Short-term changes in soil C, N, and biota following harvesting and regeneration of loblolly pine (Pinus taeda L.)

Abstract In affiliation with the USDA-FS long-term soil productivity program, a series of studies have been established in the US gulf coast region to monitor the effects of intensive silviculture on site productivity. This report presents early results of a study of the interactive effects of harvest intensity and cultural treatments on soil C, N, and biological processes following the regeneration of two stands of loblolly pine (Pinus taeda L.), a 19-year-old stand in St. Helena Pa., LA and a 27-year-old stand in Tyler Co., TX. Two harvesting intensities (MWT, mechanical whole-tree and HFBO, hand felled bole-only removed) were combined in a factorial assignment with bedding and herbaceous weed control (at St. Helena) or bedding and fertilization (at Tyler). Total C and N in 0–15 cm of soil were determined before, 9 and 21 months after harvesting. Total C and N in 0–60 cm were measured 2 years after harvesting at St. Helena and 3 years after harvesting at Tyler. At Tyler, N mineralization, soil respiration, and microbial populations were monitored before and for 2 years after harvesting. In the 19-year-old stand, MWT removed 67% of the above-ground biomass and 38% of the above-ground N compared to 46% and 10% for HFBO. In the 27-year-old stand, MWT removed 62% of the biomass and 35% of the N, while HFBO removed 48% and 13%. Harvesting method had no effect on total C or total N in surface soil but bedding resulted in higher levels of both 1 years after harvesting. The effect of bedding was still detected 3 years after treatment at Tyler but not at St. Helena. Herbaceous weed control at St. Helena had no effect on total N in the surface 60 cm of soil but significantly reduced total C at 30–60 cm 6 months after treatment. Sixteen months after application of 250 kg ha−1 of diammonium phosphate (DAP) at Tyler, the surface 60 cm of mineral soil in fertilized plots averaged 373 kg ha−1 more total N than did soil from unfertilized plots. Total C in the surface 60 cm was not effected by fertilization. Irrespective of harvesting or cultural treatment, total C and total N in the surface 15 cm of soil declined at both sites during the first year after harvesting. The declines were 8.2 Mg ha−1 total C, 361 kg ha−1 total N at St. Helena and 7.6 Mg ha−1 total C, 380 kg ha−1 total N at Tyler. After 2 years, total C and total N had returned to >90% of pre-harvest levels at both locations. Harvesting intensity had no effect on soil temperature, soil respiration, N mineralization, or microbial populations, but bedding significantly increased soil respiration and N mineralization during the first growing season after harvesting. Net N mineralization was not effected by treatments during the second year after harvesting, but was lower in the harvested plots than in the unharvested reference and inversely related to the pre-harvest rate. Fertilization with DAP increased mineral N flux for 2 months following application, but to a lessor extent than did bedding. Nitrification was significantly greater than ammonification in all treatments during the first year after harvesting but was equal to or less than ammonification during the second growing season and in unharvested plots. In these US gulf coast pine stands, harvesting and regeneration resulted in significant but transitory perturbation in soil processes. All of the processes studies returned to or near pre-harvest levels after two growing seasons. Differences in surface biomass removal, between whole-tree and bole-only harvesting, had no measurable effect on the monitored soil processes during the first two growing seasons after harvesting although differences may appear later in the rotation. Harvesting, per se, resulted in an increase in nitrification but no increase in net N mineralization. Bedding incorporated surface organic matter, accelerated microbial activity, and increased net N mineralization but before planted pine seedlings could benefit from the increased N availability. A prolonged release of nutrients from the undisturbed surface residues may be more beneficial to productivity of the pine regeneration than the rapid mineralization which followed bedding.