Inter-Rotational Effects of Fertilization and Weed Control On Growth and Soil Nutrient Availability in a 2-Year-Old Loblolly Pine Plantation in North Florida.

Long term sustainability of intensive forest management systems depends on their ability to maintain site productivity over multiple rotations without deteriorating inter-rotational soil nutrient supply and retention. Two randomized complete block experiments, each consisting of three replications, were established on the same site (Ultic alaquods) as the previous rotation using a common full-sib loblolly pine family.  The original treatments were: control (C), fertilizer only (F), weed control only (W), and fertilizer+ weed control (FW).  One experiment was actively retreated as in the previous rotation, while the second was left untreated to examine carryover effects on growth dynamics and soil nutrient availability.

The actively retreated FW plots had significantly higher aboveground biomass accumulation (10.3 Mg/ha) than the F (6.9 Mg/ha), W (5.8 Mg/ha), and C (3.6 Mg/ha) treatments.  In the untreated carryover plots, the F treatment had the highest aboveground biomass accumulation (8.4 Mg/ha) compared to the FW (4.6 Mg/ha), C (4.5 Mg/ha), and W (3.8 Mg/ha) treatment plots. In addition, early growth results from the second rotation showed an increase in height among all treatments in both experiments (1.6 to 2.2-fold in actively retreated and 1.1 to 3.3-fold in untreated carryover plots) relative to the first rotation.  While advanced genetics, elevated atmospheric CO₂, and improved site preparation (understory mulching, Panicum control, Rhyacionia control) may have contributed to the higher overall growth responses in the second rotation, the significantly higher relative yields for the untreated F carryover plots were also associated with significantly higher levels (p=0.003) of soil P availability compared to the other treatments. These results suggest that the understory vegetation in the untreated F carryover plots likely served as an important P sink or facilitated P retention in organic matter pools in the first rotation which then subsequently became a P source (through mineralization) in the second rotation.