Effects of seasonal variation and land cover on riparian denitrification along a mid-sized river

Figures & data

Figure 1. Map of study area including the four sample sites along the Licking River, KY with site 1 in the most urban area.

Table 1. All sites averaged by season ± standard error for denitrification potential (DNF) and regulating factors. Minimum and maximum values in parentheses.

	Potential DNF (ng N g dry soil^−1 hr^−1)	Soil NH4^+–N (mg N kg^−1)	Soil NO3^−–N (mg N kg^−1)	Water NH4^+–N (mg N L^−1)	Water NO3^−–N (mg N L^−1)	Soil organic matter (mg OM g dry soil^−1)	Soil moisture (g water g dry soil^−1)
Summer 2010 – drought	51.4 ± 12.4 (0.01–179)	16.4 ± 5.8 (2.00–115)	6.23 ± 0.99 (0.33–20.3)	0.05 ± 0.01 (0.02–0.16)	0.29 ± 0.06 (0.01–0.91)	53.9 ± 3.62 (29.9–92.5)	0.42 ± 0.03 (0.16–0.80)
Fall 2010 – drought	37.6 ± 6.99 (0.96–150)	22.1 ± 7.3 (1.59–126)	5.40 ± 1.31 (0.01–22.6)	0.04 ± 0.01 (0.01–0.11)	0.31 ± 0.01 (0.01–0.99)	53.6 ± 3.87 (22.4–93.8)	0.34± 0.04 (0.09–0.71)
Winter 2010–11 – flood	153 ± 40.1 (0.28–706)	10.4 ± 5.6 (0.67–118)	6.71 ± 1.56 (0.01–26.2)	0.03 ± 0.01 (0.01–0.13)	1.23 ± 0.13 (0.31–1.96)	58.3 ± 3.43 (37.0–96.5)	0.47 ± 0.04 (0.29–0.93)
Spring 2011 – flood	187 ± 37.2 (0.96–651)	3.44 ± 1.6 (0.44–38.5)	6.20 ± 1.60 (0.01–30.3)	0.05 ± 0.01 (0.01–0.24)	0.64 ± 0.03 (0.43–0.94)	55.7 ± 3.13 (27.4–79.0)	0.41 ± 0.02 (0.15–0.75)

Figure 2. Average seasonal denitrification potential and standard error among study sites. Site 1 had the greatest impervious surface.

Figure 3. Average monthly riparian potential denitrification rate for all sites along the Licking River corresponds with river discharge. River discharge data were collected from USGS gage #03254520 (USGS 2011).

Figure 4. Average seasonal soil organic matter and standard error among study sites. Site 1 had the greatest impervious surface.

Figure 5. Average seasonal water NO₃⁻–N concentration and standard error among study sites. Site 1 had the greatest impervious surface.

Figure 6. Average seasonal soil NH₄⁺–N concentration and standard error among study sites. Site 1 had the greatest impervious surface.

Table 2. Percent cover of each cover class analyzed at each of the four study sites. Site 1 was near the confluence in Newport, KY, site 2 was 30.7 km upstream near Visalia, KY, site 3 was 39.5 km upstream near Morning View, KY, and site 4 was 59.3 km upstream in Butler, KY.

Land cover class	Site 1	Site 2	Site 3	Site 4
Impervious surface	12.8	0.5	0.0	2.1
Bare soil	5.2	0.0	8.6	9.3
Fine vegetation	18.9	18.9	4.4	12.1
Course vegetation	40.1	64.5	74.1	53.5
Mixed vegetation	23.0	16.1	12.9	23.0

Figure 7. Aerial photographs with delineated land cover classes for study sites 1, 2, 3, and 4. Land cover classes were tabulated within a 1000 m long by 200 m wide segment upstream from each study site.

Table 3. Relationship between denitrification potential rate and measured potential regulators (covariates) among study sites (analyzed using ANCOVA, n = 96, df₁ = 3, df₂ = 92).

	Overall model		Denitrification		Site
	F-value	p-value	F-value	p-value	F-value	p-value
Soil organic matter	11.9	<0.01	26.4	<0.01	0.69	0.56
Soil nitrate	6.54	<0.01	8.23	<0.01	3.40	0.02
Soil ammonium	4.22	<0.01	0.38	0.54	4.42	<0.01
Water nitrate	5.85	<0.01	5.91	0.02	5.92	<0.01
Water ammonium	4.39	<0.01	0.48	0.33	5.85	<0.01
Soil water content	4.92	<0.01	2.76	0.10	4.65	<0.01

Figure 8. Relationships of regulator and covariate soil organic matter with denitrification potential (a) and soil NO₃⁻–N (b).

Table 4. Average denitrification potential (DNF) and average water nitrate concentration of soils from riparian areas and two marshes along larger rivers. Ag stands for agriculture and n.a. stands for data not available.

River	Watershed area (km^2)	Area land use	Average DNF (ng N g^−1 hr^−1)	Water NO3^−–N (mg L^−1)	Source
Licking River	9310	Urban/Ag	107.5	0.62	Current results
River Thames (upper section)	9950	Urban/Ag	6250	7.7	Burt et al. (1999)
Marne River	12,800	Urban/Ag	3370	n.a.	Garnier et al. (2010)
Baraboo River	1577	Ag/Forest	1.41	1.25	Orr et al. (2007)
Dyke Marsh, Potomac River	14,700	Urban	1557–3234	0.52	Hopfensperger et al. (2009)

Table 5. Urban riparian average denitrification potential (DNF) ± standard error (SE) of soils with depth of sample used for DNF analysis. Bold indicates data from current studyand n.a. stands for data not available.

Average DNF ± SE (ng N g^−1 hr^−1)	Depth of sample (cm)	Source
62.5 ± 292	0–30	Newham et al. (2011)
70 ± 10	0–2	Arango and Tank (2008)
134.0 ± 36.8	0–10	Current results
350 ± 120	0–10	Gift et al. (2010)
421 ± 149	5–30	Groffman et al. (2002)
1000 ± 470	n.a.	Newcomer et al. (2012)
2196 ± 63	0–5	Groffman et al. (2002)
2630 ± 1020	0–10	Groffman and Crawford (2003)

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