Nitrogen Mineralization in a Pine Plantation Fifteen Years After Harvesting and Site Preparation

Intensive site preparation for forest tree planting may result in a mid‐rotation decline in soil N availability. Such decline has not been fully documented. This study was conducted in a loblolly pine ( Pinus taeda L.) plantation in the Piedmont of North Carolina to evaluate the effects of nutrient removal during harvest and site preparation on N availability at mid‐rotation. Treatments, installed in 1981, consisted of a combination of harvest (stem‐only vs. whole‐tree) and site preparation (chop and burn vs. shear, pile, and disk), with a split‐plot of vegetation control (no herbicide vs. herbicide). In 1995 net N mineralization was examined by monthly in situ soil incubations from May through November (7 mo). Net N mineralization was approximately 3 times lower at mid‐rotation than shortly after treatment. A 5°C drop in soil temperature at 10‐cm depth helped explain ≈50% of this decline. At mid‐rotation, harvest intensity, but not site preparation intensity, affected N mineralization, with stem‐only harvest plots mineralizing 11 kg N ha −1 more than whole‐tree harvest plots during the seven months. Chop–burn–no herbicide plots mineralized 34(±3) kg N ha −1 , chop–burn–herbicide: 30(±3) kg N ha −1 , shear–pile–disk–herbicide: 28(±3) kg N ha −1 , and shear–pile–disk–no herbicide: 19(±3) kg N ha −1 in the seven months. Mid‐rotation mineralization was positively correlated with soil temperature and negatively correlated with soil P and soil C:N ratio. The effect of harvest on N mineralization was probably exerted through P nutrition, whereas the lack of site preparation effects suggested that large nutrient removals that occurred with shearing and piling did not have lasting and negative effects on N availability in this plantation.