Watershed and fire severity are stronger determinants of soil chemistry and microbiomes than within-watershed woody encroachment in a tallgrass prairie system

Fire can impact terrestrial ecosystems by changing abiotic and biotic
conditions. Short fire intervals maintain grasslands and communities
adapted to frequent, low-severity fires. Shrub encroachment that follows
longer fire intervals accumulates fuel and can increase fire severity.
This patchily distributed biomass creates mosaics of burn severities in
the landscape—pyrodiversity. Afforded by a scheduled burn of a watershed
protected from fires for 27 years, we investigated effects of woody
encroachment and burn severity on soil chemistry and soil-inhabiting
bacteria and fungi. We compared soils before and after fire within the
fire-protected, shrub-encroached watershed and soils in an adjacent,
annually burned and non-encroached watershed. Organic matter and nutrients
accumulated in the fire-protected watershed but responded less to woody
encroachment within the encroached watershed. Bioavailable nitrogen and
phosphorus and fungal and bacterial communities responded to high-severity
burn regardless of encroachment. Low-severity fire effects on soil
nutrients differed, increased bacterial but decreased fungal diversity and
effects of woody encroachment within the encroached watershed were
minimal. High-severity burns in the fire-protected watershed led to a
novel soil system state distinct from non-encroached and encroached soil
systems. We conclude that severe fires may open grassland restoration
opportunities to manipulate soil chemistry and microbial communities in
shrub-encroached habitats.