mycorrhizal plant species. Vegetation dominated by grasses and forbs with arbuscular and non-mycorrhizal associations showed highest soil free amino acid content, distinguishing them from the sites dominated by shrubs with ecto- and ericoid-mycorrhizal associations. Warming increased shrub and decreased moss cover at two sites, and by using redundancy analysis, we found that altered soil free amino acid composition was related to this plant cover change. From this, we conclude that the mycorrhizal type is important in controlling soil N cycling and that expansion of shrubs with ectomycorrhiza (and to some extent ericoid mycorrhiza) can help retain N within the ecosystems by tightening the N cycle.

Contrasting nitrogen cycling between herbaceous wetland and terrestrial ecosystems inferred from plant and soil nitrogen isotopes across China

Abstract Understanding nitrogen (N) cycling in different ecosystems is crucial to predicting and mitigating the global effects of altered N inputs. Although wetlands have always been assumed to differ largely from terrestrial ecosystems in N cycling, evidence from direct comparison from the field along wide environmental gradients is lacking. Here, we hypothesized strong coupling of plant and soil δ15N in terrestrial ecosystems due to lower N inputs and losses but weak coupling of plant and soil δ15N in wetlands because of higher N inputs and losses. We performed a large‐scale field investigation on 26 pairs of herbaceous wetland and terrestrial sites across China covering 21 degrees of latitude and determined natural abundance of nitrogen isotopes (δ15N) in soils and leaves of 346 dominant and subordinate plant species. We analysed the relationships between leaf and soil δ15N and their drivers including plant functional types in these two types of ecosystems. Plant functional types including mycorrhizal type and N2‐fixing status had consistently significant influences on leaf δ15N in herbaceous wetland and terrestrial ecosystems. Leaf δ15N increased significantly with soil δ15N within and across mycorrhizal types in both ecosystems, and, as hypothesized, the relationships were stronger and steeper in terrestrial than in wetland ecosystems. Moreover, leaf and soil δ15N were positively and significantly correlated within both N2‐fixers and non‐N2‐fixers in terrestrial ecosystems and within only non‐N2‐fixers in wetlands. At the community level, we also found more highly significant relationships between leaf and soil δ15N in terrestrial than in wetland ecosystems. Besides plant functional types, climatic and soil factors contributed to the variation in leaf δ15N in both ecosystems. Synthesis. Weaker relationships between plant and soil δ15N in wetlands at species and community levels support the hypothesis that larger N inputs and losses lead to weaker coupling in the plant–soil systems in wetlands than in terrestrial ecosystems. This provides strong evidence from a large spatial scale for contrasting N cycling in these two types of ecosystems regardless of plant functional type in terms of nutrient uptake strategy. Our findings add to our predictive power of ecosystem N dynamics under environmental changes, for example, land‐use changes and elevated N inputs.

Unraveling Nitrogen, Sulfur, and Carbon Metabolic Pathways and Microbial Community Transcriptional Responses to Substrate Deprivation and Toxicity Stresses in a Bioreactor Mimicking Anoxic Brackish Coastal Sediment Conditions

Microbial communities are key drivers of carbon, sulfur, and nitrogen cycling in coastal ecosystems, where they are subjected to dynamic shifts in substrate availability and exposure to toxic compounds. However, how these shifts affect microbial interactions and function is poorly understood. Unraveling such microbial community responses is key to understand their environmental distribution and resilience under current and future disturbances. Here, we used metagenomics and metatranscriptomics to investigate microbial community structure and transcriptional responses to prolonged ammonium deprivation, and sulfide and nitric oxide toxicity stresses in a controlled bioreactor system mimicking coastal sediment conditions. Ca. Nitrobium versatile, identified in this study as a sulfide-oxidizing denitrifier, became a rare community member upon ammonium removal. The ANaerobic Methanotroph (ANME) Ca. Methanoperedens nitroreducens showed remarkable resilience to both experimental conditions, dominating transcriptional activity of dissimilatory nitrate reduction to ammonium (DNRA). During the ammonium removal experiment, increased DNRA was unable to sustain anaerobic ammonium oxidation (anammox) activity. After ammonium was reintroduced, a novel anaerobic bacterial methanotroph species that we have named Ca. Methylomirabilis tolerans outcompeted Ca. Methylomirabilis lanthanidiphila, while the anammox Ca. Kuenenia stuttgartiensis outcompeted Ca. Scalindua rubra. At the end of the sulfide and nitric oxide experiment, a gammaproteobacterium affiliated to the family Thiohalobacteraceae was enriched and dominated transcriptional activity of sulfide:quinone oxidoreductases. Our results indicate that some community members could be more resilient to the tested experimental conditions than others, and that some community functions such as methane and sulfur oxidation coupled to denitrification can remain stable despite large shifts in microbial community structure. Further studies on complex bioreactor enrichments are required to elucidate coastal ecosystem responses to future disturbances.

Nitrogen Gain and Loss Along an Ecosystem Sequence: From Semi-desert to Rainforest

Plants and microorganisms, besides the climate, drive nitrogen (N) cycling in ecosystems. Our objective was to investigate N losses and N acquisition strategies along a unique ecosystem-sequence (ecosequence) ranging from arid shrubland through Mediterranean woodland to temperate rainforest. These ecosystems differ in mean annual precipitation, mean annual temperate, and vegetation cover, but developed on similar granitoid soil parent material, were addressed using a combination of molecular biology and soil biogeochemical tools. Soil N and carbon (C) contents, δ15N signatures, activities of N acquiring extracellular enzymes as well as the abundance of soil bacteria and fungi, and diazotrophs in bulk topsoil and rhizosphere were determined. Relative fungal abundance in the rhizosphere was higher under woodland and forest than under shrubland. This indicates toward plants' higher C investment into fungi in the Mediterranean and temperate rainforest sites than in the arid site. Fungi are likely to decompose lignified forest litter for efficient recycling of litter-derived N and further nutrients. Rhizosphere—a hotspot for the N fixation—was enriched in diazotrophs (factor 8 to 16 in comparison to bulk topsoil) emphasizing the general importance of root/microbe association in N cycle. These results show that the temperate rainforest is an N acquiring ecosystem, whereas N in the arid shrubland is strongly recycled. Simultaneously, the strongest 15N enrichment with decreasing N content with depth was detected in the Mediterranean woodland, indicating that N mineralization and loss is highest (and likely the fastest) in the woodland across the continental transect. Higher relative aminopeptidase activities in the woodland than in the forest enabled a fast N mineralization. Relative aminopeptidase activities were highest in the arid shrubland. The highest absolute chitinase activities were observed in the forest. This likely demonstrates that (a) plants and microorganisms in the arid shrubland invest largely into mobilization and reutilization of organically bound N by exoenzymes, and (b) that the ecosystem N nutrition shifts from a peptide-based N in the arid shrubland to a peptide- and chitin-based N nutrition in the temperate rainforest, where the high N demand is complemented by intensive N fixation in the rhizosphere.

Terrestrial Ecosystem Modeling with IBIS: Progress and Future Vision

Dynamic Global Vegetation Models (DGVM) are powerful tools for studying complicated ecosystem processes and global changes. This review article synthesizes the developments and applications of the Integrated Biosphere Simulator (IBIS), a DGVM, over the past two decades. IBIS has been used to evaluate carbon, nitrogen, and water cycling in terrestrial ecosystems, vegetation changes, land-atmosphere interactions, land-aquatic system integration, and climate change impacts. Here we summarize model development work since IBIS v2.5, covering hydrology (evapotranspiration, groundwater, lateral routing), vegetation dynamics (plant functional type, land cover change), plant physiology (phenology, photosynthesis, carbon allocation, growth), biogeochemistry (soil carbon and nitrogen processes, greenhouse gas emissions), impacts of natural disturbances (drought, insect damage, fire) and human induced land use changes, and computational improvements. We also summarize IBIS model applications around the world in evaluating ecosystem productivity, carbon and water budgets, water use efficiency, natural disturbance effects, and impacts of climate change and land use change on the carbon cycle. Based on this review, visions of future cross-scale, cross-landscape and cross-system model development and applications are discussed.

Response of substrate kinetics and biological mechanisms to various pH constrains for cultured Nitrobacter and Nitrospira in nitrifying bioreactor

Nitrite (NO2−) oxidation is an essential step of biological nitrogen cycling in natural ecosystems, and is performed by chemolithoautotrophic nitrite-oxidizing bacteria (NOB). Although Nitrobacter and Nitrospira are regarded as representative NOB in nitrification systems, little attention has focused on kinetic characterisation of the coexistence of Nitrobacter and Nitrospira at various pH values. Here, we evaluate the substrate kinetics, biological mechanism and microbial community dynamics of an enrichment culture including Nitrobacter (17.5 ± 0.9%) and Nitrospira (7.2 ± 0.6%) in response to various pH constrains. Evaluation of the Monod equation at pH 6.0, 6.5, 7.0, 7.5, 8.0 and 8.5 showed that the enrichment had maximum rate (rmax) and maximum substrate affinity (KS) for NO2− oxidation at pH 7.0, which was also supported by the largest absolute abundance of Nitrobacter nxrA (5.26 × 107 copies per g wet sludge) and Nitrospira nxrB (1.975 × 109 copies per g wet sludge) genes. Moreover, the predominant species for the Nitrobacter-like nxrA were N. vulgaris and N. winogradskyi, while for the Nitrospira-like nxrB, the predominant species were N. japonica, N. calida and Ca. N. bockiana. Furthermore, the rmax was strongly and positively correlated with the abundance of the Nitrobacter nxrA or Nitrospira nxrB genes, or N. winogradsk, whereas KS was positively correlated with the abundance of Nitrobacter nxrA or Nitrospira nxrB genes or Ca. N. bockiana. Overall, this study could improve basis kinetic parameters and biological mechanism of NO2− oxidation in WWTPs.

Comparative Microbial Nitrogen Functional Gene Abundances in the Topsoil vs. Subsoil of Three Grassland Habitats in Northern China.

The microbial groups of nitrogen fixers, ammonia oxidizers, and denitrifiers play vital roles in driving the nitrogen cycle in grassland ecosystems. However, the understanding of the abundance and distribution of these functional microorganisms as well as their driving factors were limited mainly to topsoil. In this study, the abundances of nitrogen functional genes (NFGs) involved in nitrogen fixation (nifH), ammonia oxidation (amoA), and denitrification (nirK, nirS, and nosZ) were investigated in both topsoil (0–10 cm, soil layer with concentrated root) and subsoil (30–40 cm, soil layer with spare root) of three grassland habitats in northern China. The abundance of NFGs decreased with soil depth except for the archaeal amoA gene and the distribution of nifH, archaeal amoA, nirK, and nirS gene was significantly impacted by grassland habitats. Moreover, the distribution of NFGs was more responsive to the vertical difference than horizontal spatial heterogeneity. Redundancy analysis revealed that the distribution pattern of overall NFGs was regulated by grassland habitats, and these regulations were more obvious in the subsoil than in the topsoil. Variance partitioning analysis further indicated that soil resource supply (e.g., organic matter) may control the vertical distribution of NFGs. Taken together, the findings in this study could fundamentally improve our understanding of the distribution of N cycling-associated microorganisms across a vertical scale, which would be useful for predicting the soil N availability and guiding the soil N management in grassland ecosystems.

Model responses to CO2 and warming are underestimated without explicit representation of Arctic small-mammal grazing.

We use a simple model of coupled carbon and nitrogen cycles in terrestrial ecosystems to examine how “explicitly representing grazers” vs. “having grazer effects implicitly aggregated in with other biogeochemical processes in the model” alters predicted responses to elevated carbon dioxide and warming. The aggregated approach can affect model predictions because grazer‐mediated processes can respond differently to changes in climate compared with the processes with which they are typically aggregated. We use small‐mammal grazers in a tundra as an example and find that the typical three‐to‐four‐year cycling frequency is too fast for the effects of cycle peaks and troughs to be fully manifested in the ecosystem biogeochemistry. We conclude that implicitly aggregating the effects of small‐mammal grazers with other processes results in an underestimation of ecosystem response to climate change, relative to estimations in which the grazer effects are explicitly represented. The magnitude of this underestimation increases with grazer density. We therefore recommend that grazing effects be incorporated explicitly when applying models of ecosystem response to global change.

Arbuscular mycorrhiza fungi increase soil denitrifier abundance relating to vegetation community

Denitrification by microorganisms in soil regulates ecosystem nitrogen availability and cycling. Although arbuscular mycorrhiza fungi (AMF) are best known as the key connectors between plant and microorganisms in below-ground soil, however, less knowledge is available about the interactive effects of AMF and vegetation community traits on soil denitrifiers. In this study, we manipulated experimental pots with or without AMF inoculation (AMF+ and AMF− treatments) under three plant richness levels and 27 different plant community compositions. Our results provided evidence that inoculation of AMF significantly increased the abundances of nirS-, nirK- and nosZ-type denitrifiers as revealed by quantitative PCR (qPCR), which inferred a positive role of AMF in N-cycling microorganisms. Plant community traits, including richness, community composition, biomass and species, were less important in influencing the abundances of soil denitrifiers, and no significant interactive effect was detected between AMF inoculation and plant richness or plant community compositions, indicating a weak direct relationship between plants and soil denitrifiers. Our study provided comprehensive insights into the roles of AMF-plant associate bio-system in driving the variation of soil denitrifiers.

Heavy rainfall in peak growing season had larger effects on soil nitrogen flux and pool than in the late season in a semiarid grassland

Increasing heavy rainfalls can strongly affect ecosystem nitrogen (N) cycling processes and thereby alter soil N fluxes and pools. However, the effects of heavy rainfalls on soil N fluxes and pools are poorly understood, particularly with regards to high rainfall timing under field conditions. We conducted a 3-year (2014–2016) manipulative experiment in which heavy rainfall was imposed in middle (plant peak growing stage) or late (plant senescent growth stage) growing season in a semiarid grassland of Inner Mongolia, China to explore the responses of N2O fluxes and soil total N contents and the underlying microbial mechanisms. Mid-season heavy rainfall promoted soil N2O emissions by 65% on average across the three years, attributable mainly to increases in denitrifying nirK and nirS abundances induced large denitrification at higher soil water contents. However, archaeal and bacterial amoA and narG genes did not change significantly due probably to counteracting effects of increased soil water content (positive) and soil pH (negative). Mid-season heavy rainfall led to 17% reduction in soil total N by the end of the last year of the study (2016), partly due to the enhanced accumulated N2O emissions over the three years. In contrast, late-season heavy rainfall did not change N2O emissions and soil total N contents even though soil water content, soil pH and nirK and nirS abundance were significantly increased, perhaps due to limitation by low temperature. Timing of the heavy rainfall events during the plant growing season strongly influenced their impacts on soil N fluxes and pools and heavy rainfalls in the peak stage of plant growth may potentially cause a positive feedback to global warming and exacerbate N limitation in terrestrial ecosystems.