Nitrogen Trace Gas Emissions from a Riparian Ecosystem in Sot&Em

CHEMOSPHERE

PERGAMON Chemosphere 49 (2002) 1389-l 398 www.elsevier.com/locate/chemosphere

Nitrogen trace gas emissions from a riparian ecosystem in sot&em Appalachia John T. Walker aP*, Christopher D. Geron a, James M. Vose b, Wayne T. Swank b ’ US Enviromental Protection Agency, Natkmal R&k Managmt Research Laboratory, iUD-63, Research Triangk Park, NC 27711, USA b US Department of Agricdture, Forest Service, Coweeta Hydrologic Laboratory, Otto, NC 28763, USA Received 11 July 2001; received in revised form 21 May 2002; accepted 19 June 2002

Abstract

In this paper, we present two years of seasonal nitric oxide (NO), ammonia (NH3), and nitrous oxide (NrO) trace gas fluxes measured in a recovering riparian zone with cattle excluded and adjacent riparian zone grazed by cattle. In the recovering riparian zone, average NO, NH3, and NrO fluxes were 5.8,2.0, and 76.7 ngNm-* s-i (1.83,0.63, and 24.19 kg N ha-’ y-t), respectively. Fluxes in the grazed riparian zone were larger, especially for NO and NHr, measuring 9.1, 4.3, and 77.6 ng Nm-* s-* (2.87, 1.35, and 24.50 kg N ha-’ y-l) for NO, NHr, and N20, respectively. On average, NzO accounted for greater than 85% of total trace gas flux in both the recovering and grazed riparian zones, though NrO fluxes were highly variable temporally. In the recovering riparian zone, variability in seasonal average fluxes was explained by variability in soil nitrogen (N) concentrations. Nitric oxide flux was positively correlated with soil ammonium (NH:) concentration, while N20 flux was positively correlated with soil nitrate (NO;) concentration. Ammonia _’ .,’ flux was positively correlated with the ratio of NH: to NO;. In the grazed riparian zone, average NH3 and NrO fluxes were not correlated with soil temperature, N concentrations, or moisture. This was likely due to high variability in soil microsite conditions related to cattle effects such as compaction and N input. Nitric oxide flux in the grazed riparian zone was positively correlated with soil temperature and NO; concentration. Restoration appeared to significantly a&t NO flux, which increased ~600% during the first year following restoration and decreased during the second year to levels encountered at the onset of restoration.- By comparing the ratio of total trace gas flux to soil N concentration, we show that the restored riparian zone is likely more efficient than the grazed riparian zone at diverting upper-soil N from the receiving stream to the atmosphere. This is likely due to the recovery of microbiological communities following changes in soil physical characteristics. Q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Nitrogen; Restoration; Agriculture; Cattle; Riparian buffers

1. Introduction organic matter input, and modification of stream physical conditions (e.g., temperature) and &&/wildlife hab- A riparian zone is the transitional boundary between itat. Thus, this zone heavily influences the function and terrestrial and aquatic ecosystems. Riparian zone fun0 quality of adjacent aquatic systems. In recent years, the tions include sediment and nutrient filtering, stream nitrogen (N) over-enrichment of fresh- and salt-water systems has become more tidespread on a global scale (Paerl, 1995). Non-point source pollution has become the dominant contributor to N entering these systems *Corresponding author. Tel.: +l-919-541-2288; fax: +1-919- and is the most diIBcult source to manage. Natural and 541-7885. manipulated riparian zones have been shown to effec- E-mail address: [email protected] (J.T. Walker). tively reduce the flux of nutrients in ground and surface 0045-6535/02/$ - see front matter 8 2002 Elsevier Science Ltd. All rights reserved. PIk SOO45-6535(02)00320-X (. 1390 J.T. Walker et al. I Chemosphere 49 (2002) 1389-1398

water to aquatic systems (Gilliam et al., 1997). For ex- termediate product of both nitrification and denitrilica- ample, forested and grass buffer strips can reduce N in tion, nitriflcation is the dominant source of soil NO subsurface. waters by 40-100% and 10-&O%, respectively under typical environmental conditions (Anderson and (Osborne and Kovacic, 1993). Natural resource man- Levine, 1986). Numerous studies have shown a positive agers, farmers, and local citizen-based groups concerned correlation between NO flux and soil N (Gasche and about water pollution are becoming more aware of the Papen, 1999; Russow et al., 2000), soil temperature value of riparian ecosystems in management of water (Slemr and Seiler, 1984, Conrad, 1996; Thornton et al., quality. Thus, the restoration of degraded riparian sys- 1997; Martin et al., 1998), and, under certain conditions, tems is becoming more widespread. Many regions in the soil moisture (Davidson, 1991; Vale& and Thornton, US have set goals to protect riparian zones of thousands 1993; Martin et al., 1998). The relative importance of of stream miles. We seek to better understand the soil, nitrification and denitrification to N20 production is water, vegetation, and atmospheric processing and ex- difficult to discern (Rester et al., 1997; Stevens et al., change of N in pasture and riparian-aquatic systems. 1997), however, anaerobic NO; reduction through Information on the emission of N trace gases from soils denitrification is considered the dominant pathway of along the pasture to stream gradient is a potentially N20 production (Russow et al., 2000). Factors known important component of these processes. to influence soil N20 emission include available N, Characterization of nitric oxide (NO), nitrous oxide temperature, soil moisture content, and pH (Conrad, (NzO), and ammonia (NHs) emissions from soils 1996). Furthermore, N20 emissions from both riparian provides important information regarding both at- zones and grazed pastures generally exhibit high vari- mospheric chemistry and soil nutrient and microbial ability in time and space (Firestone and Davidson, 1989; dynamics. While gaseous N fluxes are a$i&portant Denmead et al., 2000, Groffman et al., 2000).

component of N cycling in riparian ecosystems, these <’ _ ~ .In general, soil NH3 emissions will be highest in gases are also important to atmospheric chemistry on warm, neutral to alkaline soils with an adequate supply local, regional, and global spatial scales and on temporal of NH: and low cation exchange capacity (Langford scales ranging from hours to decades. For example, NzO et al., 1992; Whitehead and Raistrick, 1993). The partial is a greenhouse gas and is also responsible for the de- pressure of NH3 in soil increases with concentration of struction of stratospheric ozone (Crutzen, 1970; Rama- NH:, pH, and temperature (Langford et al., 1992). Soils nathan et al., 1985). Soils account for approximately will become a source of NH3 when the partial pressure 25% (10 Tg N@-N y-l; Tg = lOI* g) of global annual in the soil exceeds the atmospheric NH3 partial pressure. N20 emissions (Warneck, 1988). Nitric oxide plays ‘aii Soil receiving cattle urine will exhibit relatively large ,’ / integral role in the photochemistry of tropospheric NH3 losses to the atmosphere, following hydrolysis of ozone and is also a precursor to atmospheric nitrous and urea to ammonium carbonate. This process proceeds nitric acids (Chameides and Walker, 1973; Galloway rapidly and the resulting pool of NH: is depleted and Liens, 1981; Logan, 1983). Global estimates sug- through NH3 volatilization (Jarvis et al., 1989). gest that NO emission from soil contributes approxi- We quantified soil fluxes of NO, NzO, and NH3 as mately 15% (7 Tg NO,-N y-l) of total nitrogen oxides part of a project investigating water quality impacts of (NO,) emissions (44 Tg NO,-N y-t; Bouwman, 1990; riparian zone restoration. Seasonal flwes in a restored Lee et al., 1997). Ammonia emissions lead to the for- riparian zone and adjacent riparian zone grazed by mation of ammonium (NH:) aerosol and atmospheric cattle, measured during the first two years following deposition of NH, gas and NH:, which may cumula- restoration, are presented. System differences in relatively account for a significant fraction of N input to tionships between flux, temperature, available N, and ecosystems downwind of strong NH3 source regions soil moisture are illustrated, and the impact of restora- such as confined animal feeding operations and intensive tion on flux rates is assessed. agriculture. Estimates of NH3 emissions from grazed pasture and soils under natural vegetation are limited (Langford et al., 1992). Globally, NHs emissions from 2. Methods undisturbed ecosystems represent only 4.5% (2.4 Tg NH3-N y-t) of total emissions (Bouwman et al., 1997). 2.1. Site description Synthetic fertilizers and agricultural crops, however, together contribute 24% (12.6 Tg IQ&-N y-i) of global Fluxes were measured at a site near Dillard, Georgia NH3 emissions (Bouwman et al., 1997). (34.97” N, 83.39” W), located in the state’s Blue Ridge Emissions of NO and N20 from riparian zone soils Mountains physiographic province. Soils are Saunook are primarily regulated by the activity of nitrifying and series, line-loam (USDA, 1996). The receiving stream at denitrifying soil bacteria (F&stone et al., 1980; Tiedje this site is Sutton Branch, a first-order tributary of the et al., 1984, Anderson and Levine, 1986; Davidson et al., Little Tennessee River. Historic and contemporary 1986). Studies have shown that, although NO is an in- landuse adjacent to the stream is beef cattle grazing, and J.T. Waker et al, I Chemosphere 49 (2002) 1389-1398 1391

the riparian zone has been severely degraded by cattle. struments Incorporated (TEI) Model 17C chemilu- Characteristics of riparian zones impacted by cattle minescent NO,/NHs analyzer. The basic operating overgrazing include soil compaction, lack of vegetation, principle of this instrument involves the reaction of NO chronic nutrient input, and bank destabilization (Belsky with ozone (OS), which produces an excited nitrogen et al., 1999). In June 1998, as part of the Upper Lit- dioxide (N4) molecule that, upon decay to a lower tle Tennessee River Comprehensive Non-point Source energy state, produces a luminescence proportional to Management Project, cattle were excluded from a por- the concentration of NO present in the sample (TEI, tion of the riparian zone along a 610 m section of the 2000). The concentration of NH3 in the sample is cal- stream by fencing, establishing a 12 m wide buffer on culated as the difference between the total N (Nt = each side of the stream. Hence, fencing provided an area NO + NO2 + NHs) and NO, (NO + N4) channels of within the riparian zone that was recovering from the the instrument. To calculate Nt, the sample is passed impacts of cattle grazing (i.e., “restored”) and an area through an external thermal converter (825 “C) which that was still impacted by cattle grazing (i.e., “unre- converts NO2 + NHs to NO. The NO from these sour- stored”). Native vegetation was allowed to return, and ces is then detected in the reaction chamber, representing Salix, Pkznus, and Populus seedlings were planted at the concentration of Nt. Similarly, NO, is converted to wide (~3 m) spacing. NO as it passes through an internal molybdenum converter (325 “C). Again, the NO from these sources is 2.2. Flux calculation then detected in the reaction chamber, representing the concentration of NO,. Ammonia concentration is then Trace gas fluxes were determined using, a+ flow- determined as Nt-NO,. An external pump draws air through chamber mass balance technique (Kaplan et al., through the analyxer at a constant rate of approximately 1988; Aneja et al., 1996; Roelle et al., 1999). The cham-, __ ’ 0;650 1 min-’ . Switching between the NO,, Nt, and NO bers were transparent acrylic lined with Teflon (polytet- channels is controlled via timed solenoid valves. ratluoroethylene) fihn. The cylindrical 28.2-l chamber The analyzer was typically operated on the O-50 ppb was placed onto a stainless steel collar driven into the soil scale, and was calibrated with a TEI Model 146 dynamic approximately 25 cm. The soil collar was inserted into calibration system by mixing zero air and NO. Multi- the soil at least 12 h prior to placement of the chamber. point calibrations consisted of a zero point, a span value Dry zero grade air was pumped into the chamber at a at 80% full scale, and concentrations at 70%, 50%, 30%, constant rate (4.0-5.5 Ipm) and drawn from the chamber 20%, and 10% of full scale to generate calibration curves at approximately 1 lpm. The excess air was vented, for all three channels. Calibrations were performed at ,/ minimi&g pressure differences between the chamber the beginning and end of each measurement intensive, and the atmosphere. After the flow rate to the chamber and zero/span procedures were performed daily. The was set, the gas concentration in the chamber was al- high temperature converter efficiency was determined lowed to reach steady state between flux from the soil and by introducing a known concentration of NH3 within net removal processes (NO and NHs) in the chamber the range of observed NHs values. The efficiency of the (-30 mitt). Only measurements taken after this transition molybdenum NO, converter was tested by introducing a period were used for flux calculation. Air from the known concentration of NO2 generated by mixing NO chamber was drawn directly to analytical instriimenta- with 0s. One-minute average chamber concentrations of tion for online determination of gas concentrations. The NO and NI-Is were recorded continuously and averaged flux was determined Sy the equation: to yield 5-min average fluxes. Nitrous oxide concentrations were determined using J = MO z+L (1) a Hewlett Packard 5890 gas chromatograph with elec- ( > tron capture detection (Porapak Q column, 2 m length, where J is the gas flux (ngNme2 s-l), [Cl,, is the gas 5 ml sample loop). The system was configured for con- concentration (ngm-‘) at the chamber outlet, Q is tinuous on-lime monitoring, and the concentration of the carrier gas flow rate (ls-*), A is the soil surface N20 in the flux chamber was determined approximately (A = 0.0794 m2) enclosed by the chamber, and L (m s-t) every 12 min. The system was multipoint calibrated at represents gas loss to the chamber wall per unit area the beginning and end of each measurement intensive as (Kaplan et al., 1988). The average value of the loss term described above. All analytical and data acquisition observed in the study was L = 9.16 x 10”” ms-’ for NO equipment were housed in a temperature-controlled and 5.6 x lo4 ms-t for NI-Is. On average, this loss mobile laboratory. accounts for approximately 50% of the total flux of both species. Given its atmospheric stability, a loss term for 2.3. Soil parameters NsO was not calculated. Concentrations of NO and NHs inside the chamber Soil temperature (averaged over O-10 cm, Campbell were determined using a Therm0 Environmental In- Scientific probe Model 107) and volumetric water 1392 J.T. Walker et al I Chemosphere 49 (2002) 1389-1398

content (averaged over O-15 cm, Campbell Scientific outliers and does not assume that the sample is drawn Model 615 water content reflectometer) were measured from a particular theoretical distribution (Schlotzhauer continuously within the unrestored and restored riparian and Littell, 1987). The relationships between trace gas zones during flux measurement periods. Soil NO; and fluxes and soil temperature, moisture, and N concen- NH: concentrations (O-IO cm) were determined during trations were examined using regression analysis. Mod- flux measurement periods using a Perstorp 3500 colori- els were fit using the method of least squares (SAS, metric system following the extraction and analytical 1992). All analyses were performed using SAS statistical methods developed by the Coweeta Hydrologic Labo- software. ratory (US Forest Service, 2000). Porous cup lysimeters (30 cm soil depth) were used to sample soil solution NO; and NH: following analytical procedures developed at 3. Results and discussion Coweeta Hydrologic Laboratory for sediment and water samples (US Forest Service, 2000). Lysimeters were in- 3.1.Fluxes stalled at 3-m spacing from stream edge to inside the unrestored area along nine transects evenly spaced Overall, fluxes were slightly larger in the unrestored throughout the study area. After an equilibration period riparian zone than the restored riparian zone for all of 3-4 weeks, lysimeters were sampled weekly. Samples three gases (Table 1). Also, total flux variance was larger were composited and analyzed monthly. in the unrestored riparian zone for all species, though variation was substantial in both the restored and un- 2.4. Trace gasjlux data description restored areas (Table 1). On average, replicate mean NO, NH3, and NaO fluxes differed by 70%, 90%, and Nitric oxide and NH, fluxes were measured approx- ’ 6%, respectively, within the restored riparian zone dur- imately every three months between July 1998 and Au- ing individual measurement periods. Replicate means gust 2000. Nitrous oxi&, ,measurements began in June within the unrestored riparian zone I varied by 1 IO%, 1999. Typical measurement periods lasted from 4 ‘to ‘7 143%, and 32%, respectively, during individual mea- days. Flmes were measured in two~lo&itions within both surement periods. Larger spatial variation in the unre- the restored riparian zone and the unrestored riparian stored riparian zone was likely due to more spatially zone during each measurement period. Chambers were heterogeneous patterns of soil compaction, vegetation placed in the same general area from season to season. disturbance, and N input due to cattle grazing. Average fluxes consist of daytime values only. The av- / erage number of 5-min flux vahres in an individual ,’ 3.1.1. Ammonia measurement period was 170,200, and 100 for NO, NHs, There was considerable temporal variation in NH3 and NZO, respectively. Unrestored and restored riparian flux for both unrestored and restored riparian zone NHs values for July 1998 and unrestored NH3 values for measurements (Pig. la). Except during August 1999 April 1999 were omitted from the analysis due to mois- and February 2000, differences between unrestored and ture-induced sampling difficulties which resulted in flux restored riparian zone NHs fluxes were statistically underestimation. sign&ant (p < 0.10). Edaphic factors regulating this , temporal variation were difficult to discern in either the 2.5. Statistical ani$ysi.v unrestored or restored riparian zone. Results from linear \ regression analyses indicated no statistically significant In this study, differences in means were tested for (p > 0.10) relationship between temporal variation in statistical significance using the non-parametric Wilco- NH3 flux and either soil temperature or moisture. Av- xon rank sums test. This test is robust with respect to erage soil temperature (O-10 cm) within the restored

Table 1 Summary flux (ng N m” s-l) statistics for the unrestored and restored riparian zones

Gas LocatiOU N Mean (ng N rns2 s-l) Standard deviation MhhllUlIl Maximum NO Unrestored 8 9.1 11.5 0.7 33.1 NO Restored 8 5.8 4.0 1.8 12.2 NH3 Unrestored 6 4.3 6.2 0.3 15.8 NH3 Restored 7 2.0 1.9 0.2 6.0 N20 unrestored 5 77.7 92.1 0.0 182.0 NzO Restored 5 76.7 84.8 0.0 200.3 Means consist of daytime (7:OO AM-8:00 PM) S-min average fluxes averaged by seasonal measurement period. J.T. Walker et al I Chemosphere 49 (2002) 1389-1398

DAJ ANPA DAJ ANPA J ANPA J ANFA EPUUOEU EPUUOEU UUOBU UUOBU CRNGVBG CRNGVBG NGVBG NGVBG 9999900 9999900 99900 99900 8999900 8999900 99900 99900 Restored Unrestored Restored Unrestored

J DAJ ANFA JDAJANFA UEP UUOEU UEPUUOEU LCRNGVBG LCRNGVBG 99999900 99999900 88999900 88999900 Restored Unrestored Fig. 1. Fluxes (ng N m-* 8’) averaged by measurement period within the unrestored and restored ripsrisn zones. Bar extensions represent the upper 95% confidence limit for the mean. Asterisks during February and August 2000 in the N20 chart represent average iluxes below detection limit. riparian zone during seasonal measurement periods controlled by DNRA rather than denitrikation, which ranged from 9.7 to 26.1 “C. Throughout the study, soil may dominate when NOT-to-C ratios are high (Tiedje volumetric water in the restored riparian zone (O-l 5 cm) et al., 198% Tiedje, 1988). Schipper et al. (1994) observed ranged from 18.2% to 46%, while averaging 34.3%. lowest soil NH: concentrations at the upslope edge of Saturation volumetric water content was approximately the riparian zone where NO; concentrations and deni- 64% in the restored riparian zone after reversal of trifjing enzyme activity (DEA) were highest and maxi- compaction. mum NH: concentrations near the stream where DEA In the restored riparian zone, flux was not correlated was lowest. In the present study, we also observed de- (p > 0.10) with soil NO; or NH: concentrations but creasing soil solution NO; concentrations (30 cm lysi- was highly correlated with the ratio of NH: to NO; meters) from the edge of the restored riparian zone to (R2 = 0.84, p = 0.007). Studies have shown that ob- the stream coupled with increasing NH: concentrations served spatial patterns of NH: concentration in restored (Fig. 2). At this site, soil within the restored ripar- riparian soils may be partly related to patterns of dis- ian zone exhibits relatively low O/oN-to-%C ratios similatory NO; reduction to NH: (DORA, Schipper (%N:%C < 0.1, O-30 cm). The observed relationship et al., 1994). In soils that contain low NO;-to-carbon between NH, loss to the atmosphere and NH:-to-NO; (C) ratios, anaerobic NO; reduction may be primarily ratio in this study suggests that NHs loss may be greater J.T. Walker et aL I Chemosphere 49 (2002) 1389-1398

0.6 restored and restored sections of the riparian zone (Fig. T lb). During February 2000, average soil temperature (O- 10 cm) was 8.5 T, which would likely reduce microbial activity. During August 2000, soil volumetric water was 36% and 18% in the nnrestored and restored sections of the riparian zone, respectively. Under these moisture conditions, the soil is likely to be mostly aerobic. Anaerobic bacteria responsible for NsO production through denitrification would therefore be miuimally active. 2 4 6 8 IO 2 4 6 8 10 In the restored riparian zone, average fluxes Distance fkom Stream (m) Distance Corn Stream (m) were positively correlated with soil NO; concentration (R2 = 0.83, p = 0.02). Because N20 is a product of both Fig. 2. Average 30 cm lysimeter NO,-N and NH:-N con- the formation and reduction of NO;, this correlation centrations (mgl-‘) vs. distance from stream (m) within the restored riparian zone. Bar extensions represent the upper 95% does not indicate whether nitrification or denitrification confidence limit for the mean. Note the difference in scales was the source of N20. This does suggest, however, that hetwcen charts. the near-surface soil was active in processing N entering the restored riparian zone. Relationships between restored riparian NsO flux, temperature, and soil moisture where the NH,+-to-NO; ratio is high, signifying a were not evident (p > 0.10). If flux was NO; limited, the readily available, new pool of NH:. As illustrated in it&ence of soil moisture and temperature on flux may Fig. 2, spatial variability’in soil NH:-to+IC$ ratio, and I have been negligible. In the tmrestored riparian zone, thus NI-Is loss to the atmosphere at this site, may be at statistical relationships between average flux and soil least partially controlled by partitioning of NO; re- temperature, moisture, and N concentrations were not duction between DNRA and denitrification and varia- evident @ > 0.10). Cattle-induced variability in soil tion in the environmental controls which regulate the conditions may have confounded such relationships. corresponding microbial activity of those mechanisms. Since riparian zones have been shown to process Following this hypothesis, Fig. 2 also reflects the im- sign&ant amounts of NO; in groundwater, it has re- portance of measuring thtxes along a gradient from the cently been hypothesized that these systems may repre- restoration boundary to the stream. In this study, sent “hotspots” of NsO emissions to the atmosphere / however, fluxes inside the restored riparian zone were (GrofIinan et al., 1998, 2000). While numerous studies / consistently measured approximately 2.5 m from the have addressed the issue+ of denitritication in riparian restoration boundary. soils (see Martin et al., 1999), relatively few studies have In the unrestored riparian zone, statistical relation- addressed the issue of N20 production in groundwater ships between measurement period average NH3 flux (see Grothnan et al., 2000). Measuremerrts of NsO and soil temperature, moisture, and N concentrations emissions from riparian soils are even more limited, were not sign&ant 0, > 0.10). Given the irregular though Weller et al. (1994) published a dataset con- presence of cattle in the unrestored riparian zone and taining N20 emissions from a forested riparian zone associated variability in N inputs, vegetation distur- receiving runoff from an adjacent cornfield. At that site, bance, and com$&ion, the absence of clear relation- g&mdwater NO; concentrations were shown to de- ships between flux and edaphic factors is not surprising. crease from 2-10 to cl mg NO;-N 1-r during transit It is possible that the variation in average NHs fluxes through the riparian zone (Weller et al., 1994). Fhtxes may be more readily explained by spatial variation in measured in that study were much lower (1.5-2.8 ng cation exchange capacity or spatiotemporal variation in N2G-N mm2 s-t) than the fluxes observed in the present pH, variables which were not quantified in this study. study. This difference may be attributed to a number of Overall, the fluxes observed in this study are similar to factors. For example, Weller et al. (1994) may have fluxes measured by Roelle and Aneja (2002) in a fallow observed conditions which favor a predominance of Ns agricultural soil (3.4-26.1 ng NH3-N m-* s-r) using the rather than NzO production during denitrification dynamic chamber technique. (Grothnan et al., 2000). Weller et al. (1994) also reported relatively low concentrations of NsO in groundwater 3.1.2. Nitrous oxide during that study. The fluxes observed in our study (24.2 Excluding February and August 2000, when NsO and 24.5 kg N20-N ha-r y-l) are within the range of fluxes were below detection limits, differences between fluxes (O-41.8 kg N20-N ha-’ y-l) for mineral agricul- restored and unrestored riparian zone measurement tural soils reported in a review by Bouwman (1996). Our period means were statistically significant (p < 0.10) and results are similar to those of Weller et al. (1994), large temporal variability was observed in both the un- however, in that while both study sites show a reduction J.T. Walker et al I Chemosphere 49 (2002) 1389-1398 1395 in groundwater NO; from the riparian zone boundary Table 2 to the stream, NzO emissions are not, on average, Ratio of flux (ng N m-* s-*) to soil N [(NH:--N) + (NO;-N)] highest at the measurement location nearest the stream. within the restored and unrestored riparian zones Weller et al. (1994) attributed this to large spatial vari- Ratio Restored Unrestored ability in denitrification rates and the factors controlling NO flux/soil N’ 3.18 1.74 the ratio of Ns to NsO. NH3 fluxlsoil N 3.27 1.21 N20 flux/soil N 28.6 27.7 3.1.3. Nitric oxide * ng NO-N mv2 s-l where soil N = @‘Hz-N) + (NO;-N). The temporal pattern of NO flux in the restored mg N kg soil-’ ’ riparian zone was much different than flux in the unrestored riparian zone (Fig. lc). During all periods, re- (NO;-N)] within the restored and unrestored sections stored and unrestored riparian zone mean fluxes were of the riparian zone for NO, NH3, and NsO. Ratios of significantly different 0, < 0.10). In the restored riparian NO and NH3 flux to soil N were 1.45 and 2.1 times zone, flux increased &OO% (1.8-12.2 ng NO-N mm2 s-l) larger, respectively, in the restored riparian zone. This during the first year following restoration. During the indicates that the restored riparian zone was more effi- second year, flux decreased to levels observed at the cient than the unrestored riparian zone in exporting N onset of restoration (1.6 ng NO-N mm2 s-i). In contrast, from the soil to the atmosphere and that restoration, flux within the unrestored riparian zone did not display therefore, improved the ability of surface soils to divert this temporal pattern. N from the receiving stream, particularly through emis- Average NO flux within the restored riparian zone sions of NO and NHs. was positively correlated with soil NH:- con&&ation #-Average soil NH:-N concentrations (O-10 cm) in (R2 = 0.87,~ = 0.005). As previously mentioned, NH: is ’ the restored riparian zone experienced a net decrease necessary for’ nitrifIcation to proceed. Soil temperature; c (~~95%) from 5.11 to 0.33 mg NH:-N kg-’ (Fig. 3, moisture, and NO; concentration were not sign&ant R2 = 0.77, p < 0.001) over the period April 1999-August explanatory variables (p > 0.10). In the unrest&d rip’ arian zone, NO flux was positively correlated with soil temperature (Rz = 0.29, p = 0.098) and NO; concen- 0 10 NO; tration (P = 0.67, p = 0.030). Nitrate is a product of 9 nitrification and is the substrate necessary for denitrifi- 8 cation. Though NO is a byproduct of both processes, the 7 different relationships between NO flux and soil N in the restored and unrestored areas is characteristic of differences in microbial characteristics within the two soils. The pattern of NO fhrx in the restored riparian zone was likely the cumulative result of physical, chemical, and biological factors. The flux increase during the first year following restoration is consistent with the observed decrease in bulk density (1.15-0.95 g cme3), which likely APR99 JUL99 OCIY9 JAN00 APROO JUJ.BO OCI’OO reM in increased soil aeration. This increased aeration may have increased the activity of aerobic nitrifying bacteria, thereby stimulating NO emissions. The observed relationship between NO flux and soil NH: concentration indicates that nitrilication may be the primary source of NO in the restored riparian zone. The average NO flux values observed in this study are similar to fiux magnitudes for other agricultural soils in the southeast US (1.0-7.0 ng NO-N me2 s-t; Aneja et al., 1996).

3.2. Riparian nitrogen cycling APR99.JULB9 OCW JAN00 APROO JULOO OCIW We have shown that flux magnitudes were slightly larger in the unrestored riparian zone. A relevant ques- - Unrestored (0) - - -Restored (*) tion is whether or not restoration increased or decreased Fig. 3. Soil (O-10 cm) NH:-N and NO;-N concentrations the importance of trace gas emission as a pathway of (mgkg-*) within the mxestored and restored riparian zones. N diversion from the receiving stream. Table 2 shows Symbols represent mean values. Regression lines were fit to all ratios of flux (ng N rnw2 s-i) to soil N [(NH:--N) + data points. Note the difference in scales between charts. 1396 J.T. Walker et al. I Chemosphere 49 (2002) 1389-1398

2000, corresponding to an 87% decrease in NO flux. Two years following restoration, it does not appear Average soil NOT-N concentrations (O-IO cm) in the that this landuse change has altered N trace gas emis- restored riparian zone experienced a net decrease from sion in a way that would impact regional budgets of 0.98 to 0.02 mg NO;-N kg-i (R2 = 0.29, p = 0.032) NO, NH3, and N20, given large-scale implementation of over the same period. Concentrations of NH:-N (p = riparian restoration measures. Our results do show, 0.47) and NO;-N (p = 0.32) in the unrestored area did however, that NO flux increased significantly during the not exhibit statistically significant trends (Fig. 3). In the first year following restoration before returning to pre- restored riparian zone, concentrations at the end of the restoration levels. Also, observed N20 fluxes are higher period are lower than those reported for forest soils than previously published measurements for riparian (3.1-4.6 mg NH: kg-‘, 1.2-1.6 mg NO; kg-‘) in this zones. By comparing the ratio of total trace gas flux to region (waide et al., 1988). This suggests that residual soil N concentration, we show that the restored riparian N in the upper 10 cm of soil was quickly processed zone was more e5cient than the grazed riparian zone at following restoration, and that new N input from the diverting surface soil N from the receiving stream to the unrestored riparian zone is either efficiently processed atmosphere. within, or quickly transported through, this layer. This It is likely that the impact of restoration on N trace response in soil N concentrations likely results from gas emissions is highly site specific. For example, Groff- increased accumulation in vegetation and increased N man et al. (2000) point out that the impact of riparian processing efficiency and/or increased biomass of soil restoration on NzO emissions depends on the ratio of microbes. The observed increase in NO flux during the N20 to Nz produced during denitrification, which is first year following restoration and higher efficiency of regulated by soil pH, NO; concentration, and denitritl- the restored riparian zone in converting N,,to”“fiO and cation rate (Grown et al., 2000). To accurately assess NHs gases, is consistent with this response. , the potential impact of riparian restoration on regional or national NO, NHs, and NsO budgets, flux measurements must accurately reflect at least watershed-scale 4. conclusions variability. Fluxes will vary with site characteristics such as soil type, vegetation type, magnitude and speciation of Seasonal measurements of NO, NH,, and NzO trace N input, and hydrologic characteristics. gas 5txe.s during the first two years following restoration During this study, we observed large spatial and of a degraded riparian zone are presented, along with temporal flux variability within both the restored and fluxes from the adjacent unrestored section of the rip- unrestored sections of the riparian zone. This indicates ,/’ arian zone. In the restored riparian zone, average NO, that we should make more than two replicate measure- ;’ NHr, and N20 fluxes were 1.83, 0.63, and 24.19 kg N ments in space per treatment per measurement period ha-* y-l, respectively. Fluxes in the unrestored riparian in future experiments. We recommend a combination of zone were slightly higher, measuring 2.87, 1.35, and replicate measurements spaced systematically with re- 24.50 kg N ha-’ y-r for NO, NHs, and N20, respec- spect to the stream and a large chamber footprint to tively. Nitrous oxide flux was most important in terms of appropriately capture spatial flux variability. N loss from the restored ripariau zone, followed by NO We believe that the high degree of variability observed and NH3. in fluxes and soil N concentrations was primarily due to Variability in restored riparian seasond’ ’ average the irregularity of cattle activity in the unrestored riparian fluxes was explained by variability in soil N concentra- zone, and that this variability and lack of microbiological tions. Nitric oxide flux was positively correlated with soil information sigmficantly limited our ability to examine NH: concentration, while N20 flux was positively cor- N cycling processes at the mechanistic level. In order related with soil NO; concentration. Ammonia flux was to extend the work presented here to investigate the N positively correlated with the ratio of NH: to NG;. cycling mechanisms in restored riparian zones, we have Relationships between NH3 flux, soil temperature, and implemented controlled experiments at a second site. Our moisture were not evident. In the unrestored riparian intent during this second phase of the project is to identify zone, average NH3, and NsO fluxes were not explained and characterize the primary physical, chemical, and bio- by soil temperature, N concentrations, or moisture. This logical mechanisms regulating nutrient transport and is likely due to high variability in soil conditions related transformation within both currently degraded and re- to cattle effects, such as compaction and N input. Nitric stored riparian zones previously degraded by cattle. oxide flux in the unrestored riparian zone was positively correlated with soil temperature and NO; concentration. Restoration appeared to significantly a&et NO Acknowledgements flux, which increased r&00% during the tlrst year following restoration and, during the second year, de- The authors acknowledge the cooperation and as- creased to levels encountered at the onset of restoration. sistance of Terry Seehom of the Rabun Gap Nacoochee J. T. Walker et al I Chemosphere 49 (2002) 1389-1398 1397

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