Monday, November 5, 2007 - 2:45 PM
113-6
Timing of Mid-Rotation Silvicultural Operations and Soil Texture Impact Soil Nitrogen and Carbon Dynamics.
Improvements in loblolly pine plantation productivity require refined silvicultural systems that consider soil type, stand development, ecology, and their interactions. On 3 mid-rotation loblolly pine plantations in Louisiana located on a gradient of soil drainage classes, soil and microbial C and N dynamics were measured in response to fertilization and understory vegetation suppression. Treatments consisted of combinations of: (1) N + P fertilizer timing (No fertilizer, 1 year pre-thin, 6 months post-thin), and (2) vegetation suppression (None, complete vegetation suppression). Interactions between fertilizer timing, vegetation control, and soil type have influenced stand N and C dynamics. Pre-thin fertilization treatments significantly increased soil N mineralization at the moderately well and poorly drained sites. This mineralization increase occurred on the moderately well drained site only in tandem with vegetation suppression because the herbicide reduced understory and microbial N immobilization. Pre-thin soil N supply was markedly lower than that observed in the year following thinning. The 6 months post-thin fertilizer treatments increased soil N mineralization at all sites. Soil N mineralization was highest with vegetation suppression due to declines in soil labile C, which in turn reduced microbial N immobilization. Thinning may have had an overriding impact on soil N supply in the year following thinning at the well drained sites because stand N demands did not appreciably increase in tandem with higher stand N supply. Thus, conventional fertilization timing may exceed stand capacity to assimilate applied N on such sites.
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