Spatial patterns of soil nitrification and nitrate export from forested headwaters in the northeastern United States

Abstract

[1] Nitrogen export from small forested watersheds is known to be affected by N deposition but with high regional variability. We studied 10 headwater catchments in the northeastern United States across a gradient of N deposition (5.4 À 9.4 kg ha À1 yr À1 ) to determine if soil nitrification rates could explain differences in stream water NO 3 À export. Average annual export of two years (October 2002 À export was positively related to nitrification rates (r 2 = 0.34, p = 0.04) and the relative nitrification (r 2 = 0.37, p = 0.04). These relationships were much improved by restricting consideration to the 6 watersheds with a higher number of rate measurements (59-130) taken in transects parallel to the streams (r 2 of 0.84 and 0.70 for the nitrification rate and relative nitrification, respectively). Potential nitrification rates were also a better predictor of NO 3 À export when data were limited to either the 6 sampling points closest to the watershed outlet (r 2 = 0.75) or sampling points <250 m from the watershed outlet (r 2 = 0.68). The basal area of conifer species at the sampling plots was negatively related to NO 3 À export. These spatial relationships found here suggest a strong influence of near-stream and near-watershed-outlet soils on measured stream NO 3 À export.

The role of wetland coverage within the near‐stream zone in predicting of seasonal stream export chemistry from forested headwater catchments

“…Furthermore, although there was a correlation between catchment wetland proportion and near‐stream zone wetland proportion, the catchments that deviated from this relationship also showed the biggest deviations in NO 3 ‐N dynamics, further suggesting that differences in the near‐stream zone are important for predicting patterns of stream NO 3 ‐N concentrations. These observations are consistent with the field‐based observations by Ross et al (), where stream concentrations were more related to measurements taken near the stream than across the whole catchment. It is also important to note that the relationship between wetland proportion and NO 3 ‐N concentrations is much stronger in upland dominated catchments, because wetland‐dominated catchments generally have low and less variable NO 3 ‐N concentrations.…”

Nitrate in watersheds: Straight from soils to streams?

“…High soil solution nitrate often signals a progression toward N saturation and reduced N retention efficiency [ Magill et al ., ; Perakis and Sinkhorn , ], yet this signal of N saturation is dampened in receiving streams, suggesting that some processes that decrease nitrate concentration—such as nitrate removal, transformation, or dilution—are enhanced within the flow paths along which nitrate flows from below the rooting zone in the uplands to the stream channel. Our results are consistent with the hypothesis that stream and riparian processes are responsible for a significant fraction (up to 50%) of nitrogen removal in ecosystems [ Alexander et al ., ; Galloway et al ., ; Seitzinger et al ., ] and previous studies that have recognized the contribution of groundwater [ Gold et al ., ; Mayer et al ., ], riparian [ Gold et al ., ; McClain et al ., ; Tabacchi et al ., ; Ross et al ., ], hyporheic [ Crenshaw et al ., ; McClain et al ., ], and instream processing [ Bernhardt et al ., ] as important sinks for nutrients. To better understand and partition uptake, transformation, and dilution, we need more studies collecting data along these flow paths from upslope to stream [ Band et al ., ; Fisher et al ., ; Grimm et al ., ; Mayer et al ., ; Barnes and Raymond , ].…”

Riparian Corridors: A New Conceptual Framework for Assessing Nitrogen Buffering Across Biomes

“…Net rates of nitrification can be calculated from the changes of nitrate pool sizes over the course of in situ incubation of soils samples in polyethylene bags for several weeks (Eno, 1960;Malone et al, 2018) or even a day (Ross et al, 2006). Such measurement of net nitrification is only a potential rate (Hart et al, 1994) and can be interpreted as a proxy of potential nitrate contribution to the subjacent soil or adjacent stream (Ross et al, 2012). Therefore, the potential for nitrogen retention potential (plant and microbial uptake) and/or removal (denitrification) of the riparian system can be inferred from the difference between gross ammonification and net nitrification.…”