The role of woody vegetation in preventing ground water pollution by nitrogen from septic tank leachate

Wat. Res. Vol. 21, No. 5, pp. 605-614, 1987 Printed in Great Britain. All rights reserved

0043-1354/87 $3.00+0.00 Copyright ~'~ 1987 Pergamon Journals Ltd

THE ROLE OF W O O D Y V E G E T A T I O N IN P R E V E N T I N G G R O U N D W A T E R P O L L U T I O N BY N I T R O G E N FROM SEPTIC T A N K LEACHATE JOAN G. EHI~NFELD Doolittle Hall, Center For Coastal and Environmental Studies, Rutgers University, New Brunswick, NJ 08903, U.S.A. (Received June 1986)

ASatract--The ability of woody vegetation to remove nitrogen from septic tank leachate was studied in pine upland, oak upland and hardwood wetland habitats of the New Jersey Pinelands. The study was stimulated by the incorporation of a term for plant uptake in nutrient dilution models used for Pinelands land-use management decisions. Plant response was studied at sites involving septic tank leach fields and matched control sites for each habitat type. At each site, total biomass, net production, and tissue N concentrations for the dominant species was determined. The hardwood wetland habitat had a total biomass of 15.9MTha -~, a net primary production of 5.4MTha-tyr -t and a net N uptake 75-80 kg N ha-' yr- J. Tissue N values for wetland trees and shrubs did not show significant differences between control and experimental sites. The pine upland communities had a biomass of 55 MT ha- ~and a net production of 5.7 MT ha- ~yr- t; net N uptake ranged from 45 kg N ha- ~yr- ~ (control sites) to 56 kg N ha- ~yr- ~, (experimental sites). The oak upland communities had a biomass of 59 MT ha- Land a net primary production of 5.0 MT ha - ~yr - t; net uptake ranged from 55 kg N ha - ~yr - ~in the control sites to 69.3 kg N ha- ~yr- J. Tissue N concentrations showed significant increases for tree but not shrub species in both upland habitats. The capacity of the upland woody plants to increase N uptake and storage appears to be related to roofing depth and to the proximity of the plants to the drain tile: only plants with deep taproots, growing close to (within I m) the trench showed significant increases in uptake. It is suggested that only by augmenting appropriate natural vegetation with supplementary tree plantings can upland vegetation be utilized to reduce nitrogen movement to groundwater from septic systems in sandy soil; vegetation at uaturally-occurring densities will not have a significant effect on water quality. Key words--ground water quality, septic tank leachate, vegetation, plant nutrient uptake, nitrogen, Pinus,

wetlands, wastewater renovation, sandy soils

INTRODUCTION This paper presents a study of the ability of native woody vegetation in a variety of habitats of the New Jersey Pinelands to remove nitrogen from septic tank effluent. Given that septic tanks and cesspools add 8 × l06 gal of domestic waste to ground water each year (Gerba, 1985), and that this waste contains high levels of nitrogen as nitrate and/or ammonia, depending on soil conditions (Hansel and Machmeier, 1980; Reneau, 1977; Starr and Sawhney, 1980), there is a large potential, often realized, for ground and surface water pollution with high levels of N (Kerfoot and Skinner, 1981; Canter and Knox, 1985; Groff and Oheda, 1982). During the past two decades, considerable attention has been devoted to the use of vegetated areas to treat wastewater (e.g. Athanas et aL, 1981; Loehr, 1977; Blobaum et aL, 1979; Nichols, 1983). However, most of the work has been devoted to the study of the land application o f wastes and relatively little work has been done on the possible use of vegetation to mitigate helowground sources of nutrient pollution. Brown and Thomas (1978) have assessed the capacity of grass to reduce N leaching from septic fields, and have applied the results to the

development of a model for regulating housing density in the pinelands (Brown, 1980). Franklin (1979) has proposed using native woody upland vegetation to mitigate groundwater pollution from septic tank systems in sandy soils. Roman and G o o d (1983) have cited plant uptake of nutrient polluttbn from homing developments as a wetland characteristic in support of strict wetland protection requirements in the Pinelands. Although the problems posed by septic tank leachate have been explored in depth for the New Jersey pinelands, they apply to all other areas with extensive suburban development on sandy soils (e.g. the Atlantic and Gulf Coastal Plains) and barrier islands (Anon., 1986). Since natural vegetation is commonly retained on or near houses in suburban areas, the potential effects of such vegetation on septic tank leachate-groundwater connections deserves investigation and assessment. The variables that have been identified as important in determining the ability of a plant-soil system to renovate wastewater include site geology and soils, hydrology, rate of wastewater application, wastewater quality, and the ability of the vegetation to remove and store nutrients from the wastewater

6O5

606

JOAN G. EHRENFELD

MATERIALS AND METHODS (Odurn, 1985; Heliotis and DeWitt, 1983). In order to be effective in wastewater renovation, the plants at a Study sites A total of 13 sites was studied in both wetland and upland given site must be capable of (1) increasing their areas. For each habitat type, sites, referred to below as nutrient uptake above rates under natural conditions, "septic" sites, were selected adjacent to the septic tank leach and (2) storing the excess nutrient in a form not field of a house, and "control" sites were situated either on susceptible to rapid release. For example, although the homeowner's property but > 100 m from the house, or emergent aquatic herbaceous plants frequently show in nearby state forests or other protected areas. Each elevated nutrient concentrations in their tissues dur- combinationof septic site and matched control is referred to below as a "set." Septic sites were selected following initial ing the growing season in response to cultural en- studies of groundwater height in wells that indicated the richment (Boyd, 1970), rapid and complete decom- direction of groundwater flow (Douglas and Busscher, position during the winter returns the nutrients to the unpubl, data). The lowland sites were established in hardwood swamps, a dominant wetland community of the outflow water, thus rendering the site ineffective at long-term nutrient removal (Sloey et al., 1978; Kibby, region which consists of a red maple-forest canopy, with a dense layer of ericaceous shrubs and a variably developed 1978; Nichols, 1983; Simpson et al., 1978). Woody herbaceous layer, depending on the canopy development vegetation offers the potential for long-term storage and site history (Ehrenfeld, 1986). Wetland sites were selected so that the septic tank leach field extended to within in bole and branch tissue, and i n leaf litter that, 20 m of the edge of the wetland. Upland sites were located because it is more resistant to decomposition than in both pine-dominated and oak-dominated forest areas. In that of aquatic plants (Dickinson and Pugh, 1974), each of these habitats, one pair of sites (septic and control) will form a nutrient-retaining pool [through micro- was established so that the study area vegetation was within the leach field, and another set was established with the bial immobilization in the standing crop of litter study plot vegetation adjoining and downgradient from, but (Berg and Staaf, 1981; Fahey, 1983)]. Septic tank effluent is similar in chemical com- not over, the leach field. These two vegetation-leach field configurations represent the most common such configurposition to domestic and municipal wastewaters ations in Pinelands housing developments (Ehrenfeld, 1984). (Canter and Knox, 1985). The principal difference Figure I shows the location of the study sites and region. Results are presented below as mean values for the wetland between ecosystem treatment of such wastewaters and the potential treatment of septic tank effluent by sites in septic and control locations, and either as means or as individual sets of septic/control sites for the uplands. vegetation is that the septic tank effluent is introduced A soil pit was excavated at each upland site and represenunderground, at the base of or below the main zone tative soil samples taken for analysis. Samples were air-dried of fine roots most active in nutrient and water uptake and submitted to the Soil Testing Laboratory, New Jersey (Hermann, 1977; Russell, 1977). Thus, the capacity of Agricultural Experiment Station, for chemical and physical vegetation to affect leachate quality will depend on analyses. In the wetland sites, auger samples were removed for analyses, and to determine the depth of the organic the physiological responsiveness of the plant species substrate. to elevated nutrient levels (Chapin, 1980), the abunVegetation and nutrient sampling dance, distribution and activity of roots, the capacity At each site, an 0.1 ha plot was delimited. Within each of the root system to respond to increased nutrients plot the vegetation was studied by (I) tallying all trees by and water, the lateral and vertical flow patterns of the species and breast height diameter (DBH), (2) tallying all effluent (which in turn reflect site topography, geol- shrub stems by species and basal diameter (BD), for stems ogy and soil), and the effectiveness of the soil in over 2.0era BD, (3) clipping all above-ground vegetation within 1.0 or 0.5 m2 quadrants randomly placed within the transforming and adsorbing nutrients. larger plots (quadrat size and number depending on the In this study, plant uptake of nitrogen was examdensity of the vegetation, but always totalling 30 m~ total ined in a variety of lowland and upland habitats area clipped). The clipped material was separated by species affected by nutrient inputs from septic tank effluent into current (leaves and new twigs) and perennial (older flow, in order to determine the potential for vegeta- twigs, branches and bole) tissues, dried at 70°C for 48 h, tive uptake to provide a means of reducing nitrogen weighed, and ground in a Wiley mill. Nitrogen was determined in the ground plant material by digestion with a input to the groundwater system. The sites are all HzSO4-HzO2 mixture, followed by a semi-Kjeldhahl deterwithin the New Jersey Pinelands, a large (550,000 ha) mination (Allen et al., 1975). Two-four replicates were done portion of southern New Jersey with very sandy soils for each sample, to maintain a SE of < 10% of the mean. Biomasses of trees and large shrubs were determined by and a shallow groundwater aquifer that both prousing regression equations relating basal or breast height vides high-quality drinking water and maintains a diameter, as appropriate, to the biomass components (curvariety of high-value freshwater and coastal wetrent tissue, branches, bolewood). These regression equations lands. The Pinelands area has been previously deincluded several developed for this work, and others culled from the literature; the sources and equations are given in scribed (Forman, 1979; Good and Good, 1984) and Ehrenfeld and Gulick ( 1981) and Ehrenfeld (I 984). Biomass its sensitivity to groundwater pollution from septic of small shrubs and herbs were determined from the matank inputs is the basis of extensive stringent land-use terial in the clipping plots. This work resulted in biomass regulations and water quality standards (Good and values for principal tissue types for the major species at Good, 1984). The Pinelands region is the northern tip each site, and nitrogen concentrations for each of these tissues. Annual bolewood production was determined by of the Atlantic Coastal Plain, which is reflected in its coring 10 trees per site, measuring mean annual growth vegetation and soils; thus, the water quality issues increments for the past 10yr, and applying the mean addressed here have relevance to the rest of the increment to the regression equations to determine boleCoastal Plain province. wood growth per year.

Plant uptake of septic tank leachate

~

607

ENION

Co

\t

Oce~l n C,o '\

~,

Laket~urst

0

C h a t s w o l tt $

HH

~ 0

I

som,~o~t

~

L A N TIC CITY

Fig. 1. Location of the study sites and study area. H: hardwood swamp sites; O: oak upland sites: P: pine upland sites.

Net primary production was calculated as the sum of the current leaf and twig tissue mass plus the bolewood increment mass. Loss to herbivores and premature tissue mortality were not taken into account, nor was underground production. Thus, these net annual production values provide a relative measure for inter-site comparisons, but are only estimates of the actual value of biomass production. Estimates of nitrogen fluxes and storage were obtained by multiplying nitrogen tissue concentrations by the appropriate biomass values. Since new tissues, particularly leaves and herbaceous green plants, are the primary consumers of nitrogen during the growing season, the net primary production is here considered a measure of"uptake potential," i.e. the ability of the vegetation of a site to utilize nitrogen. Similarly, the standing crop of perennial tissues and its nutrient content is here considered a measure of the ability of the vegetation to store nitrogen on a long-term basis, referred to below as "storage potential." RESULTS

Table 1 summarizes the descriptive data for the soils of the three site types. The wetland soils are classified as muck (Markley, 1971), contain organic matter to a depth of 0.5-2 m and are highly acidic, with a very low cation exchange capacity (CEC). The muck is underlain by coarse alluvial sand and/or gravel, but these sediments are normally well below the rooting depth of the wetland plants. In the upland sites, the soils at all depths are sands or sandy loams in the Evesboro series (mesic, coated, typic quartz-

ipsamments) and also are highly acidic. These soils are poor in nutrients (Tedrow, 1979), having low levels of cations, nitrogen and phosphorus (Wang, 1984). Organic matter accumulates in the surface horizon from a mor humus, but is frequently removed by fire (Forman and Boerner, 1981). The soils at the two upland site types are thus similar in texture and composition. The composition of the vegetation in the three habitat types is shown in Table 2. Hardwood swamps contain both a higher diversity of woody plants, and higher total biomass than do the upland sites. The canopy consists primarily of red maple (Acer rubrum) and associated swamp species, but upland trees (oaks and pine) may enter the wetlands along the edges or on "islands" of slightly higher ground within the swamp. The dominant trees are 50-100 yr old, with a mean DBH of 20.8-t-3.9 crn. The tree stratum accounts for a mean of 96.2% of the total biomass, with the rest due to shrubs and a small amount of herbaceous vegetation. Of the tree biomass, 97% of the biomass is accounted for by the boles and branches. In the shrub component, 89% of the biomass is accounted for by stems and branches. The vegetation of the hardwood swamps has been described in more detail by Ehrenfeld and Gulick (1981) and Ehrenfeld (1986). The pine uplands habitat presents a strong contrast

608

JOAN G . EHRENFELD Table 1. Soil characteristics o f three habitat types Texture

Hardwood swamp Pine upland

O a k upland

Depth (cm)

% Sand

% Silt

% Clay

% Organic matter

pH

CEC*

0-50 0-20 20-40 40 0-20 20--40 40

-87 84 91 90 95 95

(Muck) 6 9 2 7 I 1

-5 8 7 3 4 4

80-90% 3.4 0.9 0.4 4.4 1.0 0.4

3.7 3,7 4.3 4.5 3.8 4.6 4.6

5-10 <4 <4 <4 <4 <4 <4

* F r o m Markley (1971)

Table 2. Biomass (kg ha -~) o f m a j o r species m three habitat types. Values are means of all sites in each type ± SE. ' - - ' indicates the species did not occur in the site type Species N u m b e r o f sites

Hardwood swamp

Pine upland

Oak upland

5

4

4

Trees

Acer rubrum Nyssa sylvatica Magnolia virgi~iana Pinus rigida Quercus spp* Other trees

83,474 + 27,840 31,055 ± 5572 12,296 ± 5642 161 __ 159 17,840 ± 13,808 7815 + 3179

---50,329 4- 4374 -384 + 384

1688 ± 432 1068 ± 478 1072 ± 383 1124 ± 431 ---1 2 8 4 + 344 80 ± 53 158,957

----2902 + 110 832 4- 458 672 4- 160 46 4- 13 4 4- 4 55,169

---I 1,276 4- 1449 43,480 4- 3041 2553 4- 1221

Shrubs

Clethra alnifolia Vaccinium corymbosum Leucothoe racemosa Rhododendron viscosum Quercus ilicifolia Gaylussacia baccata Vaccinium vacillans Other shrubs Herbs Total

----481 ± 481 671 4- 258 142 4- 96 137 ± 100 I 4- I 58,741

*Includes Quercus alba, Q. velutina, Q. prinus, Q. stellata; referred to in the text as " c a n o p y o a k s . "

to the swamp habitat in community structure. The total site biomass is about one-third that of the swamp, primarily because of the structure of the canopy. The canopy consists of a single species (pitch pine, Pings rigida) typically of small size (mean dia 6-14cm DBH, mean age of dominant individuals 39-57yr; Ehrenfeid, 1984). This canopy structure reflects the regional history of frequent fires and frequent cutting (Forman, 1979); and thus these stands can be considered typical of the Pinelands region. The shrub stratum of the pine upland stands is characterized by a dense layer of scrub oak (Quercgs ilicifolia), and a sub-layer of low-growing ericads (Gaylgssacia baccata and Vaccinium vacillans). Although the total biomass of the shrub layer (4452kgha -~) is less than that of the swamps (6236 kg ha-~), it represents a higher fraction of the site biomass ( ~ 8% vs 4% in the swamps). The total biomass in the oak upland habitat is also low compared to the swamps, but is similar to that of the pine uplands. Tree species diversity is greater than in the pine uplands or the swamps, but shrub diversity is lower. The shrub stratum is the least well developed of the three habitats. Scrub oak is sparse or absent, and the low-growing ericads contribute little biomass to the stand. The trees range in diameter from 6 to 21 cm (25-83 yr); the larger size of

the oaks, compared to the pines, reflects their more frequent occurrence in sites with a history of protection from fire. Table 3 shows the distribution of the biomass by plant tissue. Despite the great difference in total site biomass between wetlands and uplands, both upland habitat types have a total net primary production comparable to that of the swamp habitat. This results from the fact that the dominant canopy species in the swamps (A. rubrum) has an open canopy of small leaves, whereas the dominant species in the upland habitats (P. rigida and the oaks, Quercgs spp) have much denser canopies of heavy needles and large leaves, respectively. The difference in the production of tree leaves and current twigs as a function of tree size for the three species is shown in Fig. 2. This figure was generated by solving the regression equation relating stem diameter to leaf and current twig production for each species for diameters from 2.5 to 20.0cm, at 0.5cm increments. The graph shows clearly that for stands of the same number of comparably sized trees, leaf and current twig production l'or A. rubrum is about half that of either P. rigida or Quercgs, while the production for Quercgs is only about 10% greater than that of P. rigida. For example, a 20 cm dia Quercgs would produce 7.0 kg leaves and new twigs each year. A similarly sized

Plant uptake of septic tank leachate

609

Table 3. Biomassdistributionin tissue types. Means(± SE) over all sites in habitat type Pine Oak Hardwood upland upland swamp Net primaryproduction(kg ha ~yr- t) Tree leavesand new twigs 3585 + 286 3320_+108 3283 + 151 Shrub leavesand new twigs 1298+ 321 240 -+75 873 -+ 148 Herbs 4+4 I -+ I 80 + 53 Bolewood increment 1197+ 212 1236+ 174 1133-+20.7 Total* 5696+_570 4983 + 87 5372-+29.2 Standingcrop (kg ha- i) Tree wood and bark 48.071 +_4225 51,780 + 1443 149,870+ 15,332 Shrub wood and bark 4168 +465 1439-+984 5498 -+984 Total* 51,353 _+4523 52,164 + 2472 154,013+ 15,606 Litter (kg ha - J) 18,134 + 369 I 1,244 -+73 5576+ 606 *Mean of site totals, not column total.

P. rigida would produce 6.4 kg leaf and twig biomass (91%), but a 20cm DBH A. rubrum would only produce 47.5% (3.3 kg) as much leaf and twig tissue per year. Since tree leaf production is the principal contributor to total site primary production (Table 3), the pattern of leaf production for the tree population of a given stand is a prime variable determining the capacity of the site for nutrient uptake. It can also be seen from Table 3 that shrub leaf production influences total site primary production although the oaks have the highest rate of leaf production per tre¢ (Fig. 2), the oak-dominated sites have the lowest net production because of the absence of a welldeveloped shrub layer. However, as the SEs indicate, there is considerable variation among stands, and the rate of net primary production for the three habitat types can be considered equivalent. In order to further characterize the determinants of net primary production, the upland sites were separately analyzed by correlation and stepwise multiple regression in order to relate net primary production to easily measurable stand parameters. Production was found to be correlated with total site biomass (r =0.72), and more weakly with tree biomass (r = 0.57) and mean tree dia (r = 0.51). Regression analysis showed that net primary production could

be expressed as a function of total site biomass and mean diameter of the dominant tree (NPP-- - 3 2 9 . 7 +0.127 T O T B I O - 140.459 DBH, r: adj = 0.78, F = 13.8). Since total site biomass can be estimated by using available dimension analysis equations to convert stem diameters of both tree and shrub species to biomass components, it is thus possible for a single set of simple measurements (diameter of all trees and large shrubs) at a site to be used to estimate net primary production, i.e. annual uptake potential. Table 3 also shows the standing crop values of perennial tissues for the three site types. The swamp habitats have a much higher storage potential for nitrogen than do tbe upland habitats, because of the much larger amount of bole and branchwood. Although the pine upland and the swamp habitats have comparable amounts of shrub wood tissue, the shrub stratum has a greater relative importance in the pine upland (8% of the total standing crop) than in the swamp (3.7%). However, the shrub contribution to nitrogen storage remains relatively small ( < 10%) at all site types. Table 4 shows the concentration of nitrogen in leaf plus current twig tissues of the upland species, and the results of t-test comparisons of values for septicenriched sites vs control sites. Because significant variation was found among sites (one-way analysis of 7 0 variance for Pinus, Q. ilicifolia, and the canopy oaks 1[ ~ P*ne all significant at P < 0.05), the data are presented as 6o! Oak comparisons for each set of matched control and septic sites. At all sets of upland sites, pitch pine 502 ~ : showed a significant increase in needle nitrogen concentration in the presence of septic tank effluent. The m difference between control and septic site values ~ 3oranged from an ! 1% to a 49% increase in tissue 7 i = nitrogen. The canopy species of oak, which have - 2c~ much higher levels of nitrogen under background 2 conditions than do the Pinus, also showed a variable response to the effluent, depending on site. At the I , o O enriched site in set 2, concentrations for two of the three species were 10-15% higher than in the paired T r e e D*ameter ( c m ) Fig. 2. Theoretical leaf and current twig biomass as controls. Differences that were not statistically a function of tree diameter, as predicted by regression significant nevertheless trended in the expected direcequations (see te×t), for the three major tree species. tion. The shrub species showed a contrasting pattern

JOAN G. EHRENFELD

610

Table 4. N i t r o g e n concentrations ( % dry wt + SE; n = 4) of leaf and current twig tissue in upland species: comparison of septic-enriched and control sites. '--' indicates the species did not occur in the site type Oak upland

Site set

Pinus rigida Quercus prinus Quercus alba Quercus velutina

1 2 1 2 1 2 1 2

Septic 1.17 1.26 2.05 1.66 1.92 1.73 1.68 1.68

+ 0.03 + 0.02 + 0.18 _+ 0.03 + 0.02 + 0.02 + 0.22 + 0.02

Quercus ilicifolia Vaccinium vacillans Gaylussacia baccata

Pine upland

1 2

1.03 _+ 0.10 0.93 +_ 0.02

1

1.40 + O.15

2

1.15+0.04

Control

Site set

1.05 + 0.01 b* 0.92 + 0.01 c 1.79 + 0 . 1 3 1.44 + 0.01 ¢ 1.67 + 0.12 0.64 + 0.08 1.83 __ 0.10 1.52 + 0.02 ~ -1.18+_0.02 1.10 + 0.03 c 1.29 + 0.04 1.35 + 0.06"

3 4

3 4 3 4 3 4

Septic

Control

1.16-+0.04 1.14 + 0.04 --

0.78+0.01 c 0.95 -+ 0.05 a

--

--

--

--

--

1.52+0.13 1.88+0.13 1.40+0.15 1.20 _+ 0.03 1.45-+0.07 1.21 __.0.04

1.58+0.05 2.25+0.31 1.31_+0.21 1.03 + 0.07" 1.23 + 0.01 b 1.34+0.11

*t-tests between paired sites; a: P < 0 . 0 5 ; b: P < 0.01; c: P < 0.001.

of response: there was no evidence of significant differences in the predicted direction. Quercus ilicifolia showed no significant differences among site pairs, and the smaller shrubs showed as many cases of control values higher than septic site values as vice versa. Thus, these data suggest that tree species but not shrub species were sensitive to below-ground nutrient enrichment at least at some sites. Table 5 shows the concentrations of nitrogen in old twig and branch wood for the upland sites. These results are similar in pattern to those of the current leaves and twigs. Pinus rigida shows significant increases in the enriched site of each pair, except set 3; the canopy oaks have significantly elevated concentrations at site set 2 but not set I, and the shrubs show no pattern of tissue concentration response to nitrogen enrichment. Table 5 also shows the concentrations for fresh litter. Although there are differences among habitat types (reflecting the species composition of the vegetation), there is no difference between pairs of each set of septic and control sites. The nitrogen contents of plant tissues from the hardwood swamp sites are shown in Tables 6 and 7.

Concentrations are more similar between septic and control sites than was observed for plants in the upland habitats but show more within-species variability (as indicated by the higher SEs). N o perennial tissue concentrations showed significant differences between enriched and control, and leaf tissue concentrations differed, albeit in the expected direction, for only three of nine species or species groups. Table 8 gives estimates of the increase in nitrogen uptake and storage that could be anticipated at a

Table 6. Nitrogen concentrations ( % dry wt + SE) in new tissues of hardwood swamp plants

Septic sites Acer rubrum

1.89 1.74 2.48 1.79 1.51 1.94 1.63 1.91 1.90

Nyssa sylvatica Magnolia virginiana Clethra alnifolia Vaccinium corymbosum Leucotho¢ racemosa Rhododendron viscosum Gaylussacia frondosa Herbaceous spp

Control sites

+ 0.13 + 0.07 + 0.15 + 0.14 ± 0.05 + 0.09 + 0.09 -+ 0.21 + 0.09

a: P < 0.05.

Table 5. Nitrogen concentrations in twig and branch wood, upland sites (% dry wt _+ SE) n = 4 for all data Oak upland Site set

Pinus rigida Quercus prinus Quercus alba Quercus velutina

I 2 1 2 I 2 I 2

Quercus ilicifolia Vaccinium vacillan.~ Gaylussacia baccata Litter

I 2 I 2 I 2

Septic

Pine upland Control

0.69 + 0.01 0.46+0.01 0.61 + 0 . 0 6 0.58+0.01 0.47 _+ 0.05 0.50 + 0.01 0.47_+0.12 0.50 + 0.02 --

0.44 + 0.02 c* 0.29 + 0.01 c 0.59+0.09 0.37 + 0.0V 0.49 -+ 0.03 0.39 + 0.02 ~ 0.49_+0.01 0.39 -+ 0.0V --

0.50-+0.04 0.51 _+0.02 0.55 -4-_0.07 0.40-+0.01 1.14_+0.15 1.07_+0.19

0.60-+0.13 0.53_+0.04 0.46 -+ 0.08 0.49+0.01 c 1.16-+0.08 1.04_+0.05

*a: P < 0.05: b: P < 0.01; c: P < 0 . 0 0 5 .

Site set

Septic

3 4

0.31 _+ 0.03 0.53+0.07

3 4 3 4 3 4 3 4

Control 0.40 + 0.02 ~ 0.31 + 0 . 0 1 =

---

---

--

--

0.40_+0.01 0.46 _+ 0.03 0.40 -+ 0,03 0.56_+0.07 0.46 _+ 0.05 0.31 -+0.05 0.88-+0.04 0.90_+0.04

0.34_+0.03 0.43 _-!-0.04 0.43_+0.01 0.38_+0.06 0.31 + 0.0 + ~ 0.40-+0.05 0.84+0.05 0.78_+0.08

1.93 + 0.19 1.48 + 0.02 a 2.10 + 0.06 1.92 + 0.12 1.55 + 0.17 1.49 + 0.15" 1.76 + 0.12 1.34 + 0.08" 1.97 -+ 0.14

Plant uptake of septic tank leachate Table 7. Nitrogen concentrations (% + SE) in twig and branch wood of hardwood swamp plants Septic sites Control sites Acer rubrum 0.30±0.11 0.60±0.10 Nyssa sylvatica 0.62 + 0.05 0.60 + 0.09 Magnolia virginiana 0.52 ± 0.12 0.68 ± 0.12 Clethra alnifolia 0.65 + 0.09 0.71 ± 0.05 Vaccinium corymbosum 0.58 + 0.08 0.49 + 0.19 Leucothoe racemosa 0.36 + 0.03 0.40 ± 0.07 Rhododendron viscosum 0.31 _+0.11 0.48 ± 0.16 Gaylu.~saciafrondosa 0.43 ± 0.07 0.41 ± 0.16 Litter 1.34 + 0.06 1.36 _+0.14 given site, based on the data presented above. The baseline values in Table 8 give the observed nitrogen quantities in the toal net primary production ( = uptake) and in perennial woody tissue ( = storage). These net N uptake values are very similar to the values of net N mineralized in these ecosystems (38-53 kg N ha -j yr-L; Poovaradom, 1986). The uptake values were derived by considering only the control sites in each habitat type, in order to get an indication of uptake capacity under undisturbed conditions. The nitrogen contents for each tissue and species at each site were then increased by the amount (as a percentage) observed at the matched enriched site for that tissue and species; this increase in N uptake was calculated only for those comparisons for which significant changes were recorded (Tables 4-7). Thus, the "if enriched" column in Table 8 shows the nitrogen uptake and perennial storage which could occur assuming that the changes in nitrogen concentrations observed between matched sites would occur if an undisturbed site were subjected to septic tank effluent. Table 8 shows that both the absolute and relative increase in nitrogen uptake and storage in a site subjected to septic tank leachate vary greatly both within and between site types. Wetlands show the poorest ability to utilize excess nitrogen in vegetative uptake. Because no woody tissues and few herbaceous tissues showed significant elevations in concentration, there was little impact on ecosystem rates of uptake and storage. In the upland communities, differences in plant response to nutrient additions, noted above, combined with differences in biomass distribution among species to yield very different results for the control sites in each habitat type. Although the pine uplands as a group were more capable of storing nitrogen than were the oak sites, there was high variation between the sets of sites in Table 8.

Mean

Site type

both vegetation types. Similar variation was also seen for N uptake. This variation among sites appears to reflect the juxtaposition of the plants and the drain pipes. The site sets with the larger increase in N utilization between septic and control were those with the sampled plants within the drain field, and within I m of the trench. The site sets with little or no response were those with the sampled vegetation at the edge of but not within the drain field. Thus, in these sandy soils, the effluent must move vertically to the water table and experience there sufficient dilution that even over relatively short horizontal differences, nutrient enrichment of the goundwater is not experienced by the phreatophytic woody plants. DISCUSSION The results demonstrate that native woody vegetation in a variety of habitat types has some ability to increase N uptake in the presence of excess N from septic tank drain fields, but that this ability varies greatly both with plant species, and with the spatial relationship between the plant location and the drainage field. In general, wetland species, and thus wetland habitats, show less capacity to respond to septic tank inputs than do upland habitats. In the uplands, pine responds more reliably over a range of site types than do the other species, and thus habitats with a dominance of pine in the canopy are more likely to show a change in net uptake than are habitats with pine mixed with other species. Finally, upland vegetation situated directly above a septic drain field shows a greater response than does vegetation located adjacent to the drain field. The failure of the wetland habitat to show a response to septic tank effluent was surprising, in view of the extensive evidence supporting the use of wetlands for wastewater renovation (e.g. Kadlec, 1979; Sloey et al., 1978; Heliotis and DeWitt, 1983), However, the results reported here were also obtained in a separate study of Atlantic white-cedar ( C h a m aecyparis thyoides) swamp response to septic tank effluent inputs (Ehrenfeid and Schneider, 1983). This supports the inference that wetland species do not experience changes in nutrient regime, or cannot increase N uptake, under these conditions. Since wetland plants do show increases in uptake in response to directly applied wastewater, it is likely that the mode of wastewater application is a critical

annual uptake and annual storageo f nitrogen (kg ha - ~ yr- n) at control sites, and projected values if subject to enrichment from septic tank leachates Baseline If enriched Increase Uptake Storage Uptake Storage Uptake % Storage %

Hardwood swamp

Pine upland 1 2 Oak upland I 2

611

75.7

4.6

80.2

4.6

4.5

5.9

0

0

45.6 44.7

4.1 2.3

56.4 52.0

4.1 3.9

10,8 7.3

23.7 16.3

0 1.6

0 69.6

68. I 54.8

7.9 5.1

69.3 66.4

8.4 7.5

1.2 11.6

1.8 21.1

0.5 2.4

6.3 47.1

612

JOANG. EHKENFELD

variable in determining plant response. In a typical lands soils (as well as in loam and clay soils); they wastewater application to a wetland, sewage effluent found that NH4-N accumulated below and adjacent is sprayed or sludge is spread over the wetland to the septic line and NO3-N accumulated at about surface, or is introduced into surface water flooding 75 cm from the line and below the line, but fell off the wetland. Thus, available nutrients enter the eco- rapidly with horizontal distance. Brown (1980) has system at the site of plant uptake (surface roots and, also extensively reviewed the literature on the vertical for submerged aquatic vegetation, the leaves). Under and horizontal movement of nutrients in septic tank such conditions, herbaceous vegetation in marshes leachate, and has reported that nitrate may move up can show two to five-fold increases in nutrient con- to 100 m laterally (more commonly, 10 m), and amtent (Slocy et al., 1978; Kibby, 1978; Odum, 1985; monium has been reported to move up to 12m Boyd, 1970), and woody vegetation has also been laterally. This movement, however, is within the shown to increase in nitrogen content in some studies groundwater. Brown and Thomas (1978) also found (Richardson et al., 1976). Nichols (1983) reviewed that for grass growing in lysimeter systems equipped several studies that demonstrated a switch from soil with septic drain tile, increased uptake extended only absorption of nutrients to foliar uptake in submerged 75 cra from the septic line. Thus, in order to utilize septic tank nutrients, root systems must either be vegetation subjected to nutrient-enriched floodwater. However, septic tank effluent entering a wetland has located within 1 m of the drain tile and at sufficient moved through both the soil absorption system depth to intercept the plume at and below the tile, or (which primarily converts NH4-N to NO3-N), and must be able to intersect the ground water table. The through the wetland substrate, which usually pro- results of this study suggest that both the spatial motes denitrifieation (Nichols, 1983). Thus, it is likely juxtaposition of the drain lines and the trees, and the that nitrogen from a septic tank effluent is signifi- natural rooting patterns of particular species detercantly reduced in quantity before the effluent water mine the capacity of woody vegetation to remove reaches the upper layers of wetland substrate, where nutrients from septic tank effluent. the bulk of the absorptive roots are located. In The amount of N removed (net uptake, Table 8) is addition, it should be noted that many of the woody small compared to that from repeatedly harvested species found in the wetlands studied here either grass systems (Brown and Thomas, 1978). In the cannot utilize nitrate or prefer ammonia as a source latter study, Bermuda grass on a comparable sandy of N (Nichols, 1983; Wall et al., 1982). Thus, the lack soil removed 194 kg N ha-~, whereas the ecosystems of response of the wetland woody species to septic studied here removed 45-75kgha -~yr -m. In the tank effluent nitrogen is likely to be the result of both Brown and Thomas study, the grass was repeatedly restricted N availability to the plants and inap- harvested during the long (March through October) propriate chemical form of the influent N. growing season. Clearly, the lack of ability to harvest The distribution of roots with depth may also be native woody vegetation and the shorter growing important in explaining the variation observed season contribute to the much smaller uptake rates. among the upland species of woody plants. Pinus In the absence of site-specific information about N rigida showed the most consistent increases in tissue loading to the soil from the septic systems studies N concentration; needle N levels were significantly here, the value of Brown et al. (1984) of elevated at all upland enriched sites, and wood N 2170 kg N ha -myr -~ can be used as a rough approxilevels were significantly elevated at three of the four mation of N input from the septic system. At this sites. This tree typically produces a long taproot, rate, the woody upland vegetation accounted for only extending 15-30 m (McQuilken, 1935), and sends out 2.5% (pine) and 3.1% (oak) removal. lateral roots at depths of 20-30 cm, below the mass The analysis of biomass production and tissue of roots of the shrub species. The canopy oaks distribution with tree size (Table 3; Fig. 2) suggests, showed significant increases in tissues N in both however, that site vegetation could be designed to leaves and wood at one of the enriched sites, but not increase this rate of N removal. For example, some at the other. These trees usually have shorter tap- pine-dominated ecosystems in the Pinelands region roots, and produce laterals at shallower depths in the support high densities of small trees (2600 stems ha- t, surface soil. The shrub species, including both the tall with a mean dia of 8 cm DBH). By removing the shrub Q. ilicifolia and the short shrubs V. vaciilans shrub layer vegetation and doubling the density of and G. baccata, showed virtually no response to the pine trees by means of planting within the area of the effluent. The roots of these species are concentrated drain field, the uptake could be doubled. Removal of in the organic layer and surface horizon of the leaf litter over the site could also be considered as an mineral soil, with much shorter taproots extending up analog of harvest, since leaf and and small twig to I m in length (Laycock, 1967). In the upland sites, production constitutes most of the net primary pro59% of the roots are in the top 10 cm of the soil, and duction and thus the tissue responsible for N uptake. 83% are within 30cm of the surface (Ehrenfeld, In conclusion, the results of this study suggest that 1984). native woody vegetation is capable of responding to Brown et al. 0984) studied the lateral and vertical nutrient additions from septic tank effluent by inmovement of N in sandy soils comparable to Pine- creases in uptake. However, the magnitude of the

Plant uptake of septic tank leachate uptake and storage is low, compared to herbaceous plants, because of the inability to repeatedly harvest the vegetation during the growing season and because of the relatively low nutrient levels characteristic of these plants. Removal of nitrogen from septic tank effluent by vegetation has been factored into the mathematical model now used to regulate housing density in the Pinelands (Anon., 1980), and has been suggested as a method to reduce N inputs to groundwater from high-density developments (Franklin, 1979). The results reported here do not support either of these applications of the idea, because of the low potential for N removal in forest biomass. REFERENCES

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