Reduction of N2O emission by biochar and/or 3,4-dimethylpyrazole phosphate (DMPP) is closely linked to soil ammonia oxidizing bacteria and nosZI-N2O reducer populations

Soil health hazards of di(2-ethylhexyl) phthalate: New perspectives on earthworms from different ecological niches DNA damage, gut microbial disruption and soil enzyme changes

Di(2-ethylhexyl) phthalate (DEHP) is perceived an emerging threat to terrestrial ecosystem, however, clear and accurate studies to fully understander ecotoxicity and underlying mechanisms of DEHP on the soil fauna remain poorly understood. Therefore, this study conducted a microcosm experiment of two earthworm ecotypes to investigate the ecological hazards of DHEP from multiple perspectives. The results showed that DEHP significantly increased the 8-hydroxy-deoxyguanosine (8-OHdG) content both in Eisenia foetida (13.76-133.0%) and Metaphire guillelmi (11.01-49.12%), leading to intracellular DNA damage. Meanwhile, DEHP negatively affected the expression of functional genes (ATP-6, NADH1, COX), which may be detrimental to mitochondrial respiration and oxidative stress at the gene level. The two earthworm guts shared analogous dominant bacteria however, the incorporation of DEHP drastically suppressed the homogeneity and diversity of the gut microbes, which further disrupted the homeostasis of the gut microbial ecological network. The keystone species in the gut of E. foetida decreased under DEHP stress but increased in the gut of M. guillelmi. Moreover, DEHP presented detrimental effects on soil enzyme activity, which is mainly associated with pollutant levels and earthworm activity. Collectively, the findings expand the understanding of soil ecological health and reveal the underlying mechanisms of the potential exposure risk to DEHP.

Effects of the three amendments on NH3 volatilization, N2O emissions, and nitrification at four salinity levels: An indoor experiment

The marked salinity and alkaline pH of coastal saline soil profoundly impact the nitrogen conversion process, leading to a significantly reduced nitrogen utilization efficiency and substantial gaseous nitrogen loss. The application of soil amendments (e.g. biochar, manure, and gypsum) was proved to be effective for the remediation of saline soils. However, the effects of the three amendments on soil nitrogen transformation in soils with various salinity levels, especially on NH3 volatilization and N2O emission, remain elusive. Here, we reported the effects of biochar, manure, and gypsum on NH3 volatilization and N2O emission under four natural salinity gradients in the Yellow River Delta. Also, high-throughput sequencing and qPCR analysis were performed to characterize the response of nitrification (amoA) and denitrification (nirS, nirK, and nosZ) functional genes to the three amendments. The results showed that the three amendments had little effect on NH3 volatilization in low- and moderate-salinity soils, while biochar stimulated NH3 volatilization in high-salinity soils and reduced NH3 volatilization in severe-salinity soils. Spearman correlation analysis demonstrated that AOA was significantly and positively correlated with the NO3--N content (r = 0.137, P < 0.05) and N2O emissions (r = 0.174, P < 0.01), which indicated that AOA dominated N2O emissions from nitrification in saline soils. Structural equation modeling indicated that biochar, manure, and gypsum affected N2O emission by influencing soil pH, conductivity, mineral nitrogen content, and functional genes (AOA-amoA and nosZ). Two-way ANOVA further showed that salinity and amendments (biochar, manure, and gypsum) had significant effects on N2O emissions. In summary, this study provides valuable insights to better understand the effects of gaseous N changes in saline soils, thereby improving the accuracy and validity of future GHG emission predictions and modeling.

Global warming potential assessment under reclaimed water and livestock wastewater irrigation coupled with co-application of inhibitors and biochar

The application of nitrification inhibitors (nitrapyrin) and urease inhibitors (N-(N-butyl) thiophosphoric triamide) under conventional water resources has been considered as an effective means to improve nitrogen utilization efficiency and mitigate soil greenhouse gas emissions. However, it is not known whether the inhibitors still have an inhibitory effect under unconventional water resources (reclaimed water and livestock wastewater) irrigation and whether their use in combination with biochar improves the mitigation effect. Therefore, unconventional water resources were used for irrigation, with groundwater (GW) control. Nitrapyrin and N-(N-butyl) thiophosphoric triamide were used alone or in combination with biochar in a pot experiment, and CO2, N2O, and CH4 emissions were measured. The results showed that irrigation of unconventional water resources exacerbated global warming potential (GWP). All exogenous substance treatments increased CO2 and CH4 emissions and suppressed N2O emissions, independent of the type of water, compared to no substances (NS). The inhibitors were ineffective in reducing the GWP whether or not in combination with biochar, and the combined application of inhibitors with biochar further increased the GWP. This study suggests that using inhibitors and biochar in combination to regulate the greenhouse effect under unconventional water resources irrigation should be done with caution.

Nitrogen (N) "supplementation, slow release, and retention" strategy improves N use efficiency via the synergistic effect of biochar, nitrogen-fixing bacteria, and dicyandiamide

Irrational nitrogen (N) fertilizer management and application practices have led to a range of ecological and environmental problems that seriously threaten food security. In this study, an effective N fertilizer management strategy was established for improving N fertilizer utilization efficiency (NUE). Biochar, N2-fixing bacteria (Enterobacter cloacae), and a nitrification inhibitor (dicyandiamide, DCD) were simultaneously added to the soil during maize cultivation. The goal was to increase soil ammonium nitrogen content and NUE by regulating the relative abundance, enzyme activity, and functional gene expression of N conversion-related soil microbes. Biochar combined with E. cloacae and DCD significantly increased soil N content, and the NUE reached 46.69 %. The relative abundance of Burkholderia and Bradyrhizobium and the activity of nitrogenase increased significantly during biological N2 fixation. Further, the abundance of the nifH gene was significantly up-regulated. The relative abundance of Sphingomonas, Pseudomonas, Nitrospira, and Castellaniella and the activities of ammonia monooxygenase and nitrate reductase decreased significantly during nitrification and denitrification. Moreover, the abundance of the genes amoA and narG was significantly down-regulated. Correlation analyses showed that the increase in soil N2 fixation and the suppression of nitrification and denitrification reactions were the key contributors to the increase in soil N content and NUE. Biochar combined with E. cloacae and DCD synergistically enabled the supplementation, slow release, and retention of N, thus providing adequate N for maize growth. Thus, the combination of biochar, E. cloacae, and DCD is effective for mitigating the irrational application of N fertilizers and reducing N pollution.

Effects of nitrification and urease inhibitors on ammonia-oxidizing microorganisms, denitrifying bacteria, and greenhouse gas emissions in greenhouse vegetable fields

Soil microorganisms and N cycling are important components of biogeochemical cycling processes. In addition, the study of the effects of nitrification and urease inhibitors on N and microorganisms in greenhouse vegetable fields is essential to reducing N loss and greenhouse gas emissions. The effects of nitrification inhibitors [2-chloro-6-(trichloromethyl) pyridine (CP), dicyandiamide (DCD)], and urease inhibitor [N-(n-butyl) thiophosphoric triamide (NBPT)] on soil inorganic N (NH4+-N, NO2--N and NO3--N) concentrations and the production rates of greenhouse gases (N2O, CH4, and CO2) in greenhouse vegetable fields were investigated via indoor incubation experiments. Polymerase chain reaction amplification and high-throughput sequencing technology (Illumina Miseq) were used to explore the community structure and abundance changes of ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB), and denitrifying bacteria (nirK and nirS). The results showed that CP and DCD obviously inhibited NH4+-N conversion, and NO2--N, and NO3--N accumulation, NBPT slowed down urea hydrolysis and NH4+-N production, and the apparent nitrification rates of soil were in the following order: NBPT > DCD > DCD + NBPT > CP + NBPT > CP. Compared with urea treatment, the peak N2O production rate of inhibitor treatment decreased by 73.30-99.30%, and the production rate of CH4 and CO2 decreased by more than 66.16%. DCD and CP reduced the abundance of AOA and AOB, respectively. Furthermore, NBPT hindered the growth of ammonia-oxidizing microorganisms and nirS-type denitrifying bacteria, and urea and nitrification inhibitors were detrimental to the growth of Ensifer and Sinorhizobium in the nirK community. Nitrification and urease inhibitors can effectively slow down nitrification and greenhouse gas emissions, reduce N loss and improve soil quality by inhibiting the growth of ammonia-oxidizing microorganisms and denitrifying bacteria.

Effects of different remediation methods on phosphorus transformation and availability

The effects of different heavy metal pollution remediation methods on soil nutrient transformation and soil health remain unclear. In this study, the effects of phytoextraction (PE) and passivation remediation (PR) on Cd-polluted soil phosphorus transformation and availability were compared by pot experiment. The results showed that PE significantly reduced the concentrations of total and available Cd (both H2O-Cd and DTPA-Cd) in soil, PR also decreased available Cd content but had no significant effect on total Cd content. PE slightly increased soil pH and NH4+-N content, while PR significantly increased soil pH, NO3--N and AK content. PE promoted the conversion of stable P (including HCl-Pi and residual-Pt), and increased the content of labile P (including H2O-Pi, NaHCO3-Pi and NaHCO3-Po) and the proportion of moderately labile P (including NaOH-Pi and NaOH-Po), while PR showed the opposite trend. PE showed a higher soil phoC gene abundance and acid phosphatase (ACP) activity, while PR showed a higher phoD gene copies and alkaline phosphatase (ALP) activity. Soil bacteria and phoD-harboring bacteria community was significantly affected by remediation methods and soil types. Compared with PR, PE reduced phoD-harboring bacterial diversity but significantly increased the abundance of genera associated with P dissolution (Streptomyces) and P conversion (Bradyrhizobium and Frankia), both of which were significantly positively correlated with labile P or moderately labile P. In general, compared with PR, PE can effectively remove soil Cd pollution, while maintaining a higher content of labile P and a higher proportion of moderately labile P, which can be considered as a green and sustainable remediation strategy conducive to soil quality.

Characteristics of denitrification activity, functional genes, and denitrifying community composition in the composting process of kitchen and garden waste

N₂O can be easily produced during the co-composting of kitchen waste (KW) and garden waste (GW). This study investigated the effects of the co-composting of KW and GW at different ratios (1:2, 1:1.5, 1:1, and 1.5:1) on the denitrifying activities, functional genes, and community composition of denitrifiers. The results showed that the denitrification activity of KW and GW at a 1:2 ratio was the lowest. The gene abundances of nirS, nirK, nosZI, and nosZII were high on days 12 and 28 under the four different ratios. Network analysis demonstrated that nosZ-type denitrifiers could construct a complex and reciprocal bacterial network to promote the reduction of N₂O to N₂. Mantel test results revealed that nirS-, nirK-, nosZI-, and nosZII-type denitrifiers were significantly positively correlated with pH, C/N, and moisture content. These findings demonstrated that composting with appropriate proportions of KW and GW could reduce N₂O emissions caused by denitrification.

Niche Differentiation Among Canonical Nitrifiers and N2O Reducers Is Linked to Varying Effects of Nitrification Inhibitors DCD and DMPP in Two Arable Soils

The efficacy of nitrification inhibitors (NIs) dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) varies with soil types. Understanding the microbial mechanisms for this variation may lead to better modelling of NI efficacy and therefore on-farm adoption. This study addressed the response patterns of mineral nitrogen, nitrous oxide (N₂O) emission, abundances of N-cycling functional guilds and soil microbiota characteristics, in relation to urea application with or without DCD or DMPP in two arable soils (an alkaline and an acid soil). The inhibition of nitrification rate and N₂O emission by NI application occurred by suppressing ammonia-oxidizing bacteria (AOB) abundances and increasing the abundances of nosZI-N₂O reducers; however, abundances of ammonia-oxidizing archaea (AOA) were also stimulated with NIs-added in these two arable soils. DMPP generally had stronger inhibition efficiency than DCD, and both NIs' addition decreased Nitrobacter, while increased Nitrospira abundance only in alkaline soil. N₂O emissions were positively correlated with AOB and negatively correlated with nosZI in both soils and AOA only in acid soil. Moreover, N₂O emissions were also positively correlated with nirK-type denitrifiers in alkaline soil, and clade A comammox in acid soil. Amendment with DCD or DMPP altered soil microbiota community structure, but had minor effect on community composition. These results highlight a crucial role of the niche differentiation among canonical ammonia oxidizers (AOA/AOB), Nitrobacter and Nitrospira, as well as nosZI- and nosZII-N₂O reducers in determining the varying efficacies of DCD and DMPP in different arable soils.

Proper organic substitution attenuated both N2O and NO emissions derived from AOB in vegetable soils by enhancing the proportion of Nitrosomonas

The ammonia oxidation process driven by microorganisms is an essential source of nitrous oxide (N₂O) and nitric oxide (NO) emissions. However, few evaluations have been performed on the changes in the community structure and abundance of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) under substituting portion of chemical fertilizers with organic manure (organic substitution) and their relative contribution to the ammonia oxidation process. Here, five long-term fertilization strategies were applied in field (SN: synthetic fertilizer application; OM: organic manure; M1N1: substituting 50 % of chemical N fertilizer with organic manure; M1N4: substituting 20 % of chemical N fertilizer with organic manure; and CK: no fertilizer). We investigated the response characteristics of AOB and AOA community structures by selective inhibitor shaking assays and high-throughput sequencing and further explained their relative contribution to the ammonia oxidation process during three consecutive years of vegetable production. Compared to SN and M1N4, the potential of ammonia oxidation (PAO) was significantly reduced by 26.4 % and 22.3 % in OM and 9.5 % and 4.4 % in M1N1, resulting in N₂O reductions of 38.9 % and 30.8 % (OM) and 31.2 % and 21.1 % (M1N1), respectively, and NO reductions of 45.0 % and 34.1 % (OM) and 40.1 % and 28.3 % (M1N1). RDA and correlation analyses showed that the soil organic carbon and ammonium nitrogen content increased while AOB gene abundance and diversity significantly decreased with increasing organic replacement ratio; however, the relative abundance of Nitrosomonas in AOB increased in OM and M1N1, which further demonstrates that AOB are the main driver in vegetable soils. Therefore, the appropriate proportion of organic substitution (OM and M1N1) could decrease the N₂O and NO emissions contributed by AOB by affecting the soil physicochemical properties and AOB community structure.

Meta-analysis of the effect of nitrification inhibitors on the abundance and community structure of N2O-related functional genes in agricultural soils

Application of nitrification inhibitors (NIs) in agricultural systems is an important strategy to enhance fertilizer nitrogen use efficiency and mitigate soil nitrous oxide (N₂O) emissions. Here, we conducted a global meta-analysis of 88 published studies to assess the response of N₂O-related functional gene and transcript abundances, and community structure to NIs application. Application of NIs significantly reduced the abundance of ammonia-oxidizing bacteria ammonia monooxygenase (AOB amoA) genes, AOB amoA transcript and nitrite reductase (nirS and nirK) genes. The effectiveness of NIs on reducing the AOB amoA abundance was influenced by N form, soil texture, soil pH and the experimental type (field vs. laboratory). Specifically, NIs were more effective when a mixed inorganic and organic N source was applied to a medium-textured soils. The NIs effectiveness increased with increasing soil pH. The response of AOB amoA abundance to NIs application was not affected by NI type, N rate, soil moisture, soil temperature and soil organic carbon (SOC). The inhibitory effect of NIs on nirS abundance increased with increasing soil temperature. NIs decreased soil nitrifying enzyme activity (NEA) and denitrifying enzyme activity (DEA) by 34.5 % and 27.0 %, respectively, leading to an overall 63.6 % reduction of N₂O emissions. Soil NEA correlated positively with the abundance and community structure of AOB amoA but not with AOA amoA. Decrease in DEA with NIs application coincided with the decreasing nirS and nirK abundances. This global-scale assessment demonstrates that the effectiveness of NIs in reducing N₂O emissions was attributed to the inhibiting effects on AOB amoA, nirS and nirK genes. Our findings highlight that NIs' inhibition effects on bacterial ammonia-oxidizing community and the encode enzymes in transformation of nitrite to nitric oxide are the main mechanisms for mitigation of N fertilizer-induced N₂O emissions.

The synergistic toxicity effect of di(2-ethylhexyl)phthalate and plant growth disturbs the structure and function of soil microbes in the rhizosphere

Di-(2-ethylhexyl) phthalate (DEHP) is a dominant phthalic acid ester in the environment and commonly occurs at high concentration in agricultural soils. Its influence on the soil microbial community has been widely reported, while research related to its effects on microbial structure, function, and interactions in the rhizosphere, a microbial hotspot region in the terrestrial ecosystem, is still limited. This study investigated the response of microbes in the rhizosphere to DEHP contamination. DEHP reduced microbial quantity, shifted the microbial community structure, and enriched the soil bacteria with potential DEHP degraders. Although the rhizosphere can alleviate DEHP toxicity, DEHP still played an important role in microbial community construction in the rhizosphere. Interestingly, some microbes were influenced by the synergistic toxicity effect of DEHP addition and plant growth, and there were significant differences in their relative abundance and alpha diversity in soil treated with both DEHP and planting compared to soils with just DEHP spiking or planting. The genes related to cell motility, metabolism of terpenoids and polyketides, protein families, genetic information processing, and replication and repair pathways changed only in soil treated with both DEHP and planting further proved the existence of synergistic toxicity. Anyway, the impact of DEHP on microbial function in the rhizosphere was important with 52.42‰ of the genes being changed. The change in cell motility, biofilm formation, and genes related to the quorum sensing pathway might affect the relationship between microbes, which play a crucial role in ecosystem function. This was further proven by changes in the microbial co-occurrence pattern. Our results can benefit risk evaluation of DEHP to microbial community in the rhizosphere, which is important for the effective function of terrestrial ecosystems and soil health.

The Effects of N Enrichment on Microbial Cycling of Non-CO2 Greenhouse Gases in Soils-a Review and a Meta-analysis

Terrestrial ecosystems are typically nitrogen (N) limited, but recent years have witnessed N enrichment in various soil ecosystems caused by human activities such as fossil fuel combustion and fertilizer application. This enrichment may alter microbial processes in soils in a way that would increase the emissions of methane (CH₄) and nitrous oxide (N₂O), thereby aggravating global climate change. This review focuses on the effects of N enrichment on methanogens and methanotrophs, which play a central role in the dynamics of CH₄ at the global scale. We also address the effects of N enrichment on N₂O, which is produced in soils mainly by nitrification and denitrification. Overall, N enrichment inhibits methanogenesis in pure culture experiments, while its effects on CH₄ oxidation are more complicated. The majority of previous studies reported that N enrichment, especially NH₄⁺ enrichment, inhibits CH₄ oxidation, resulting in higher CH₄ emissions from soils. However, both activation and neutral responses have also been reported, particularly in rice paddies and landfill sites, which is well reflected in our meta-analysis. In contrast, N enrichment substantially increases N₂O emission by both nitrification and denitrification, which increases proportionally to the amount of N amended. Future studies should address the effects of N enrichment on the active microbes of those functional groups at multiple scales along with parameterization of microbial communities for the application to climate models at the global scale.

Synergistically mitigating nitric oxide emission by co-applications of biochar and nitrification inhibitor in a tropical agricultural soil

Agricultural soils are the hotspots of nitric oxide (NO) emissions, which are related to atmospheric pollution and greenhouse effect. Biochar application has been recommended as an important countermeasure, however, its mitigation efficiency is limited as biochar, under certain conditions, can stimulate soil nitrification. Therefore, biochar co-applied with nitrification inhibitor could optimize the mitigation potential of biochar. Herein, a laboratory-scale experiment was conducted to investigate the effects of co-application of biochar and nitrification inhibitor on NO emission, nitrogen cycling function and bacterial community in a tropical vegetable soil. Results showed that a single application of biochar or nitrification inhibitor significantly decreased NO emissions, and this mitigation effectiveness was amplified by their co-applications. Soil NO₂^--N intensity, along with abundances of AOB-amoA and nirK were significantly and positively correlated with cumulative NO emissions. The stimulated activity of ammonia monooxygenase and growths of AOB and total comammox Nitrospira by biochar were weakened by nitrification inhibitor, implying decreased nitrification-driven NO production. The nitric oxide reductase activity and related qnorB abundance in nitrification inhibitor-added soils were increased by biochar, indicating promoted NO consumption during denitrification. The nirK abundance and NO₂^--N intensity were decreased more by co-applications of biochar or nitrification inhibitor. Moreover, both biochar and nitrification inhibitor changed bacterial β-diversity, and their co-application synergistically enriched Armatimonadetes and Verrucomicrobia abundances and decreased WPS-2 abundance. This study highlights that co-applications of biochar and nitrification inhibitor can make their respective advantages complementary to each other, thereby achieving a larger mitigation of NO emissions from agricultural soils in tropical regions.

A meta-analysis to examine whether nitrification inhibitors work through selectively inhibiting ammonia-oxidizing bacteria

Nitrification inhibitor (NI) is often claimed to be efficient in mitigating nitrogen (N) losses from agricultural production systems by slowing down nitrification. Increasing evidence suggests that ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) have the genetic potential to produce nitrous oxide (N₂O) and perform the first step of nitrification, but their contribution to N₂O and nitrification remains unclear. Furthermore, both AOA and AOB are probably targets for NIs, but a quantitative synthesis is lacking to identify the “indicator microbe” as the best predictor of NI efficiency under different environmental conditions. In this present study, a meta-analysis to assess the response characteristics of AOB and AOA to NI application was conducted and the relationship between NI efficiency and the AOA and AOB amoA genes response under different conditions was evaluated. The dataset consisted of 48 papers (214 observations). This study showed that NIs on average reduced 58.1% of N₂O emissions and increased 71.4% of soil NH4+ concentrations, respectively. When 3, 4-dimethylpyrazole phosphate (DMPP) was applied with both organic and inorganic fertilizers in alkaline medium soils, it had higher efficacy of decreasing N₂O emissions than in acidic soils. The abundance of AOB amoA genes was dramatically reduced by about 50% with NI application in most soil types. Decrease in N₂O emissions with NI addition was significantly correlated with AOB changes (R² = 0.135, n = 110, P < 0.01) rather than changes in AOA, and there was an obvious correlation between the changes in NH4+ concentration and AOB amoA gene abundance after NI application (R² = 0.037, n = 136, P = 0.014). The results indicated the principal role of AOB in nitrification, furthermore, AOB would be the best predictor of NI efficiency.

DMPP and Polymer-Coated Urea Promoted Growth and Increased Yield of Greenhouse Tomatoes

Improvements in nitrogen (N) use efficiency reduce stress on the environment and improve tomato production. A two-year trial was conducted in greenhouse tomatoes with a split-plot design, in which one factor was the N application rate (150 kg·ha−1, N1; 200 kg·ha−1, N2; and 250 kg·ha−1, N3) and two other factors were the type of urea applied (urea, T1; slow-release (polymer-coated) urea, T2, and nitrification inhibitors (3,4-dimethylpyrazole phosphate, DMPP) + urea, T3); no N fertilizer was applied in the control. The effects of the nitrogen (N) application rate and type of urea applied on the root morphology indexes, growth indexes, photosynthetic parameters, yield (Y), water use efficiency (WUE), and nitrogen agronomic efficiency (NAE) of greenhouse tomatoes were investigated. The results show that an appropriate N application rate (200 kg·ha−1) can improve tomato growth and net photosynthetic rate (Pn). With T3, the Y and WUE of greenhouse tomatoes first increased and then decreased as the N application rate increased, but with T1 and T2, the Y and WUE increased as the N application rate increased. The NAE of greenhouse tomatoes was significantly lower with N3 than with N2. The root growth, plant growth, Pn, Y, WUE, and NAE of the tomatoes were improved with T2 and T3 compared to T1. These findings can be used to promote N conservation and increase the Y of facility agriculture crops.

Silicon fertilization influences microbial assemblages in rice roots and decreases arsenic concentration in grain: A five-season in-situ remediation field study

Microbial mechanism of in-situ remediation of arsenic (As) in As-contaminated paddy fields by silicon (Si) fertilization has been rarely reported, especially under continuous rice cultivation and Si applications. In this study, two Si fertilizers were applied for three phases in five consecutive rice seasons to investigate the long-lasting impacts on in-situ remediation of As, and the underpinning microbial mechanism of root-associated compartments (bulk soil, rhizosphere and endosphere) was explored using the last double-cropping rice. Repeated application of Si fertilizers as base manure had a long-lasting effect on reducing As concentrations in rice grains. Application of Si fertilizer at an adequate amount resulted in an extended in-situ remediation effect from endosphere to rhizosphere. The microbial diversity and richness in rhizosphere soil and endosphere were significantly impacted by Si fertilization, the effects depending on application doses and prolonged seasons. Si fertilization can immobilize As in the root or rhizosphere, and Fe concentrations and the As- and Fe-transforming microorganisms (i.e. Geobacteraceae) are the determinants of As uptake in rice. We recommend more extensive supplementation of Si fertilizer at a higher rate to decrease grain As concentration for in-situ remediation. This study sheds light on the microbial-mediated mechanism underlying Si fertilization effect on decreased As uptake in paddy fields.

The inhibitory efficacy of procyanidin on soil denitrification varies with N fertilizer type applied

Denitrification is a major process of the nitrogen (N) cycle by converting nitrate (NO₃^-) back to gaseous nitrogen (N₂), which leads to massive losses of N, including fertilizer N, from agricultural systems. One mitigation strategy for these N losses involves denitrification inhibition by plant-derived biological denitrification inhibitors (BDIs). Procyanidin was recently identified as a new class of BDI in root extracts from Fallopia spp. However, the efficacy of this compound on soil denitrification under different N fertilizer sources is not well understood. Here, a 14-day microcosm experiment was conducted using three nitrate-based fertilizers (NH₄NO₃, KNO₃, and Ca(NO₃)₂) to investigate the impact of procyanidin on soil denitrification and associated microbial pathways. Results showed that procyanidin inhibited denitrification activity regardless of the source of N fertilizer applied, but the inhibitory efficacy of procyanidin varied with N fertilizer types. Addition of procyanidin had greater denitrification inhibition in the soils applied with NH₄NO₃ than with other types of N fertilizer. Moreover, nitrate reductase activity was significantly suppressed by procyanidin addition across all three N fertilizers tested. Quantification of denitrifying functional genes (nirS, nirK, and nosZ) demonstrated that procyanidin inhibited the activity and growth of nirS- and nirK-type denitrifiers, but stimulated the growth of nosZI-containing denitrifiers. These findings indicate that the inhibition of soil denitrification by procyanidin was mainly a result of the suppression of nitrate reductase activity and nirS- and nirK-type denitrifiers abundance. The use of procyanidin together with N fertilizers, especially NH₄NO₃, can be an effective way to reduce the N losses by denitrification.

Combined biochar and double inhibitor application offsets NH3 and N2O emissions and mitigates N leaching in paddy fields

The effects of combined biochar and double inhibitor application on gaseous nitrogen (N; nitrous oxide [N₂O] and ammonia [NH₃]) emissions and N leaching in paddy soils remain unclear. We investigated the effects of biochar application at different rates and double inhibitor application (hydroquinone [HQ] and dicyandiamide [DCD]) on NH₃ and N₂O emissions, N leaching, as well as rice yield in a paddy field, with eight treatments, including conventional urea N application at 280 kg N ha^-1 (CN); reduced N application at 240 kg N ha^-1 (RN); RN + 7.5 t ha^-1 biochar (RNB1); RN + 15 t ha^-1 biochar (RNB2); RN + HQ + DCD (RNI); RNB1 + HQ + DCD (RNIB1); RNB2 + HQ + DCD (RNIB2); and a control without N fertilizer. When compared with N leaching under RN, biochar application reduced total N leaching by 26.9-34.8% but stimulated NH₃ emissions by 13.2-27.1%, mainly because of enhanced floodwater and soil NH₄⁺-N concentrations and pH, and increased N₂O emission by 7.7-21.2%, potentially due to increased soil NO₃^--N concentrations. Urease and nitrification inhibitor addition decreased NH₃ and N₂O emissions, and total N leaching by 20.1%, 21.5%, and 22.1%, respectively. Compared with RN, combined biochar (7.5 t ha^-1) and double inhibitor application decreased NH₃ and N₂O emissions, with reductions of 24.3% and 14.6%, respectively, and reduced total N leaching by up to 45.4%. Biochar application alone or combined with double inhibitors enhanced N use efficiency from 26.2% (RN) to 44.7% (RNIB2). Conversely, double inhibitor application alone or combined with biochar enhanced rice yield and reduced yield-scaled N₂O emissions. Our results suggest that double inhibitor application alone or combined with 7.5 t ha^-1 biochar is an effective practice to mitigate NH₃ and N₂O emission and N leaching in paddy fields.

DMPP reduces nitrogen fertilizer application rate, improves fruit quality, and reduces environmental cost of intensive apple production in China

In China, excessive application of nitrogen (N) fertilizer is common in intensive apple production. To resolve issues of benefit reduction and environmental pollution caused by excessive N, a two-year trial was conducted in an apple orchard with a split-plot design, in which the main factor was the N level (500, 400, 300, and 200 kg N ha^-1 year^-1, expressed as TN, TN80%, TN60%, and TN40%, respectively) and the deputy factor was whether or not to add 3,4-dimethylpyrazole phosphate (DMPP, expressed as +D). The effects of N reduction combined with DMPP on soil N transformation, fruit quality, economic benefits, and environmental effects were investigated. The results showed that DMPP reduced the production of nitrate and its vertical migration by inhibiting the abundance of AOB amoA and decreased N₂O emission by reducing nirKC1 levels. Moreover, N reduction combined with DMPP improved N use efficiency (26.67-49.35%) and reduced N loss rate (15.25-38.76%). Compared with TN, TN60% + D increased the content of anthocyanin and soluble sugar by 21.15% and 13.09%, respectively, and decreased environmental costs caused by NH₃ volatilization and N₂O emission by 33.84%, while maintaining yield and N utilization rate at relatively high levels. Considering the agronomic, economic and environmental benefits, on the basis of traditional N application rate, 40% N reduction combined with DMPP (TN60% + D) could ensure target yield, corresponding quality and economic benefits, maintain soil N fertility, and reduce the risk of N losses to the environment. The present research could provide references for green, efficient, and sustainable development of apple production.

Di-(2-ethylhexyl) phthalate (DEHP) exposure suppressed the community diversity and abundance of ammonia-oxidizers and mitigated N2O emissions in an alkaline soil

Phthalic acid esters (PAEs) pollution has become an increasing issue worldwide, but little is known about its effects on ammonia-oxidizers and nitrous oxide (N₂O) in the soil environment. Here, a 50-day soil microcosm experiment was conducted using low and high doses bis (2-Ethylhexyl) phthalate (DEHP) to estimate the effect of DEHP exposure on soil N₂O emissions and the nitrifying community in calcareous soil. The results showed that DEHP exposure at 10 and 100 mg kg^-1 doses significantly reduced N₂O by 67.5% and 73.6%, respectively, relative to the DEHP absent treatment. The microbial biomass carbon and nitrogen (MBC and MBN) were consistently and significantly decreased with DEHP exposure at 5, 22, and 50 days after incubation, especially with high-dose. The bipartite association networks showed that DEHP exposure changed the compositions of both ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) communities. Moreover, the AOA and AOB amoA gene abundances were significantly decreased by DEHP addition to the soil (P < 0.05). Random forest modeling showed that the AOB Shannon index and pH were the most important biotic and abiotic factors affecting N₂O emissions in the soil with DEHP exposure, respectively. Partial least squares path modeling (PLS-PM) demonstrated that the reduction of N₂O emissions due to DEHP exposure was mainly ascribed to the changes of the AOB community. The results from this study highlight the toxicity of DEHP on the ammonia oxidizers, and its mitigating effect on N₂O emissions in the soil where ammonia-oxidation is largely driven by AOB.

Effect of P availability on straw-induced priming effect was mainly regulated by fungi in croplands

Phosphorus (P) accumulation in croplands resulting from ever-increasing input of P fertilizer strongly influences soil microbial growth and activities, which is expected to alter the soil priming effect (PE) induced by plant residue. However, the effect of P availability on the magnitude and direction of PE remains largely unexplored and the underlying microbial mechanisms are still unclear. Therefore, a 40-day incubation experiment was established by adding C₄-maize straw to C₃-soils with or without long-term P fertilizer inputs to investigate PE and accompanied dynamics of microbiota. The results revealed that in both soils, straw application caused positive real PEs via a "microbial co-metabolism" mechanism, accompanied by a microbial succession from the dominance of r- and K-strategists to K-strategists (mainly fungi). In addition, long-term amendment with P increased PE by 83.2% compared with no P fertilization control, which was mainly mediated by K-strategists, especially the fungal families Chaetomiaceae and Myrmecridiaceae. The increased PE was accompanied by enhanced microbial biomass carbon, extracellular enzyme activities, and bacterial gene abundance, confirming the "stoichiometric decomposition" theory. Meanwhile, deviating from the conventional paradigm, higher phosphatase activity and lower enzymatic stoichiometry of carbon (C)-to-P ratios in high-P soil compared with that in low-P soil suggested stronger "P mining" with high-P availability.

Effects of sulfoxaflor on greenhouse vegetable soil N2O emissions and its microbial driving mechanism

The wide application of pesticides ensures the safety of food production, but it also has a serious impact on soil ecosystem. Although sulfoxaflor as a pesticide has great potential for application due to its excellent insecticidal activity and low crossresistance, little is known about its soil environmental safety risks. In this study, the effects of sulfoxaflor on N₂O emissions and microorganisms in greenhouse vegetable soils were studied by indoor simulation culture experiments. Dynamic changes of soil main inorganic N and N₂O emission rate were tested, and the abundance and community of total bacteria and microorganisms related to N cycle were analyzed. The results indicated that soil microorganisms rapidly degraded sulfoxaflor, and the N₂O emissions rate and ammonium nitrogen (NH₄⁺-N) content significantly increased, while nitrate nitrogen (NO₃^--N) content was significantly decreased. Sulfoxaflor significantly changed the abundance and community of total bacteria, nitrite reducing and nitrous oxide reducing bacteria, but had no significant effect on ammoxidation microorganisms. The N₂O emission rate was positively correlated with gene abundance of denitrifying microorganisms. Under 65% soil maximum water holding capacity, sulfoxaflor may broke the dynamic balance of N₂O production and consumption in the denitrification process, which caused a significant increase in N₂O emission. Therefore, the application of sulfoxaflor had a certain effect on N cycling and utilization in greenhouse vegetable soil.

Win-win: Application of sawdust-derived hydrochar in low fertility soil improves rice yield and reduces greenhouse gas emissions from agricultural ecosystems

As a good soil synergist, biochar has a wide prospect in improving soil fertility and crop production. Although hydrochar, produced by hydrothermal carbonization process has attracted attention due to production advantages, hydrochar application in low fertility soils as well as its impact to the associated greenhouse gas (GHG) emissions in farmlands is rarely reported. To advance our understanding on the effect of hydrochar addition on grain yield from low fertility soils and the corresponding CH₄ and N₂O emissions, a soil-column experiment, with two hydrochar types (sawdust-derived hydrochar (SDH), microbial-aged hydrochar (A-SDH)) at two application rates (5‰, 15‰; (w/w)), was conducted. The results showed that hydrochar addition evidently increased rice yield. The N₂O emissions were mainly related to the substrate supply of the hydrochar itself and less affected by the denitrifiers (functional genes) present. Hydrochar amendment at low application rate (5‰; SDH05, A-SDH05) significantly decreased the cumulative N₂O emissions by 26.32% ~ 36.84%. Additionally, hydrochar amendment could not increase the CH₄ emissions due to the substrate limitation; the cumulative emissions were similar with those from the control, ranging between 11.1-12.8 g m^-2. Regarding grain yield and global warming potential, greenhouse gas intensity from the soils subjected to hydrochar (SDH05, A-SDH05, A-SDH15) were significantly lower than that of the control, observation attributed to the high yield and low N₂O emissions. Overall, hydrochar addition is an effective strategy to ensure grain yield in low fertility soils with relatively low/controlled GHG emissions, especially when the amendment is applied at low application rate.

The limited effects of carbonaceous material amendments on nitrite-oxidizing bacteria in an Alfisol

Carbonaceous materials are soil conditioners that affect nitrogen cycles. However, how carbonaceous materials influence nitrite-oxidizing bacteria (NOB) is yet unclear. In this study, we investigated the NOB community and its potential activities under different treatments (control, biochar, straw, limestone, biochar + limestone, and straw + limestone) in an Alfisol, a type of arable soil depleted in calcium carbonate but enriched in aluminum- and iron-bearing minerals. Treatments with limestone increased soil pH, and straw inputs caused an increment of available potassium (AK). Ammonia (NH₄⁺) was inversely changed under the straw and biochar + limestone amendments. None of the treatments significantly impacted the abundance of Nitrobacter (nxrA) or the potential nitrite oxidation activity (PNO). The abundance of Nitrospira (nxrB) increased in the biochar + limestone-treated samples and was significantly correlated with PNO, pH, and AK. High-throughput sequencing results showed that the α-diversity of NOB did not change in response to the treatments. The dominant Nitrobacter OTUs were affiliated within the Clusters 3, 4, 8, and 9 (a new cluster named in this study), while those of Nitrospira were in the lineage II and Namibian soil cluster 2. The limited compositional variation for Nitrobacter was explained by pH, and that for Nitrospira by pH, TN, and NH₄⁺. Among all available data in this study, the richness of Nitrospira was the most important predictor (73%) for PNO. Therefore, we assumed that the community of nitrite oxidizers (Nitrospira) could be relatively redundant in function, supported by the observation that the carbonaceous inputs did not impact either the potential activity or the α-diversity but did affect the abundance and community composition.

Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate Application During the Later Stage of Apple Fruit Expansion Regulates Soil Mineral Nitrogen and Tree Carbon-Nitrogen Nutrition, and Improves Fruit Quality

In order to solve the problems of nitrogen (N) losses and fruit quality degradation caused by excessive N fertilizer application, different dosages of the nitrification inhibitor, 3,4-dimethylpyrazole phosphate (DMPP) (0, 0.5, 1, 2, and 4 mg kg^-1 soil), were applied during the later stage of 'Red Fuji' apple (Malus domestica Borkh.) fruit expansion in 2017 and 2018. The effects of DMPP on soil N transformation, carbon (C)-N nutrition of tree, and fruit quality were investigated. Results revealed that DMPP decreased the abundance of ammonia-oxidizing bacteria (AOB) amoA gene, increased the retention of NH₄ ⁺-N, and decreased NO₃ ^--N concentration and its vertical migration in soil. DMPP reduced ¹⁵N loss rates and increased ¹⁵N residual and recovery rates compared to the control. ¹³C and ¹⁵N double isotope labeling results revealed that DMPP reduced the capacity of ¹⁵N absorption and regulation in fruits, decreased ¹⁵N accumulation in fruits and whole plant, and increased the distribution of ¹³C from vegetative organs to fruits. DMPP increased fruit anthocyanin and soluble sugar contents, and had no significant effect on fruit yield. The comprehensive analysis revealed that the application of 1 mg DMPP kg^-1 soil during the later stage of fruit expansion effectively reduced losses due to N and alleviated quality degradation caused by excessive N fertilizer application.