Carryover Effects of Fertilization and Weed Control Treatments on Soil Respiration and Organic Matter Decomposition in a Second Rotation Pinus Taeda (L.) Stand Growing on a Florida Spodosol.

A replicated (n=3), randomized, complete block design study was established in north Florida on a poorly drained Ultic Alaquod to examine the carryover effects of first rotation fertilization and weed control treatments on soil respiration and organic matter decomposition in a second rotation, juvenile loblolly pine (P. taeda L.) stand.  The first rotation carryover treatments were: control (C_C), fertilizer only (C_F), weed control only (C_W), and fertilizer+ weed control (C_FW). Soil respiration was measured repeatedly for over one year using four PVC collars installed in each treatment plot. Two collars were placed on the mineral soil to estimate total soil respiration (Rs), and the remaining two collars were placed in the mineral soil of two trenched sites (50 cm deep) to estimate heterotrophic soil respiration (Rh). At age 2 years, the mean Rs for both the historical fertilization (C_F, 4.56 µg CO₂/cm²/sec) and the combined fertilizer+weed control treatments (C_FW, 4.49 µg CO₂/cm²/sec) increased by 29% and 27%, respectively, compared to the control (C_C, 3.53 µg CO₂/cm²/sec). The historical weed control treatment (C_W, 2.97 µg CO₂/cm²/sec), however, had significantly lower mean Rh compared to the C_F (4.02 µg CO₂/cm²/sec) and C_FW (3.66 µg CO₂/cm²/sec) treatments. Soil temperature explained 33% of the variation in Rs, and 41% of the variation in Rh. Soil moisture had no significant effect on Rs, and explained only 2% of the total variation in Rh. Significantly higher decomposition rates, using a common organic substrate [Betula papyrifera (Marsh.) tongue depressors], was observed in the C_F compared to the C_W treatment (p=0.027). Results suggest that historical fertilization treatments increased soil respiration rates in these young stands, apparently by increasing OM decomposition rates in the next and also potentially because in the previous rotation there were larger pools of forest floor, roots, and dead woody debris that carried over from the previous rotation. In addition, significant correlations were observed between mean Rh and Mehlich III soil extractable Mn (r=0.66, p<0.05), Zn (r=0.63, p<0.05), and Cu (r=0.61, p<0.05) concentrations (0-10 cm), suggesting that micronutrient supply may be an important driver of Rh rates for these nutrient limited soils.