Gross N2O Production Process, Not Consumption, Determines the Temperature Sensitivity of Net N2O Emission in Arable Soil Subject to Different Long-Term Fertilization Practices

nirS and nosZII bacterial denitrifiers as well as fungal denitrifiers are coupled with N2O emissions in long-term fertilized soils

Fertilizer application plays a critical role in soil fertility and crop yield and has been reported to significantly affect soil denitrification. However, the mechanisms by which denitrifying bacteria (nirK, nirS, nosZI, and nosZII) and fungi (nirK and p450nor) affect soil denitrification are poorly understood. Therefore, in this study, we investigated the effect of different fertilization treatments on the abundance, community structure, and function of soil denitrifying microorganisms in an agricultural ecosystem with long-term fertilization using mineral fertilizer or manure and their combination. The results showed that the application of organic fertilizer significantly increased the abundance of nirK-, nirS-, nosZI-, and nosZII-type denitrifying bacteria as the soil pH and phosphorus content increased. However, only the community structure of nirS- and nosZII-type denitrifying bacteria was influenced by the application of organic fertilizer, which led to a higher contribution of bacteria to nitrous oxide (N2O) emissions than that observed after inorganic fertilizer application. The increase in soil pH reduced the abundance of nirK-type denitrifying fungi, which may have presented a competitive disadvantage relative to bacteria, resulting in a lower contribution of fungi to N2O emissions than that observed after inorganic fertilizer application. The results demonstrated that organic fertilization had a significant impact on the community structure and activity of soil denitrifying bacteria and fungi. Our results also highlighted that after organic fertilizer application, nirS- and nosZII-denitrifying bacteria communities represent likely hot spots of bacterial soil N2O emissions while nirK-type denitrifying fungi represent hot spots for fungal soil N2O emissions.

Biochar amendments and climate warming affected nitrification associated N2O and NO production in a vegetable field

Soil nitrification driven by ammonia-oxidizing microorganisms is the most important source of nitrous oxide (N₂O) and nitric oxide (NO). Biochar amendment has been proposed as the most promising measure for combating climate warming; both have the potential to regulate the soil nitrification process. However, the comprehensive impacts of different aged biochars and warming combinations on soil nitrification-related N₂O and NO production are not well understood. Here, 1-octyne and acetylene were used to investigate the relative contributions of ammonia-oxidizing bacteria (AOB) and archaea (AOA) to potential nitrification-mediated N₂O and NO production from the fertilized vegetable soil with different aged biochar amendments and soil temperatures in microcosm incubations. Results demonstrated that AOB dominated nitrification-related N₂O and NO production across biochar additions and climate warming. Biochar amendment did not significantly influence the relative contribution of AOB and AOA to N₂O and NO production. Field-aged biochar markedly reduced N₂O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N₂O yield while fresh- and lab-aged biochar produced negligible effects on AOB-dependent N₂O yield. Climate warming significantly increased N₂O production and AOB-dependent N₂O yield but less so on NO production. Notably, the relative contribution of AOB to N₂O production was enhanced by climate warming, whereas AOB-derived NO showed the opposite tendency. Overall, the results revealed that field-aged biochar contributed to mitigating warming-induced increases in N₂O and NO production via inhibiting AOB-amoA gene abundance and AOB-dependent N₂O yield. Our findings provided guidance for mitigating nitrogen oxide emissions in intensively managed vegetable production under the context of biochar amendments and climate warming.

Assembly of root-associated N2O-reducing communities of annual crops is governed by selection for nosZ clade I over clade II

The rhizosphere is a hotspot for denitrification. The nitrous oxide (N2O) reductase among denitrifiers and nondenitrifying N2O reducers is the only known N2O sink in the biosphere. We hypothesized that the composition of root-associated N2O-reducing communities when establishing on annual crops depend on soil type and plant species, but that assembly processes are independent of these factors and differ between nosZ clades I and II. Using a pot experiment with barley and sunflower and two soils, we analyzed the abundance, composition, and diversity of soil and root-associated N2O reducing communities by qPCR and amplicon sequencing of nosZ. Clade I was more abundant on roots compared to soil, while clade II showed the opposite. In barley, this pattern coincided with N2O availability, determined as potential N2O production rates, but for sunflower no N2O production was detected in the root compartment. Root and soil nosZ communities differed in composition and phylogeny-based community analyses indicated that assembly of root-associated N2O reducers was driven by the interaction between plant and soil type, with inferred competition being more influential than habitat selection. Selection between clades I and II in the root/soil interface is suggested, which may have functional consequences since most clade I microorganisms can produce N2O.

Plants Mitigate Nitrous Oxide Emissions from Antibiotic-Contaminated Agricultural Soils

Vegetable production systems are hotspots of nitrous oxide (N₂O) emissions and antibiotic pollution. However, little is known about the interconnections among N₂O emissions, vegetable growth, and antibiotic contamination. To understand how plants regulate N₂O emissions from enrofloxacin (ENR)-contaminated soils, in situ N₂O emissions were measured in pot experiments with cherry radish and pakchoi. Gross N₂O production and consumption processes were discriminated based on an acetylene inhibition experiment. Results indicated that vegetable growth decreased the cumulative N₂O flux from 0.71 to -0.29 kg ha^-1 and mitigated the ENR-induced increase in N₂O emissions. Radish displayed better mitigation of N₂O emissions than pakchoi. By combining the analysis of N₂O flux with soil physicochemical and microbiological properties, we demonstrated that growing vegetables could either promote gross N₂O consumption or decrease gross N₂O production, primarily by interacting with soil nitrate, clade II nosZ (nosZII)-carrying bacteria, and Deinococcus-Thermus. ENR inhibited N₂O consumption more than N₂O production, with the nosZII-carrying bacteria, represented by Gemmatimonadetes, as the main inhibition target. However, increasing nosZII-carrying bacteria by growing radish offsets the inhibitory effect of ENR. These findings provide new insights into N₂O emissions and antibiotic pollution in vegetable-soil ecosystems and broaden the options for mitigating N₂O emissions.