Fertiliser use has multi-decadal effects on microbial diversity and functionality of forest soils

The productivity and sustainability of planted forest ecosystems are dependent on the provision of soil nutrients and nutrient cycling. In turn, these are supported by the microbiome present in soils and their interactions with the wider biotic and abiotic environments (i.e. expression of microbiome x edaphic x management x environment interactions). This study aims to explore the extent of management-induced disruption on the soil microbiome and how this disruption in turn impacts the microbiome's future ability to respond to environmental change. This was tested by determining if land-use prior to afforestation, had an enduring effect on the soil microbial community responses to the management practices typically employed in plantation forestry. Samples were collected from the Berwick long-term site productivity (LTSP) trial at the end of a second rotation of Pinus radiata (similar to 60 years in continuous forestry). Prior to afforestation the land-use had been pastoral farming to support livestock production. Forestry operation treatments associated with the removal of organic matter (tree and residue management) and the use of fertilisers had no significant long-term effect on bacterial or fungal microbial communities. However, when comparisons were made with similar LTSP sites that had no history of pastoral farming history, significant variations in microbiome properties were apparent with significantly higher total abundance of ectomycorrhizal species (ECM) at the site with a pastoral history. We suggest that that ecological memory associated with the initial states of the sites drove the differing response of the microbial communities to the fertiliser additions used to remove nutrient limitations for the growing P. radiata crop. Essentially, the closer a site was to this nutrient threshold, the less the sensitivity of the microbiome; therefore, at the more nutrient rich ex-pasture site the habitat conditions were re-enforced by fertiliser use rather than being changed by it.