Legacies of agriculture and forest regrowth in the nitrogen of old-field soils

In the Carolina Piedmont of the USA, agricultural and forest management in the 19th and 20th centuries has greatly altered soil organic nitrogen (N). The objective of this study is to evaluate effects of two centuries of land use on N in upland Piedmont soils that are derived from the region's most common bedrock, granitic gneiss. Effects of agriculture on total soil N wer e examined by comparing soils cropped mainly for cotton since about 1800 wi th soils that remained under hardwood forest without cultivation or fertili zation. Effects of forest regrowth on the N of old-field soils were examine d in eight permanent plots resampled on seven occasions from 1962 to 1997 a t the Calhoun Experimental Forest in South Carolina. Together, the soil-comparative study and the four-decade field experiment i llustrate how soil N in the southern Piedmont has been altered by agricultu ral management during the 19th and 20th centuries. Not only have agricultur al harvests removed considerable N from Piedmont soils, but soil organic ma tter has been enriched in N by agricultural fertilization, a practice that has now contributed greatly to N cycles of many old-field forests in the re gion. In old-field pine stands (Pinus taeda) at the Calhoun Experimental Forest, 40 years of forest growth accumulated 366 kg ha(-1) of N (CV=9.3%) in tree biomass and 740 kg ha(-1) (CV=9.7%) in forest floor between planting in 195 7 and the last sampling in 1997. In the four decades, mineral-soil N was di minished by 823 kg ha(-1) (CV=39.5%), a reduction in N accompanied by subst antial decreases in mineralizable N as well. On the other hand, N accretion in the whole forest ecosystem averaged 5.9 kg ha(-1) per year over this pe riod (significant at a probability of <0.07), an accretion attributed mainl y to atmospheric N deposition rather than N-2 fixation, Despite the N accre tion and legacy of agricultural fertilization, the 40-year-old Calhoun fore st has grown into a state of acute N deficiency. Future N research should i nclude support for a network of long-term field studies which investigates N dynamics in forest floor and logging slash, and estimates N-use and N-ret ention efficiencies of fertilized pine-forest ecosystems. (C) 2000 Elsevier Science B.V. All rights reserved.