Assessing the Linkages between Tree Species Composition and Stream Water Nitrate in a Reference Watershed in Central Appalachia
| Authors: | Mark B. Burnham, Martin J. Christ, Mary Beth Adams, William T. Peterjohn |
| Year: | 2021 |
| Type: | Scientific Journal |
| Station: | Northern Research Station |
| DOI: | https://doi.org/10.3390/f12081116 |
| Source: | Forests |
Abstract
Many factors govern the flow of deposited nitrogen (N) through forest ecosystems and into stream water. At the Fernow Experimental Forest in WV, stream water nitrate (NO3-) export from a long-term reference watershed (WS 4) increased in approximately 1980 and has remained elevated despite more recent reductions in chronic N deposition. Long-term changes in species composition may have altered forest N demand and the retention of deposited N. In particular, the abundance and importance value of Acer saccharum have increased since the 1950s, and this species is thought to have a low affinity for NO3-. We measured the relative uptake of NO3- and ammonium (NH4+) by six important temperate broadleaf tree species and estimated stand uptake of total N, NO3-, and NH4+. We then used records of stream water NO3- and stand composition to evaluate the potential impact of changes in species composition on NO3- export. Surprisingly, the tree species we examined all used both mineral N forms approximately equally. Overall, the total N taken up by the stand into aboveground tissues increased from 1959 through 2001 (30.9 to 35.2 kg N ha-1 yr-1). However, changes in species composition may have altered the net supply of NO3- in the soil since A. saccharum is associated with high nitrification rates. Increases in A. saccharum importance value could result in an increase of 3.9 kg NO3--N ha-1 yr-1 produced via nitrification. Thus, shifting forest species composition resulted in partially offsetting changes in NO3- supply and demand, with a small net increase of 1.2 kg N ha-1 yr-1 in NO3- available for leaching. Given the persistence of high stream water NO3- export and relatively abrupt (~9 year) change in stream water NO3- concentration circa 1980, patterns of NO3- export appear to be driven by long-term deposition with a lag in the recovery of stream water NO3- after more recent declines in atmospheric N input.