SOIL-NITROGEN MINERALIZATION POTENTIAL IN A FERTILIZED LOBLOLLY-PINE PLANTATION

Nitrogen deficiency is the most common nutrient deficiency in natural pine (Pinus spp.) stands and plantations of the southern USA. This study was conducted to identify a soil test of available N that correlates well with tree growth volume response to N fertilization. We evaluated the utility of 98-d aerobic incubation for assessing changes in soil N availability resulting from fertilization of a loblolly pine (P. taeda L.) plantation. Results were compared with results from a 30-d incubation and correlated with tree volume growth. Seven months after fertilization with 224 kg N ha-1 as urea, significant increases (P > 0.0312) in N-mineralization potential (N-phi) were found in surface (0-15 cm) soils, but not subsurface (15-50 cm) soils. The increase was also significant in subsequent incubations up to 16 mo after fertilization. A concurrent short-term aerobic-incubation study did not detect significant increases due to fertilization after 7 mo, illustrating the greater sensitivity of the long-term incubation to differences in available N. No differences in N-phi were attributed to P fertilization at 56 kg ha-1, although significant increases occurred when P was included in nutrient leaching solutions in the incubated soils following CaCl2 extraction for inorganic N. Differences in N-phi occurred among seasons, with maximum mineralization occurring in spring and minimum in autumn. Nitrogen-mineralization potential correlated with 2-yr tree volume growth, with the best correlation at the spring sampling date (r = 0.81, P > 0.0002). The linear relationship between volume growth and N-phi was improved with the addition of tree stand stocking as a covariate (r = 0.85). Nitrogen-mineralization potential was also correlated with soil Kjeldahl N, organic C, and exchangeable acidity.