PLANT AND SOIL RESPONSES TO CHRONIC NITROGEN ADDITIONS AT THE HARVARDFOREST, MASSACHUSETTS

Data are presented on changes in plant and soil processes in two fores t types (red pine plantation and oak-maple forest) at the Harvard Fore st, Petersham, Massachusetts, in response to 3 yr of chronic N fertili zation. The hardwood stand exhibited greater N limitation on biologica l function than the pine stand prior to fertilization as evidenced by a lower het N mineralization rate, nearly undetectable rates of net ni trification, and very low foliar N content. N additions were made in s ix equal applications throughout the growing season, and consisted of 5 and 15 g. m-2 . yr- 1 of N as ammonium nitrate. The pine stand showe d larger changes than the hardwood stand for extractable N, foliar N, nitrification, and N leaching loss. Retention of added N was essential ly 100% for all but the high application pine plot from which signific ant N leaching occurred in the 3rd yr of application. From 75 to 92% o f N added to fertilized Plots was retained in the soil, with larger fr actions retained in the hardwood stand than the pine stand for all tre atments. As hypothesized, the stands are exhibiting highly nonlinear p atterns of nitrogen output in response to continuous nitrogen inputs. The implications of this nonlinearity for regional eutrophication of s urface waters and atmospheric deposition control policy are discussed.