Changes in nitrogen functional genes in soil profiles of grassland under long-term grazing prohibition in a semiarid area

Effects of grazing exclusion on soil microbial diversity and its functionality in grasslands: a meta-analysis

Grazing exclusion (GE) is considered an effective strategy for restoring the degradation of overgrazed grasslands on the global scale. Soil microbial diversity plays a crucial role in supporting multiple ecosystem functions (multifunctionality) in grassland ecosystems. However, the impact of grazing exclusion on soil microbial diversity remains uncertain. Here, we conducted a meta-analysis using a dataset comprising 246 paired observations from 46 peer-reviewed papers to estimate how GE affects microbial diversity and how these effects vary with climatic regions, grassland types, and GE duration ranging from 1 to 64 years. Meanwhile, we explored the relationship between microbial diversity and its functionality under grazing exclusion. Overall, grazing exclusion significantly increased microbial Shannon (1.9%) and microbial richness (4.9%) compared to grazing group. For microbial groups, GE significantly increased fungal richness (8.6%) and bacterial richness (5.3%), but decreased specific microbial richness (-11.9%). The responses of microbial Shannon to GE varied among climatic regions, grassland types, and GE duration. Specifically, GE increased microbial diversity in in arid, semi-arid, and dry sub-humid regions, but decreased it in humid regions. Moreover, GE significantly increased microbial Shannon in semidesert grasslands (5.9%) and alpine grasslands (3.0%), but not in temperate grasslands. Long-term (>20 year) GE had greater effects on microbial diversity (8.0% for Shannon and 6.7% for richness) compared to short-term (<10 year) GE (-0.8% and 2.4%). Furthermore, grazing exclusion significantly increased multifunctionality, and both microbial and plant Shannon positively correlated with multifunctionality. Overall, our findings emphasize the importance of considering climate, GE duration, and grassland type for biodiversity conservation and sustainable grassland ecosystem functions.

Metagenomics study of soil microorganisms involved in the carbon cycle in a saline-alkaline meadow steppe in the Songnen Plain in Northeast China

Soil microorganisms play an important role in regulating and contributing to carbon cycling processes in grassland ecosystems. Soil salinization is one of the major problems causing soil degradation, and its effects on carbon cycle immobilization-related functional genes in soil microorganisms remain unknown. Therefore, we took Songnen salinization grassland as the research object, selected grasslands with different salinization levels, and explored the diversity of soil microorganisms and functional genes related to carbon cycling in Songnen grassland with different salinization levels through metagenomic technology. The results showed that with the increase of salinity, the relative abundance of Ascomycetes increased, while the relative abundance of Proteus and Firmicutes decreased. In addition, the relative abundance of functional genes related to carbon cycling fixation has also decreased. As the degree of soil salinization increases, the relative abundance of glycoside hydrolases (GH)130 family significantly increases, while the relative abundance of soil carbohydrate enzymes belonging to GH3 and GH55 families significantly decreases. Using structural equation modeling (SEM), it was found that soil pH and conductivity (EC) have a significant impact on soil microbial diversity and functional genes related to carbon cycling fixation. The increase in soil pH directly reduces the Shannon diversity of soil microbial diversity and functional genes related to carbon cycling fixation. Therefore, it can be concluded that the intensification of grassland salinization reduces the diversity of bacteria and fungi, and affects the diversity of functional genes related to carbon cycling fixation by reducing the total diversity of bacteria. The increase in salinity has a negative feedback effect on grassland soil carbon cycling. This study provides a theoretical framework for grassland soil carbon sequestration and degradation restoration.

Microbial nitrogen mineralization is slightly affected by conversion from farmland to apple orchards in thick loess deposits

Organic nitrogen mineralization, indispensable to soil carbon and nitrogen cycles, is the largest contributor to nitrate reservoirs in deep vadose zones. The microbial nitrogen mineralization (MNM) within deep soils, particularly in regions with intensive agricultural activities and thick soil horizons, has been largely disregarded. As such, this study aims to address this knowledge gap by investigating the chiA-harboring microbial structure and network within nine 10-m profiles beneath cultivated farmland and two apple orchards. The results showed that apple orchards, compared to farmland, had considerable water deficit and nitrogen accumulation within deeper soil layers due to well-developed root systems and the overuse of chemical fertilizers. However, the chiA-harboring microbial diversity, composition, and abundance all exhibited significant variations with soil depths rather than being influenced by different land use types. Moreover, the diversity indices and gene abundances decreased with soil depths, and the related soil microbes included 19 phyla, 29 classes, 72 orders, 114 families, and 197 genera, with Actinobacteria and Proteobacteria being the two major bacterial phyla. The microbial co-occurrence network was simper beneath apple orchards. The chiA-harboring microorganisms within deep unsaturated zones were greatly influenced by the depth-dependent soil nutrients, such as total nitrogen, organic carbon, and available potassium. The limited plant root biomass and the inhibitory effects of dried soil layers both restricted the availability of carbon sources, which further interfered with the MNM processes within deep soils insignificantly. Therefore, despite the considerable plant-induced ecohydrological consequences, the depth-dependent MNM processes were slightly affected after the transformation from farmland to apple orchards within thick loess deposits. This study offers crucial insights into microbial dynamics of the deep biosphere, thereby contributing to our understanding of depth-dependent biogeochemical cycles within global deep unsaturated zones.

Arbuscular mycorrhizal fungi alleviate erosional soil nitrogen loss by regulating nitrogen cycling genes and enzymes in experimental agro-ecosystems

Nutrient losses from agricultural ecosystems are increasingly threatening global environmental and human health. Although arbuscular mycorrhizal (AM) fungi have the potential to regulate soil nitrogen (N) loss by enhancing plant uptake and soil particle immobilization, the microbial mechanism behind such mycorrhizal effect is unknown. Herein, by conducting a simulated erosion experiment, we compared the effects of exogenous AM fungal inoculation (Funneliformis mosseae) on the gene abundances and enzyme activities of N-cycling processes, and associated such effect to N uptake and loss. The experiment was composed of combinations of two AM fungal treatments (control vs. AM fungal inoculation), two crops (maize vs. soybean) and two slopes of the plots (6° vs. 20°). The experimental plots subjected to natural rainfalls to simulate the erosion events. We showed that the effects of AM fungi were greater in the maize soils than in the soybean soils. In the maize soils, AM fungi increased the abundances of N-fixing (+81.1 %) and nitrifying genes (+200.7 %) and N cycling enzyme activity (+22.3 %). In the soybean soils, AM fungi increased the N-fixing gene abundance (+36.9 %) but decreased the abundance of nitrifying genes (-18.9 %). The abundance of N-fixing gene was positively correlated with N uptake but negatively correlated with N loss. Additionally, AM fungi enhanced the effects of mycorrhizal colonization and moisture but decreased the effects of nutrients on soil microbial metrics related to N-cycling processes. Therefore, AM fungal inoculation enhanced N uptake and reduced N loss by increasing N-fixing gene abundance, and that AM fungi should be preferably used for the low N environments or for the ecosystems highly limited by or competing for N.

Alteration of microbial carbon and nitrogen metabolism within the soil metagenome with grazing intensity at semiarid steppe

Grazing causes changes in microbiome metabolic pathways affecting plant growth and soil physicochemical properties. However, how grazing intensity affects microbial processes is poorly understood. In semiarid steppe grassland in northern China, shotgun metagenome sequencing was used to investigate variations in soil carbon (C) and nitrogen (N) cycling-related genes after six years of the following grazing intensities: G0, control, no grazing; G1, 170 sheep days ha-1 year-1; G2, 340 sheep days ha-1 year-1; and G3, 510 sheep days ha-1 year-1. Taxa and functions of the soil microbiome associated with the C cycle decreased with increasing grazing intensity. Abundances of genes involved in C fixation and organic matter decomposition were altered in grazed sites, which could effects on vegetation decomposition and soil dissolved organic carbon (DOC) content. Compared with the control, the abundances of nitrification genes were higher in G1, but the abundances of N reduction and denitrification genes were lower, suggesting that light grazing promoted nitrification, inhibited denitrification, and increased soil NO3- content. Q-PCR further revealed that the copies of genes responsible for carbon fixation (cbbL) and denitrification (norB) decreased with increasing grazing intensity. The highest copy numbers of the nitrification genes AOA and AOB were in G1, whereas copy numbers of the denitrification gene nirK were the lowest. A multivariate regression tree indicated that changes in C fixation genes were linked to changes in soil DOC content, whereas soil NO3- content was linked with nitrification and denitrification under grazing. Thus, genes associated with C fixation and the N cycle affected how C fixation and N storage influenced soil physicochemical properties under grazing. The findings indicate that grazing intensity affected C and N metabolism. Proper grassland management regimes (e.g., G1) are beneficial to the balances between ecological protection of grasslands and plant production in the semiarid steppe.

Metagenomics reveals taxon-specific responses of soil nitrogen cycling under different fertilization regimes in heavy metal contaminated soil

Soil deficiency, cyclic erosion, and heavy metal pollution have led to fertility loss and ecological function decline in mining areas. Fertilization is an important way to rapidly replenish soil nutrients, which have a major influence on the soil nitrogen cycling process, but different fertilization regimes have different impacts on soil properties and microbial functional potentials. Here, metagenomic sequencing was used to investigate the different responses of key functional genes of microbial nitrogen cycling to fertilization regimes and explore the potential effects of soil physicochemical properties on the key functional genes. The results indicated that AC-HH (ammonium chloride-high frequency and concentration) treatment significantly increased the gene abundance of norC (13.40-fold), nirK (5.46-fold), and napA (5.37-fold). U-HH (urea-high frequency and concentration) treatment significantly increased the gene abundance of hao (6.24-fold), pmoA-amoA (4.32-fold) norC (7.00-fold), nosZ (3.69-fold), and nirK (6.88-fold). Functional genes were distributed differently among the 10 dominant phyla. The nifH and nifK genes were distributed only in Proteobacteria. The hao gene was distributed in Gemmatimonadetes, Nitrospirae and Proteobacteria. Fertilization regimes caused changes in functional redundancy in soil, and nirK and nirB, which are involved in denitrification, were present in different genera. Fertilization regimes with high frequency and high concentration were more likely to increase the gene abundance at the genus level. In summary, this study provides insights into the taxon-specific response of soil nitrogen cycling under different fertilization regimes, where changes in fertilization regimes affect microbial nitrogen cycling by altering soil physicochemical properties in a complex dynamic environment.

Symbiotic diazotrophs in response to yak grazing and Tibetan sheep grazing in Qinghai-Tibetan plateau grassland soils

Grazing by local livestock is the traditional human practice in Qinghai-Tibetan Plateau grassland, and moderate intensity grazing can maintain high productivity and diversity of alpine grassland. Grazing ecosystems are often nitrogen-limited, but N2-fixing communities in response to yak grazing and Tibetan sheep grazing in Qinghai-Tibetan Plateau grassland have remained underexplored. In this study, we applied quantitative PCR quantitation and MiSeq sequencing of nifH under yak grazing and Tibetan grazing through a manipulated grazing experiment on an alpine grassland. The results showed that the grazing treatments significantly increased the soil ammonium nitrogen (AN) and total phosphorus (TP), but reduced the diazotrophs abundance. Compared with no grazing treatment, the composition of diazotrophs could be maximally maintained when the ratio of yak and Tibetan sheep were 1:2. The foraging strategies of grazing livestock reduced the legumes biomass, and thus reduced the diazotrophs abundance. Data analysis suggested that the direct key factors in regulating diazotrophs are AN and TP, and the changes of these two soil chemical properties were affected by the dung and urine of herbivore assemblages. Overall, these results indicated that the mixed grazing with a ratio of yak to Tibetan sheep as 1:2 can stabilize the soil diazotrophsic community, suggesting that MG12 are more reasonable grazing regimes in this region.

Effects of grazing prohibition on nirK- and nirS-type denitrifier communities in salt marshes

Introduction

Grazing prohibition is an effective management practice to restore salt marsh functioning. However, the effects of grazing exclusion on denitrifying microbial communities and their controlling factors in salt marshes remain unclear.

Methods

In this study, we surveyed soil physicochemical properties and above- and below-ground biomass and using quantitative polymerase chain reaction and Illumina MiSeq high-throughput sequencing technology to determine the relative abundance, composition, and diversity of nitrite reductase nirS- and nirK-type denitrifying bacterial communities associated with grazing prohibition treatments and elevations.

Results

The abundance of nirS-type denitrifiers increased with grazing prohibition time, whereas the abundance of nirK-type denitrifiers remained unaltered. Moreover, nirS-type denitrifiers were more abundant and diverse than nirK-type denitrifiers in all treatments. Grazing prohibition significantly altered the operational taxonomic unit richness, abundance-based coverage estimator, and Chao1 indices of the nirS-type denitrifying bacterial communities, whereas it only minimally affected the structure of the nirK-type denitrifying bacterial community.

Discussion

The results imply that the nirS community, rather than nirK, should be the first candidate for use as an indicator in the process of salt marsh restoration after grazing prohibition. Substances of concern, total nitrogen, and salinity were the key environmental factors affecting the abundance and community composition of nirS and nirK denitrifiers. The findings of this study provide novel insights into the influence of the length of grazing prohibition and elevation on nirS- and nirK-type denitrifying bacterial community composition in salt marshes.

Pronounced temporal changes in soil microbial community and nitrogen transformation caused by benzalkonium chloride

As one typical cationic disinfectant, quaternary ammonium compounds (QACs) were approved for surface disinfection in the coronavirus disease 2019 pandemic and then unintentionally or intentionally released into the surrounding environment. Concerningly, it is still unclear how the soil microbial community succession happens and the nitrogen (N) cycling processes alter when exposed to QACs. In this study, one common QAC (benzalkonium chloride (BAC) was selected as the target contaminant, and its effects on the temporal changes in soil microbial community structure and nitrogen transformation processes were determined by qPCR and 16S rRNA sequencing-based methods. The results showed that the aerobic microbial degradation of BAC in the two different soils followed first-order kinetics with a half-life (4.92 vs. 17.33 days) highly dependent on the properties of the soil. BAC activated the abundance of N fixation gene (nifH) and nitrification genes (AOA and AOB) in the soil and inhibited that of denitrification gene (narG). BAC exposure resulted in the decrease of the alpha diversity of soil microbial community and the enrichment of Crenarchaeota and Proteobacteria. This study demonstrates that BAC degradation is accompanied by changes in soil microbial community structure and N transformation capacity.

Effects of the fungicide trifloxystrobin on the structure and function of soil bacterial community

Trifloxystrobin has been widely applied to prevent fungal diseases because of its high efficiency and desirable safety characteristics. In the present study, the effects of trifloxystrobin on soil microorganisms were integrally investigated. The results showed that trifloxystrobin inhibited urease activity, promoted dehydrogenase activity. Downregulated expressions of the nitrifying gene (amoA), denitrifying genes (nirK and nirS), and carbon fixation gene (cbbL) were also observed. Soil bacterial community structure analysis showed that trifloxystrobin changed the abundance of bacteria genera related to nitrogen and carbon cycle in soil. Through the comprehensive analysis of soil enzymes, functional gene abundance, and soil bacterial community structure, we concluded that trifloxystrobin inhibited both nitrification and denitrification of soil microorganisms, and also diminished the carbon-sequestration ability. Integrated biomarker response analysis showed that dehydrogenase and nifH were the most sensitive indicators of trifloxystrobin exposure. It provides new insights about trifloxystrobin environmental pollution and its influence on soil ecosystem.

Enhanced foliar 15 N enrichment with increasing nitrogen addition rates: Role of plant species and nitrogen compounds

Determining the abundance of N isotope (δ¹⁵ N) in natural environments is a simple but powerful method for providing integrated information on the N cycling dynamics and status in an ecosystem under exogenous N inputs. However, whether the input of different N compounds could differently impact plant growth and their ¹⁵ N signatures remains unclear. Here, the response of ¹⁵ N signatures and growth of three dominant plants (Leymus chinensis, Carex duriuscula, and Thermopsis lanceolata) to the addition of three N compounds (NH₄ HCO₃ , urea, and NH₄ NO₃ ) at multiple N addition rates were assessed in a meadow steppe in Inner Mongolia. The three plants showed different initial foliar δ¹⁵ N values because of differences in their N acquisition strategies. Particularly, T. lanceolata (N₂ -fixing species) showed significantly lower ¹⁵ N signatures than L. chinensis (associated with arbuscular mycorrhizal fungi [AMF]) and C. duriuscula (associated with AMF). Moreover, the foliar δ¹⁵ N of all three species increased with increasing N addition rates, with a sharp increase above an N addition rate of ~10 g N m^-2  year^-1 . Foliar δ¹⁵ N values were significantly higher when NH₄ HCO₃ and urea were added than when NH₄ NO₃ was added, suggesting that adding weakly acidifying N compounds could result in a more open N cycle. Overall, our results imply that assessing the N transformation processes in the context of increasing global N deposition necessitates the consideration of N deposition rates, forms of the deposited N compounds, and N utilization strategies of the co-existing plant species in the ecosystem.

New insights into the effects of chlorpyrifos on soil microbes: Carbon and nitrogen cycle related microbes in wheat/maize rotation agricultural field

Chlorpyrifos, a broad-spectrum organophosphorus insecticide, has been widely detected worldwide and is a potential neurotoxin and endocrine disruptor. Besides, chlorpyrifos has been proven that have a negative effect on soil microbes. In the present study, chlorpyrifos formulation (LORSBAN®, 45% emulsifiable concentrate) was applied in an agricultural field at the recommended dose (R dose, 270.0 and 337.5 g a.i. ha^-1 for wheat and maize respectively) and double recommended (DR) dose. Chlorpyrifos residue level and effect on soil microbes related to soil carbon and nitrogen cycle function were analyzed. Results showed that the half-lives of chlorpyrifos in wheat and maize field soil were 7.23-8.23 and 1.45-1.77 d, respectively. Application of chlorpyrifos at even DR dose did not result in unacceptable residual chlorpyrifos, where the final residual chlorpyrifos in wheat/maize (leaf, stem, and grain) was meet the requirement of the maximum residual limit (0.5 mg kg^-1 for wheat and 0.05 mg kg^-1 for maize) in China. Chlorpyrifos enhanced the activity of β-glucosidase by increasing the relative abundance of Sphingosinicella and promoted the carbon cycle in wheat field. The changes of cbbLR and cbbLG gene abundance also confirmed that chlorpyrifos could affect the import and export of soil carbon pool. The effect of chlorpyrifos on soil N cycle was determined by changes in the abundance of the bacterial genus Gemmatimonas, which is associated with denitrification. Further analysis of N-cycle functional genes and urease activity showed that chlorpyrifos inhibited nitrogen fixation in wheat field, but promoted nitrogen fixation in maize field. In general, bacterial abundance, urease, and AOA-amoA gene could be early warning markers of chlorpyrifos contamination. The results demonstrated the negative effects of chlorpyrifos on soil microbes especially on soil C and N cycle in actual agricultural field. It provides new insights about chlorpyrifos environmental pollution and its effect on soil ecosystems.

Structural, metabolic, and functional characteristics of soil microbial communities in response to benzo[a]pyrene stress

Understanding the characteristics of soil microbes responding to benzo[a]pyrene (BaP) helps to deepen the knowledge of the risks of BaP to soil ecosystem. In this study, the structural, metabolic, and functional responses of soil microbial communities to BaP (8.11 mg kg^-1) were investigated. Analysis of microbial community structure based on 16 S rRNA and ITS gene sequencing indicated that BaP addition enriched microbes associated with aromatic compound degradation (Sphingomonas, Bacilli, Fusarium) and oligotrophs (Blastocatellaceae, Rokubacteriales), but inhibited Cyanobacteria involved in nitrogen-fixing process. Network analysis showed that the bacterial community enhanced intraspecific cooperation, while fungal community mainly altered the keystone taxa under BaP stress. Biolog EcoPlate assay demonstrated that microbial metabolism of carbon sources, especially nitrogen-containing sources, was stimulated by BaP addition. Functional analysis based on enzyme activity tests, functional gene quantification, and function annotation showed that nitrogen-cycling processes, especially nitrogen fixation, were significantly inhibited. These results suggest that BaP-tolerant microbes may establish cooperative relationships and compete for resources and ecological niches with sensitive microbes, especially those associated with nitrogen cycling, ultimately leading to enhanced carbon source utilization and restricted nitrogen cycling. This study clearly elucidates the adaptation strategies and functional shifts of soil microbial communities to BaP contamination.

Effects of cloransulam-methyl and diclosulam on soil nitrogen and carbon cycle-related microorganisms

Cloransulam-methyl and diclosulam are applied to soybean fields to control broad-leaved weeds. These herbicides have become a focus of attention because of their low application dose and high-efficiency advantages. However, the effects of these two herbicides on soil microorganisms are unknown. The present study investigated the effects of 0.05, 0.5, and 2.5 mg kg^-1 of cloransulam-methyl or diclosulam on soil microbes after 7, 14, 28, 42, and 56 days of exposure. The results showed that the two herbicides increased the abundances of functional bacteria related to pesticide degradation. Based on the genetic expression results, we speculated that 0.05 mg kg^-1 of these two herbicides inhibited the nitrification reaction but promoted the denitrification reaction. Diclosulam at a concentration of 0.5 mg kg^-1 may enhance the ability of microbes to fix carbon. β-glucosidase activity was activated by the two herbicides at a concentration of 2.5 mg kg^-1. Diclosulam had a positive effect on urease, but cloransulam-methyl activated urease activity only at concentrations of 0.05 and 0.5 mg kg^-1. The results of the integrated biomarker response showed that the toxicity of diclosulam was greater than that of cloransulam-methyl. Our research provides data for evaluating the environmental risks of cloransulam-methyl and diclosulam.

Coupling Between the Responses of Plants, Soil, and Microorganisms Following Grazing Exclusion in an Overgrazed Grassland

Grazing exclusion is an effective management practice to restore grassland ecosystem functioning. However, little is known about the role of soil microbial communities in regulating grassland ecosystem functioning during long-term ecosystem restorations. We evaluated the recovery of a degraded semiarid grassland ecosystem in northern China by investigating plant and soil characteristics and the role of soil microbial communities in ecosystem functioning after 22 years of grazing exclusion. Grazing exclusion significantly increased the alpha diversity and changed the community structure of bacteria, but did not significantly affect the alpha diversity or community structure of fungi. The higher abundance of copiotrophic Proteobacteria and Bacteroidetes with grazing exclusion was due to the higher carbon and nutrient concentrations in the soil, whereas the high abundance of Acidobacteria in overgrazed soils was likely an adaptation to the poor environmental conditions. Bacteria of the Sphingomonadaceae family were associated with C cycling under grazing exclusion. Bacteria of the Nitrospiraceae family, and especially of the Nitrospira genus, played an important role in changes to the N cycle under long-term exclusion of grazing. Quantitative PCR further revealed that grazing exclusion significantly increased the abundance of nitrogen fixing bacteria (nifH), ammonia oxidizers (AOA and AOB), and denitrifying bacteria (nirK and nosZ1). Denitrifying enzyme activity (DEA) was positively correlated with abundance of denitrifying bacteria. The increase in DEA under grazing exclusion suggests that the dependence of DEA on the availability of NO₃ ^- produced is due to the combined activity of ammonia oxidizers and denitrifiers. Our findings indicate that decades-long grazing exclusion can trigger changes in the soil bacterial diversity and composition, thus modulating the restoration of grassland ecosystem functions, carbon sequestration and soil fertility.

Bioelectric field accelerates the conversion of carbon and nitrogen in soil bioelectrochemical systems

Soil bioelectrochemical systems (BESs) utilize indigenous microorganisms to generate biocurrent/electric fields that stimulate the degradation of organic pollutants, exhibiting great potential in the removal of petroleum hydrocarbons from soils. In this study, a horizontal bioelectric field was constructed to investigate the conversion of carbon and nitrogen in a soil BES. After 182 days, the degradation rates of total petroleum hydrocarbons, alkanes, and aromatics were promoted by 52 %, 38% and 136%, respectively. Meanwhile, the bioelectric field accelerated NH₄⁺-N production near the cathode, whereas NH₄⁺-N consumption near the anode indicated that the bioelectric field promoted the cathode-dominated ammoniation process and the anode-dominated denitrification process. Additionally, a distinctive microbial community was formed under the bioelectric field, and the improved degradation on the cathode and the anode relied on special functional bacteria (typically, cathode, Alcanivorax; anode, Marinobacter). The dramatic enrichment in anodic denitrifying bacteria, including Pontibacillus, Sediminimonas, Georgenia, etc., explained the enhanced denitrification process under the bioelectric field. This study simultaneously clarified the carbon and nitrogen conversion processes and corresponding bacterial community occurring under the bioelectric field for the first time, helping to form regulation strategies in the practical application of soil BESs and providing a new perspective for removing petroleum hydrocarbons from soils.

Nitrogen fertilizer and Amorpha fruticosa leguminous shrub diversely affect the diazotroph communities in an artificial forage grassland

Soil diazotrophs have been known to be essential in biological nitrogen (N) fixation, which contributes to the sustainability of agricultural ecosystems. However, there remains an inadequacy of research on the effects of different N inputs from N fertilization and from symbiotic N fixation associated with legumes on the diazotroph communities in agricultural ecosystems. Hence, we investigated the variations in diazotroph abundance and community composition as well as the soil properties with different N inputs in the Guimu-1 hybrid elephant grass cultivation on karst soils in China. We conducted six different N treatments: control, Amorpha fruticosa planting at a spacing of 1.5 × 2 m (AFD1), A. fruticosa planting at a spacing of 1 × 2 m (AFD2), N fertilization (N), A. fruticosa planting at a spacing of 1.5 × 2 m with N fertilization (AFD1N), and A. fruticosa planting at a spacing of 1 × 2 m with N fertilization (AFD2N). Our results showed that the interaction between sampling time and N fertilization significantly affected the diazotroph abundance. In July, the diazotroph abundance significantly decreased in the N fertilization treatments: N, AFD1N, and AFD2N, compared to that in the control. The richness and Chao1 estimator of diazotrophs significantly increased in AFD2N and AFD1 correspondingly in December and July, relative to those in the control. Co-occurrence networks showed species-species interactions with high negative correlations that occurred more in the control than in the N input plots. The N input from N fertilization and legume planting directly increased the ammonium N and nitrate N and consequently affected the dissolved organic N and pH of the soil, thereby altering the diazotroph abundance and richness. Our findings demonstrated that both N fertilization and legumes could reduce the interspecific competition among diazotroph species by providing greater N availability in the forage grass.