Symptoms of nitrogen saturation in an aggrading forested watershed in western Maryland

Abstract

The objective of this study was to evaluate the nitrogen (N) biogeochemistry of an 18-22 year old forested watershed in western Maryland. We hypothesized that this watershed should not exhibit symptoms of N saturation. This watershed was a strong source of nitrate (NO 3 À ) to the stream in all years, with a mean annual export of 9.5 kg N ha À1 year À1 and a range of 4.4-18.4 kg N ha À1 year À1 . During the 2001 and 2002 water years, wet deposition of inorganic N was 9.0 kg N ha À1 year À1 and 6.3 kg N ha À1 year À1 , respectively. Watershed N export rates in 2001 and 2002 water years were 4.2 kg N ha À1 year À1 and 5.3 kg N ha À1 year À1 , respectively. During the wetter water years of 2003 and 2004, the watershed exported 15.0 kg N ha À1 year À1 and 18.4 kg N ha À1 year À1 , rates that exceeded annual wet deposition of N by a factor of two (7.5 kg N ha À1 year À1 in 2003) and three (5.5 kg N ha À1 year À1 in 2004). Consistent with the high rates of N export, were high concentrations (2.1-3.3%) of N in foliage, wood (0.3%) and fine roots, low C:N ratios in the forest floor (17-24) and mineral soil (14), high percentages (83-96%) of the amount of mineralized N that was nitrified and elevated N concentrations (up to 3 mg N l À1 ) in soil solution. Although this watershed contained a young aggrading forest, it exhibited several symptoms of N saturation commonly observed in more mature forests.

Do indicators of nitrogen retention and leaching differ between coniferous and broadleaved forests in Denmark?

“…When accounting for N accumulation in aboveground biomass of >10 kg N ha À1 yr À1 , the net N loss from the soil amounts to >10-40 kg N ha À1 yr À1 in these stands planted on former arable land. We only identified one site in the literature showing a similar phenomenon; a young aggrading mixed broadleaved forest in a catchment in Maryland, USA with C/Nmin = 14 that leached 15-18 kg N ha À1 yr À1 to the stream (Castro et al, 2007). We have no simple explanation and do not know if this is only a transient phenomenon while more long-term studies are needed to better explain these observations of excess N-out fluxes.…”

Dynamics of Dissolved Forms of Carbon and Inorganic Nitrogen in Small Watersheds of the Coastal Atlantic Forest in Southeast Brazil

“…9). For this comparison, we selected studies conducted in: minimally disturbed forested tropical watersheds, indicated as tropical (Lewis et al 1999;McDowell and Asbury 1994;Neill et al 2006); minimally disturbed temperate watersheds predominantly forested and/or with low N atmospheric inputs, indicated as temperate (low) (Cairns and Lajtha 2005;Campbell et al 2000;Lewis et al 1999;Lewis 2002;Vanderbilt et al 2003); temperate watersheds predominantly forested with some anthropogenic impact and/or with high N atmospheric inputs, indicated as temperate (high) (Boyer et al 2002;Campbell et al 2004;Castro et al 2007); and minimally disturbed watersheds dominated by N 2 -fixing red alder (Alnus rubra), indicated as temperate (red alder) (Compton et al 2003; Fig.9). …”

“…9 Log-log relationship between annual runoff (millimeters per year) and annual export (kilograms per hectare per year) of DIN, NO 3 − , and NH 4 + in the Indaiá stream (present study, IND) and in a set of temperate and tropical watersheds. Data extracted from the following published studies: Temperate (low), minimally disturbed watersheds predominantly forested and/or with low N atmospheric inputs: Cairns and Lajtha (2005), Campbell et al (2000), Lewis (2002), Lewis et al (1999) and Vanderbilt et al (2003); Temperate (high), watersheds predominantly forested with some anthropogenic impact and/or with high N atmospheric inputs: Boyer et al (2002), Campbell et al (2004) and Castro et al (2007); Temperate (red alder), minimally disturbed watersheds predominantly composed of N 2 -fixing red alder (Alnus rubra): Compton et al (2003); Tropical, minimally disturbed forested watersheds: Lewis et al (1999), McDowell and Asbury (1994) and Neill et al (2006) streams. However, there is no evidence of N saturation of the Atlantic Forest.…”

Ecological risks in anthropogenic disturbance of nitrogen cycles in natural terrestrial ecosystems

“…But actual forests do not always follow this model because responses to N load are site-specific. For example, although this model predicts that the risk of N saturation is high at old-growth forests, Castro et al (2007) reported that even young aggrading forests exhibited several symptoms of N saturation. Afforested stands of former arable land also tend to increase nitrate leaching in young stages of forest development (Hansen et al 2007).…”