Excess nitrogen (N) in terrestrial ecosystems can arise from increased atmospheric N deposition, a phenomenon common in eastern US forests. In spite of decreased anthropogenic N emissions, atmospheric concentrations of reactive N remain high in areas within this region. Excess N in forests has been shown to alter biogeochemical cycling of essential plant nutrients primarily via enhanced production and leaching of NO3-, which leads to loss of base cations from the soil. The purpose of our study was to investigate this phenomenon using a multifaceted approach to examine foliar nutrients of two herbaceous layer species in one N-treated watershed (WS3—receiving aerial applications of 35 kg N/ha/yr as (NH4)2SO4, from 1989 to the present) and two untreated reference watersheds at the Fernow Experimental Forest, WV, USA. In 1993, we analyzed foliar tissue of Viola rotundifolia, a dominant herb layer species and prominent on all seven sample plots in each watershed. In 2013 and 2014, we used foliar tissue from Rubus allegheniensis, which had become the predominant species on WS3 and had increased, to a lesser extent, in cover on both reference watersheds.
Results/Conclusions
Foliar N and potassium (K) were higher and foliar calcium (Ca) was lower on WS3 than on the reference watersheds for both species. Foliar magnesium (Mg) was lower on WS3 for Viola, but was not different among watersheds for Rubus. Results support the hypothesis that excess N lowers plant-available Ca and, to a lesser degree, Mg, but not K. Foliar manganese (Mn) of Rubus averaged >4 times that of Viola, and was >50% higher on WS3 than on the reference watersheds. Kriging maps revealed increases from 1991 to 2011 in both concentration and spatial heterogeneity in extractable Mn in surface (top 5 cm) soil on WS3. Nitrogen-mediated increases in Rubus cover during this period were decidedly patchy within WS3, the result of heterogeneity in light regime from spatial variation in canopy gap fraction, with higher cover associated with greater gap fraction. Because plots with high Rubus cover/high gap fraction were spatially associated with plots with high soil Mn (and vice versa), we suggest that the increased spatial heterogeneity in surface soil Mn arose from nutrient redistribution via uptake of Mn from throughout the rooting zone and accumulation at high concentrations in foliar tissue, followed by additions of Mn to surface soils via decomposition of high-Mn Rubus litter.