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For both wetlands, soil redox status affected P release and assimilatory ... Institute, Louisiana State University, Baton Rouge, Louisiana 70803-75 ll.(Prcsenl ...
WATER RESOURCES BULLETIN VOL.

28, N O . 4

A U G U S T 1992

AMERICAN WATER R E S O U R C E SASSOCIATION

PHOSPHORUS R E L E A S E AND ASSIMILATORY CAPACITY OF TWO LOWER MISSISSIPPI V A L L E Y FRESHWATER WETLAND SOILSi

P. H . M a s s c h e l e y n , J . H . P a r d u e , R . D . D e L a u n e , a n d W. H . P a t r i c k , J r .

e s t u a r i n e biogeochemical n u t r i e n t cycles. A l t h o u g h freshwater wetlands are thought to play an important role i n the flux and transformations of nutrients, the nature a n d significance of t h e i r role is still controvers i a l . Some have suggested that freshwater wetlands export large q u a n t i t i e s of n i t r o g e n a n d phosphorus ( D u x b u r y a n d P e v e r l y , 1978; P o m e r o y et a l , 1969), e n h a n c i n g the p r o d u c t i v i t y of d o w n s t r e a m ecosystems. O n the other h a n d , others often hypothesize t h a t freshwater wetland serve as a sink for nutrients (Boyt et a l , 1977; B r i n s o n et a l , 1984; L i n d a u et a l , 1988; N i c h o l s , 1 9 8 3 ; P e t e r j o h n a n d C o r r e l l , 1984; V a l i e l a et a l , 1976) r e d u c i n g e u t r o p h i c a t i o n of the aquatic environment. T h e idea of wetlands as a n u t r i ent sink has led to the increased use of wetlands for disposal and treatment of nutrient-laden agricultural, i n d u s t r i a l , a n d domestic wastewaters. A full unders t a n d i n g of the biogeochemical f u n c t i o n i n g of wetland ecosystems i s of critical importance i n assessing their n u t r i e n t release or a s s i m i l a t i o n capacity.

A B S T R A C T : P h o s p h o r u s fluxes a n d w a t e r q u a l i t y f u n c t i o n s of a bottomland hardwood a n d freshwater

m a r s h w e t l a n d soil were

c o m p a r e d . T h e effect of soil p h y s i c o c h e m i c a l c o n d i t i o n s , p h o s p h o r u s l o a d i n g r a t e , a n d diffusive e x c h a n g e b e t w e e n soils a n d t h e o v e r l y ing

flood w a t e r c o l u m n o n p h o s p h o r u s r e l e a s e a n d r e t e n t i o n were

s t u d i e d . T h e p r e d o m i n a n t l y m i n e r a l s w a m p forest soil d i s p l a y e d greater phosphorus sorption potential t h a n the organic

freshwater

m a r s h s o i l . M o r e o v e r , d u e to its l o w b u l k d e n s i t y (0.11 g c m } , t h e - 3

f r e s h w a t e r m a r s h soil surface a r e a r e q u i r e d for p h o s p h o r u s t i o n is v e r y l a r g e c o m p a r e d to t h e b o t t o m l a n d h a r d w o o d

reten-

wetland

s o i l . F o r b o t h w e t l a n d s , s o i l r e d o x s t a t u s affected P r e l e a s e a n d a s s i m i l a t o r y c a p a c i t y . T h e more r e d u c i n g the soils, the s m a l l e r their phosphorus

retention capacity (greater their

release).

P h o s p h o r u s r e m o v a l from t h e o v e r l y i n g w a t e r c o l u m n into the wetl a n d soils followed a

first-order

kinetic model. U n d e r similar hydro-

logical c o n d i t i o n s , p h o s p h o r u s w a s f o u n d to diffuse 1.2 t i m e s faster to t h e b o t t o m l a n d h a r d w o o d soil t h a n i n t h e f r e s h w a t e r m a r s h soil. R e s u l t s i n d i c a t e t h a t w h i l e t h e b o t t o m l a n d h a r d w o o d w e t l a n d soil w i l l serve as a s i n k for p h o s p h o r u s e n t e r i n g s u c h w e t l a n d , p h o s p h o rus will be released a n d exported from t h e freshwater

m a r s h soil

i n t o adjacent e c o s y s t e m s . (KEY

2

T E R M S : w e t l a n d s ; p h o s p h o r u s ; s o r p t i o n ; biogeochemistry.)

T h i s p a p e r f o c u s e s on p h o s p h o r u s ( P ) r e l e a s e and/or sorption from two different w e t l a n d types i n the L o w e r M i s s i s s i p p i Valley. M i c r o b i a l a n d plant uptake are reported to regulate short-term P wetland biogeochemistry by r e m o v a l of dissolved P from the water column. W h i l e the m i c r o b i a l pool i s s m a l l and q u i c k l y becomes s a t u r a t e d ( N i c h o l s , 1983), the wetl a n d vegetation t a k e s up s u b s t a n t i a l P q u a n t i t i e s , but, after tissue death, most of the p l a n t P i s released again (Nichols, 1983). L o n g - t e r m storage or release of P depends on soil a d s o r p t i o n - d e s o r p t i o n r e a c t i o n s , and the i n c o r p o r a t i o n - m i n e r a l i z a t i o n of organic P i n peat soil (Nichols, 1983; R i c h a r d s o n , 1985). We, therefore, identified and quantified soil biogeochemical processes r e g u l a t i n g P release a n d assimilatory capacity

INTRODUCTION The Lower M i s s i s s i p p i Valley, i n c l u d i n g the M i s s i s s i p p i r i v e r deltaic p l a i n , contains vast areas of freshwater wetlands, i n c l u d i n g bottomland hardwood swamp forests a n d f r e s h w a t e r m a r s h e s . T h e s e wetlands serve as a l i n k between terrestrial a n d aquatic systems, a n d r e p r e s e n t i m p o r t a n t ecosystems w i t h numerous beneficial functions. T h e y provide detritus for the food web a n d h a b i t a t a n d n u r s e r y grounds for aquatic organisms, waterfowl, and furbearers (De L a C r u z , 1 9 7 3 ; C r o w a n d M c D o n a l d , 1978). W e t l a n d s a r e a n i n t e g r a l c o m p o n e n t of g l o b a l a n d

' P a p e r N o . 9 2 0 0 4 of t h e W a t e r R e s o u r c e s B u l l e t i n . D i s c u s s i o n s a r e o p e n u n t i l A p r i l 1, 1992. R e s p e c t i v e l y , A s s i s t a n t P r o f e s s o r - R e s e a r c h , R e s e a r c h A s s o c i a t e , Professor, a n d B o y d P r o f e s s o r a n d D i r e c t o r , W e t l a n d

Biogeochemistry

I n s t i t u t e , L o u i s i a n a S t a t e U n i v e r s i t y , B a t o n R o u g e , L o u i s i a n a 70803-75 l l . ( P r c s e n l a d d r e s s for M a s s c h e l e y n i s : L a b o r a t o r y for A n a l y t i c a l a n d Agrochemistry, C o u p u r e L i n k s 653, B-9000 G e n t , Belgium.)

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WATER RESOURCES BULLETIN

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p r e p a r e d by m i x i n g 340 g of d r y soil w i t h 1700 m L d i s t i l l e d water so that the final soil to w a t e r ratio was 1:5. In the e x p e r i m e n t s w i t h the f r e s h w a t e r m a r s h soil, the final soil to w a t e r ratio was 1:18. A 1:18 ratio was the m i n i m u m ratio that allowed continuous s t i r r i n g of the freshwater m a r s h soil suspensions, due to the h i g h content of p l a n t debris. F o u r sets of samples f r o m each s o i l were e q u i l i b r a t e d a t d i f f e r e n t E ^ s : - 2 0 0 , 0, +200, a n d +500 m i l l i v o l t s (mV). A f t e r m a i n t a i n i n g the s o i l - w a t e r s u s p e n s i o n s a t t h e s p e c i f i c r e d o x p o t e n t i a l for t w o w e e k s , P w a s a d d e d (as K H P 0 ) to m a k e the total P load equal to 2.5 m g P kg d r y soil. A f t e r 24 h o u r s of e q u i l i b r a t i o n , a soil suspension aliquot was w i t h d r a w n , centrifuged, a n d filtered through a 0.45 urn filter, u n d e r an i n e r t argon atmosphere to m a i n t a i n r e d u c i n g conditions. T h e P c o n c e n t r a t i o n of the s u p e r n a t a n t was m e a s u r e d as d e s c r i b e d below. T h e n , the s o i l P l o a d i n g r a t e w a s increased to 25 m g P k g dry soil. A t the end of a 24h o u r e q u i l i b r a t i o n p e r i o d , the s a m p l i n g p r o c e d u r e was repeated. In a stepwise m a n n e r , the P l o a d i n g rate was increased to 50, 100, a n d 250 m g P k g dry soil. T h e amount of P sorbed (removed from solution) by the soil was calculated from the difference i n the concentration of phosphorus added a n d the concentration r e m a i n i n g i n solution after the e q u i l i b r a t i o n p e r i od.

of two different freshwater wetland types. T h e P processing capacity of a forested b o t t o m l a n d h a r d w o o d and a P a n i c u m h e m i t o m o n (maiden cane) dominated f r e s h w a t e r m a r s h s o i l were s t u d i e d i n l a b o r a t o r y microcosms a n d mesocosms, a n d field studies. O n the b a s i s of t h e r e s u l t s , the s u i t a b i l i t y of the s t u d i e d freshwater wetlands for P removal a n d water quality improvement is evaluated.

MATERIALS AND METHODS

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Soils W e t l a n d soil a n d flood water samples were collected f r o m t w o f i e l d s i t e s i n t h e L o w e r M i s s i s s i p p i V a l l e y : (1) a b o t t o m l a n d h a r d w o o d s w a m p f o r e s t ( S p r i n g B a y o u W i l d l i f e M a n a g e m e n t A r e a , Avoyelles P a r i s h , L o u i s i a n a ) a n d (2) a P a n i c u m h e m i t o m o n f r e s h w a t e r m a r s h (St. J o h n the B a p t i s t P a r i s h , Louisiana). T h e two wetlands represent a contrast i n hydrology, vegetation, and soil physicochemical characteristics. T h e forested b o t t o m l a n d h a r d w o o d wetl a n d is seasonally flooded by the M i s s i s s i p p i a n d R e d R i v e r s . T h e w a t e r table r i s e s i n the l a t e f a l l a n d , d e p e n d i n g on t h e y e a r , r e m a i n s i n the soil profile through the w i n t e r a n d into the growing season. T h e d o m i n a n t canopy vegetation include b a l d cypress ( T a x o d i u m d i s t i c h u m ) , honey locust ( G l e d i t s i a t r i a c a n t h o s ) , a n d oaks ( Q u e r c u s s p . ) . T h e clayey-silt soil has a p H of 5.7, b u l k density of 0.95 g c m , a n d an organic carbon (C) a n d phosphorus (P) content of 28 and 0.5 g k g s o i l , respectively. T h e c o n t i n u o u s l y flooded freshwater m a r s h borders L a c des A l l e m a n d s , a 65 k m f r e s h w a t e r l a k e i n t h e n o r t h e r n e n d of B a r a t a r i a B a s i n . P a n i c u m h e m i t o m o n is the predomin a n t p l a n t species, of which a large percentage makes up floating m a r s h . T h e freshwater m a r s h represents a h i g h l y organic soil with a p H of 6.3, bulk density of 0.11 g c m a n d c o n t a i n e d 230 g C k g s o i l , 15 g nitrogen k g soil, and 0.85 g P k g soil.

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A n additional experiment was performed u s i n g the controlled E ^ - p H microcosms to determine the critical redox potential for P release to the i n t e r s t i t i a l water in the bottomland hardwood soil. D e t a i l s of the experi m e n t a l p r o c e d u r e h a v e been p r e s e n t e d elsewhere and w i l l not be repeated here ( M a s s c h e l e y n et a l . , 1992). Briefly, suspensions were incubated u n d e r aerobic conditions (+500 m V ) , sampled, centrifuged, and filtered as described above. S u s p e n s i o n s were t h e n sequentially reduced by 100 m V increments u s i n g the redox c o n t r o l f e a t u r e s of the m i c r o c o s m s . S a m p l e s w e r e t a k e n at each i n c r e m e n t u n t i l a f i n a l redox potential of - 2 0 0 m V was reached.

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Phosphorus Diffusion from Overlying Flood Into the Wetland Soils

Waters

Sorption Studies L a b o r a t o r y S t u d i e s . P h o s p h o r u s diffusion from the o v e r l y i n g flood waters into the w e t l a n d soils was studied u s i n g intact flooded w e t l a n d soil cores. N i n e 25-cm l o n g soil cores from each s t u d y site were collected w i t h 15-cm d i a m e t e r P V C c y l i n d e r s . T h e core bottoms were sealed with a P V C cap a n d watertight silicon sealant, a n d transported to the laboratory. In the laboratory, cores were flooded to a 12-cm h e i g h t by a d d i n g or r e m o v i n g some of the flood water. A f t e r one m o n t h of e q u i l i b r a t i o n i n the laboratory at 25'C, the supernatant water was loaded w i t h P. T h e

T h e relationship between P removal and P l o a d i n g rate and soil physicochemical conditions was studied i n l a b o r a t o r y m i c r o c o s m s . S o i l m a t e r i a l f r o m the study sites was equilibrated at various redox potentials ( E ) u s i n g a redox control system (Patrick et a l . , 1973). C o n t r o l l i n g the redox potential allows for the duplication of a range of n a t u r a l l y - o c c u r r i n g physicochemical conditions t h a t exist in w e t l a n d soils. F o r the bottomland hardwood soil, the suspensions were h

WATER RESOURCES BULLETIN

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Phosphorus Release a n d Assimilatory C a p a c i t y of Two L o w e r M i s s i s s i p p i Valley F r e s h w a t e r W e t l a n d Soils

or without orthophosphate-P) was applied at the end of each system so that the daily effluent volume was 300 m L . The influent volume ranged from 300 to 320 m L d a y . A l l four systems were preconditioned during a six-day period by a p p l y i n g distilled water. T h i s allowed the soil to o b t a i n near field c h a r a c t e r i s t i c s ; i.e., a bulk density of 0.14 g c m , a n d a flood water depth of approximately 1 cm. A f t e r the p r e - e q u i l i b r a tion period, two of the four systems were loaded w i t h increasing amounts of P. The other two systems were used as controls. In the treated freshwater systems, the P loading was as follows: 0.095 m g P k g dry soil d a y for seven consecutive days, followed by 0.95 m g P k g dry soil d a y for another seven consecutive days, and 9.55 m g P k g dry soil d a y for three consecutive days. A t the end of P a p p l i c a t i o n , d i s t i l l e d water was passed over the treated system for one day. The total P loading rate was 36.02 m g k g d r y soil. Distilled water was passed over the control plots during the 25-day study period. Effluents were analyzed d a i l y for dissolved i n o r g a n i c a n d total P concentrations.

f o l l o w i n g P loadings were a p p l i e d : 0 m g P L (control), 1 m g P L - flood water (0.12 g/m ), and 10 m g P L flood water (1.2 g/m ). Phosphorus was added as KH2PO4. T h r e e replicates were performed for each t r e a t m e n t . T h e decline i n P c o n c e n t r a t i o n s i n the flood water was monitored over time. Periodically, 8 m L a l i q u o t s were w i t h d r a w n from the o v e r l y i n g waters at a u n i f o r m height (6 cm above soil surface). One h o u r before each s a m p l i n g , the flood water was gently m i x e d , to provide a u n i f o r m P concentration i n the w a t e r w i t h no r e s u s p e n s i o n of solids. S a m p l e s were collected i n glass vials a n d stored for P analysis. A t the e n d of the e x p e r i m e n t , the r e m a i n i n g flood water was siphoned off, and the P contaminated soils were allowed to become aerobic (oxidized) for a period of two m o n t h s . T h e n , d i s t i l l e d water was added as floodwater a n d the release of P from the soil into the water determined d u r i n g a two week period. _ 1

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M e s o c o s m S t u d i e s . F i e l d mesocosm studies were i n i t i a t e d to measure actual P soikwater exchange for c o m p a r i s o n s to l a b o r a t o r y s t u d i e s . In A p r i l 1991, sealed field enclosures (1.44 m ) were constructed at the b o t t o m l a n d hardwood w e t l a n d study site u s i n g P V C plastic sheeting (0.3 cm thick). The bottom of the enclosures were forced into the soil (4-6 cm) a n d were h i g h enough to prevent flow over the top of the mesocosm. A f t e r securing the enclosures, the flood water column h e i g h t i n the mesocosms was measured and the water column loaded w i t h P. Flood water height v a r i e d from 21-26 cm. Two different P loading rates were a p p l i e d i n duplicated enclosures: 10 and 20 m g L r flood water. The total P loading rate ranged from 1.85 to 2.11 g P m - and from 4.46 to 4.93 g P n r " , for the 10 a n d 20 m g P L r treatments, respectively. Two control plots (no P added) were also included. Several times d u r i n g a 16-day period water samples were collected a n d analyzed for P. Water depth and temperature i n the plots were also measured. 2

A n a l y t i c a l M e t h o d s . Redox measurements were made u s i n g bright p l a t i n u m electrodes w i t h a calomel electrode as reference. Inorganic P (orthophosphateP) was determined c o l o r i m e t r i c a l l y (detection l i m i t , 10 ug/L; U S E P A , 1979). Total P was determined with an i n d u c t i v e l y coupled argon emission spectrometer (ICP). The detection l i m i t of our I C P for P i s 30 pg L . T h e a m o u n t of P p r e s e n t a s o r g a n i c P (+ p o l y p h o s p h a t e s ) w a s c a l c u l a t e d as the d i f f e r e n c e between the total P a n d PO4-P a n a l y s i s . D i s s o l v e d organic carbon i n the flood waters was d e t e r m i n e d a c c o r d i n g to M o o r e (1985). S o i l s were d i g e s t e d for total metal and P content u s i n g A q u a R e g i a . M e t a l s and major cations i n soil extracts a n d flood waters were determined with an ICP. _ 1

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P h o s p h o r u s release a n d storage capacity of the f r e s h w a t e r m a r s h was s t u d i e d i n an o v e r l a n d flow system. The use of this system allowed for the s i m u l a tion of the P processing capacity of the f r e s h w a t e r m a r s h soil under near field conditions. The design of the overflow system was s i m i l a r to an earlier overl a n d flow treatment system (Pardue e t a i , 1988). The i n s i d e d i m e n s i o n s of the P l e x i g l a s overflow system were 51 x 11.5 x 5 cm (L x W x H ) . The subsurface effluent collection port was sealed and the aspirator was disconnected. The overflow systems were placed h o r i z o n t a l l y w i t h no s l o p e . T h e s u r f a c e e f f l u e n t p a s s e d over a 1 cm weir, i n t o a f u n n e l c o n t a i n i n g ( W h a t m a n 42) filter paper a n d was collected i n graduated c y l i n d e r s . A p p r o x i m a t e l y 330 g of u n d i s t u r b e d freshwater m a r s h soil was placed into each of the four systems. D u r i n g a 25-day period, distilled water (with

RESULTS A N D DISCUSSION Bottomland Hardwood

Soil

S o r p t i o n - d e s o r p t i o n and/or p r e c i p i t a t i o n - d i s s o l u t i o n of i n o r g a n i c s o i l P, a n d d i f f u s i o n e x c h a n g e between soil and flood water column, and vice versa, w e r e the b i o g e o c h e m i c a l p r o c e s s e s r e g u l a t i n g P behavior i n the bottomland hardwood w e t l a n d soil. P h o s p h o r u s S o r p t i o n S t u d y . For the bottomland h a r d w o o d soil, redox p o t e n t i a l was f o u n d to be an i m p o r t a n t physicochemical p a r a m e t e r c o n t r o l l i n g P uptake and retention (Figure 1). U n d e r reduced soil

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conditions (0 a n d - 2 0 0 m V ) , a s u b s t a n t i a l amount of indigenous P was released from the soil (0.9 a n d 2.72 mg P k g dry soil, respectively). How&Ver, w h e n the soil was loaded w i t h P, removal of P by the soil significantly reduced w a t e r soluble P concentrations. Soils equilibrated u n d e r oxidized a n d moderately reduced conditions (+500 a n d +200 m V ) removed from 90 to 98 percent of the P added to the water, depending on the P load. U n d e r reduced conditions (0 a n d - 2 0 0 m V ) , 28 to 74 percent of the P load was removed by the soil. H i g h e r percentages of added P was removed by the soil at h i g h e r P loads. Phosphorus sorption and release from the m i n e r a l bottomland hardwood wetland soil was governed largely by the F e chemistry of the soil, w h i c h , i n t u r n , was greatly influenced by the oxidation-reduction status of the soil. F i g u r e 2 i l l u s t r a t e s how soluble Fe and P concentrations were linearly correlated (R =0.94) as a function of soil redox p o t e n t i a l . C r i t i c a l soil redox potentials for M n a n d N 0 reduction (between +200 to +300 m V ) i n the w e t l a n d soil agree w i t h previously described sequences ( P a t r i c k a n d D e L a u n e , 1977). The sequence of N 0 , M n , a n d F e redox reactions also occurred i n order of t h e i r t h e r m o d y n a m i c possibility (denitrification followed by M n reduction followed by Fe reduction). T h e amounts of water soluble F e and P increased w i t h a decrease i n redox p o t e n t i a l below 200 mV. T h e increase of soluble P as a r e s u l t of soil

flooding is well documented, and i s generally a t t r i b u t ed to the d i s s o l u t i o n of s l i g h t l y soluble f e r r i c phosphate compounds and i r o n oxyhydroxide solid phases containing coprecipitated P (Carignan and Flett, 1981; K r o m and B e r n e r , 1980; P a t r i c k and M a h a p a t r a , 1968; P o n n a m p e r u m a , 1972). U p o n floodi n g , m i c r o b i a l d i s s i m i l a t o r y reduction processes w i l l dissolve slightly soluble F e solid phases a n d thereby release any P associated w i t h the i r o n . M i c r o b i a l reduction of Fe(III) solid phases also determined to a great extent the P sorption capacity of the soil. In oxidized m i n e r a l wetland soils, Fe(III)-oxides tenaciously b i n d P. W i t h the development of anaerobic conditions a n d the s i m u l t a n e o u s r e d u c t i o n of the Fe(III) compounds, the sorptive capacity of the soil for P d r a s t i c a l l y d e c r e a s e s . T h e e x t e n t of F e r e d u c t i o n , characterized by the w e t l a n d soil redox status, determines, to a great extent, the d i s t r i b u t i o n of P between the soil solid a n d water soluble phase. A s the reduction i n t e n s i t y (decrease i n E^) of the soil increases, the P a s s i m i l a t o r y capacity of the studied bottomland hardwood soil decreased (Figure 1).

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3

Movement Between Floodwater and Soil: L a b o r a t o r y M e s o c o s m a n d F i e l d S t u d y . Results f r o m the sorption s t u d i e s i n d i c a t e t h a t the b o t t o m l a n d hardwood wetland soil could serve as a s i n k for P e n t e r i n g the w e t l a n d . I n o r d e r to f u r t h e r test t h i s

F i g u r e 1. P h o s p h o r u s R e l e a s e , a n d R e m o v a l C a p a c i t y as A f f e c t e d b y S o i l R e d o x P o t e n t i a l a n d P L o a d i n g R a t e for the B o t t o m l a n d H a r d w o o d W e t l a n d S o i l ( n e g a t i v e v a l u e s r e p r e s e n t release o f P by the soil).

WATER RESOURCES BULLETIN

766

P h o s p h o r u s R e l e a s e a n d A s s i m i l a t o r y C a p a c i t y of T w o L o w e r M i s s i s s i p p i V a l l e y F r e s h w a t e r W e t l a n d S o i l s

90

Soli

redox

potential

(mV)

F i g u r e 2. C r i t i c a l Soil R e d o x L e v e l f o r N 0 , M n , F e R e d u c t i o n , a n d P 3

R e l e a s e i n the B o t t o m l a n d H a r d w o o d W e t l a n d S o i l .

h y p o t h e s i s , a l a b o r a t o r y mesocosm study a n d field study were conducted.

C = C ( e ) , where C represents the i n i t i a l P concentration added, and C the flood water P concentration after a certain time t (in days). Removal of P from the water column was a first-order process with a h a l f life of 3.9 days. The constant k equaled 0.176 ± 0.027 (± s.e.). The P flux into the soil was 0.76 a n d 6.42 p.g P c m d for the 1 a n d 10 m g P / L of floodwater experim e n t s , respectively. T h e flooded w e t l a n d s o i l t h u s served as a sink for P. A t the end of the experiment, the r e m a i n i n g flood water was siphoned off a n d the surface of the P contaminated soils was allowed to become aerobic (oxidized) for a p e r i o d of two m o n t h s . T h e n , d i s t i l l e d water was added as flood water a n d the release of P from the soil into the water determined d u r i n g a twoweek period. Phosphorus and Fe concentrations i n the water overlying the soil increased gradually over time and r e a c h e d a m a x i m u m of 0.09 a n d 0.28 m g I r , respectively. A p p a r e n t l y , some of the soil F e a n d P became solubilized a n d diffused back into the overlyi n g water column. t

0

_kt

0

t

L a b o r a t o r y M e s o c o s m S t u d y . Intact bottomland hardwood wetland soil cores were flooded with flood water collected from the field site (containing 0.12 m g P L ) , to a uniform height (12 cm above soil surface) and equilibrated for one m o n t h i n the laboratory. A t the end of the 30-day e q u i l i b r a t i o n period, the flood water column characteristics were as follows: pH=6.9 ± 0.2 (range), a n d Eh=380 ± 30 m V (range). The surface soil was reduced (Eh=+50 m V ) a n d h a d a p H of 6.8. U n d e r these experimental conditions, release of P from the bottomland hardwood w e t l a n d soil into the overlying flood water occurred. A t the end of the equil i b r a t i o n p e r i o d , the flood w a t e r c o l u m n contained 0.78 ± 0.20 a n d 0.16 ± 0.16 m g L r F e a n d P, respectively. A s s u m i n g the area of the soil surface responsib l e f o r P r e l e a s e to be e q u a l to t h e i n t e r n a l cross-sectional area of the P V C cores (177 cm ), the P flux into the overlying water flood water column was 1.92 ug P c m over the 30-day equilibration period. W h e n the water column o v e r l y i n g the bottomland hardwood wetland soil was loaded w i t h P, diffusion of P from the flood water to the soil was found to be the c r i t i c a l biogeochemical process c o n t r o l l i n g P c h e m i s t r y (Figure 3). U s i n g a n o n l i n e a r regression techn i q u e ( S A S , 1987), t h e d a t a sets were f i t t e d by: - 1

- 2

1

- 1

1

2

- 2

F i e l d S t u d y . T h e f i r s t - o r d e r P r e m o v a l process from the flood water column was confirmed i n a field study using confined mesocosms. A t the start of the experiment, the P e q u i l i b r i u m concentration i n the control and treated plots was approximately 0.6 m g L flood water, and r e m a i n e d practically unchanged - 1

767

WATER RESOURCES BULLETIN

Masscheleyn, Pardue, DeLaune, and Patrick

over t h e 1 6 - d a y s t u d y p e r i o d i n the c o n t r o l mesocosms. T h e k i n e t i c s of P diffusion from the water colu m n into the s w a m p forest w e t l a n d soil i n the treated plots i s shown i n F i g u r e 4. U n d e r field conditions, P removal k i n e t i c s ( C = C ( e ) ) was characterized by a decay constant (k) of 0.091 ± 0.01, l e a d i n g to a h a l f t

0

r e m o v a l t i m e of 7.6 days. P h o s p h o r u s r e m o v a l was slower u n d e r field conditions t h a n i n the laboratory diffusion columns (Figure 3 vs. F i g u r e 4). Conformity of the P r e m o v a l curves to a first-order k i n e t i c reaction i m p l i e s t h a t the reaction rate is proportional to the q u a n t i t y of P r e m a i n i n g i n t h e flood w a t e r

_kt

c IB O

c o u 3 i—

O J=

o. in

O

a. IB

nj 73 O Q U.

0

2

4

6

Time

8

10

12

(days)

14

F i g u r e 3. D i f f u s i v e E x c h a n g e B e t w e e n F l o o d W a t e r C o l u m n P a n d B o t t o m l a n d H a r d w o o d W e t l a n d S o i l ( r e s u l t s from l a b o r a t o r y

study).

F i g u r e 4. D i f f u s i v e E x c h a n g e B e t w e e n F l o o d W a t e r C o l u m n P a n d B o t t o m l a n d H a r d w o o d W e t l a n d S o i l ( r e s u l t s from field s t u d y ) .

WATER RESOURCES BULLETIN

768

16

P h o s p h o r u s R e l e a s e a n d A s s i m i l a l o r y C a p a c i t y o f T w o L o w e r M i s s i s s i p p i V a l l e y F r e s h w a t e r W e t l a n d Soils

solution phase when the solution concentration of P w a s low, a n d removed P f r o m s o l u t i o n w h e n the P c o n c e n t r a t i o n was h i g h . E x c e p t u n d e r o x i d i z e d soil conditions (+500 m V ) , P was released at the lower P l o a d i n g rates (0-50 m g P k g d r y soil) studied. W h e n the soil was loaded w i t h 100 a n d 250 m g P k g dry soil, approximately 18 and 60 percent of the added P was removed by the soil, irrespective of soil redox status. Compared to the P retention capacity of the bott o m l a n d h a r d w o o d w e t l a n d s o i l ( F i g u r e 1), t h e organic freshwater m a r s h soil has a very low P a s s i m ilatory capacity. A s i n the bottomland hardwood wetl a n d soil, increased P concentrations i n solution were correlated w i t h increased soluble F e concentrations, i n d i c a t i n g the i m p o r t a n c e of F e a n d soil o x i d a t i o n reduction status i n controlling P chemistry.

column. T h e observed difference between the l a b o r a tory diffusion a n d field study m a y be due to the difference i n flood w a t e r c o l u m n h e i g h t . W h i l e the flood water table h e i g h t i n the laboratory study was 12 cm, the water c o l u m n height i n the field mesocosms varied from 21 to 26 cm. Differences i n P removal rate probably indicate incomplete m i x i n g of the water colu m n i n the f i e l d study. W h e n the water c o l u m n is incompletely m i x e d , the water depth may become an i m p o r t a n t v a r i a b l e for P r e m o v a l i n wetlands. T h i s f i n d i n g i l l u s t r a t e s the importance of wetland h y d r o logic conditions for removal of flood water P. In genera l , as the depth of the water i n the wetland increases, the level of P removal decreases, i f m i x i n g is incomplete.

- 1

- 1

T h e h i g h content of dissolved organic carbon ( D O C ) in the freshwater m a r s h soil f u r t h e r complicates P behavior. L i t t l e is k n o w n about the f i x a t i o n of P by organic soils compared to the v o l u m i n o u s l i t e r a t u r e t h a t exists on the fixation of P by m i n e r a l soils. The formation of organic P is believed to be the dominant form of P a c c u m u l a t i o n i n peat w e t l a n d s a n d u l t i mately controls the rate of P storage i n these wetland types (Nichols, 1983). M i n e r a l i z a t i o n of soil-organic P and the subsequent release of dissolved organic P or i n o r g a n i c P w i l l , therefore, g r e a t l y i n f l u e n c e the P assimilatory capacity of freshwater m a r s h soils.

Freshwater M a r s h Soil The P release and retention capacity of the organic freshwater m a r s h soil were evaluated i n a laboratory s o r p t i o n , d i f f u s i o n , a n d overflow study. S o i l redox potential, P loading rate, a n d diffusion processes dictate whether the freshwater peat soil will serve as a sink or source of P. P h o s p h o r u s S o r p t i o n S t u d y . G e n e r a l l y , the f r e s h w a t e r m a r s h s o i l s e r v e d as a s o u r c e f o r P (Figure 5). T h e organic m a r s h soil released P to the

100

2.5

25 P load (mg/kg dry

soil)

F i g u r e 5. P h o s p h o r u s R e l e a s e , a n d R e m o v a l C a p a c i t y as Affected by Soil Redox S t a t u s a n d P L o a d i n g R a t e for the F r e s h w a t e r M a r s h W e t l a n d Soil ( n e g a t i v e v a l u e s represent release of P by the soil).

769

WATER RESOURCES BULLETIN

Masscheleyn, Parduc, D e L a u n e , and Patrick

Movement Between Flood Water and Laboratory Mesocosm Studies.

B e c a u s e w h o l e - s e d i m e n t m o l e c u l a r d i f f u s i o n coefficients i n freshwater sediments of low a n d h i g h porosity can be v e r y s i m i l a r ( S w e e r t s et a l . , 1991), t h e a m o u n t of soil surface sites available for P sorption w i l l determine P concentration gradients at the s o i l flood w a t e r surface a n d , thereby, the k i n e t i c s of the diffusion process. In the bottomland h a r d w o o d forest, more soil is available on a surface area (volume) b a s i s to remove P from the w a t e r column. Moreover, the P sorption capacity of this p r e d o m i n a n t l y m i n e r a l soil is greater t h a n for the freshwater m a r s h soil (Figure 1 vs. F i g u r e 5).

Soil:

L a b o r a t o r y M e s o c o s m S t u d i e s . W h e n the freshwater m a r s h soil cores were flooded for 30 days, the e q u i l i b r i u m P content of the flood w a t e r was 0.11 m g L , w h i c h corresponds to a P flux of 1.32 ug P c m - . The flooded w e t l a n d soils h a d a n E h of +80 ± 60 m V and a p H e q u a l to 6.2. R e s u l t s from the P diffusion study are shown i n F i g u r e 6. A s w i t h the bottomland hardwood forest soil, the removal of P from the overl y i n g flood w a t e r column by diffusion to the soil could be d e s c r i b e d b y a f i r s t - o r d e r [ C = C ( e - ) ] k i n e t i c model. F o r the freshwater m a r s h soil, the constant k was equal to 0.14 ± 0.05, leading to a h a l f life of P of 4.7 days. T h e P flux into the freshwater wetland soil was 0.73 a n d 7.09 ug P c m - d for the 1 and 10 p p m P experiment, respectively. _ 1

2

t

2

0

T h e observed difference i n P diffusion k i n e t i c s from the flood water column to the wetland soils i l l u s t r a t e s the importance of the retention time of P loaded water i n the wetland. A s the retention time i n the w e t l a n d is decreased, less P w i l l be removed from the i n c o m i n g flood water. Based on differences i n h a l f r e m o v a l times for P, the residence time of P loaded water i n the freshwater m a r s h w e t l a n d needs to be 1.2 times t h a t of the residence t i m e i n the b o t t o m l a n d h a r d wood forest w e t l a n d i n order to o b t a i n a s i m i l a r P r e m o v a l efficiency. It is also i m p o r t a n t to realize t h a t the previous estimates were obtained u s i n g one p a r t i c u l a r flood water c o l u m n h e i g h t (12 cm above s o i l surface). If m i x i n g is incomplete, then as the depth of the water i n a wetland decreases (increases, reactions between flood water P a n d the wetland soil increases (decreases) thereby decreasing (increasing) the t i m e necessary for n u t r i e n t removal.

k t

_ 1

In the freshwater m a r s h flood water c o l u m n , P was removed more slowly t h a n from the bottomland h a r d wood forest flood water. T h i s could be expected on the basis of t h e difference i n b u l k d e n s i t y between the two w e t l a n d soils. C o m p a r e d to the bottomland h a r d wood forest soil, the freshwater m a r s h soil is charact e r i z e d by a v e r y l o w b u l k d e n s i t y (0.11 vs. 0.95 g c m for the b o t t o m l a n d hardwood soil). A l t h o u g h i n both s t u d i e s t h e s o i l s u r f a c e a r e a a v a i l a b l e for P removal is the same, the weight of soil available for P sorption i n the freshwater m a r s h is almost nine times less t h a n i n the bottomland hardwood forest wetland. - 3

Time

(days)

F i g u r e 6. D i f f u s i v e E x c h a n g e B e t w e e n F l o o d W a t e r C o l u m n P a n d F r e s h w a t e r W e t l a n d Soil ( r e s u l t s f r o m l a b o r a t o r y

WATER RESOURCES BULLETIN

770

study).

P h o s p h o r u s R e l e a s e a n d A s s i m i l a t o r y C a p a c i t y o f T w o L o w e r M i s s i s s i p p i V a l l e y F r e s h w a t e r W e t l a n d Soils

from the data on P concentration a n d flood water volu m e t h a t passed over the w e t l a n d soil overflow system. T h e difference between the inorganic P ( O r t h o p h o s p h a t e - P ) a n d the t o t a l P flux represents the flux of organic P (+ possible polyphosphates) from the w e t l a n d soil system. T h e c o n t r o l s y s t e m s (no P a d d e d , F i g u r e 7 A ) r e l e a s e d s u b s t a n t i a l a m o u n t s of both i n o r g a n i c and

F i e l d M e s o c o s m S t u d y . Release a n d sorption of P from the freshwater m a r s h soil was evaluated, under n e a r f i e l d c o n d i t i o n s , i n a n o v e r l a n d flow s y s t e m . F i g u r e s 7 A a n d 7 B show the release of inorganic and total P for the control a n d P treated m a r s h soil, respectively. U n d e r the e x p e r i m e n t a l conditions, the redox a n d p H of the flooded w e t l a n d soil were - 9 0 ± 50 m V a n d 6.2, respectively. F l u x e s are c a l c u l a t e d 14 12 ~10 -

o 8

«

o

Orthophospate-P



Total P-C

a. -a X O)

6

a E 42 T —I——I——I——I—'—I——I——I——I——I——I—I— 2 4 6 8 1 0 12 1 4 16 18 2 0 2 2 2 4 1

1

1

1

1

Time

14

1

1

1

1

— r

(days)

B

12 10 -

o

"S3 o» 41

S >< Q. X

-a

o

8 -

o

Orthophosphata-P



Total-P

64 -

Q. CJ

E

0

i

I**

2 -

g ?

2

i

i

oo o o o o 9 1—i—i—'—i—"—i—