We analyzed changes in soil nitrogen (N) and in stem growth of three important tree species in a mixed deciduous forest over 10 years (1999-2009) in response to nitrogen fertilization. The study was conducted at the Fernow Experimental Forest, West Virginia, on two watersheds supporting stands of similar age: WS3 (fertilized with ammonium sulfate annually since 1989) and WS7 (untreated). Stem diameter growth was measured in Acer rubrum (ACRU), Liriodendron tulipifera (LITU), and Prunus serotina (PRSE) on each watershed in 1999, 2003, and 2009. Soil was sampled to 5 cm from around the base of 7 stems of each species in each watershed, and analyzed for inorganic N pools. Subsamples were incubated at 25 C to compare relative N flux rates.
Results/Conclusions
In 2009, soil pH was substantially lower in WS 3 than in WS 7 (pH 3.83 and 4.18, respectively). Inorganic soil N pools (nitrate plus ammonium) were significantly higher in WS3 than in WS 7 (P = 0.05); however, total mineralization in incubated samples was 23% lower in WS 3 (P = 0.08). Between 1999 and 2009, total stem growth averaged 35% lower in WS 3 relative to WS 7. In recent years (2003-2009), mean annual stem diameter growth in WS 7 declined an average of 29% for all three species relative to growth in the earlier years (1999-2003). In WS 3, there was a similar decline in growth rate over the same period for PRSE, but not for either ACRU or LITU. Relatively high rates of ambient N deposition in these mid-Appalachian forests may be leading toward convergence of some aspects of soil chemistry in the control watershed (WS 7) with those of the artificially fertilized watershed (WS 3). In addition, divergent growth responses of tree species to soil changes driven by excess N could lead to long-term shifts in species composition.
