Long-term effects of weed control and fertilization on the carbon and nitrogen pools of a slash and loblolly pine forest in north-central Florida

The effects of fertilization, weed control, and fertilization plus weed control on vegetation and soil C and N pools were examined for a loblolly pine (Pinus taeda L.) and slash pine (Pinus elliottii var. elliottii Engelm.) forest at ages 18 and 26 years (at the end of rotation). The total C accumulated in fertilized forests without weed control was 20% (slash pine) and 40% (loblolly pine) greater than in the control forests at the end of rotation. Weed control increased pine C pools at 18 years, but by the end of rotation, weed control effectively resulted in no gain in ecosystem C. When the two treatments were combined, weed control slightly subtracted from the net C benefit produced by fertilization. This result occurred because of decreased forest floor and soil C in the weed control plots. Fertilization significantly increased stem, foliage, forest floor, and soil N pools, and N retention was 63% and 103% of the applied N in the slash and loblolly pine forests, respectively. Weed control with fertilization reduced ecosystem N retention efficiency, but 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.