Soil pH is a strong regulator for activity as well as for size and composition of denitrifier communities. Low pH not only lowers overall denitrification rates but also influences denitrification kinetics and gaseous product stoichiometry. N2O reductase is particularly sensitive to low pH which seems to impair its activity post-transcriptionally, leading to higher net N2O production. Little is known about how complex soil denitrifier communities respond to pH change and whether their ability to maintain denitrification over a wider pH range relies on phenotypic redundancy. In the present study, we followed the abundance and composition of an overall and transcriptionally active denitrifier community extracted from a farmed organic soil in Sweden (pHH2O = 7.1) when exposed to pH 5.4 and drifting back to pH 6.6. The soil was previously shown to retain much of its functioning (low N2O/N2 ratios) over a wide pH range, suggesting a high functional versatility of the underlying community. We found that denitrifier community composition, abundance and transcription changed throughout incubation concomitant with pH change in the medium, allowing for complete reduction of nitrate to N2 with little accumulation of intermediates. When exposed to pH 5.4, the denitrifier community was able to grow but reduced N2O to N2 only when near-neutral pH was reestablished by the alkalizing metabolic activity of an acid-tolerant part of the community. The genotypes proliferating under these conditions differed from those dominant in the control experiment run at neutral pH. Denitrifiers of the nirS-type appeared to be severely suppressed by low pH and nirK-type and nosZ-containing denitrifiers showed strongly reduced transcriptional activity and growth, even after restoration of neutral pH. Our study suggests that low pH episodes alter transcriptionally active populations which shape denitrifier communities and determine their gas kinetics.

Rising atmospheric CO concentrations are expected to increase nitrous oxide (N O) emissions from soils via changes in microbial nitrogen (N) transformations. Several studies have shown that N O emission increases under elevated atmospheric CO (eCO ), but the underlying processes are not yet fully understood. Here, we present results showing changes in soil N transformation dynamics from the Giessen Free Air CO Enrichment (GiFACE): a permanent grassland that has been exposed to eCO , +20% relative to ambient concentrations (aCO ), for 15 years. We applied in the field an ammonium-nitrate fertilizer solution, in which either ammonium (NH4+) or nitrate (NO3-) was labelled with N. The simultaneous gross N transformation rates were analysed with a N tracing model and a solver method. The results confirmed that after 15 years of eCO the N O emissions under eCO were still more than twofold higher than under aCO . The tracing model results indicated that plant uptake of NH4+ did not differ between treatments, but uptake of NO3- was significantly reduced under eCO . However, the NH4+ and NO3- availability increased slightly under eCO . The N O isotopic signature indicated that under eCO the sources of the additional emissions, 8,407 μg N O-N/m during the first 58 days after labelling, were associated with NO3- reduction (+2.0%), NH4+ oxidation (+11.1%) and organic N oxidation (+86.9%). We presume that increased plant growth and root exudation under eCO provided an additional source of bioavailable supply of energy that triggered as a priming effect the stimulation of microbial soil organic matter (SOM) mineralization and fostered the activity of the bacterial nitrite reductase. The resulting increase in incomplete denitrification and therefore an increased N O:N emission ratio, explains the doubling of N O emissions. If this occurs over a wide area of grasslands in the future, this positive feedback reaction may significantly accelerate climate change.

Organic fertilizers have been shown to stimulate CH4 uptake from agricultural soils. Managing fertilizer application to maximize this effect and to minimize emission of other greenhouse gasses offers possibilities to increase sustainability of agriculture. To tackle this challenge, we incubated an agricultural soil with different organic amendments (compost, sewage sludge, digestate, cover crop residues mixture), either as single application or in a mixture and subjected it to different soil moisture concentrations using different amounts of organic amendments. GHG fluxes and in vitro CH4 oxidation rates were measured repeatedly, while changes in organic matter and abundance of GHG relevant microbial groups (nitrifiers, denitrifiers, methanotrophs, methanogens) were measured at the end of the incubation. Overall the dynamics of the analyzed GHGs differed significantly. While CO2 and N2O differed considerably between the treatments, CH4 fluxes remained stable. In contrast, in vitro CH4 oxidation showed a clear increase for all amendments over time. CO2 fluxes were mostly dependent on the amount of organic residue that was used, while N2O fluxes were affected more by soil moisture. Several combinations of amendments led to reductions of CO2, CH4, and/or N2O emissions compared to un-amended soil. Most optimal GHG balance was obtained by compost amendments, which resulted in a similar overall GHG balance as compared to the un-amended soil. However, compost is not very nutrient rich potentially leading to lower crop yield when applied as single fertilizer. Hence, the combination of compost with one of the more nutrient rich organic amendments (sewage sludge, digestate) provides a trade-off between maintaining crop yield and minimizing GHG emissions. Additionally, we could observe a strong increase in microbial communities involved in GHG consumption in all amendments, with the strongest increase associated with cover crop residue mixtures. Future research should focus on the interrelation of plants, soil, and microbes and their impact on the global warming potential in relation to applied organic amendments.

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