Nitrogen cycling and availability of two forest ecosystems in the Upper Peninsula of Michigan

Nitrogen cycling in northern temperate forests has received considerable attention because available N levels often limits tree growth on many sites. The objectives of my study were: (1) to evaluate N cycling and availability in two second-growth northern hardwood stands in relation to stand net primary production in the Upper Peninsula of Michigan, (2) to evaluate N cycling and availability in two young red pine plantations which were established after clearcutting previous hardwood stands, (3) to develop a process model for soil N mineralization, and (4) to use this model to simulate N mineralization in both the hardwood stands and red pine plantations.; Net N mineralization and nitrification rates in two northern hardwood stands were significantly different in the surface mineral soil horizons (0-10 cm) (p {dollar}{dollar} 0.05). Other aspects of N cycling, such as throughfall N inputs, and forest floor and mineral soil leaching losses, were generally not different between the hardwood stands or the plantations. Clear seasonal trends of net N mineralization and nitrification in both the forest floor and surface mineral soils were obvious in both stand types. Soil temperature and organic matter C:N ratio were major factors related to the seasonal variations, but available water was not. A significant relationship between stand NPP and soil N availability was found in the hardwood stands, but not in the red pine plantations. This indicated that soil N availability limited the productivity in the hardwood stands, but did not limit red pine growth.; Agreement was found between the N mineralization model's results and field observations in both stand types. The model also adequately described the seasonal trends of observed net nitrification values, but agreement was not as good as for seasonal N mineralization. The results of sensitivity analysis showed that increasing temperature and organic N content increased N mineralization and nitrification rates in both the red pine plantations and the hardwood stands. The sensitivity of the model to fluctuations in soil moisture and C:N ratio was different for the forest floor and the mineral soil; the model was most sensitive to the moisture changes in the forest floor. In contrast, C:N ratio was influential in the mineral soil, but had little effect on N mineralization in the forest floor.; Using this model, net N mineralization and nitrification rates in the forest floor and surface mineral soil (0-15 cm) were simulated for six years (May 1986 to April 1992) in both the hardwood stands and the red pine plantations. The model indicated that converting northern hardwood stands to red pine plantations significantly decreased net N mineralization and nitrification rates. Over a six-year period N mineralization rates in the forest floor decreased by 55%, while nitrification rates decreased by 43%. Net N mineralization and nitrification rates in the surface mineral soil (0-15 cm) also decreased after the establishment of red pine plantations, but the effect was smaller. Changes in C:N ratios, litterfall, and fine roots after timber harvesting were major factors causing the decrease of N mineralization and nitrification rates in the forest floor and the surface mineral soil.