SOIL CARBON AND NITROGEN IN A PINE-OAK SAND PLAIN IN CENTRAL MASSACHUSETTS - ROLE OF VEGETATION AND LAND-USE HISTORY

Over the last 150 years much of the landscape of eastern North America has been transformed from predominantly agricultural lands to forest. Although cultivation strongly affects important ecosystem processes s uch as biomass accumulation, soil organic matter dynamics, and nitroge n cycling, recovery of these processes after abandonment is insufficie ntly understood. We examined soil carbon and nitrogen pools and nitrog en dynamics for 16 plots on a central Massachusetts sand plain, over 8 0% of which had been cultivated and subsequently abandoned at least 40 years ago. The two youngest old-field forests, located on sites aband oned 40-60 years prior to our sampling, had the lowest mineral soil ca rbon content (0-15 cm), 31% less than the average of unplowed soils. S oil carbon concentration and loss-on-ignition were significantly highe r in unplowed soils than in all plowed soils, but these differences we re offset by the higher bulk density in formerly plowed soils, leading to no significant differences in C content between plowed and unplowe d soil. Soil C:N ratios were lower in formerly plowed soils (26.2) tha n in unplowed soils (28.0). While soil N content was not affected by l and-use history or vegetation type, net N mineralization showed much g reater variation. In situ August net nitrogen mineralization varied ne arly 40-fold between stand types: lowest in pitch pine and white pine stands (-0.13 and 0.10 kg N ha(-1) 28 day(-1)), intermediate in scrub oak stands (0.48 kg N ha(-1) 28 day(-1)) and highest in aspen and mixe d oak stands (1.34-3.11 kg N ha(-1) 28 day(-1)). Mineralization was mo re strongly related to present vegetation than to land-use history or soil N content. Appreciable net nitrification was observed only in the most recently abandoned aspen plot (0.82 kg N ha(-1) 28 day(-1)), sug gesting that recent disturbance and residual agricultural lime stimula ted nitrification. Carbon:nitrogen ratios increased and pH declined wi th stand age. Higher bulk density, lower loss-on-ignition and C:N rati os, and slightly lower C concentrations in the surface mineral soil ar e the persistent legacies of agriculture on soil properties. Short-ter m agricultural use and the low initial C and N concentrations in these sandy soils appear to have resulted in less persistent impacts of agr iculture on soil C and N content and N cycling.