The End of Rotation Effects of Weed Control and Fertilization On the Carbon and Nitrogen Dynamics of a Slash and Loblolly Pine Forest in North Central Florida.

The effects of fertilization, weed control, and fertilization+weed control on vegetation and soil carbon (C) and nitrogen (N) pools were examined for a loblolly pine (Pinus taeda) and slash pine (Pinus elliottii) forest growing on a Florida Spodosol.  C and N pools were also compared for the loblolly forest at 18 and 25-26 years of age (the end of rotation).  The total C accumulated in fertilized forests without weed control was 22% (slash pine) and 39% (loblolly pine) greater than in the control forests at the end of rotation.  Weed control alone increased pine C content at 18 years, but by the end of rotation weed control effectively resulted in no gain in ecosystem C and even subtracted from the net C benefit produced by fertilization when the two treatments were combined.  This result occurred because of decreased forest floor and soil C in the weed control plots.  Fertilization significantly increased forests N pools, and N retention was 63% and 103% of the applied N in the slash and loblolly pine forests, respectively.  Although weed control with fertilization reduced ecosystem N retention efficiency; weed control alone did not negatively affect ecosystem N accumulation.  These results suggest that the optimal treatment for increasing C accumulation and N retention in these ecosystems is fertilization without weed control.