Elevated plant colonisation shifts soil microbes towards faster‐growing decomposition‐oriented communities

Colonisation by previously absent plants is increasing under global environmental change. Colonist establishment can alter plant community composition and could subsequently affect soil microbial communities. Using a 9‐year seed addition experiment, we ask: (i) how do colonists influence plant community composition; and (ii) do these vegetation changes drive shifts in soil prokaryote and fungal communities?

We introduced 23 colonist species, divided into ‘fast’ and ‘slow’ resource acquisition strategies, into two grassland sites that differed in successional age and community composition. We manipulated local disturbance and colonist seed additions and examined how site and colonist characteristics influenced microbial community composition. Plant and microbial communities were surveyed 9 years after initial seed addition.

Colonist cover was higher in the early succession site. Across both sites, evidence indicated that higher colonist cover led to communities with faster growth strategies, altered belowground resource acquisition strategies, and—at the early succession site—lowered taxonomic diversity. Through these changes in plant community composition, higher colonist cover was associated with less diverse prokaryote communities characterised by elevated metabolic activity and cellular replication, and a higher abundance of fungal saprotrophs.

Additional associations between colonists and microbial composition depended on site characteristics and colonist characteristics. Colonist cover was only weakly linked to other microbial functional groups at the early succession site, with no observed links to obligate fungi, despite colonist abundance being highest there. At the late succession site, higher cover of fast‐growing colonists was associated with prokaryote communities exhibiting lower metabolic activity and greater obligate fungal abundance. Also at the late succession site, plots with more colonist seed additions supported higher prokaryote diversity and greater abundance of fungal obligates than unsown plots, regardless of current plant composition.

Synthesis : Our findings indicate that elevated plant colonisation drives shifts towards faster‐growing, decomposition‐oriented microbial communities. However, associations between plant colonists and microbial communities were stronger at the late succession site, even though colonist cover was higher in the early succession site. Practitioners should therefore consider belowground impacts of plant colonisation across all grasslands, and not only where effects on plant community composition are most visible.