Forest ecosystem recovery following disturbance is often constrained by nitrogen (N) availability. Biological nitrogen fixation (BNF) can replenish N, but in temperate forests BNF is transitory following disturbance. A critical but unanswered question is how much of the N required for biomass accumulation by N-fixing trees is supplied by BNF, and how the dynamics of BNF correspond to the dynamics of biomass N gain over many decades. We quantified BNF from measures of nodule biomass and activity from 120 stems of Robinia pseudoacacia across 11 stands that ranged in age from 2 to 75 years post-cutting at the Coweeta Hydrologic Laboratory and Nantahala National Forest. We estimated aboveground biomass and biomass N from measures of stem diameter, height and tissue N concentrations applied to allometric equations.
Results/Conclusions
BNF by Robinia pseudoacacia peaked 15 years following disturbance and declined rapidly over time. In contrast, aboveground biomass growth rates increased exponentially and then plateaued after 50 years of recovery. Thus, BNF preceded peak demands for N by several decades, indicating that soil N accounts for most of the biomass N in old trees. In addition, the relative investment in BNF (BNF/aboveground biomass) declined exponentially over time, indicating that trees less than 5 years old allocate the most to BNF. Our findings suggest that BNF is a significant source of N for R. pseudoacacia less than 5 years old, but soils become an increasingly important N source for building biomass as trees age. The dynamics of BNF and N cycling closely correspond to changes in resource availability and forest community dynamics following disturbance.