Nitrogen dynamics after site preparation in three loblolly pine plantations on the Virginia Piedmont

Abstract

Intensive site preparation practices and their effect on nitrogen cycling have been implicated as possible causes of productivity declines on forest sites in Australia and New Zealand. This study was initiated in order to determine the effects of site preparation intensity upon N distribution and availability in loblolly pine (Pinus taeda L.) plantations in Virginia. In the summer of 1982, three forest sites at the Reynolds Homestead Research Center on the Virginia Piedmont were clearcut. In the fall of the same year all three sites were prepared for planting using one of the following treatments: 1. shear, rake, disk (S,R,D) (3-passes); 2. shear-disk (S-D) (1-pass); and 3. chop, burn (C,B) (high intensity burn). During March of 1983, 1-0 genetically improved loblolly pine seedlings were planted on all sites. Pine biomass was greatest on the S,R,D area after three growing seasons. Total biomass and N content (NCONT) of native vegetation and forest floor were greatest in the S-D area. Total N in the upper 15 cm of mineral soil was also greatest in the S-D area. Total system N was highest in the S-D area and this treatment is more N-conservative than either of the more intensive treatments. During the third growing season potentially mineralizable N levels were highest on the two disked treatment areas, 157 and 144 kg N/ha for the s-o, and S,R,D areas, respectively. Pine foliar nutrient concentrations determined after the second and third growing seasons provided no evidence of existing or impending nutrient deficiencies. These results show that short-term pine nutrition and growth were not adversely affected by reductions of N capital on these sites. However, if wasteful practices, such as raking and burning with high intensity fires, are also used to establish subsequent stands on these same sites, cumulative losses of N could result in productivity declines.