1. Eucalyptus regnans grows rapidly from seed after wildfires, out-competing other species, thereby
forming pure stands of mature forests that rank amongst the world’s most carbon dense. By global
standards, these forests grow on infertile soils. It is unclear how E. regnans is able to obtain large
amounts nitrogen (N) from these infertile soils to support its rapid growth after fire.
2. We measured carbon (C) and N stored in plant biomass and photosynthetic rates of E. regnans
2 years after a wildfire and examined whether E. regnans stimulated its own N supply through rootinduced
increases in microbial decomposition and N mineralization. We compared microbial biomass,
gross N mineralization rates and soil C in trenched and rooted plots.
3. Photosynthetic rates of E. regnans seedlings were high and comparable to photosynthetic rates
observed in fertilized crops. Presence of roots of E. regnans and allied microflora enhanced gross N
mineralization more than fivefold compared to soil without roots present. Soil microbial biomass
was more than doubled by root presence. The soil N pulse caused by the fire and N mineralization
rates in the absence of roots were too small to account for the large amount of N stored in E. regnans
2 years after the fire.
4. Our results suggest that E. regnans facilitated its rapid growth by enhancing microbial activity
and N mineralization. This enhanced microbial activity also contributed to a substantial loss of soil
C (~62% of carbon gained in plant biomass was concurrently lost from soil).
5. Synthesis. At the ecosystem scale, the synergistic effects of plant growth and soil N mineralization
need to be carefully assessed against costs to soil C for forests regenerating after disturbance.