The importance of in-stream uptake for regulating stream concentrations and outputs of N and P from a forested watershed: evidence from long-term chemistry records for Walker Branch Watershed

Abstract

Long-term, weekly measurements of streamwater nitrogen and phosphorus concentrations in the West Fork of Walker Branch, a 1st order forested stream in eastern Tennessee, were used to assess the importance of in-stream processes for controlling stream concentrations and watershed exports. Over the period from 1991 to 2002, there was a slight declining trend in watershed export of dissolved inorganic N via streamflow, despite relatively high and constant wet N deposition rates (5 kg/ha/y). The watershed retains >90% of N deposition inputs. Concentrations of NO 3 À and soluble reactive phosphorus (SRP) showed distinct seasonal patterns with autumn and early spring minima and summer maxima. An end-member mixing analysis indicated that these seasonal concentration patterns were largely a result of seasonal variations in in-stream uptake processes, with net uptake of NO 3 À and SRP having the greatest impact on streamwater concentrations in November (reductions of 29 lg N/l and 2.5 lg P/l, respectively). This was likely a result of high rates of uptake by microbes colonizing new inputs of leaf detritus. For NO 3 À there was a secondary peak in net uptake in March and April (about 9 lg N/l) resulting from increased uptake by stream algae and bryophytes. Summer was a period of net release of NO 3 À to streamwater (peaking at 9 lg N/l in July) and minimal net effects on SRP concentrations. On average, in-stream processes resulted in removal of about 20% of the NO 3 À and 30% of the SRP entering the stream from the catchment annually. This study, as well as other recent work, suggests that in-stream processes are important buffers on stream nutrient concentrations and exports reducing the effects of changes in inputs and retention in terrestrial portions of watersheds.

Assimilatory uptake rather than nitrification and denitrification determines nitrogen removal patterns in streams of varying land use

“…We recognize that lab assays of nitrification and denitrification were measured under optimum redox conditions, but note that inorganic N availability represented ambient conditions. Additionally, whole-stream NO { 3 uptake may be underestimated because we used short-term nutrient additions that increased NO { 3 concentrations and thus could have saturated demand (Mulholland et al 2002). Therefore, comparing these rates represents a maximum estimate of the contribution of nitrification and denitrification to whole-stream NO { 3 dynamics.…”

“…Although we calculate assimilatory U 2NO3 as the fraction of total NO { 3 removal not accounted for by denitrification, this fraction may not be solely attributable to assimilatory demand. Other processes, such as dissimilatory NO Long-term fate of assimilated N-Although assimilatory N uptake is only temporary, it slows the downstream flux of NO { 3 by removing it from the water column and transforming it into a particulate organic form (Mulholland 2004). Previous studies have rarely considered the ultimate fate of this assimilated N, but a synthesis of denitrification studies indicates the removal of assimilated N following remineralization and coupled nitrification/ denitrification when water residence times are long (Seitzinger et al 2006).…”

Nitrogen and Phosphorus Attenuation within the Stream Network of a Coastal, Agricultural Watershed

“…Williams et al (2004) developed a watershed mass balance for the Ipswich River basin and determined that the stream network retained 9% of the total N entering the river. Mulholland (2004) found that a 300-m reach of Walker Branch, Tennessee, decreased annual NO 3 2 and PO 4 32 inflow by 20 and 30%, respectively. Several reaches of the Neversink River, New York, were a yearround net sink for NO 3 2 , attenuating 3 to 29% of the nutrient load entering them (Burns, 1998).…”

“…In a study of watershed stream networks in the northeastern USA, it was estimated that 76% of the N entering the stream networks may have been permanently removed via denitrification or temporarily retained through biotic sequestration (Seitzinger et al, 2002). Alteration of nutrient concentrations by riverine processes during transport also changes the timing of nutrient delivery and the quality (coarse vs. fine particulate organic matter and organic vs. inorganic dissolved nutrients) of nutrients exported to downstream ecosystems (Meyer and Likens, 1979;Mulholland, 2004).…”

The saturation of N cycling in Central Plains streams: 15N experiments across a broad gradient of nitrate concentrations

“…In-stream processes influence the transport, retention, and removal of N from the landscape (Peterson et al 2001;Mulholland 2004;Bernhardt et al 2005). Within the context of larger river networks, low-order streams can have a disproportionately large impact on the rate at which N is retained and attenuated within streams (Alexander et al 2000).…”