Disturbance, Nitrogen Availability, and Nitrogen Losses in an Intensively Managed Loblolly Pine Plantation

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Abstract
The interactive effects of harvest intensity, site preparation, and herbicide treatment were evaluated in a clear—cut Piedmont site in North Carolina. Forest harvesting caused increased nitrogen mineralization and nitrification in all treatments, but harvesting without additional treatment had little effect on nitrate—nitrogen pool sizes and losses. The removal of most surface organic material during intensive site preparation led to greatly increased nitrate pool sizes and losses. Treatment with herbicide accentuated this effect, and in combination such removals and herbicide applications led to accumulations of nitrate—nitrogen in surface soil of 27 and 24 kg/ha in the first and second summers following site preparation. Nitrogen losses by leaching, denitrification, and erosion were also greatest in the plots where organic residues had been removed and herbicides applied. These results suggest that microbial immobilization controlled nitrogen pool sizes and losses, and this suggestion was confirmed using 15N. Soils from the plots without residue removal or herbicide treatment immobilized >90% of added 15N in 28 d, while those from the residue removal/herbicide treated plots immobilized <70%. Microbial activity was the predominant process regulating nitrogen availability and losses following disturbance in this site; site—preparation practices that removed important substrates for microbial metabolism caused elevated nitrogen losses.