Carbon and nitrogen mass balance during primary succession on Mount St. Helens

Background/Question/Methods

Carbon (C) and nitrogen (N) accumulation in soils is an essential component of ecosystem development during primary succession. Mount St. Helens provides an excellent opportunity to understand development of C and N cycles because researchers have maintained long-term datasets on soil properties, vegetation cover, and trophic interactions since the eruption. A C and N mass balance was constructed for soil and vegetation to gain insight into ecosystem development at the very earliest stages of primary succession. Soil to 1m depth and vegetation were collected and analyzed for C and N content and isotope composition.

Results/Conclusions

Soil carbon content ranged from 90.5 to 811.8 kg C ha ^-1 and was positively associated with vegetation diversity. Soil δ¹³C ranged from ‑27.2 to ‑19.7‰, consistent with input from C₃ vegetation. Soil N content ranged from 1.7 to 57.4 kg N ha^‑1 across cover types. Soil δ¹⁵N ranged from ‑2.7 to ‑1.0‰ for all depths in vegetated sites, suggesting that N₂-fixation may be the dominant source of N input. Barren sites had δ¹⁵N from -5.2‰ to -1.5‰, indicating that atmospheric N deposition may have influenced soil development. Plant C content ranged from 196.2 to 1018.6 kg C ha^‑1, with the largest storage in moss. Mean vegetation δ¹³C was ‑27.6 to ‑27.0‰ for vegetated and barren sites, respectively. Nitrogen content ranged from 4.4 to 28.3 kg N ha^‑1 and was positively associated with Lupinus lepidus cover. Plant δ¹⁵N averaged ‑2.9% in sites with L. lepidus dominance. Mean plant δ¹⁵N was ‑4.2‰ in sites without L. lepidus cover. Our results indicate that N deposition contributes to ecosystem N balance in the very earliest stages of succession, while development of N₂-fixing communities rapidly accelerates C and N accumulation.