Fig. 3. Net soil CO₂ fluxes as affected by soil disturbance and ammonium nitrate fertilizer (100 kg N ha^–1) in mature forests, mid-successional communities, and no-till corn (Zea mays L.) fields at the KBS LTER site. Vertical bars are standard errors of mean (s.e., n = 3 sites x 7 sample dates). Different higher and lowercase letters represent significant differences (P < 0.05) for treatments among sites and among treatments within the same site, respectively.