Long term farming systems affect soils potential for N 2 O production and reduction processes under denitrifying conditions

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Soil Biology and Biochemistry, ISSN: 0038-0717, Vol: 114, Page: 31-41

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Hans-Martin Krause; Cécile Thonar; Wolfram Eschenbach; Reinhard Well; Paul Mäder; Sebastian Behrens; Andreas Kappler; Andreas Gattinger
Elsevier BV
Immunology and Microbiology; Agricultural and Biological Sciences
article description
N 2 O is a potent greenhouse gas with an atmospheric lifetime of 114 years which also contributes to ozone layer destruction. Mitigating N 2 O emissions is especially challenging to the agricultural sector that is responsible for the majority of anthropogenic N 2 O release. In order to develop effective mitigation strategies, a detailed understanding of drivers for N 2 O production and reduction in agriculturally managed soils is needed. Denitrification is recognized as one of the most important source processes for N 2 O emissions from soils. However, the last step in denitrification, the reduction of N 2 O to N 2 is the only known sink for N 2 O 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 N 2 O 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 NH 4 15 NO 3 equivalent to a moderate fertilization event in the field with 40 kg N ha −1. In order to assess soil's potential for N 2 O production and reduction, we combined direct measurements of denitrification end products N 2 O and N 2 with molecular analysis of functional denitrifying communities involved in NO 2 − and N 2 O 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 NO 3 − consumption rates. Results revealed increased NO 3 − consumption and greatest potential for N 2 O emissions in BIOORG as a result of increased soil organic carbon contents. Production of N 2 was similar in BIOORG and CONMIN and significantly lower in NOFERT, most likely due to significantly decreased pH inhibiting N 2 O reduction. This caused the greatest N 2 O/(N 2 O + N 2 ) ratios in NOFERT (0.88 ± 0.02) followed by BIOORG (0.79 ± 0.01) and CONMIN (0.68 ± 0.02) (p < 0.001). Lowest N 2 O/(N 2 O + N 2 ) ratios in CONMIN were reflected by lowest N 2 O emissions and coincided with elevated nosZ transcript copies in the beginning of incubation. Although highest N 2 O 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 N 2 O production and reduction processes during denitrification.