Relationships between nitrogen cycling microbial community abundance and composition reveal the indirect effect of soil pH on oak decline

Tree decline is a global concern and the primary cause is often unknown. Complex interactions between anthropomorphically generated fluctuations in nitrogen and acidifying compounds have been proposed as factors destabilising nutrient availability and decreasing stress tolerance in oak trees. Microbial communities are crucial in soil nitrogen (N) transformations, yet little is known about the relationships between soil N cycling and tree health. Here, we combined high throughput sequencing, qPCR analysis of key nitrification and denitrification functional genes with soil chemical analyses to characterise ammonia-oxidising bacteria (AOB), archaea (AOA) and denitrifying communities in soils associated with symptomatic (decline) and asymptomatic oak trees (Quercus robur and Q. petraea). We showed that asymptomatic oak trees positively correlated with an increase in AOB abundance that is driven positively by soil pH. Conversely, no relationship was found between AOA abundance and tree health; however AOA abundance was driven by lower concentrations of NH4+, supporting niche differentiation with AOA and low soil NH4+ concentration. Denitrifier abundance was influenced primarily by soil C:N ratio, and correlations with AOB regardless of tree health. These findings indicate that amelioration of soil acidification by balancing C:N may affect AOB abundance driving N transformations, reducing stress on declining oak trees.