Postfire Phosphorus Enrichment Mitigates Nitrogen Loss in Boreal Forests.

Net nitrogen mineralization (Nmin) and nitrification regulate soil N availability and loss after severe wildfires in boreal forests experiencing slow vegetation recovery. Yet, how microorganisms respond to postfire phosphorus (P) enrichment to alter soil N transformations remains unclear in N-limited boreal forests. Here, we investigated postfire N-P interactions using an intensive regional-scale sampling of 17 boreal forests in the Greater Khingan Mountains (Inner Mongolia-China), a laboratory P-addition incubation, and a continental-scale meta-analysis. We found that postfire soils had an increased risk of N loss by accelerated Nmin and nitrification along with low plant N demand, especially during the early vegetation recovery period. The postfire N/P imbalance created by P enrichment acts as a "N retention" strategy by inhibiting Nmin but not nitrification in boreal forests. This strategy is attributed to enhanced microbial N-use efficiency and N immobilization. Importantly, our meta-analysis found that there was a greater risk of N loss in boreal forest soils after fires than in other climatic zones, which was consistent with our results from the 17 soils in the Greater Khingan Mountains. These findings demonstrate that postfire N-P interactions play an essential role in mitigating N limitation and maintaining nutrient balance in boreal forests.

Myricetin Acts as an Inhibitor of Type II NADH Dehydrogenase from Staphylococcus aureus.

BACKGROUND: Staphylococcus aureus is a common pathogenic microorganism in humans and animals. Type II NADH oxidoreductase (NDH-2) is the only NADH:quinone oxidoreductase present in this organism and represents a promising target for the development of anti-staphylococcal drugs. Recently, myricetin, a natural flavonoid from vegetables and fruits, was found to be a potential inhibitor of NDH-2 of S. aureus. The objective of this study was to evaluate the inhibitory properties of myricetin against NDH-2 and its impact on the growth and expression of virulence factors in S. aureus. RESULTS: A screening method was established to identify effective inhibitors of NDH-2, based on heterologously expressed S. aureus NDH-2. Myricetin was found to be an effective inhibitor of NDH-2 with a half maximal inhibitory concentration (IC50) of 2 µM. In silico predictions and enzyme inhibition kinetics further characterized myricetin as a competitive inhibitor of NDH-2 with respect to the substrate menadione (MK). The minimum inhibitory concentrations (MICs) of myricetin against S. aureus strains ranged from 64 to 128 µg/mL. Time-kill assays showed that myricetin was a bactericidal agent against S. aureus. In line with being a competitive inhibitor of the NDH-2 substrate MK, the anti-staphylococcal activity of myricetin was antagonized by MK-4. In addition, myricetin was found to inhibit the gene expression of enterotoxin SeA and reduce the hemolytic activity induced by S. aureus culture on rabbit erythrocytes in a dose-dependent manner. CONCLUSIONS: Myricetin was newly discovered to be a competitive inhibitor of S. aureus NDH-2 in relation to the substrate MK. This discovery offers a fresh perspective on the anti-staphylococcal activity of myricetin.

The induced and intrinsic resistance of Escherichia coli to sanguinarine is mediated by AcrB efflux pump.

IMPORTANCE: The use of plant extracts is increasing as an alternative to synthetic compounds, especially antibiotics. However, there is no sufficient knowledge on the mechanisms and potential risks of antibiotic resistance induced by these phytochemicals. In the present study, we found that stable drug resistant mutants of E. coli emerged after repetitive exposure to sanguinarine and demonstrated that the AcrB efflux pump contributed to the emerging of induced and intrinsic resistance of E. coli to this phytochemical. Our results offered some insights into comprehending and preventing the onset of drug-resistant strains when utilizing products containing sanguinarine.

Viral lysing can alleviate microbial nutrient limitations and accumulate recalcitrant dissolved organic matter components in soil.

Viruses are critical for regulating microbial communities and biogeochemical processes affecting carbon/nutrient cycling. However, the role of soil phages in controlling microbial physiological traits and intrinsic dissolved organic matter (DOM) properties remains largely unknown. Herein, microcosm experiments with different soil phage concentrates (including no-added phages, inactive phages, and three dilutions of active phages) at two temperatures (15 °C and 25 °C) were conducted to disclose the nutrient and DOM dynamics associated with viral lysing. Results demonstrated three different phases of viral impacts on CO2 emission at both temperatures, and phages played a role in maintaining Q10 within bounds. At both temperatures, microbial nutrient limitations (especially P limitation) were alleviated by viral lysing as determined by extracellular enzyme activity (decreased Vangle with active phages). Additionally, the re-utilization of lysate-derived DOM by surviving microbes stimulated an increase of microbial metabolic efficiency and recalcitrant DOM components (e.g., SUV254, SUV260 and HIX). This research provides direct experimental evidence that the "viral shuttle" exists in soils, whereby soil phages increase recalcitrant DOM components. Our findings advance the understanding of viral controls on soil biogeochemical processes, and provide a new perspective for assessing whether soil phages provide a net "carbon sink" vs. "carbon source" in soils.

Linkages of nitrogen-cycling microbial resistance and resilience to soil nutrient stoichiometry under dry-rewetting cycles with different fertilizations and temperatures in a vegetable field.

Multiple dry-rewetting (DRW) cycles occur in intensively managed vegetable fields due to frequent tillage and irrigation. Soil nitrogen (N) cycling depends on the resistance and resilience of related microbial populations to DRW cycles, which could be closely related to soil nutrient status. However, the linkage of N-cycling microbial resistance and resilience and soil nutrient stoichiometry remains unknown in vegetable field. Here, we established four fertilization treatments in a four-year greenhouse vegetable field: no N fertilization, synthesized N fertilization, substituting 50% of chemical N with organic fertilizer or biofertilizer. Then, we set up an 85-day DRW-cycling incubation at 15, 25 and 35 °C including a 55-day fluctuating moisture for microbial resistance and then a 30-day constant moisture for microbial resilience. The results showed that microbial resistance was high (resistance index = 0.87- 0.99) in response to DRW cycles, but microbial resilience was generally low (resilience index = -0.36- 0.76), especially in 50% organic substitution or 15 °C. N-cycling microbes showed an important trade-off between their resistance and resilience to DRW cycles. Furthermore, most treatments showed microbial carbon limitation and N abundance during DRW cycles and recovered gradually to the undisturbed state. Microbial resistance was significantly related to the soil nutrient stoichiometry of carbon, N and phosphorus, while microbial resilience was mainly correlated with carbon-related indicators. In conclusion, N-cycling microbes presented good stability with oligotrophic strategy to frequent DRW cycles, which was linked to not only the historical legacy effect of DRW cycles but also soil nutrient stoichiometry in the vegetable field.

Biochar amendment mitigated N2O emissions from paddy field during the wheat growing season.

Biochar may variably impact nitrogen (N) transformation and N-cycle-related microbial activities. Yet the mechanism of biochar amendment on nitrous oxide (N2O) emissions from agricultural ecosystems remains unclear. Based on a 6-year long-term biochar amendment experiment, we applied a dual isotope (15N-18O) labeling technique with tracing transcriptional genes to differentiate the contribution of nitrifier nitrification (NN), nitrifier denitrification (ND), nitrification-coupled denitrification (NCD) and heterotrophic denitrification (HD) pathway to N2O production. Then the field experiment provided quantitative data on dynamic N2O emissions, soil mineral N and key functional marker gene abundances during the wheat growing season. By using 15N-18O isotope, biochar decreased N2O emission derived from ND (by 45-94%), HD (by 35-46%) and NCD (by 30-64%) compared to the values under N application. Biochar increased the relative contribution of NN to total N2O production as evidenced by the increase in ammonia-oxidizing bacteria, but did not influence the cumulative NN-derived N2O. The field experiment found that the majority of the N2O emissions peaked following fertilization, in parallel with soil NH4+ and nitrite dynamics. Soil N2O emissions during the wheat growing stage were effectively decreased (by 38-48%) by biochar amendment. Based on the correlation analyses and random forest analysis in both microcosm and field experiments, the decrease in nitrite concentration (by 62-65%) and increase in N2O consumption were mainly responsible for net N2O mitigation, as evidenced by the decrease in the ratios of nitrite reductase genes/transcripts (nirS, nirK and fungal nirK) and N2O reductase gene/transcripts (nosZI and nosZII). Based on the extrapolation from microcosm to field, biochar significantly mitigated N2O emissions by weakening the ND processes, since NCD and HD contributed little during the N2O emission "peaks" following urea fertilization. Therefore, emphasis should be put on the ND process and nitrite accumulation during N2O emission peaks and extrapolated to all agroecosystems.

Organic substitutions aggravated microbial nitrogen limitation and decreased nitrogen-cycling gene abundances in a three-year greenhouse vegetable field.

Partially substituting chemical fertilizer with organic fertilizer has substantially changed the stoichiometric imbalances of carbon (C), nitrogen (N) and phosphorus (P) between microbial communities and their available resources in agroecosystems. However, how organic substitution alters microbial nutrient limitation and then affects soil N cycle in intensive greenhouse vegetable ecosystem, remain unknown. Thus, we performed a three-year greenhouse vegetable field experiment in China with different fertilization strategies: no N fertilization, chemical N fertilization, and substituting 20% (1M4N) or 50% (1M1N) of chemical N with organic fertilizer (organic substitutions). Our results demonstrated that the microbial communities presented N limitation, accompanying with a strong N:P but a weak C:N (or P) microbial homeostasis in response to high N:P imbalance among all treatments. Organic substitutions at 1M1N and 1M4N significantly aggravated microbial N limitation and decreased the gene abundances of nitrification and denitrification by 4.7%-27.3% than that of chemical N fertilization. Microbial N limitation was strongly influenced by N:P stoichiometric imbalance illustrated from regression analysis. The N-cycling gene abundances were not only dependent on the inorganic N pool and soil physicochemical properties (i.e. pH and electrical conductivity), but also affected by microbial nutrient limitation inferred from random forest analysis. Furthermore, the 1M1N treatment performed better than the 1M4N in terms of improved crop yield and less microbial N limitation. Overall, these results highlight the importance of ecological stoichiometry in regulating soil N cycle under different fertilization strategies for intensive greenhouse vegetable ecosystem.

The effect of long-term biochar amendment on N2O emissions: Experiments with N15-O18 isotopes combined with specific inhibition approaches.

Numerous studies reporting a transient decrease in soil nitrous oxide (N2O) emissions after biochar amendment have mainly used short-term experiments. Thus, long-term field trials are needed to clarify the actual impact of biochar on N2O emissions and the underlying mechanisms. To address this, both a 15N18O labeling technique and gene analyses were applied to investigate how N2O production pathways and microbial mediation were affected by long term biochar amendment in field. Then, 1-octyne and 2-phenyl l-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO) were used in combination with potassium chlorate to evaluate the relative contribution of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential ammonia oxidation (PAO) and the associated N2O production. Acidic and alkaline greenhouse vegetable soils that had each received two separate treatments were collected (control, no biochar amendment; biochar, biochar amended in the field after 2 or 7 years). The results showed that biochar decreased N2O emissions by 48% in acidic soils and by 22% in alkaline soils compared to those in control. These results were explained by decreases in nitrifier denitrification- (by 74%) and heterotrophic denitrification-derived N2O production (by 58%), as further evidenced by a decrease in NO2- (by 87%) and the (nirK+nirS+fungal nirK):(nosZ-I + nosZ-II) ratio (by 5%) in both greenhouse vegetable soils. However, biochar increased nitrifier nitrification-derived N2O in both soils because of increases in pH and PAO, which were attributed to an increased abundance of AOB rather than AOA. The contribution of AOB to PAO (or N2O) exceeded 69% (or 68%) of the total in acidic soil and 88% (or 85%) of the total in alkaline soil after biochar amendment. Our findings demonstrated that the mitigation of N2O by biochar is linked to specific N2O production pathways.

Biochar can mitigate methane emissions by improving methanotrophs for prolonged period in fertilized paddy soils.

Biochar application to fertilized paddy soils has been recommended as an effective countermeasure to mitigate methane (CH4) emissions, but its mechanism and effective duration has not yet been adequately elucidated. A laboratory incubation experiment was performed to gain insight into the combined effects of fresh and six-year aged biochar on potential methane oxidation (PMO) in paddy soils with ammonium or nitrate-amendment. Results showed that both ammonium and nitrate were essential for CH4 oxidation though high ammonium (4 mM) inhibited PMO as compared to low ammonium (1 mM and 2 mM), and that nitrate was better in promoting PMO than ammonium. Moreover, ammonium-amendment promoted type I pmoA, and nitrate-amendment enhanced type II pmoA abundance. Both fresh and aged biochar increased PMO as well as nitrification by enhancing the total, type I and type II methanotrophs as compared to the control. Increased soil PMO with mineral N input in both six-year aged biochar and fresh biochar amendment, indicating that biochar mitigated CH4 by promoting PMO for prolonged period in fertilized paddy soils.