Activity of alkaline phosphatase of the field-moist soil as a function of sonication time. Data are means ± SD (<i>n</i> = 3).

<p>Activity of alkaline phosphatase of the field-moist soil as a function of sonication time. Data are means ± SD (<i>n</i>  =  3).</p

Activity of alkaline phosphatase of the autoclaved soil and the borax-borate buffer (control) with 50 mU external alkaline phosphatase as functions of sonication time.

<p>Activity of alkaline phosphatase of the autoclaved soil and the borax-borate buffer (control) with 50 mU external alkaline phosphatase as functions of sonication time.</p

Absorbance (410 nm) of supernatants of soil slurries after 300 s sonication at different extraction ratios as a function of power density.

<p>Absorbance (410 nm) of supernatants of soil slurries after 300 s sonication at different extraction ratios as a function of power density.</p

Total bacteria number of field-moist soil as a function of sonication time.

<p>Total bacteria number of field-moist soil as a function of sonication time.</p

Temperature of 100 ml deionized water as a function of sonication power.

<p>Temperature of 100 ml deionized water as a function of sonication power.</p

Temporal in situ synmics of N20 reductase activity as affected by nitrogen fertilization and implications for the N20/(N20+N2) product ratio and N20 mitigation.

In vitro, high nitrate (NO3) concentrations significantly inhibit N2O reductase activity. However, little information is available on the in situ temporal effects of excessive N fertilization on soil N2O reductase activity and the regulation of the N2O/(N2 + N2O) product ratio in agricultural soil. This study examined the monthly in situ dynamics of NO3 − concentration, N2O reductase activity, and N2O/(N2 + N2O) product ratio for 2 years in loamy soil that had received either continuous N fertilizer at 400 kg N ha−1 year−1 for 15 years (N400) or no N fertilizers (CK). N2O reductase activity was significantly lower under the N400 treatment than under the CK and correlated negatively with soil NO3 − concentration. The decrease in N2O reductase activity resulted in the N2O/(N2 + N2O) product ratio increasing. These results demonstrate that excessive N fertilization has the potential to increase N2O emissions by reducing N2O reductase activity in soils. These results highlight the need for N2O mitigation options to embrace the reduction of soil NO3 − concentrations

Irrigation of DOC-rich liquid promotes potential denitrification rate and decreases N2O/(N2O+N2) product ratio in a 0–2 m soil profile

Lack of dissolved organic carbon (DOC) is generally one of the key factors limiting denitrification in subsoil beneath the root zone. Despite a number of laboratory DOC amendment studies, the effects of in situ DOC infiltration on subsoil denitrification, and on subsequent end product composition, are less understood. Here, we report on the effects of in situ infiltration of a DOC-rich liquid, derived from decomposing straw, on potential denitrification rate (PDR), N2O/(N2O + N2) product ratio, and nitrate stock in a 0–2 m soil profile. The results showed that in situ infiltration with a DOC-rich liquid (100 mm, 2 ton DOC ha−1) significantly increased the DOC concentration and PDR, and significantly decreased the N2O/(N2O + N2) product ratio in the soil profile. Up to 70% of the nitrate accumulated in the 0–2 m soil profile disappeared within three weeks following the infiltration of the DOC-rich liquid. The majority of the nitrate removed could be accounted for by denitrification. The predominant end product of denitrification was N2. The mass ratio between the consumed DOC and nitrate-N was about 5. Our results demonstrate the significant potential for removing subsoil nitrate by in situ introduction of DOC generated from the above-ground crop biomass

Perturbation-free measurement of in situ di-nitrogen emissions from denitrification in nitrate-rich aquatic ecosystems

Increased production of reactive nitrogen (Nr) from atmospheric di-nitrogen (N2) has greatly contributed to increased food production. However, enriching the biosphere with Nr has also caused a series of negative effects on global ecosystems, especially aquatic ecosystems. The main pathway converting Nr back into the atmospheric N2 pool is the last step in the denitrification process. Despite several attempts, there is still a need for perturbation-free methods for measuring in situ N2 fluxes from denitrification in aquatic ecosystems at the field scale. Such a method is needed to comprehensively quantify the N2 fluxes from aquatic ecosystems. Here we observed linear relationships between the δ15N-N2O signatures and the logarithmically transformed N2O/(N2+N2O) emission ratios. Through independent measurements, we verified that the perturbation-free N2 flux from denitrification in nitrate-rich aquatic ecosystems can be inferred from these linear relationships. Our method allowed the determination of field-scale in situ N2 fluxes from nitrate-rich aquatic ecosystems both with and without overlaying water. The perturbation-free in situ N2 fluxes observed by the new method were almost one order of magnitude higher than those by the sediment core method. The ability of aquatic ecosystems to remove Nr may previously have been severely underestimated

Electron shuttle potential of biochar promotes dissimilatory nitrate reduction to ammonium in paddy soil

Enhancing dissimilatory nitrate reduction to ammonium (DNRA) is environmentally and agronomically beneficial due to DNRA improving nitrogen (N) retention in soil. However, the rate of DNRA is generally considerably lower than that of denitrification because DNRA requires more electron donors than denitrification. Biochar has been increasingly reported to act as an “electron shuttle” to facilitate electron transfer and to promote redox reactions in soil. Thus, this study aimed to investigate whether and how biochar could enhance the DNRA process in a paddy soil. The results showed that, compared with the no-biochar control, the application of rice straw biochar increased the DNRA rate from 0.2 to 0.7 mg NH4+-N kg−1 dry soil d−1. As well, biochar simultaneously, increased the relative abundance of DNRA functional microbes (nrfA-type microbes) and functional gene (nrfA) expression levels. Biochar's enhancement of DNRA was positively correlated with the biochar properties relevant to electron shuttling (e.g., specific capacitance). In contrast, the application of electron shuttle-weakened biochar (oxidized by H2O2) did not increase, or even decreased, the DNRA rate in the paddy soil. These results demonstrate that biochar can act as an electron shuttle to enhance electron availability for DNRA functional microorganisms and consequently promote the DNRA process in paddy soil. Our results indicate that amendment of paddy soil with biochar containing a high-capacity electron shuttle function is beneficial for preserving N by transforming the mobile nitrate anion into the less mobile ammonium cation in paddy soils

Enhancement of subsoil denitrification using an electrode as an electron donor

Laboratory culture studies have demonstrated that some microbial strains can use electrons generated by electrodes in the denitrification reaction. To test whether the native soil microbiota can use electrode electrons for denitrification, a subsoil slurry was incubated under an electric potential treatment. A potentiostat-poised (−500 mV) electrode served as an electron donor. The electric potential treatment enriches the electroactive denitrifying bacteria and accelerates the nitrate reduction in the subsoil slurry, with N2 as the dominant end product. These results demonstrate that an electrode can serve as an electron donor to enhance the subsoil denitrification. This finding supports the future development of a technique to remove accumulated nitrate in subsoils and reduce nitrate contamination in groundwater