Long- and Short-Term Changes in Soil Carbon and Nutrient Availability Following Commercial Sawlog Harvest.

The clearcut harvest of Coweeta Hydrologic Lab's WS 7 was undertaken as an interdisciplinary study of watershed level responses to forest management.  Vegetation and soil sampling plots were established on the 59 ha watershed in 1975 and sampled for 2 years before harvest.  Following harvest, 10 soil plots were re-established along with 4 additional plots on adjacent reference WS 2.  Soil sample collections (0-10 and 10-30 cm depths) occurred each year, from 1977 until 1985, then again in 1992, 1993, 1998, and 2008.  Total soil C and N and soil exchangeable Ca, Mg, and K increased significantly only in surface soils for 2 to 3 years after harvest, compared to pre-harvest levels.  Following this increase, soil C and N content returned to pre-harvest levels, while cation content remained elevated.  Laboratory soil incubations from 1977 to 1982 showed higher soil potential N mineralization and nitrification as well as, extractable NO₃-N and NH₄-N concentrations in the harvested watershed than the reference.  WS 7 stream NO₃-N concentrations increased during harvest in 1977, as expected, followed by a decline in 1981.  In 1989, stream NO₃-N concentrations began to increase again and have continued a pattern of increase and decline through the present.  Continued stream NO₃-N export is attributed to changes in N availability due to shifts in the vegetation community during secondary succession.