Long term farming systems affect soils potential for N₂O production and reduction processes under denitrifying conditions

N₂O is a potent greenhouse gas with an atmospheric lifetime of 114 years which also contributes to ozone layer destruction. Mitigating N₂O emissions is especially challenging to the agricultural sector that is responsible for the majority of anthropogenic N₂O release. In order to develop effective mitigation strategies, a detailed understanding of drivers for N₂O production and reduction in agriculturally managed soils is needed. Denitrification is recognized as one of the most important source processes for N₂O emissions from soils. However, the last step in denitrification, the reduction of N₂O to N₂ is the only known sink for N₂O in soil. Although the impact of single parameters on denitrification is quite well documented, there is still a knowledge gap when it comes to the impact of complex farming systems on N₂O production and reduction. In this experiment, we incubated soil samples from the DOK long term field trial in Therwil/Switzerland comparing organic (BIOORG) and conventional (CONMIN) farming systems with an a non-fertilized control (NOFERT). Soil samples were incubated under 90% WFPS after fertilization with NH₄¹⁵NO₃ equivalent to a moderate fertilization event in the field with 40 kg N ha⁻¹. In order to assess soil's potential for N₂O production and reduction, we combined direct measurements of denitrification end products N₂O and N₂ with molecular analysis of functional denitrifying communities involved in NO₂⁻ and N₂O reduction on DNA and mRNA levels. In order to monitor N cycling processes under the chosen conditions, stable isotope tracing was employed to quantify nitrification and NO₃⁻ consumption rates. Results revealed increased NO₃⁻ consumption and greatest potential for N₂O emissions in BIOORG as a result of increased soil organic carbon contents. Production of N₂ was similar in BIOORG and CONMIN and significantly lower in NOFERT, most likely due to significantly decreased pH inhibiting N₂O reduction. This caused the greatest N₂O/(N₂O + N₂) ratios in NOFERT (0.88 ± 0.02) followed by BIOORG (0.79 ± 0.01) and CONMIN (0.68 ± 0.02) (p < 0.001). Lowest N₂O/(N₂O + N₂) ratios in CONMIN were reflected by lowest N₂O emissions and coincided with elevated nosZ transcript copies in the beginning of incubation. Although highest N₂O emissions in BIOORG were detected, the incubation setup cannot directly be translated to field conditions. Nevertheless, our results emphasize that farming system induced changes on soil geochemical parameters like soil pH and soil organic carbon affect microbial N₂O production and reduction processes during denitrification.