Microbial N Transformations and N2O Emission after Simulated Grassland Cultivation: Effects of the Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate (DMPP).

Grassland cultivation can mobilize large pools of N in the soil, with the potential for N leaching and N2O emissions. Spraying with the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) before cultivation was simulated by use of soil columns in which the residue distribution corresponded to plowing or rotovation to study the effects of soil-residue contact on N transformations. DMPP was sprayed on aboveground parts of ryegrass and white clover plants before incorporation. During a 42-day incubation, soil mineral N dynamics, potential ammonia oxidation (PAO), denitrifying enzyme activity (DEA), nitrifier and denitrifier populations, and N2O emissions were investigated. The soil NO3- pool was enriched with 15N to trace sources of N2O. Ammonium was rapidly released from decomposing residues, and PAO was stimulated in soil near residues. DMPP effectively reduced NH4+ transformation irrespective of residue distribution. Ammonia-oxidizing archaea (AOA) and bacteria (AOB) were both present, but only the AOB amoA transcript abundance correlated with PAO. DMPP inhibited the transcription of AOB amoA genes. Denitrifier genes and transcripts (nirK, nirS, and clades I and II of nosZ) were recovered, and a correlation was found between nirS mRNA and DEA. DMPP showed no adverse effects on the abundance or activity of denitrifiers. The 15N enrichment of N2O showed that denitrification was responsible for 80 to 90% of emissions. With support from a control experiment without NO3- amendment, it was concluded that DMPP will generally reduce the potential for leaching of residue-derived N, whereas the effect of DMPP on N2O emissions will be significant only when soil NO3- availability is limiting. IMPORTANCE: Residue incorporation following grassland cultivation can lead to mobilization of large pools of N and potentially to significant N losses via leaching and N2O emissions. This study proposed a mitigation strategy of applying 3,4-dimethylpyrazole phosphate (DMPP) prior to grassland cultivation and investigated its efficacy in a laboratory incubation study. DMPP inhibited the growth and activity of ammonia-oxidizing bacteria but had no adverse effects on ammonia-oxidizing archaea and denitrifiers. DMPP can effectively reduce the potential for leaching of NO3- derived from residue decomposition, while the effect on reducing N2O emissions will be significant only when soil NO3- availability is limiting. Our findings provide insight into how DMPP affects soil nitrifier and denitrifier populations and have direct implications for improving N use efficiency and reducing environmental impacts during grassland cultivation.

Activity of Type I Methanotrophs Dominates under High Methane Concentration: Methanotrophic Activity in Slurry Surface Crusts as Influenced by Methane, Oxygen, and Inorganic Nitrogen.

Livestock slurry is a major source of atmospheric methane (CH), but surface crusts harboring methane-oxidizing bacteria (MOB) could mediate against CH emissions. This study examined conditions for CH oxidation by in situ measurements of oxygen (O) and nitrous oxide (NO), as a proxy for inorganic N transformations, in intact crusts using microsensors. This was combined with laboratory incubations of crust material to investigate the effects of O, CH, and inorganic N on CH oxidation, using CH to trace C incorporation into lipids of MOB. Oxygen penetration into the crust was 2 to 14 mm, confining the potential for aerobic CH oxidation to a shallow layer. Nitrous oxide accumulated within or below the zone of O depletion. With 10 ppmv CH there was no O limitation on CH oxidation at O concentrations as low as 2%, whereas CH oxidation at 10 ppmv CH was reduced at ≤5% O. As hypothesized, CH oxidation was in general inhibited by inorganic N, especially NO, and there was an interaction between N inhibition and O limitation at 10 ppmv CH, as indicated by consistently stronger inhibition of CH oxidation by NH and NO at 3% compared with 20% O. Recovery of C in phospholipid fatty acids suggested that both Type I and Type II MOB were active, with Type I dominating high-concentration CH oxidation. Given the structural heterogeneity of crusts, CH oxidation activity likely varies spatially as constrained by the combined effects of CH, O, and inorganic N availability in microsites.

Inhibition of methane oxidation in a slurry surface crust by inorganic nitrogen: an incubation study.

Livestock slurry is an important source of methane (CH). However, depending on the dry matter content of the slurry, a floating crust may form where methane-oxidizing bacteria (MOB) and CH oxidation activity have been found, suggesting that surface crusts may reduce CH emissions from slurry. However, it is not known how MOB in this environment interact with inorganic nitrogen (N). We studied inhibitory effects of ammonium (NH), nitrate (NO), and nitrite (NO) on potential CH oxidation in a cattle slurry surface crust. At headspace concentrations of 100 and 10,000 ppmv, CH oxidation was assayed at salt concentrations up to 500 mM. First-order rate constants were used to evaluate the strength of inhibition. Nitrite was the most potent inhibitor, reducing methanotrophic activity by up to 70% at only 1 mM NO. Methane-oxidizing bacteria were least sensitive to NO, tolerating up to 30 mM NO at 100 ppmv CH and 50 mM NO at 10,000 ppmv CH without any decline in activity. The inhibition by NH increased progressively, and no range of tolerance was observed. Methane concentrations of 10,000 ppmv resulted in 50- to 100-fold higher specific CH uptake rates than 100 ppmv CH but did not change the inhibition patterns of N salts. In slurry surface crusts, MOB maintained activity at higher concentrations of NH and NO than reported for MOB in soils and sediments, possibly showing adaptation to high N concentrations in the slurry environment. Yet it appears that the effectiveness of surface crusts as CH sinks will depend on inorganic N concentrations.

Catch Crop Residues Stimulate N2O Emissions During Spring, Without Affecting the Genetic Potential for Nitrite and N2O Reduction.

Agricultural soils are a significant source of anthropogenic nitrous oxide (N2O) emissions, because of fertilizer application and decomposition of crop residues. We studied interactions between nitrogen (N) amendments and soil conditions in a 2-year field experiment with or without catch crop incorporation before seeding of spring barley, and with or without application of N in the form of digested liquid manure or mineral N fertilizer. Weather conditions, soil inorganic N dynamics, and N2O emissions were monitored during spring, and soil samples were analyzed for abundances of nitrite reduction (nirK and nirS) and N2O reduction genes (nosZ clade I and II), and structure of nitrite- and N2O-reducing communities. Fertilization significantly enhanced soil mineral N accumulation compared to treatments with catch crop residues as the only N source. Nitrous oxide emissions, in contrast, were stimulated in rotations with catch crop residue incorporation, probably as a result of concurrent net N mineralization, and O2 depletion associated with residue degradation in organic hotspots. Emissions of N2O from digested manure were low in both years, while emissions from mineral N fertilizer were nearly absent in the first year, but comparable to emissions from catch crop residues in the second year with higher precipitation and delayed plant N uptake. Higher gene abundances, as well as shifts in community structure, were also observed in the second year, which were significantly correlated to NO 3 - availability. Both the size and structure of the nitrite- and N2O-reducing communities correlated to the difference in N2O emissions between years, while there were no consistent effects of management as represented by catch crops or fertilization. It is concluded that N2O emissions were constrained by environmental, rather than the genetic potential for nitrite and N2O reduction.