| In forest plantations, the period from harvest through replanting is when soils are most subject to changes in nutrient availability. Soil nitrogen (N) dynamics following harvest are commonly characterized by increased mineralization rates and extractable mineral N levels. These effects have been attributed to several factors including increased decomposition of forest floor and harvest residues from the previous rotation, increased soil temperature and moisture, and reduced N uptake caused by tree removal. More recently, it has been hypothesized that higher available N levels may result from reduced microbe immobilization due to lower levels of available carbon (C) from fresh litter inputs and root exudates following harvest. Thus, heterotrophic soil microbes, which are mainly responsible for N immobilization-mineralization, may be limited by energy sources and may not require as much N as before the harvest.;In loblolly pine plantations of the Southeast US, N and phosphorus (P) fertilizers are commonly used to increase wood production, which is realized in part by increasing the amount of foliage. Through litterfall, this foliage accumulates in the forest floor forming significant C and nutrient pools. The objectives of this project were to determine if post-harvest retention of the forest floor and its incorporation into the mineral soil could affect the magnitude and timing of N supply to the subsequent stand, and to examine if C limitations in the soil microbial population may be linked to these dynamics.;Forest floor decomposition and nutrient release, mineral soil C, N, and P pools, and foliar nutrition and tree growth of the regenerating stand were examined following harvest in a loblolly pine plantation on the Coastal Plain of North Carolina. Treatments included three forest floor retention levels (0, 15, and 30 Mg ha-1) combined with two levels of incorporation (mixed, non-mixed) in a factorial design. After two years, the forest floor lost 84% and 78% of its mass, 80% and 69% of its N content, and 85% and 79% of its P content from the control and doubled treatments, respectively, using the 0 retention treatment as a reference. Total C and N pools in the mineral soil increased 20 and 21% respectively, and available C, N, and P pools increased 46, 47, and 49% respectively, by doubling the forest floor. A post-harvest flush of available soil N was observed throughout the first two growing seasons and doubling the forest floor caused a full year delay in peak N availability as compared to the removed treatment. The incorporation treatment had a transient effect, with available C, N, and P pools showing significantly higher levels only during the first month of sampling. Tree growth was not affected by forest floor retention treatments, but it was affected by the incorporation treatment showing 17 % more volume growth in the mixed treatment by year 3. In general, foliar nutrient concentrations increased at year 1 as compared to initial levels, but decreased to initial levels by year 3.;A laboratory experiment that measured the microbial respiration response to addition of C and water showed higher respiration responses to C additions from soils of the removed treatment, as compared to the control and the doubled treatments. Furthermore, additions of C decreased the extractable N, across field treatments and sampling dates, by 94% as compared to additions of water, confirming the strong control that C availability exerts on N release. |
