Relatively little is known about the herb layer’s contributions to nitrogen cycling in temperate forests and how these contributions change under different tree species known to differ in their association with soil N availability, elevated nitrogen deposition, or differing land use history. This study a) quantified nitrogen reductase activity (NRA) of two tree species (Acer saccharum vs A. rubrum) and common herbaceous species directly surrounding them and b) assessed how these assemblages affect the concentration of nitrogen in the soil water beneath these trees. Data was collected from 72 plots in four watersheds (WS) at the Fernow Experimental Forest in West Virginia. Watersheds were paired by their different levels of nitrogen deposition (unfertilized vs fertilized, 50 yr-old stands), stand age (50 yr vs ~100 yr), and stand composition (references watershed, both ~100 yr). Nine pairs of circular plots around Acer saccharum and A. rubrum trees were established in each watershed. In each plot, NRA assays were conducted in the field using foliage of the maples and common herbaceous species during the summer of 2018. Starting in September 2018, soil water was collected monthly from a tension lysimeter in each plot and analyzed colorimetrically for nitrate and ammonium concentrations.
Results/Conclusions
Preliminary analyses showed that NRA was significantly (P<0.05) lower under red (2.7 mmol g-1h-1) than sugar maple (14 mmol g-1h-1) in the reference watershed pair; that NRA was higher (21.5 mmol g-1h-1) in watersheds with younger trees (3 and 7) irrespective of species; and that fertilization had no effect on either red or sugar maple NRA. Of the understory species, Viola spp. and Christmas fern responded to fertilization with significantly higher NRA. These results indicate that, while trees seem unable to respond to additional N inputs, herbaceous plants may and thus likely play a key role in N retention. Soil water nitrate concentrations in the two reference watersheds were low (2.7 ppm, SD=1.2), regardless of tree species. The younger unfertilized watershed (WS7) had higher values: (8 ppm, SD=3.5) than the older watersheds without differences between lysimeters under red and sugar maple. In the fertilized watershed, nitrate concentrations were 19.8 ppm (SD=2.0) under sugar maples whereas soil water under red maples did respond to fertilization (6.4 ppm, SD=0.3). The absence of a tree species effect appears to contradict previous findings on soil N availability near red and sugar maple but may also reflect the season of data collection (Sep-Dec).