Disturbance mediated changes in nitrogen cycling in the New Jersey Pinelands

Background/Question/Methods

Disturbance events such as fire and insect defoliation have profound effects on the storage and movement of nitrogen in ecosystems, and are both prevalent disturbances in the New Jersey Pinelands (NJPL). The present study aims to quantify the impact and interaction of these disturbance events on nitrogen dynamics in the NJPL. To assess fire disturbance, 20 total plots were established: 10 from an area that would not be burned and 10 that would be in the area of a scheduled prescribed burn. To assess defoliation disturbance, 5 plots were chosen from gypsy moth (Lymantria dispar) defoliation events that occurred in 2006, 2007, and 2008. We determined pools of nitrogen present in disturbed areas by taking soil cores in burn plots the summer before the burn, immediately before and after the burn, and three months after the burn. Soil cores were taken from one oak-dominated and one pine-dominated area in each defoliation plot. To determine nitrogen availability, mixed bed resins were interred at each burn site for 5 months before the fire, and replaced by new resins immediately after the fire in 3 month intervals at the burn sites and interred and retrieved in 5 month intervals in the defoliation sites.

Results/Conclusions

Available nitrate is significantly higher in pine dominated areas than oak dominated areas within the defoliation plots (p = .049), but available ammonium does not differ between pine and oak dominated areas.  Plots defoliated in 2006 and 2007 demonstrated a 64% and 62% decrease, respectively, in available ammonium compared to plots defoliated in 2008, indicating a transient increase in available ammonium after insect defoliation. Data on fire mediated changes in available ammonium and nitrate is forthcoming pending a scheduled prescribed burn in late February 2012. Some of our burn sites are also in areas of past defoliation events, which will provide a unique insight to the interactions of these two disturbance events as well as their effects individually on nitrogen cycling, which has not yet been examined in this ecosystem.