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Soil nitrate-nitrogen in forested versus non-forested ecosystems ina mixed-use watershed

G.L. Bruland a,⁎, C.M. Bliss b, S. Grunwald c, N.B. Comerford c, D.A. Graetz c

a Natural Resources and Environmental Management Department, University of Hawai'i Mānoa, 1910 East-West Rd., Honolulu, HI 96822, USAb United States Department of Agriculture Forest Service, Southern Research Station, 2500 Shreveport Hwy., Pineville, LA 71360, USAc Soil and Water Science Department, University of Florida, 2169 McCarty Hall, P.O. Box 110290, Gainesville, FL 32611-0290, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 22 January 2008Received in revised form 16 July 2008Accepted 14 October 2008Available online 8 November 2008

Keywords:FloridaLand-useNitrate-nitrogenSoilWatershed

In tracking nutrients that enter the Gulf of Mexico via the Suwannee Basin, a disproportionate amount of thenitrate-nitrogen (NO3-N) has been shown to originate in the Santa Fe River Watershed (SFRW). This studyinvestigated soil NO3-N distributions across the range of land-use and soil order combinations that exist inthe SFRW with a focus on comparing NO3-N levels in forested versus non-forested land-uses. The SFRWconsists of 52% forested land-uses (i.e. pine plantation, forest regeneration, upland forest, and forestedwetland), 47% non-forested land-uses (i.e. agriculture, rangeland, and urban), and 1% water. Soil sampleswere collected from four depth intervals (0–30, 30–60, 60–120, 120–180 cm) at 101 to 141 sites with astratified-random design in six sampling events (Sept. 2003, Jan. 2004, May 2004, Jan. 2005, May 2005, andSept. 2005). No samples were collected in Sept. 2004 due to flooding associated with two hurricanes. Nitrate-nitrogen was significantly lower in forested than non-forested land-uses across all sampling events, depthintervals, and for profile average data. Within the non-forested land-use category, NO3-N levels were highestin row crop agriculture and improved pasture sites. In terms of soil order, NO3-N values were generallyhighest in Ultisols and Spodosols, but soil order explained less of the variation in the NO3-N data than didland-use or sampling date. Nitrate-N concentrations were considerably altered by Hurricanes Frances andJeanne which passed over the SFRW in late summer of 2004. In the post-hurricane sampling events, NO3-Nwas significantly lower in both forested and non-forested sites. A year later, however, NO3-N concentrationsin forested sites remained quite low, while concentrations in non-forested sites had begun to increase.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Human activity has dramatically increased the flux of nitrogen (N)and phosphorus (P) to coastal zones (Rabalais et al., 2002). For example,the change in N flux from pre-industrial times to the present has beenestimated as fourfold for the Mississippi River, eightfold in rivers of thenortheastern United States, and tenfold in rivers draining to the NorthSea (Howarth et al., 1996). This increased nutrient loading to coastalzones tracks increases in population, agricultural expansion in riverbasins, and increasing food and energy consumption (Nixon 1995;Howarth et al.,1996; Vitousek et al.,1997; Bennett et al., 2001). Globally,nitrate-nitrogen (NO3-N) is themain contributor to increased N loadingin larger rivers (Turner et al., 2003). Since the 1950s, increased marineproductivity and hypoxic conditions in the Gulf of Mexico parallel theincreases in NO3-N loads to the Gulf (Rabalais et al., 2002; Turner et al.,2006). In the summer of 2002, the hypoxic zone in the Gulf of Mexicoextended over an area of 22,000 km2 (Turner et al., 2006).

Hypoxia is just one of many nutrient enrichment symptoms in coastalzones. Other symptoms include reduced light penetration, increasedabundance of nuisance macro algae, loss of aquatic habitat such as sea-grass beds, noxious and toxic algal blooms, and shifts in trophic in-teractions and foodwebs (Schramm1999; Anderson et al., 2002; Rabalaiset al., 2002). With the growing world population, more fertilizer will beneeded for greater crop production; as a result, watershed disturbancesand nutrient pollution will likely intensify (Carpenter et al., 1998). Thus,there is a current need to identify terrestrial sources of nutrients such asNO3-N at the watershed scale, and assess their contribution to thedegradation of downstream aquatic and coastal ecosystems.

The Suwannee River Basin has experienced population growth,increases in agricultural production, and subsequent increases innutrient loading to surface waters, groundwater, and coastal zones(Pittman et al., 1997; Hornsby and Ceryak, 1999; Katz et al., 2001; Katzet al., 2004). The basin covers an area of 25,900 km2 that spans parts ofsouthern Georgia and north-central Florida (Fig. 1). Water qualitymonitoring data from the major rivers in the Suwannee Basin haveshown a significant increasing trend in NO3-N concentrations over aperiod of two decades (Ham and Hatzell, 1996). More recently, theSuwannee Basinwas shown to have the largest area of elevated NO3-N

Geoderma 148 (2008) 220–231

⁎ Corresponding author. Tel.: +1 808 956 8901; fax: +1 808 956 6539.E-mail address: bruland@hawaii.edu (G.L. Bruland).

0016-7061/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.geoderma.2008.10.005

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concentrations in the Floridian Aquifer system (SRWMD, 2002).According to a groundwater monitoring program, NO3-Nwas elevatedin the Santa Fe River Watershed (SFRW), a subwatershed within thelarger Suwannee River Basin (SRWMD, 2002). Although the SFRW(3,585 km2) comprises only 13.8% of the greater Suwannee Basin, itaccounted for 22.2% (2,676 t NO3-N) of the total N load in 2000 and19.7% (2,971 t NO3-N) of the total N load in 2003 (SRWMD, 2000;

SRWMD, 2003). Thus, based on its areal extent, the SFRW hascontributed a disproportionate amount of NO3-N from the SuwanneeBasin to the Gulf ofMexico. Recent studies in the Suwannee Basin haveindicated that the dominant NO3-N sources include inorganicfertilizers and organic animal wastes (Katz et al., 1999).

The SFRW also contains a number of groundwater springs thatsupport threatened aquatic ecosystems. These springs are biodiversity

Fig.1. Location of the Suwannee River Basin in south Georgia and north-central Florida (a). Location of the streamnetwork and sampling sites (circles) in the Santa Fe RiverWatershed(b). Map of the distribution of forested and non-forested land-uses across the Santa Fe River Watershed (c).

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hotspots in the north-central region of Florida that support variousreptile, amphibian, fish, and bird species, as well as threatened andendangered species such as the Gulf Sturgeon (Acipenser oxyrhynchusdesotoi) and theWest IndianManatee (Trichechusmanatus). The springsin the Middle Suwannee Watershed have surface water NO3-N con-centrations ranging from 1.2 to 17.0 mg L−1 (SRWMD, 2002). Such highNO3-N concentrations can lead to nuisance levels of algae andmacrophytes in many of the spring-fed rivers of this region (Joneset al., 1996; Hornsby and Mattson, 1998). Thus, NO3-N loading not onlycauses remote effects beyond terrestrial watershed boundaries, it alsocauses local changes in the freshwaterecosystemswithin thewatershed.

The increases in nutrient loading within and from the SFRW can belinked to land-use changes within the watershed. Recently there hasbeen an increasing trend of converting less intensively-managed tomore intensively-managed land-uses (Katz, 2004; Sabesan, 2004). Forexample, in the SFRW from 1990 to 2003 the areal extent of pineplantations decreased from 29.5 to 23.3% and rangeland (defined asareas subject to grazing that are not mowed or plowed and do notreceive fertilizer inputs) decreased from 15.1 to 4.3% (Sabesan, 2004).During this same period, agricultural land increased from 23.5 to 37.3%(Sabesan, 2004). These shifts in land-use have major implications forexport of NO3-N from the watershed if the tight nutrient cycles offorested ecosystems such as pine plantations, upland forests, and wet-lands retain inorganic nutrients such as NO3-N and serve as N sinks atthewatershed scale (Bandet al., 2001;Groffmanet al., 2004).Other non-forested ecosystems such as agriculture, improved pasture, and urbanareas can become saturated with NO3-N and serve as NO3-N sources atthe watershed scale (Freifelder et al., 1998). Soils along with land-useplay a critical role in the transport, cycling, and retention of nitrogenwithin a watershed. Thus, the overall goal of this project was toinvestigate spatial and temporal variability of soil NO3-N across therange of land-use and soil order combinations that exist in the SFRW.The specific objectives of this study are as follows: 1) to compare soilNO3-N levels in forested versus non-forested land-uses since there is alocal concern that forested land uses which dominate the area areimportant contributors to N increases in the Santa Fe and SuwanneeRivers; 2) to compare NO3-N levels in the SFRW before and after the2004 hurricanes Frances and Jeanne; and 3) to compare NO3-N levelsacross the different soil orders that are found in the SFRW.

2. Materials and methods

2.1. Study Area

The SFRW spans across eight counties in north-central Florida(Fig. 1a) and comprises the southeastern section of the greaterSuwannee Basin. In 1990, the dominant land-uses in the SFRWincluded pine plantation (29.5%), agriculture (including row crops andimproved pasture, 25.3%), rangeland (15.1%), wetland (13.7%), uplandforest (8.9%), and urban (5.5%). The remaining 2.0% was classified aswater or could not be classified due to cloud cover (Sabesan, 2004). By2003, the land-use distribution had shifted as follows: pine plantation(23.3%), agriculture (37.3%), rangeland (4.3%), wetland (17.9%), uplandforest (10.5%), and urban (5.6%). The remaining 1.1% was classified aswater or could not be classified due to cloud cover (Sabesan, 2004).The soils of the SFRW are predominantly sandy in texture althoughthere are areas with loamy to clayey deposits, organic soils, and karstterrain (Lamsal et al., 2006). The dominant soil orders in thewatershed include Ultisols (36.7%), Spodosols (25.8%), and Entisols(14.7%) with smaller areas of Histosols (2.0%), Inceptisols (1.1%), andAlfisols (1.0%) (Lamsal et al., 2006).

Elevation in the SFRW ranges from 3 to 91 m above mean sea level(Grunwald et al., 2006). Generally, the land is level (0–2% slopes) orgently sloping andundulating (2–5% slopes)with themajor exception tothis pattern being the moderately and strongly sloping land (5–12%slopes) along the Cody Scarp. The twomainphysiographic regions in the

watershed are the Gulf Coastal Lowlands and the Northern Highlands,which are separated from one another by the Cody Scarp (Schmidt,1997). Underlying geologic units in the SFRW include Eocene limestone,which occurs near the ground surface in the high-recharge, stronglykarst-influenced Gulf Coastal Lowlands. The Eocene limestone is cappedby Miocene sediments, which tend to be clayey and phosphatic(occurring at or near the surface along the Cody Scarp). The Miocenesediments are capped by Pliocence and Pleistocene-Holocene sedi-ments, which tend to be sandy at the surface but have loamy subsoils atvarying depths (Brown et al., 1990; Randazzo and Jones, 1997).

The stream network reflects the underlying geology as it shows amuch higher density in the eastern part of the watershed (Fig. 1b). Thepresence of the karst terrain and sinkholes has prevented theestablishment of such a stream network in the western part of thewatershed (Grunwald et al., 2006). The Santa Fe River actually flowsunderground for approximately 3 km as it passes through the CodyScarp. Based on the National Oceanic and Atmospheric Administration(NOAA) records at seven monitoring stations located within or nearbythe watershed, the mean annual precipitation calculated for theperiod from 1971 to 2000 was 1,334 mm and the mean annual tem-perature was 20.4 °C (NCDC, 2006).

2.2. Land-use reclassification

For the purposes of this study, upland forest, pine plantation,regenerating pine forest, and forestedwetlandwere reclassified into thebroader category of forested ecosystems. In the SFRW, these forestedecosystems experience little to no managed ungulate activity, andexcept for periods of harvest, are not subject to mechanical manipula-tions such as tillage or mowing. Typically the pine plantations are, atmost, fertilized twice during a 20 year rotation periodwith a cumulativeinput of less than 300 kg N ha−1; but often they are not fertilized at all. Itis also important to point out that the majority of the wetlands in theSFRWare forested systems as opposed to emergent freshwatermarshes.In contrast, non-forested sites in this watershed are more intensivelymanaged than the forested sites. For example, both the frequency (i.e.seasonal fertilization,mowing, and tillage) and the intensity of inputs tothe site (i.e. application of inorganic fertilizers, concentrated ungulateactivity, and heavy-machinery operation) are distinctly different. Thus,we reclassified the agriculture (including row crops and improvedpasture), rangeland, and urban systems into the broader category ofnon-forested ecosystems. Fig. 1c shows the distribution of forestedversus non-forested lands across the SFRW.

2.3. Field data collection

We selected 141 sampling locations using a stratified-randomsampling design. Strata were derived from land-use soil order combina-tions in ArcGIS 9.1 (Environmental Systems Research Institute, Redlands,CA). This design targeted sampling locations that were representative ofthe range of soil-landscape conditions that existed in the SFRW. As such,multiple sites within each of the land-uses, soil orders, within the sameland-use but different soil orders, and within the same soil order butdifferent land uses were sampled to ensure adequate representation ofthe land-use soil order combinations that existed in the SFRW. See Lamsalet al. (2006) for further information about the stratified-random design.Soil samples were collected during Sept. 2003, Jan. 2004, May 2004, Jan.2005,May2005, andSept. 2005. Thesedates represented typical seasonalpatterns with “wet/end of cropping season” (Sept.), “dry winter season”(Jan.), and “dry spring season” (May). Samples could not be collected inSept. 2004 due to the flooding of sites and access roads caused byHurricanes Frances (Sept. 4–8, 2004) and Jeanne (Sept. 26–30, 2004).

A representative plot was selected at each site, and the center pointof the plot was mapped with global positioning unit so that the plotcould be located in future sampling events. Soil cores were collectedwithin a 1 m distance from the center point with a 6.5-cm diameter

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auger. Four soil cores were collected from the 0 to 30 cm depth, threesoil cores were collected from the 30 to 60 cm depth and 60 to 120 cmdepths, and two soil cores were collected from the 120 to 180 cmdepth. The cores were composited for each respective depth and asubsample was collected for analysis. This sampling protocol ensuredconstant sample support in the different depth layers.

Not all sampling depths could be sampled in each event due to fieldconditions (e.g. high water table during rainy season). Likewise, not allsites could be sampled during each event as land owner permissionwas not granted for every sampling event. Therefore, the number ofsamples collected varied slightly by season and depth (Table 1).

2.4. Laboratory analyses

The soil samples were stored in a cooler on ice until being returnedto the laboratory. In the laboratory, samples were placed in arefrigerator and extracted within 24 h of collection. The extractionswere conducted with 2M potassium chloride (KCl) (Keeney andNelson, 1982). Extracts were then analysed for NO3-N content with anautoanalyser. Soil moisture content of the samples was determined bydrying a subsample for 24 h at 105 °C. Final NO3-N concentrationswere expressed as µg NO3-N g−1 of dry soil. Detection limits rangedfrom 0.02–0.05 ppm NO3-N (1.4–3.6 µM NO3-N). Samples below thedetection limit were assigned the mean value (derived from averagingthe lowest observed value in the respective sampling event and Null).Profile average NO3-N values (za) were derived according to Eq. (1),

za =∑4

i = 1zðxiÞ⁎di

∑4

i = 1di

ð1Þ

where z(xi) is themeasured NO3-N in μg g−1 soil at sampling sites (xi) forthe ith layer (i=1, 2, 3 and 4) and di is the thickness in cmof the ith layer.

2.5. Statistical analyses

Our objective was to investigate the effects of land-use, samplingdate (climatic component), soil order, and their combinations on soilNO3-N. The NO3-N data from the individual layers (i.e. 0–30, 30–60,60–120, 120–180 cm) as well as the profile average NO3-N data werelog transformed and analyzed with a series of mixed models (ProcMixed) (SAS Institute, Raleigh, NC). These mixed models includedfixed effects (land-use and soil order), random effects (site), andsampling date as a type one autoregressive repeated measure effect.Proc Mixed also allowed us to use separate variance components forthe forested versus the non-forested sites as we expected greatervariability in the NO3-N values from the non-forested sites than fromthe forested sites.

This model used a RestrictedMaximum Likelihood (REML) methodto estimate the mixed model parameters. The Akaike Information

Criteria (AIC) was used to select among possible mixed models. Meanvalues were compared across the different sample groupings with theleast squared (LSMeans) procedure in SAS. The NO3-N data was logtransformed so that it would better conform to the assumptions of themixed model. We added a constant (+1) to the NO3-N values beforethe log transformation to keep the log values N1. While the statisticalanalyses were conducted with the transformed data, the figures showthe raw data for ease of interpretation (i.e. raw numbers instead of log-scale).

3. Results and discussion

3.1. Forested versus non-forested sites across all dates

There were distinct trends in the NO3-N data in forested versusnon-forested sites, across the six sampling events, and with depth inthe soil profile. All events showed positively-skewed NO3-N distribu-tions for both forested and non-forested sites. Skewness was higherfor the forested sites across all sampling events as many of the NO3-Nmeasurements were below the detection limit for the forested land-uses (Table 1). It is a common phenomenon that environmentaldatasets show many more low than high values (Saito and Goovaerts,2000).

Prior to the hurricanes, the majority of forested and non-forestedsites had NO3-N levels above the detection limit. After the hurricanesthere was a dramatic shift, with none of the forested sites having NO3-N levels above the detection limit in the Jan. 2005 event, and less thanhalf of the forested sites having values above the detection limit in theMay and Sept. 2005 events. The decreases in NO3-N were most likelydue to the intense precipitation from Hurricanes Frances (Sept. 4–8,

Table 2Results of the mixed model ANOVAs testing for significance of the forest versus non-forest (FvNF), date, and the FvNF by date interaction effects for NO3-N across the fourlayers and for the profile average data

Layer Source of Variation Degrees of freedoma F p-value

0–30 cm FvNF 1, 140 61.7 b0.0001Date 5, 604 17.1 b0.0001FvNF⁎Date 5, 604 7.8 b0.0001

30–60 cm FvNF 1, 140 35.9 b0.0001Date 5, 595 27.1 b0.0001FvNF⁎Date 5, 595 3.8 0.002

60–120 cm FvNF 1, 137 25.0 b0.0001Date 5, 545 17.1 b0.0001FvNF⁎Date 5, 545 6.0 b0.0001

120–180 cm FvNF 1, 134 28.4 b0.0001Date 5, 448 12.2 b0.0001FvNF⁎Date 5, 448 1.5 0.19

Profile Average FvNF 1, 140 42.3 b0.0001Date 5, 604 22.4 b0.0001FvNF⁎Date 5, 604 10.1 b0.0001

a Reported as numerator degrees of freedom, denominator degrees of freedom.

Table 1Summary statistics for the depth-averaged soil NO3-N (μg g−1) from the forested (F) and non-forested (NF) sites in the six different sampling events

Date and site type na Mean Standard error of mean Median Minimum Maximum Standard deviation Skewness Kurtosis

9/03 F 64 0.32 0.08 0.17 0.01 4.59 0.62 5.50 36.529/03 NF 37 1.38 0.30 0.38 0.01 6.54 1.83 1.46 1.281/04 F 77 0.08 0.01 0.01 0.01 0.57 0.12 2.21 5.581/04 NF 44 9.39 3.58 0.30 0.01 103.12 23.76 3.00 8.645/04 F 81 0.31 0.3 0.22 0.11 1.79 0.30 3.45 13.635/04 NF 47 2.64 0.60 0.56 0.14 19.92 4.12 2.48 6.791/05 F 80 0.03 0.003 0.03 0.03 0.25 0.03 7.30 57.291/05 NF 60 1.17 0.31 0.08 0.03 13.47 2.39 3.25 12.575/05 F 81 0.05 0.02 0.01 0.01 1.23 0.19 6.14 37.095/05 NF 60 1.16 0.31 0.07 0.01 14.82 2.41 3.70 17.449/05 F 80 0.03 0.005 0.01 0.01 0.25 0.04 3.34 12.539/05 NF 55 0.95 0.21 0.16 0.01 6.19 1.56 2.27 4.95

an=number of sites sampled per event.

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2004) and Jeanne (Sept. 26–30, 2004), which passed directly over theSFRW. These storms caused a decrease in soil NO3-N concentration byboth direct and indirect processes. First, the inputs of precipitationleached NO3-N from all four layers of the soil profile and caused it tobe lost in surface water runoff and groundwater flow. Second, andmore indirectly, the precipitation inputs raised local water tables andwere assumed to stimulate high rates of denitrification of theremaining NO3-N in surface soil horizons.

According to the mixed model, forested versus non-forested statusaccounted for a greater proportion of the variance than did samplingdate across each of the four individual depths as well as for the profileaverage data (Table 2). The effect of forested versus non-forested

status was greatest in the upper 0–30 cm and tended to decrease withdepth (Table 2). Other studies have shown that the effects ofagriculture are greatest at the surface and decrease with depth inthe soil profile (Braekke, 1999; Compton and Boone, 2000; Brulandet al., 2003). While sampling date had less explanatory power thanforested versus non-forested status, sampling date did account for asignificant proportion of the variance in the NO3-N data across each ofthe depth categories as well as with the profile average data (Table 2).The forest versus non-forest by sampling date interaction term wasalso significant across each layer and for the profile averaged data(Table 2) indicating that differences in NO3-N between forested andnon-forested sites were not consistent across all sampling dates. For

Fig. 2.Mean concentrations of nitrate-nitrogen in the forested and non-forested sites in the upper 0–30, 30–60, 60–120, 120–180 cm, and the profile average during the six samplingevents. Error bars represent one standard error.

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example, while mean NO3-N was lower in forested than non-forestedsites for each sampling event and across all depths and the profileaverage data (Table 1, Fig. 2), the magnitude of these differencesvaried considerably from approximately 1 μg g−1 in the 2005 events to15 μg g−1 in the January 2004 event (Fig. 2). The highest NO3-N valueswere observed in January 2004 followed by May 2004, and this wasconsistent across all four depths.

Mean NO3-N was also significantly higher in the forested sitesacross all depths and in the profile average data in theMay 2004 eventwhen compared to the three 2005 events that followed HurricanesFrances and Jeanne. Mean NO3-N values in the non-forested sites weresignificantly higher in the pre-hurricane (2003–2004) events thanthey were in the post-hurricane (2005) events (Fig. 2) and thiswas most pronounced in the 0–30 cm soil layer. It also appeared thatNO3-N was not being retained in the 30–60 cm soil layer. In this zone,a combination of root uptake, denitrification, and leaching may havebeen responsible for the low observed NO3-N concentrations. Thelower organic matter content in this zone may have also resulted inless NO3-N retention and lower in-situ mineralization rates. Interest-ingly, in Jan. 2004, NO3-N values increased in the 60–120 and 120–180 cm layers indicating that NO3-N may have accumulated in thesehorizons due to changes in soil properties with depth or the presenceof deeper spodic horizons. While mean NO3-N values in the non-forested sites showed considerable variability prior to the hurricanes,ranging between 1–14 μg g−1, across the four soil depth categories,mean NO3-N values across all depths weremuch less variable after thehurricanes, ranging from 0.25–2.0 μg g−1 (Fig. 2).

The profile average median NO3-N values (often considered morerobust than mean values when dealing with somewhat skeweddatasets) were also lower in the forested than in the non-forested sitesacross the sampling dates (Table 1). Median NO3-N values wereactually highest for both forested and non-forested sites in the May2004 event, while mean values were highest in the Jan. 2004 events.This was largely due to a few very high (50–110 μg g−1) NO3-N valuesmeasured during the Jan. 2004 event. These high values observed inJan. 2004 may have been caused by a number of factors includingrecent fertilization of the non-forested crops and pastures, low plantuptake and microbial immobilization during the winter period, andlow precipitation that stimulated high rates of nitrification.

The lower mean NO3-N content during Sept. 2003 and in all three2005 sampling events could be attributed to the influence ofsubstantial precipitation from tropical storms and hurricanes(Table 3). High rainfall during the fall of 2003, and especially highrainfall associated with Hurricanes Frances and Jeanne in late summerof 2004 flushed NO3-N from soils in the SFRW. Large storm eventshave been shown to lead to redistribution and export of N in otherwatersheds (Welter et al., 2005). In the case of the SFRW in fall 2004,

NO3-N was most likely lost from the soils by some combination ofsurface runoff, shallow groundwater flow, and deep groundwatermovement into the underlying aquifer. However, this increased exportof soil NO3-N was most likely diluted by the large volumes of surfacewater flow that were generated from the hurricane rainfall. Thus,despite the large pulse of NO3-N from soils, concentrations of NO3-N insurface and groundwater following the hurricanes may have actuallybeen less than during baseflow due to the effect of dilution.

Interestingly, as water tables remained near the soil surface in muchof thewatershed for severalweeks following the hurricanes,we assumethe subsequent anaerobic conditions that most likely developed in bothupland and wetland areas stimulated denitrification and resulted infurther transformation of NO3-N to N2 gas and loss from the soil. Similarresults were found in another study that documented NO3-N leachingdue to rainfall-runoff events (Guo et al., 2001).

Another study of water quality in the Neuse River Watershed inNorth Carolina reported that three hurricanes in September 1999flushed nutrients out of the soil profile (Dukes and Evans, 2006). Thesethree events resulted in 584 mm of precipitation over a 21 day period(Dukes and Evans, 2006). By comparison, in Sept. 2004 the SFRWreceived an average of 368 mm of precipitation (based on 7 weatherstations) over a 26 day period. These results indicated that extremeweather events in the south-eastern United States have the potential toalter soil NO3-N status at the watershed scale. While the N cycle is quitecomplex and involves numerous interactivemechanisms, it appears thatextremeclimatic events in thiswatershed,whichoccurat low frequency,override other process that control soil NO3-N, such as fertilization,mineralization,microbial immobilization, andnitrification. For example,years with heavy rainfall (often from hurricanes) have been shown tocauseNO3-N tobeflushed/denitrified fromthe soil,while yearswith lowrainfall cause NO3-N to accumulate in the soil (Katz et al., 2001). Thus,climatic variabilitymay play an important role in NO3-N export from theSFRW. For example, rainfall at the Live Oak Station just north of theSFRWwas 1,890mm in 1991 and 840 in 1995, a difference of N1000mm(Katz et al., 2001); NO3-N biogeochemical cycling and export in these2 years were most likely very different.

3.2. Specific land-uses across all dates

According to the mixed model, both specific land-use (i.e.,agriculture, improved pasture, rangeland, urban, upland forest, pineplantation, forest regeneration, and wetland) and sampling dateaccounted for a significant proportion of the variance in the NO3-Ndata across each soil depth category and the profile average data(Table 4). The specific land-use by sampling date interaction was

Table 3Average precipitation data for different time periods during the 2003–2005 periodbased on data from seven climatic stationsawithin or near the Santa Fe RiverWatershed(NCDC 2006)

Month andyear

Monthly average(mm)

Previous 3-monthaverage (mm)

Annual average(mm)

Sept. 2003 93.9 565.8 2003: 1,403.5Jan. 2004 52.4 153.2May 2004 52.8 191.1Sept. 2004b 387.1 645.6 2004: 1,523.2Jan. 2005 44.2 194.1May 2005 139.5 389.9Sept. 2005 76.1 511.7 2005: 1,424.8

a The seven stations included Gainesville Regional Airport (Station ID 83326; AlachuaCo.), Glen St. Mary 1 W (Station ID 83470; Baker Co.), High Springs (Station ID 83956;Alachua Co.) , Lake Butler (Station ID 84723; Union Co.), Lake City 2 E (Station ID 84731;Columbia Co.), Live Oak (Station ID 85099; Suwanee Co.), and Starke(Station ID 88529;Bradford Co.).

b High monthly average due to rainfall from Hurricanes Frances and Jeanne.

Table 4Results of the mixed model ANOVAs testing for significance of the specific land-use(LU), date, and the LU by date interaction effects for NO3-N across the four layers and forthe profile average data

Layer Source of Variation Degrees of freedoma F p-value

0–30 cm LU 7, 135 23.2 b0.0001Date 5, 573 13.5 b0.0001LU⁎Date 35, 573 3.1 b0.0001

30–60 cm LU 7, 135 15.8 b0.0001Date 5, 564 25.9 b0.0001LU⁎Date 35, 564 2.3 b0.0001

60–120 cm LU 7, 132 12.5 b0.0001Date 5, 514 15.9 b0.0001LU⁎Date 35, 514 2.1 0.0002

120–180 cm LU 7, 129 12.2 b0.0001Date 5, 417 10.0 b0.0001LU⁎Date 35, 417 1.03 0.42

Profile Average LU 7, 135 25.7 b0.0001Date 5, 572 22.3 b0.0001LU⁎Date 35, 572 2.7 b0.0001

a Reported as numerator degrees of freedom, denominator degrees of freedom.

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significant for the upper three depths and the profile average data butnot for the 120–180 cm layer (Table 4). Overall, when specific land-uses were considered rather than the more general forested versusnon-forested classification, the F values for the effects of specific land-usewere less than the F values for forested versus non-forested status.Despite the decrease in the F statistic, the effect of specific land-usecontinued to havemore explanatory power than sampling date for the0–30 cm (F=23.2), 120–180 cm (F=12.2), and the profile average data

(F=25.7). Interestingly, sampling date had more explanatory powerfor the 30–60 cm (F=25.9) and 60–120 cm depths (F=15.9) suggestingthat different N transformation, retention, and loss mechanisms wereoccurring at each of the different depths or that the rates of thesedifferent mechanisms varied with depth.

The effect of specific land-use decreased consistently with depthwhile the effect of samplingdate did not (Table 4). Thismay be related toa number of factors including a decrease in sample sizewith the specific

Fig. 3. Mean concentrations of nitrate-nitrogen across the specific land-uses in the upper 0–30, 30–60, 60–120, 120–180 cm, and the profile average during the six sampling events.Error bars represent one standard error.

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land-uses and higher within class variability among the specific land-use classes. Despite the drop in the F value with the inclusion of thespecific land-uses, there were striking and consistent differences acrossthe land-use categories over time. For example, across all depths and allsix sampling events, the highest mean NO3-N values were generallyfound in the rowcrop agricultural sites (Fig. 3). The highestmeanNO3-Nvalues occurred in the Jan. 2004 event in the 0–30 cm (15 μg g−1), 60–120 cm (24 μg g−1), and 120–180 cm (25 μg g−1) layers. Interestingly,for the Jan. 2004 and May 2004 events, improved pasture siteshad higher mean NO3-N values than row crop agricultural sites in the0–30 (Jan.=26 μg g−1, May=10 μg g−1) and 30–60 cm (Jan.=5 μg g−1,May=3 μg g−1) depths. This suggested that in certain seasons and insurface soil depths, N inputs from a combination of fertilizer and animalwastes in improvedpasturesmayexceed those of N inputs of fertilizer torowcropagriculture. After rowcropsand improvedpastures, rangelandshad the next highest mean NO3-N values.

Profile average mean NO3-N values in row crop agricultural siteswere higher in the three events prior to Hurricanes Frances and Jeanne(Sept. 2003=1.90 μg g−1, Jan. 2004=17.6 μg g−1, May 2004=3.86 μg g−1)than in the three events that followed (Jan. 2005=2.25 μg g−1, May2005=2.10 μg g−1, Sept. 2005=2.56 μg g−1). Following the hurricanes,mean NO3-N values were highest in the agricultural and improvedpasture sites, many of which would have been fertilized and orreceived N inputs from animal waste (urine or fecal material).Rangelands, urban areas, and all forested sites showed low, if notundetectable, NO3-N values. The fact that NO3-N concentrations hadnot increased in the rangelands after the hurricanes suggests thatgrazing and natural fertilization in these sites was less intense than inimproved pastures. Woodard et al. (2002) compared NO3-N in soilwater below the rooting zone and forage N removal for two foragesystems on thermic, uncoated Typic Quartzipsamments comparableto soils and forage systems in the SFRW. They found that perennialbermudagrass (Cynodon spp.) was superior at preventing NO3-N fromleaching below the rooting zone than a rotation of corn (Zea mays L.),forage sorghum (Sorghum bicolor L.), and rye (Secale cereale L.). Thus,the process of crop rotation results in greater NO3-N leaching, as theplant species are being established, in contrast to a system like arangeland in which vegetative cover and plant uptake of NO3-N occuron a more continuous basis.

Throughout all sampling events and soil depths, pine plantations,forest regeneration, and upland forest sites showed consistently lowmean NO3-N values. Typical fertilization regimes for pine plantationsin this region provide (if utilized) 45–50 kg N ha−1 once during thefirst 5 years (forest regeneration phase) and as much as 200–300 kg Nha−1 applied once or in two applications of about 150 kg N ha−1 duringthe following 15 years (Pritchett and Comerford, 1982; Pritchett andComerford, 1983; Jokela, 2004). The low NO3-N values in the majorityof the forested sites were representative of upland forested ecosys-tems in Florida that are characterized by their tight cycling of N andlow fertilizer input. Forest fertilization, while an increasingly commonmanagement option, currently involves the application of no morethan 300 kg N ha−1 during a 20 to 25 year periodwith asmuch as 30 to50% of the fertilizer being removed from solution by plant uptake(Johnson, 1992; Blazier et al., 2006). That is in contrast to agriculturaloperations that will add that much N in a single year to 2 years.Furthermore, pine forests continue to absorb nutrients such as NO3-Nfrom the soil during the winter (Comerford et al., 1987) unlikedeciduous, hardwood forests. The upland forests sampled most likelyreceived no fertilizer during the study or for long periods prior to thestudy.

Although wetlands are positioned in the landscape to receivenutrient inputs from upslope areas (Bruland and Richardson, 2004;Bruland and Richardson, 2006; Grunwald et al., 2007), the wetlands inthe SFRW did not show high NO3-N concentrations. For example, lowmean NO3-N values were observed in each of the depth categories.This lead to the observation of low profile average NO3-N values across

the six sampling events. The low NO3-N concentrations in wetlandswere assumed to be due to receiving water from low NO3 pineplantation surrounding the wetlands, denitrification of NO3, andmixing of water from the river and groundwater by water levelfluctuations. McMahon and Böhlke (1996) showed that the medianNO3-N concentration in groundwater from adjacent floodplaindeposits (468 µmol L−1) and riverbed sediments (461 µmol L−1)were lower than the median concentration in the terrace deposits(1,857 µmol L−1). The authors estimated that 15–30% of the differencebetween floodplain/riverbed and terrace deposits was accounted forby denitrification and the rest by mixing of river and floodplainwater.

A chronological assessment of nutrient loading using age datingtechniques showed that the NO3-N concentrations of springs of theSuwannee River Basin have responded to increased N loads fromvarious sources in the basin for decades (Katz et al., 2001). Long-termtrends of NO3-N in the basin showed that the increasing NO3-Nconcentrations in spring waters followed the steady increase infertilizer use over time (Katz et al., 1999). Katz (2004) found thatinorganic fertilizers were the dominant source of N in spring waters inthe Suwannee River Basin based on δ15 N isotope tracers. This studyconfirmed that land-use is closely related to soil NO3-N and that non-forested land-uses were probably contributing to the majority of theNO3-N export from the watershed.

3.3. Soil orders across all dates

According to the mixed model, both soil order and sampling dateaccounted for a significant proportion of the variance in the NO3-Ndata across each soil depth category and the profile average data(Table 5). The mixed models indicated that sampling date had moreexplanatory power than soil order across the four depths and theprofile average data. This indicated that climatic factors associatedwith different seasons such as droughts, wet periods, and hurricanesmay have a greater effect on soil NO3-N than soil type in thiswatershed. The soil order by sampling date interaction was notsignificant at any depth nor for the profile average data (Table 5). Thenon-significant interaction term indicated that the differences inmean values across the soil orders were generally consistent acrossthe sampling dates. Mean NO3-N values were generally higher inUltisols across all depths and sampling dates (Fig. 4). The oneexception to this pattern was that mean NO3-N in the 120–180 cmlayer was highest in Spodosols in the January 2004 event. This wasmost likely due to retention of NO3-N in the spodic horizon. For theprofile average data, the highest mean value for any order was forUltisols in the Jan. 2004 event (7.5 μg g−1). Entisols had the next

Table 5Results of the mixed model ANOVAs testing for significance of the soil order, date, andthe order by date interaction effects for NO3-N across the four layers and for the profileaverage data

Layer Source of Variation Degrees of freedoma F p-value

0–30 cm Order 4, 137 4.2 0.003Date 5, 589 6.2 b0.0001Order⁎Date 20, 589 1.6 0.05

30–60 cm Order 4, 137 2.9 0.02Date 5, 580 9.8 b0.0001Order⁎Date 20, 580 0.75 0.77

60–120 cm Order 4, 134 2.8 0.03Date 5, 530 3.2 0.008Order⁎Date 20, 530 1.4 0.13

120–180 cm Order 4, 131 3.9 0.005Date 5, 438 5.9 b0.0001Order⁎Date 20, 438 0.5 0.95

Profile Average Order 4, 137 3.4 b0.0001Date 5, 590 5.4 0.01Order⁎Date 20, 590 0.96 0.51

a Reported as numerator degrees of freedom, denominator degrees of freedom.

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highest mean NO3-N values followed by Spodosols and then byAlfisols, although these differences were not significant according tothe post hoc LSD test. Again, the highest values were observed in the0–30 cm layer, followed by the 60–120 cm layer, and the 120–180 cmlayer. The 30–60 cm layer appeared to be a zone of NO3–N transportand leaching rather than a zone of retention across all soil orders. Thefinal category, titled “Other” in Fig. 4, included soils from the Inceptisoland Mollisol soil orders. Soils in this category had high mean NO3-Nvalues in the Sept. 2003 and May 2004 events, but low values in all

other sampling events (Fig. 4). Previous studies have shown thatagricultural practices increase NO3-N concentrations in shallowgroundwater particularly in areas with sandy soils (Dukes andEvans, 2006). The profile average mean values in the Ultisols,Spodosols, and Entisols were also significantly higher in May 2004prior to the hurricanes then they were in Jan. 2005.

We compared the mean profile average NO3-N values in theforested and non-forested sites stratified by soil order in the twoevents that occurred before and after Hurricanes Frances and Jeanne

Fig. 4.Mean concentrations of nitrate-nitrogen across the five soil orders in the upper 0–30, 30–60, 60–120,120–180 cm, and the profile average during the six sampling events. Errorbars represent one standard error.

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(Fig. 5). In the non-forested sites mean NO3-N was significantly lowerafter the hurricanes for the Ultisol and Entisol soil orders, while theSpodosols did not show much change, and Alfisols showed littlechange or even a slight increase. In the forested sites, NO3-N wasconsiderably lower in all four of the major soil orders following thehurricanes. We did not have any non-forested sites that wereclassified as Mollisols or Inceptisols, therefore this comparison couldnot be made for the final category (Other). In Jan. 2005, mean NO3-Nvalues in the non-forested sites for the four soil orders were almostidentical, all having mean NO3-N values around 1 μg g−1. For theUltisols and Entisols, this represented a drop in average soil NO3-Nconcentrations by about 2 μg g−1. By comparison, the NO3-N values inthe forested sites had mean NO3-N values of essentially zero. Thissuggests that extremeweather events and land-use control soil NO3-N

concentrations in the SFRW to a greater degree than soil order. Themajority of the non-forested sites would have experienced some sortof external NO3-N inputs from sources such as fertilization (in theagriculture and improved pasture) or animal wastes (from urinationand fecal deposits in the improved pasture and rangeland sites). It hasbeen argued that unlike cows and humans, the other biota in awatershed function largely by recycling nutrients (Freifelder et al.,1998). Thus, non-forested ecosystems dominated by humans, cows, orboth, would be expected to show higher N concentrations and export,while forested ecosystems would be expected to have lower Nconcentrations and export; this pattern existed in the SFRW.

Entisols in this part of Florida are characterized by sandy soilprofiles with low organic matter content (Grunwald et al., 2006).Using ArcGIS, we mapped areas of agricultural land-use that werelocated on Entisols (Fig. 6). Entisols would have the greatest potentialfor NO3-N export as most of these soils in the SFRW are excessivelydrained, andwater movement through these sandy soil profiles wouldbe much faster than in the more clay-rich Ultisols or the Spodosolswith their spodic horizons of illuvial organic material and sesqui-oxides. By slowing the movement of water through the soil profile,ponding water above the soil surface, or perching water aboveconfining subsurface layers, soil orders with fine-textured soils (i.e.Ultisols, Spodosols) would allow microbes a greater opportunity todenitrify porewater NO3 and prevent it from reaching the Gulf ofMexico through groundwater transport. The shaded sections in Fig. 6show the areas in the SFRW of agricultural land-use located onEntisols and represent areas that land managers and policy makerscould target for the implementation of best management practices toprevent loss of NO3-N to surface and groundwater in the western partof the watershed. Across the SFRW as a whole, land-use is a moreimportant determinant of soil NO3-N concentration than soil orderand best management practices should be implemented for row cropagricultural and improved pasture sites regardless of the soil order onwhich they occur. It has been shown that 90% of the NO3-N inputs to

Fig. 5. Mean concentrations of nitrate-nitrogen in the different land-use soil ordercombinations before (May 2004) and after (Jan. 2005) Hurricanes Frances and Jeanne(Ult NF=Ultisol Non-forested, Utl F=Ultisol Forested, Spod NF=Spodosol Non-forested,Spod F=Spodosol Forested, Ent NF=Entisol Non-forested, Ent F=Entisol Forested, AlfNF=Alfisol Non-forested, Alf F=Alfisol Forested).

Fig. 6. Areas of Santa Fe River Watershed in agricultural land-use and located on Entisols.

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the Mississippi Basin are from nonpoint sources, of which 74%are agricultural in origin (Rabalais et al., 2002). The SFRW appears tomirror this pattern of nonpoint source NO3-N export from agriculturalareas. If the majority of NO3-N exports within the SFRWwatershed arederived from agricultural areas, and particularly those located onEntisols, this suggests that these are the areas that need to be targetedfor BMP development. Identifying these areas and working withlandownersmayenable us to significantly reduce the amountofNO3-Nexported to coastal zones (Fig. 6).

4. Conclusions

In each of six sampling events spanning a period from 2003–2005,soil NO3-N was significantly lower in forested than non-forested land-uses across all four depths and in the profile average data. Whenhurricanes in Sept. 2004 passed over the SFRW, NO3-N was flushed/denitrified from the soil, resulting in some of the lowest NO3-N con-centrations observed in the forested and non-forested sites. However, insampling events following the hurricanes (2005), NO3-N concentrationsin forested and non-forested sites followed different trajectories. Non-forested sites, especially those in agricultural and improved pastureland-uses, showed significantly higher NO3-N concentrations bothin the upper 0–30 cm of the profile but also at depths up to 180 cm,indicating that inputs from subsequent fertilization and animals wereelevating soil NO3-N in the SFRW. Across the SFRW, land-use is a moreimportant contributor to soil NO3-N concentration than soil order andbestmanagementpractices should be implemented for agricultural sitesregardless of the soil order on which they occur. In particular, certainintensive land-uses (i.e., row crop agriculture and improved pasture) oncertain soil types (i.e., Entisols) may result in the export of NO3-N fromthe soils and eventually into the Gulf ofMexico. This type of informationcan help land managers and policy makers identify critical sourceareas and guide the establishment of best management practices or thedevelopment of incentive programs to change land-use practices in theidentified high-risk areas in the SFRW.

Acknowledgements

This project was supported by the USDA grant #2002–00501funded through the “Nutrient Science for Improved WatershedManagement” program. We would like to thank Dr. M.W. Clark, Dr.R.B. Brown for contacts with land owners and crucial advice duringthe field sampling phase. Wewould also like to thank K. Comerford, A.Comerford, J. Hall, S. Lamsal, A. Stoppe, S. Wu, D. Lucas and Dr. S. Satofor help with the field sampling and laboratory analysis.

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