Nitrogen-fixing Soil Research Articles

Enterobacter-infecting phages in nitrogen-deficient paddy soil impact nitrogen-fixation capacity and rice growth by shaping the soil microbiome

Bacteriophages (“phage”) play important roles in nutrient cycling and ecology in environments by regulating soil microbial community structure. Here, metagenomic sequencing showed that a low relative abundance of nitrogen-fixing bacteria but high abundance of Enterobacter-infecting phages in paddy soil where rice plants showed nitrogen deficiency. From soil in the same field, we also isolated and identified a novel virulent phage (named here as Apdecimavirus NJ2) that infects several species of Enterobacter and characterized its impact on nitrogen fixation in the soil and in plants. It has the morphology of the Autographiviridae family, with a dsDNA genome of 39,605 bp, 47 predicted open reading frames and 52.64 % GC content. Based on genomic characteristics, comparative genomics and phylogenetic analysis, Apdecimavirus NJ2 should be a novel species in the genus Apdecimavirus, subfamily Studiervirinae. After natural or sterilized field soil was potted and inoculated with the phage, soil nitrogen-fixation capacity and rice growth were impaired, the abundance of Enterobacter decreased, along with the bacterial community composition and biodiversity changed compared with that of the unadded control paddy soil. Our work provides strong evidence that phages can affect the soil nitrogen cycle by changing the bacterial community. Controlling phages in the soil could be a useful strategy for improving soil nitrogen fixation.

Endophytic fungus regulates the root secretion of IAA and ABA to increase rice nitrogen accumulation by promoting soil nitrogen fixation

Endophytic fungus regulates the root secretion of IAA and ABA to increase rice nitrogen accumulation by promoting soil nitrogen fixation

Global soil metagenomics reveals distribution and predominance of Deltaproteobacteria in nitrogen-fixing microbiome

BackgroundBiological nitrogen fixation is a fundamental process sustaining all life on earth. While distribution and diversity of N2-fixing soil microbes have been investigated by numerous PCR amplicon sequencing of nitrogenase genes, their comprehensive understanding has been hindered by lack of de facto standard protocols for amplicon surveys and possible PCR biases. Here, by fully leveraging the planetary collections of soil shotgun metagenomes along with recently expanded culture collections, we evaluated the global distribution and diversity of terrestrial diazotrophic microbiome.ResultsAfter the extensive analysis of 1,451 soil metagenomic samples, we revealed that the Anaeromyxobacteraceae and Geobacteraceae within Deltaproteobacteria are ubiquitous groups of diazotrophic microbiome in the soils with different geographic origins and land usage types, with particular predominance in anaerobic soils (paddy soils and sediments).ConclusionOur results indicate that Deltaproteobacteria is a core bacterial taxon in the potential soil nitrogen fixation population, especially in anaerobic environments, which encourages a careful consideration on deltaproteobacterial diazotrophs in understanding terrestrial nitrogen cycling.D657cgwaNSF-wLxbnJEePrVideo

Limited dependence on soil nitrogen fixation as subtropical forests develop

Soil nitrogen (N) fixation, driven by microbial reactions, is critical to support the entrance of nitrogen in nutrient poor and pioneer ecosystems. However, how and why N fixation and soil diazotrophs evolve as forests develop remain poorly understood. Here, we used a 60-year forest rewilding chronosequence and found that soil N fixation activity gradually decreased with increasing forest age, experiencing dramatic drops of 64.8% in intermediate stages and 93.0% in the oldest forests. Further analyses revealed loses in diazotrophic diversity and a significant reduction in the abundance of important diazotrophs (e.g., Desulfovibrio and Pseudomonas) as forest develops. This reduction in N fixation, and associated shifts in soil microbes, was driven by acidification and increases in N content during forest succession. Our results provide new insights on the life history of one of the most important groups of soil organisms in terrestrial ecosystems, with consequences for understanding the buildup of nutrients as forest soil develops.

Effect of Seaweed Application on the Growth, Yield and Physiological Parameters in the Intercropping Farming System.

The present study demonstrates the application of biostimulants during the cultivation of wheat and chickpeas in intercropping farming. This study examined the effect of seaweed on the increasing amount of nitrogen, yield, and nutrient quality of wheat and chickpeas. In north India, rabi crops were grown for three seasons from 2019 to 2021 in the intercropping farming system. The main crop (wheat) was sown with chickpea (legumes) to enhance the yield of two crops in one season as chickpea also helps in nitrogen fixation in soil. There was a total of 36 rows each of 6 m, of which nine rows each were for wheat and chickpea and the other nine were for one row of wheat and another of chickpea. Results of the study, exhibit the significant effect of the amount of nitrogen which was a maximum of 4.33 mg/kg in intercropping treated with seaweed as compared to intercropping with control 4.23 mg/kg. AE (Agronomic Efficiency) in the intercropping with seaweed treatment was 3.27 kg/kg as compared to 3.23 kg/kg in the control. The yield and harvest index of seaweed with intercropping was higher than intercropping in control with chemical fertilizers like urea. Hence, biostimulants along with intercropping were found to be effective in increasing the yield and nutritional value of crops.

Soil environment, carbon and nitrogen cycle functional genes in response to freeze-thaw cycles and biochar

Freeze-thaw cycles (FTCs) are a common phenomenon in seasonal frozen soil areas, which can significantly affect the changes in the soil environment and functional genes of microorganisms. To explore the effect of biochar application on soil carbon and nitrogen cycling under FTCs, an indoor soil column simulation test was used in this study. The experimental setup included different application rates of biochar (0%, 1%, 2%), with the carbon and nitrogen cycle functional genes serving as indicator genes. Real-time fluorescence quantitative PCR technology was employed for analysis. Based on the correlation heat map and redundancy analysis (RDA), the response mechanism of soil environment changes and carbon and nitrogen cycle functional genes before and after FTCs was analyzed. The results revealed that the application of biochar significantly increased soil's water-holding capacity held available nutrients in soil, and increased the ability of environmental change. Due to the influence of soil moisture and aeration, biochar reduced the variation of soil carbon sequestration gene cbbL, cbbM, and nitrogen fixation gene nifH abundance after FTCs. The application of 2 % biochar significantly increased the abundance of the nitrification gene amoA and denitrification gene nirK and enhanced the respiration of soil microorganisms. Soil moisture (SM) is the main environmental factor that determines the abundance changes of the soil carbon sequestration genes cbbL and cbbM. Soil organic matter (SOM), pH, nitrate nitrogen (NO3−-N), and ammonium nitrogen (NH4+-N) were the main environmental factors determining the abundance of soil nitrogen fixation gene nifH, nitrification gene amoA, and denitrification gene nirK. This study can provide new insights into the internal mechanism of the soil carbon and nitrogen cycle in seasonal frozen soil areas, and provide some reference for the rational application of biochar and the efficient utilization of water and soil resources in seasonal frozen soil areas in the future.

Precipitation affects soil nitrogen fixation by regulating active diazotrophs and nitrate nitrogen in an alpine grassland of Qinghai-Tibetan Plateau

Soil asymbiotic nitrogen (N) fixation provides a critical N source to support plant growth in alpine grasslands, and precipitation change is expected to lead to shifts in soil asymbiotic N fixation. However, large gaps remain in understanding the response of soil asymbiotic N fixation to precipitation gradients. Here we simulated five precipitation gradients (10 % (0.1P), 50 % (0.5P), 70 % (0.7P), 100 % (1.0P) and 150 % (1.5P) of the natural precipitation) in an alpine grassland of Qinghai-Tibetan Plateau and examined the soil nitrogenase activity and N fixation rate for each gradient. Quantitative PCR and high-throughput sequencing were used to measure the abundance and community composition of the soil nifH DNA (total diazotrophs) and nifH RNA reverse transcription (active diazotrophs) gene. Our results showed that the soil diazotrophic abundance, diversity and nifH gene expression rate peaked under the 0.5P. Soil nitrogenase activity and N fixation rate varied in the range 0.032–0.073 nmol·C2H4·g−1·h−1 and 0.008–0.022 nmol·N2·g−1·h−1 respectively, being highest under the 0.5P. The 50 % precipitation reduction enhanced the gene expression rates of Azospirillum and Halorhodospira which were likely responsible for the high N fixation potential. The 0.5P treatment also possessed a larger and more complex active diazotrophic network than the other treatments, which facilitated the resistance of diazotrophic community to environmental stress and thus maintained a high N fixation potential. The active diazotrophic abundance had the largest positive effect on soil N fixation, while nitrate nitrogen had the largest negative effect. Together, our study suggested that appropriate precipitation reduction can enhance soil N fixation through promoting the abundance of the soil active diazotrophs and decreasing soil nitrate nitrogen, and soil active diazotrophs and nitrate nitrogen should be considered in predicting soil N inputs in the alpine grassland of Qinghai-Tibetan Plateau under precipitation change.

Improving the Key Enzyme Activity, Conversion Intensity, and Nitrogen Supply Capacity of Soil through Optimization of Long-Term Oilseed Flax Rotation Planting Patterns in Dry Areas of the Loess Plateau of China

Various crop rotation patterns can result in differences in nutrient consumption and the accumulation of toxic substances in the soil, indirectly impacting the soil environment and its nutrient supply capacity. Implementing optimized crop planting practices is beneficial for maintaining the favorable physical and chemical properties of farmland soil in the arid area of northwestern China. This study aimed to establish a crop rotation pattern to improve key enzyme activities and soil nitrogen conversion efficiency, as well as understand the underlying mechanism for enhancing nitrogen supply capacity. A field experiment was conducted to study the effect of four flax planting patterns, which included 13 crop rotation patterns with different crop frequencies: 100% Flax (Cont F), 50% Flax (I) (WFPF, FPFW, PFWF, FWFP), 50% Flax (II) (FWPF, WPFF, PFFW, FFWP), 25% Flax (WPWF, PWFW, WFWP, FWPW), on the key enzyme activities and the rate of soil nitrogen conversion, as well as the nitrogen supply capacity. Here, F, P, and W represent oilseed flax, potato, and wheat, respectively. The results indicated that the wheat stubble significantly increased the intensity of soil ammonification and denitrification before planting. Additionally, the activity levels of soil nitrate reductase and nitrite reductase under wheat stubble were significantly increased by 66.67% to 104.55%, while soil urease activity significantly decreased by 27.27–133.33% under wheat stubble compared to other stubbles. After harvest, the activities of soil nitrate reductase and nitrite reductase under the wheat stubble decreased significantly, and the intensity of soil ammonification, nitrification, and denitrification reduced significantly by 7.83–27.72%. The WFWP and FWFP treatments led to a significant increase in soil nitrogen fixation intensity under various crop rotations after harvest and significantly increased the levels of inorganic nitrogen in the soil before the planting of the next crop. This study suggests that the long-term rotation planting patterns WFWP and FWFP can significantly enhance the key enzyme activities of soil nitrogen conversion and significantly improve soil nitrogen conversion before crop sowing. This may increase the rate of soil nitrogen transfer and raise the available nitrogen content of the soil. These findings are crucial for reducing soil nitrogen loss and improving soil nitrogen nutrient supply capacity in dry areas of the Loess Plateau of China.

Litter leachates transform soil bacterial composition enhancing nitrogen fixation in alpine meadow

Litter leachates have the potential to considerably regulate biogeochemical cycles in terrestrial ecosystems. Yet the effects of litter leachates on the soil microbial composition have so far not been adequately evaluated. Litter leachates of Elymus nutans, Kobresia setchwanensis, Ligularia virgaurea, were sprayed onto alpine meadow to assess their effects on community structure. Specific functional genes of soil bacteria were also analyzed to explore their potential ecological functions. Results indicate that application of litter leachates to alpine meadow on the Qinghai-Tibet Plateau over three consecutive growing seasons, modifies plant communities and soil bacterial composition. Plant biomass, soil moisture, and C:N ratio of litter leachate treated soils were lower compared to the control, but soil microbial biomass carbon was greater. Increased plant and soil bacterial α-diversities were accompanied by alterations in soil physico-chemical properties and plant community structures, including higher soil bacterial β-diversity. Soil moisture, organic carbon, microbial biomass carbon, inorganic nitrogen, and C:N ratio being abiotic factors, were closely related to bacterial composition in the topsoil (0–20 cm), while plant biomass and diversity were the major biotic driving forces of soil bacterial composition in both topsoil and subsoil (20–40 cm). L. virgaurea litter leachate application increased soil nitrogen fixation potential, E. nutans and K. setchwanensis litter leachates reduced soil nitrification and denitrification however. These results indicate that litter leachates stimulate the nitrogen cycle in alpine meadow by altering bacterial composition and function of plants and soil bacteria. This ecologically significant contribution supports strategic application of litter leachates on alpine pasture.

More is Different: Metabolic Modeling of Diverse Microbial Communities.

Microbial consortia drive essential processes, ranging from nitrogen fixation in soils to providing metabolic breakdown products to animal hosts. However, it is challenging to translate the composition of microbial consortia into their emergent functional capacities. Community-scale metabolic models hold the potential to simulate the outputs of complex microbial communities in a given environmental context, but there is currently no consensus for what the fitness function of an entire community should look like in the presence of ecological interactions and whether community-wide growth operates close to a maximum. Transitioning from single-taxon genome-scale metabolic models to multitaxon models implies a growth cone without a well-specified growth rate solution for individual taxa. Here, we argue that dynamic approaches naturally overcome these limitations, but they come at the cost of being computationally expensive. Furthermore, we show how two nondynamic, steady-state approaches approximate dynamic trajectories and pick ecologically relevant solutions from the community growth cone with improved computational scalability.

Effect of different additions of low-density polyethylene and microplastics polyadipate/butylene terephthalate on soil bacterial community structure.

The stress produced from biodegradable plastics on soil ecosystem is a rising global concern. However, effects of such microplastics (MPs) on soil ecology are still debatable. In this study, the biodegradable microplastic PBAT (polyadipate/butylene terephthalate) was used as the target object, compared with the traditional microplastic LDPE (low-density polyethylene). A pot experiment and was high-throughput sequencing analysis used to determine the effect of different additions of MPs on soil bacterial community structure and the correlation between soil bacterial community structure and chemical properties was investigated. Compared with LDPE,the results showed that EC, TN, TP, NH4+-N, and NO3--N changed obviously with the increasing of PBAT addition (p < 0.05), but pH changed little and the community richness was significantly higher in soils with low PBAT addition than that with higher PBAT addition. PBAT is beneficial to soil nitrogen fixation, but it will significantly reduce the soil P content and affect the nitrification and denitrification reaction. It suggested that addition of PBAT MPs and its addition amount result in changes in soil fertility, community abundance, and structure and composition of bacterial communities in soil samples, while the presence of PBAT MPs might affect soil carbon-nitrogen cycle.

Selenium enhanced nitrogen accumulation in legumes in soil with rhizobia bacteria

The loss of nitrogen in soil directly leads to ecological pollution such as water eutrophication. Urgent need to develop green and efficient approaches to increase nitrogen absorption in plants benefits to improve soil nitrogen use efficiency. Selenium is one of the beneficial elements of plants and has been proven to promote efficient nitrogen uptake by plants. However, previous studies focused on the effects of selenium on plant physiological functions while ignoring the regulation of selenium on the soil nitrogen cycle. This study aims to investigate the regulatory effect of selenium on legume rhizosphere microecology and soil nitrogen cycle process, and the key to legume biomass and nitrogen accumulation, soil physicochemical factors, microbiome characteristics and soil nitrogen cycle process. The results showed that selenium application increased the biomass (increased by 7.26%∼21.85% compared with CK) and nitrogen accumulation of legumes (increased by 18.29% ∼ 54.09% compared with CK). By 16S rRNA sequencing of rhizosphere soil and roots, we found selenium increased nitrogen-fixing bacteria such as Rhizobiaceae and Frankia and nitrifying bacteria such as Proteobacteria, Firmicutes and Chloroflex in rhizosphere. Moreover, selenium up-regulated soil nitrogen fixation genes (nifH) and nitrification genes (Arch-amoA, nxrA), and down-regulated denitrification genes (narG, nirS). These changes lead to a significant decrease in soil NO3−/NH4+, which was helpful for the uptake and utilization of legumes. When soil inoculated with rhizobia, the nitrogen accumulation in legumes promoted by Se was much higher than soil non-inoculated with rhizobia bacteria. This study explored the feasibility and great potential of selenium to promote nitrogen accumulation in legumes from the perspective of rhizosphere microecology, proposes and proves the approach to reduce the application of large amounts of elements by introducing selenium, which is conducive to reducing the use of nitrogen fertilizer from the source and alleviating nitrogen pollution.

Effects of Chemical Fertilizer Reduction Combined with Straw Application on Diazotrophic Communities in a Double Rice Cropping System

Based on a three-year field experiment, the effects of reduced chemical fertilizer combined with straw application on paddy yield, soil fertility properties, and community structure of diazotrophs in a double-rice cropping field three years after straw application were examined. Three treatments were applied:conventional fertilizer application (CF), chemical fertilizer reduction combined with a low straw application rate (CFLS, 3 t·hm-2), and a high straw application rate (CFHS, 6 t·hm-2). The results showed that CFLS and CFHS did not significantly reduce rice grain yield (P>0.05); significantly neutralized soil acidification; increased soil microbial biomass carbon and nitrogen, dissolved organic carbon, and organic carbon content (P<0.05); and significantly reduced soil redox potential, ammonium nitrogen, and nitrate nitrogen contents (P<0.05). This was more conducive to improve soil nitrogen use efficiency. Compared with those under the CF treatment, the natural nitrogen fixation functional communities of CFLS and CFHS increased the Shannon, PD, and Evenness indexes (P<0.05) due to the improvement of conditions such as the increase in soil carbon storage and the decrease in acidification degree. The relative abundance of microbial communities with nitrogen fixation, carbon fixation, and plant growth promotion functions such as Ferrigenium, Sulfurivermis, Methylomonas, Methylovulum, Ectothiorhodospira, and Nostoc increased significantly (P<0.05). In conclusion, the reduction in chemical fertilizer combined with 3 t·hm-2 and 6 t·hm-2 straw application was an effective measure to improve the community structure of soil diazotrophs and the potential of soil nitrogen fixation.

Study of soil nitrogen cycling processes based on the 15N isotope tracking technique in the black soil areas

In recent years, biochar has been widely used for soil improvement, but there has been some variation in its effectiveness; the mechanism of regulating nutrient cycling in soil is unclear due to different soil textures and physicochemical properties of biochar. Therefore, in this paper, the effects of different tillage patterns and biochar gradients on the nitrogen use efficiency (NUE) and soil nitrogen fixation capacity of soybean were monitored in a phased manner by field trials using 15N isotope tracer technology. The results of the study showed that (1) biochar increases the soil's nitrogen fixation capacity by improving the structural stability of the soil. Total nitrogen (TN) content of the soil increased by 42% after deep ploughing (DP) application of 500 kg/acre of biochar. (2) Biochar improved the nitrogen use efficiency of soybean plants and increased soybean yield. Compared to that of shallow tillage (SP), deep tillage application of 500 kg/acre of biochar increased the nitrogen use efficiency of soybean by 0.62–3.45%. 15N isotope tracking technology showed that soybean plants with deep tillage application of 500 kg/acre of biochar had the highest 15N content at flowering stage; compared to no biochar application, the 15N content of root, stem and leaf fractions increased by 65.4–86.9%. (3) Biochar increased soil CO2 emission fluxes, reduced soil CH4 emission fluxes and soil N2O emission fluxes, with better GHG emission reduction under DP treatment. The regulation model of deep ploughing application of 500kg/acre of biochar provides a reference and guiding meaning for future soil fertility maintenance and farmland soil tillage in black soil areas.

Genetic Variability, Correlation and Path Analysis in Mung Bean Genotypes (Vigna radiata L. Wilczek): An Experimental Investigation

Mung bean (Vigna radiata L. Wilczek) is one of the most important pulse crops due to the high content of protein as well as the ability to nitrogen fixation in soil. Seed yield is the most complex trait which highly influenced by other traits and also the environment. In this view, the study was aimed to evaluate the relationship between yield and yield attributing traits of sixty genotypes of Mung bean. The genotypes exhibited significant variability in respect to all the fourteen quantitative characters studied. Genetic variability studies showed presence of good amount of variation among the sixty genotypes. Highest PCV and GCV was recorded by seed yield followed by number of seeds per plant and primary branch length. The characters, seed yield per plant, number of seeds per plant, primary branch length, biological yield, harvest index, 100 seed weight, number of pods per plant, number of clusters per plant, plant height and number of primary branches had shown high heritability coupled with high genetic advance as percent mean. Seed yield per plant had shown highly significant and positive correlation with number of clusters per plant, number of pods per plant, pod length, number of seeds per pod, number of seeds per plant, biological yield and harvest index at both phenotypic and genotypic levels. Path analysis showed that biological yield and harvest index were the most important characters for yield improvement.

Degradation reduces the diversity of nitrogen-fixing bacteria in the alpine wetland on the Qinghai-Tibet Plateau.

Biological nitrogen fixation is a key process in the nitrogen cycle and the main source of soil available nitrogen. The number and diversity of nitrogen-fixing bacteria directly reflect the efficiency of soil nitrogen fixation. The alpine wetland on the Qinghai-Tibet Plateau (QTP) is degrading increasingly, with a succession toward alpine meadows. Significant changes in soil physicochemical properties accompany this process. However, it is unclear how does the soil nitrogen-fixing bacteria change during the degradation processes, and what is the relationship between these changes and soil physicochemical properties. In this study, the nifH gene was used as a molecular marker to further investigate the diversity of nitrogen-fixing bacteria at different stages of degradation (none, light, and severe degeneration) in the alpine wetland. The results showed that wetland degradation significantly reduced the diversity, altered the community composition of nitrogen-fixing bacteria, decreased the relative abundance of Proteobacteria, and increased the relative abundance of Actinobacteria. In addition to the dominant phylum, the class, order, family, and genus of nitrogen-fixing bacteria had significant changes in relative abundance. Analysis of Mantel test showed that most soil factors (such as pH, soil water content (SWC), the organic carbon (TOC), total nitrogen (TN), and soil C:P ratio) and abundance had a significant positive correlation. TOC, TN, total phosphorus (TP), soil C:P ratio and Shannon had a significant positive correlation with each other. The RDA ranking further revealed that TOC, SWC, and TN were the main environmental factors influencing the community composition of nitrogen-fixing bacteria. It is found that the degradation of the alpine wetland inhibited the growth of nitrogen-fixing bacteria to a certain extent, leading to the decline of their nitrogen-fixing function.

Nutrient status of integrated rice-crayfish system impacts the microbial nitrogen-transformation processes in paddy fields and rice yields.

Increasing rice yield is essential for alleviating global food crisis. High soil nutrient level guarantees high rice yields in conventional rice monoculture (RM) systems, but excessive unconsumed nutrients act as pollutants and can even threaten rice growth. The integrated rice-crayfish (IRC) system aims to transfer the excess nutrients from crayfish to paddy fields to improve the comprehensive utilization rate of nutrients and create additional profits, while the responding characteristics of IRC microbial communities in paddy fields and rice yields to the nutrient status remain unclear. Considering the crucial roles of microbiomes in promoting nutrient cycling for crop absorption in rice production progresses, the composition and functional characteristics of soil microbial communities from six IRC farms with variant nutrient statuses in the Yangtze River Delta were surveyed in this study. Compared with RM systems, IRC systems with appropriately improved (p < 0.05) soil quality created favorable nutrient (FN) status accompanied by 15% rice yields increase, while IRC systems with extremely high nutrients (HN) status (p < 0.01) accompanied by 14% rice yields reduction. Soil microbial diversity and network complexity were maintained in FN-IRC systems, but declined in HN-IRC systems, with the Shannon index significantly decreased by 9.2% and network density decreased from 0.135 (in RM) to 0.062. In the FN-IRC systems, the keystone taxa identified by co-occurrence networks displayed inextricably positive correlations with soil nitrification potential (calculated by normalization of amoA gene abundance) and rice yields. While in HN-IRC systems, the large loss of keystone taxa might limit soil nitrogen fixation potential (calculated by normalization of nifH gene abundance), and further rice yields. Our study indicates that soil nutrient management in IRC systems claim attention, and the improvement of nitrogen metabolism is the key to realize agricultural cleaner production.

Long-Term Straw Return with Reducing Chemical Fertilizers Application Improves Soil Nitrogen Mineralization in a Double Rice-Cropping System

The partial replacement of chemical fertilizer with straw return is considered an effective method for improving the accumulation of organic matter and soil fertility, but the characteristics of soil nitrogen fixation and mineralization in a double-cropped rice paddy system are unclear. Based on a 12-year field experiment, we conducted a waterlogged incubation experiment for 49 days to determine the effect of long-term straw return combined with reducing chemical fertilizer application on the dynamic changes of mineralized soil nitrogen (N) content and mineralized N rate under the treatments, including NPK (chemical fertilizers application with straw removal), SBR (straw burned return), and SR (straw return). Results showed that, compared with SBR and NPK, SR significantly increased available nitrogen by 7.4% and 16.5%, respectively, due to the higher ammonium nitrogen and nitrate nitrogen, as well as the total carbon, available phosphorus, and slowly available potassium, suggesting that it could stock a sufficient nitrogen source. During the incubation period, the amount of N mineralization was relatively higher under SR than under SBR and NPK treatments, especially during the later mineralization time, whereas there was no difference in the N mineralization rate. In addition, SR significantly increased soil cumulative N mineralization and N mineralization potential. However, SBR significantly decreased the soil mineralizable N ratio compared with SR and NPK, which may result in a worsening of the N mineralization potential. The results indicated that long-term straw return combined with reducing chemical fertilizer application could significantly improve the N supply capacity of paddy rice field soil to better coordinate the soil N supply and immobilization.

Soil microbial nitrogen-cycling gene abundances in response to crop diversification: A meta-analysis

Single planting structure has a significant impact on the maintenance of nitrogen in managed ecosystems. Although the effect of crop diversity on soil nitrogen-cycling microbes is mainly related to the influence of environmental factors, there is a lack of quantitative research. This study aims to determine the effect of diversified cropping mode on the abundance of functional genes in the soil nitrogen cycle based on the quantitative integration of a meta-analysis database containing 189 observation data pairs. The results show that the soil nifH (nitrogenase coding gene), nirS and nirK (nitrite reductase coding gene), and narG (nitrate reductase coding gene) abundances are positively affected by the diversity of plant species, whereas the amoA (ammonia monooxygenase coding gene) and nosZ (nitrous oxide reductase coding gene) show no response. Diversification duration and ecosystem type are important factors that regulate soil nitrogen fixation and nitrification gene abundances. Denitrification genes are mainly affected by categorical variables such as the planting pattern, soil layer, application species, duration, and soil texture. Among them, the long-term continuous diversification is mainly manifested in the reduction of soil nifH and increase of nirK abundances. Soil organic carbon and nitrogen linearly affect the responses of nifH, amoA, nirS, and nirK. Therefore, to maintain the soil ecological function, diversity of planting patterns needs to be applied flexibly by regulating the abundance of nitrogen-cycling genes. Our study draws conclusions in order to provide theoretical references for the sustainability of nitrogen and improvement of management measures in the process of terrestrial managed ecosystem diversification.

The effects of grazer exclosure duration on soil microbial communities on the Qinghai-Tibetan Plateau

While determining the response of soil microbes to grazer exclosure duration is critical to understanding ecosystem restoration processes, few studies have focused on this issue. With seasonal grazing as a control, microbes of alpine grassland soils under 5, 13, 22, and 39 years of grazer exclosure situated in the eastern part of the Qinghai-Tibetan Plateau, were examined. Microbial diversity was determined through Illumina high-throughput sequencing of the 16S rRNA gene and an internal transcription spacer (ITS). We found that soil bacterial α-diversity showed insignificant differences between seasonal grazing and grazer exclosure and among the grazer exclosures of different durations, while fungal α-diversity under the 5-year grazer exclosure was significantly different from those under the other treatments. Soil microbial community profiles under the 13-, 22-, and 39-year grazer exclosures were significantly different compared to those under the seasonal grazing or 5-year grazer exclosure. Briefly, longer exclosure durations led to a higher relative abundance of multiple copiotrophic microbial lineages (e.g., β-Proteobacteria, Rhizobiales, and Frankiales), whereas several oligotrophic microbial lineages (e.g., Chloroflexi, Leotiomycetes, and Xylariales) gradually and significantly decreased. Functional predictions suggest that as grazer exclosure duration was extended, the relative abundance of nitrogen fixers increased, while the proportions of plant pathogenic fungi decreased. This indicates that long-term grazer exclosure duration may contribute to enhanced soil nitrogen fixation and grassland health by maintaining plant growth and decreasing the risk of plant disease. However, this may have a resource cost as plant productivity and soil organic carbon both decreased with the extension of grazer exclosure duration. Therefore, the agroecology effect of grazer exclosure duration on the diversity and abundance of soil nitrogen fixing bacteria and plant pathogen fungi, should be given more attention in the cold and humid portion of the Qinghai-Tibetan Plateau.