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 such as biomass accumulation, soil organic matter dynamics, and nitrogen cycling, recovery of these processes after abandonment is insufficiently understood. We examined soil carbon and nitrogen pools and nitrogen dynamics for 16 plots on a central Massachusetts sand plain, over 80% of which had been cultivated and subsequently abandoned at least 40 years ago. The two youngest old-field forests, located on sites abandoned 40-60 years prior to our sampling, had the lowest mineral soil carbon content (0-15 cm), 31% less than the average of unplowed soils. Soil carbon concentration and loss-on-ignition were significantly higher in unplowed soils than in all plowed soils, but these differences were offset by the higher bulk density in formerly plowed soils, leading to no significant differences in C content between plowed and unplowed soil. Soil C:N ratios were lower in formerly plowed soils (26.2) than in unplowed soils (28.0). While soil N content was not affected by land-use history or vegetation type, net N mineralization showed much greater variation. In situ August net nitrogen mineralization varied nearly 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 mixed oak stands (1.34-3.11 kg N ha-1 28 day-1). Mineralization was more 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), suggesting that recent disturbance and residual agricultural lime stimulated nitrification. Carbon:nitrogen ratios increased and pH declined with stand age. Higher bulk density, lower loss-on-ignition and C:N ratios, and slightly lower C concentrations in the surface mineral soil are the persistent legacies of agriculture on soil properties. Short-term agricultural use and the low initial C and N concentrations in these sandy soils appear to have resulted in less persistent impacts of agriculture on soil C and N content and N cycling.