Mineral and organic fertilisation influence ammonia oxidisers and denitrifiers and nitrous oxide emissions in a long-term tillage experiment

Abstract

Nitrous oxide (N2O) emissions from different agricultural systems have been studied extensively to understand the mechanisms underlying their formation. While a number of long-term field experiments have focused on individual agricultural practices in relation to N2O emissions, studies on the combined effects of multiple practices are lacking. This study evaluated the effect of different tillage [no-till (NT) vs. conventional plough tillage (CT)] in combination with fertilisation [mineral (MIN), compost (ORG), and unfertilised control (CON)] on seasonal N2O emissions and the underlying N-cycling microbial community in one maize growing season. Rainfall events after fertilisation, which resulted in increased soil water content, were the main triggers of the observed N2O emission peaks. The highest cumulative emissions were measured in MIN fertilisation, followed by ORG and CON fertilisation. In the period after the first fertilisation CT resulted in higher cumulative emissions than NT, while no significant effect of tillage was observed cumulatively across the entire season. A higher genetic potential for N2O emissions was observed under NT than CT, as indicated by an increased (nirK + nirS)/(nosZI + nosZII) ratio. The mentioned ratio under NT decreased in the order CON > MIN > ORG, indicating a higher N2O consumption potential in the NT-ORG treatment, which was confirmed in terms of cumulative emissions. The AOB/16S ratio was strongly affected by fertilisation and was higher in the MIN than in the ORG and CON treatments, regardless of the tillage system. Multiple regression has revealed that this ratio is one of the most important variables explaining cumulative N2O emissions, possibly reflecting the role of bacterial ammonia oxidisers in minerally fertilised soil. Although the AOB/16S ratio aligned well with the measured N2O emissions in our experimental field, the higher genetic potential for denitrification expressed by the (nirK + nirS)/(nosZI + nosZII) ratio in NT than CT was not realized in the form of increased emissions. Our results suggest that organic fertilisation in combination with NT shows a promising combination for mitigating N2O emissions; however, addressing the yield gap is necessary before incorporating it in recommendations for farmers.