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Relative contribution of different decay processes to the decomposition of Eichhornia crassipes (Mart.) solms
Aquatic Botany, 42 (1992) 265-272 Elsevier Science Publishers B.V., Amsterdam
265
Relative contribution of different decay processes to the decomposition of Eichhornia crassipes (Mart.) Solms P.K. Singhal, S. G a u r and L. Talegaonkar Department of Biological Sciences, R.D. L'niversity, Jabalpur 482 OOl, India (Accepted 12 August 1991 )
ABSTRACT Singhal, P.K., Gaur, S. and Talegaonkar,L., 1992. Relativecontribution of different decay processes to the decompositionofEichhornia crassipes (Mart.) Solms.Aquat. Bot., 42: 265-272. The relative contribution of different decay processeswere detenained for Eichhornia crassipes (Man.) solms. The dry weight loss in the initial 4 days of decay was solely due t¢ non-raicrobial processesand thereafter, the contribution of microbialprocessesincreasedand that of non-microbial processesdecreasedexponentially.The greenlitter recordedsignificantlyhigherrate constantsof nonmicrobial and microbial decay than the brown litter. In 30 days of decay, the dry mass loss due to physical,autolytic and microbialleachingswas 27%, 5% and 20%, respectively,in the greenlitter, and 18%, 2% and 7%, respectively,in the brown litter. The non-~,icrobiai decomposition solelycaused 31-33% dry mass loss in the E. crassipes litter.
INTRODUCTION T h e d e c o m p o s i t i o n o f aquatic m a c r o p h y t e s involves three f u n d a m e n t a l steps which m a y o c c u r simultaneously o r after t i m e lags, i.e. physical, autolytic a n d microbial leachings. T h e leaching o f d;~ssolved inorganic a n d organic m a t t e r through abiotic d e c o m p o s i t i o n facilitates microbial colonization a n d catabolism o f the decaying litter ( M a n n , 1972; Olah, 1972; Mason a n d Bryant, 1975; G o d s h a l k a n d Wetzel, 1978; Olehlov~, 1978). As the m i c r o o r g a n i s m s d e p o l y m e r i z e structural materials (cellulose, hemicellulose a n d lignin), the rates a n d relative c o n t r i b u t i o n o f different decay processes m a y p r o v i d e a better insight into the flux o f energy b e t w e e n decaying lignocellulosic litter a n d organisms o f detritus food webs in aquatic e n v i r o n m e n t s . However, there is little i n f o r m a t i o n on this aspect o f d e c o m p o s i t i o n o f aquatic macrophytes. In the present paper, an a t t e m p t has been m a d e to quantify various decay processes o f E i c h h o r n i a crassipes ( M a r t . ) Solms. © 1992 Elsevier Science Publishers B.V. All fights reserved 0304-3770/92/$05.00
266
P.K. SINGHAL El" AL.
MATERIALS AND METHODS
The E. crassipes leaves, used in decomposition experiments, were harvested in Robertson Lake (described by Sharma and Singhal, 1988) in 1989. The leaf litter collected immediately after blooming was green with yellow patches and fresh, while that collected 45 days after blooming was brown and dry. Both green and brown litters were air dried at room temperature (28 _+2 °C) for 3 days, to achieve constant dry weight (the coefficient of variat;on (CV) of the mean was less than 2%), prior to their use in decomposition experiments. A portion of leaf litter ( 10 g) was placed in polythene bottles (approximately I 1) containing 500 ml lake water, filtered through a sieve of I mm mesh, for incubation in the laboratory (in vitro experiments) or in nylon bags of I mm mesh for incubation in the lake (in situ experiments). The following combinations of treatments were used to determine the relative contribution of various decay processes of coarse Eichhornia detritus. Treatment 1: air dried leaf litter and lake water without antibiotics (C). Treatment 2: air dried leaf litter and lake water with antifungal and antibacterial agents (CABF ). Treatment 3: oven dried leaf litter (dried at 105°C for 24 h) and lake water without antibiotics. Cyclohexamide and chloroamphenicol (both supplied by Hi media, Bombay) were used as antifungal and antibacterial agents, respectively. Antibiotics were added in each bottle to a final concentration of 60 mg 1- l, and were renewed weekly. This antibiotic dose was identical to that applied by Padgett et al. (1985). The bacterial and fungal suppression in the decaying li'~ter and the lake water was determined by using standard plate count procedures. A portion of the litter was macerated in an alc0hol-sterilized blender at low speed. The macerated litter and the lake water were passed through a 60/~m Nitex screen and divided into several portions of antibiotic treatments. The effectiveness of cyclohexamide for inhibition of fungi was tested by plvfing 0.1-1.0 ml portions of the control and treated samples ~n potato dextrose agar containing 100 mg 1- ~chloramphenicol, and that of chloramphenicol for inactivation of bacteria by plating the sample aliquots on nutrient agar containing 100 mg 1-~ cyclohexamide. The bacterial counts were made after 48 h and fungal counts after 120 h incubation at 28 _+2 ° C. The average suppression of bacteria and fungi was 91 _+7% and 94 _+4°/0, respectively (n = 9 ). The mouth of each bottle was covered by a coarse cotton cloth, and the bottles were then incubated in the dark at 28 + 2°C. Each bottle was shaken twice a day, and its water level was maintained by weekly addition of sterile glass distilled water. The residual dry weight of detritus was analyzed in triplicate after various decay periods. The detritus was dried at 60°C for 72 h in an oven for its dry weight, and was ashed at 550°C for 3 h in a Muffle furnace for its ash free dry weight (AFDW).
DIFFERENTDECAYPROCESSESOF EICHHORNIA CRASSIPES
267
It was assumed that the dry mass loss in the C treatment was due to physical, autolytic and microbial leaching, that in the C^BF treatment was due to physical and autolytic leachings, and that in the C~os treatment was due to physical and microbial leachings. The Ctos treatment was used to inactivate the enzymes which cause autolysis of plant litter after senescence (Brock, 1984). The relative contribution of different decay processes was calculated by the following methods.
Physical leaching. (i) Dry mass loss in Clos-dry mass loss due "to microbial leaching; or (ii) Dry mass loss in C-dry mass loss due to autolytic and microbial leachings.
Autolytic leaching. (i) Dry mass loss in C-dry mass loss in Clos; or (ii) Dry mass loss in CABF-dry mass loss due to physical leaching.
Microbial leaching. (i) Dry mass loss in C-dry mass loss in CABF; o r (ii) Dry mass loss in Cl05-dry mass loss due to physical leaching. The weight losses due to the above mentioned decay processes were quantified by using the first method, while the second method was used mainly to verify the results obtained from the first method. The results obtained from both the methods were significantly in congruence (Fp> 0.05). The decay rate constant (k) was calculated by fitting the values of the remaining dry weight into an exponential function of the type IV,= Woe-k' (cf. Olson, 1963 ), where IV, is the mass remaining after a time interval t (in days), and Wo is the initial mass. RESULTS The dry weight loss of green leaf litter was best explained by a simple exponential curve having a significantly pronounced rate profile for the first 50 days compared with that of the remaining decay period (Fig. 1 ). In the in vitro experiments, the dry weight loss was relatively higher than in the in situ experiments in the first 80 days, but thereafter it was relatively higher in the in situ experiments. Hence, for further decomposition experiments, the incubation period was restricted to 30-45 days. The green litter lost dry mass significantly faster than the brown litter (Fig. 2). The loss of dry mass was very similar in the initial 4 days of decay under different treatments (Fp> 0.05). After 4 days of decay, the differences in the extent of dry mass loss in both types of litter became significant between C and CABFtreatments (Fp<0.01), but remained insignificant between C and Cio5 treatments (Fp > 0.05). The rate constants of the overall decay (C treatment) and the decay due to physical and microbial leachings (C~o5 treat-
268
P.K. SINGHAL ET AL.
.~ 100 c
"5 E
~L 80
g ~ 60 "D
40
=
20
<
10
2b
6
4b
40
8'o
16o
~o
Days
Fig. 1. Pattern of in situ and in vitro decay of leaf litter o f E. crassipes (mean + SD ). Green litter
Brown litter
100
go, .tO
80
E ABF
T: 7o 60-
5040o
lb
2'0 Days
30 Days
Fig. 2. Pattern of decay in leaf litter o f E. crassipes under different treatments (mean + S D ) . C, air dried litter; CASE, air dried litter with antibacterial and antifungal agents; Cws, oven dried litter.
ment) were almost three times higher in the green litter than in the brown litter (Table 1 ). The rate constant of the non-microbial decay (CABF treatment) was only two times higher in the green litter than in the brown litter. The dry mass loss due to physical leaching was rapid in the initial 4 days of decay, but became slow later on (Fig. 3). It, the initial 4 days of decay, b(~th types of litter lost similar quantities of dry mass by physical leaching (Fp> 0.05). These results indicate that the predrying of the litter had no significant impact on the rate of physical leaching as suggested previously by some workers (Godshalk and Wetzel, 1978; Larsen, 1982; Rogers and Breen,
DIFFERENTDECAYPROCESSESOF EICHHORNIA CRASSIPES
269
TABLE 1 Decay rate constants of leaf litter of E. crassipes under different treatments
El/2
Treatment
Type of litter
n
k
SE
CD
C (air dried) Cios (oven dried)
Green Brown Green Brown
I0 34 10 34
- 0.023 -0.008 -0.02 - 0.007
0.004 0.0014 0.0028 0.0013
0.94 0.82 0.96 0.8;
31 86 34 99
G ~een Brown
10 34
- 0.0 ! 1 -0.0046
0.005 0.0017
0.72 0.55
63 150
CABF ( air dried with antibiotics)
n, number of observations; k, decay rate constant; SE, standard error of/c, CD, coefficient of determination; t=/2, time in days for 50% turnover of litter. The data fit the simple exponential curve ifCD is more than 0.80 for both the litters, and when the values of k/SE are more than 5.03 and 3.65 for green and brown litters, respectively. Green l i t t e r
Brown litter 0
50.
40.
0
_o 30,
P
p
t-
"~ 20'
r~ 10,
A
b
10
2'0 Days
3"0
b
1'5
:30
4'5
Days
Fig. 3. Pattern o f decay o f leaf litter o f E. crassipes due to different decay processes: O, overall decay; P, decay due to physical leaching; A, decay due to autolytic leaching; M, decay due to microbial leaching.
1982; Brock et al., 1982; Neely and Davis, 1985). This was further confirmed by almost similar decay rate constants in both C and Clos treatments (Table 1 ). The green litter lost dry mass at a faster rate owing to physical leaching than the brown litter between 4 and 30 days of decay (Fp<0.01). After 30 days of decay, the dry mass loss due to physical leaching was 27% in the green litter and only 18% in the brown litter. The dry mass loss dt~v to autolytic
270
P.K. SINGHALET AL.
TABLE 2 Decay rate constants of leaf litter ofE. crassipesby different decay processes Decay process
Type of litter
n
k
SE
CD
h/2
Physical leaching Non-microbial leaching Microbial leaching
Green Brown Green Brown Green Brown
10 34 l0 34 10 34
- 0.0087 - 0.0047 -0.0106 - 0.005 -0.0074 -0.0024
0.003 0.0016 0.005 0.0016 0.001 0.00026
0.78 0.64 0.69 0.60 0.97 0.95
79 147 65 138 93 288
Overall
Green Brown
l0 34
-0.022 - 0.008
0.004 0.0014
0.94 0.82
31 86
See footnotes to Table 1.
leaching was meagre: 5% in the green and 2% in the brown litter. These results suggest that the brown leaf litter lost a substantial portion of its leachable non cell wall fraction during its drying in the air in the lake, most possibly by downward translocation. The dry mass loss by microbial leaching was negligible in the initial 4 days of decay, and increased exponentivlly thereafter in both types of litter (Fig. 3). The green litter lost dry mass significantly faster than the brown litter owing to microbial leaching (Fp ~ 0.001 ). The rate constant of microbial decay of the green litter was three times higher than that of the brown litter (Table 2). This shows that the drying of litter in the lake for 45 days might have substantiaily increased its refractivity to the microbial decay. In our in vitro experiments, the fungi which could readily grow on the green leaves also failed to grow on the brown leaves. However, the insignificant differences between the decay rate constants of C and Clos litters indicate that the drying of litter did not change its lability to the microbial decay. As the non cell wall fraction of the litter, the labile fraction to microbes, does not leach out completely during the initial phase of decay, it is plausible that it plays a crucial role in the colonization and catabolism by microbes on the litter. Thus, the higher microbial decay of the green litter in comparison with the brown litter might have been due to the hig~ler quantities of leachable non cell wall fraction in the green litter even after the initial phase of decay (see Fig. 2 ). DISCUSSION
The patterns of overall decay and microbial decay were best explained by the simple exponential curve, which, however, did not explain the non-microbial decay (Tables 1 and 2). The pattern of non-microbial decay was best explained by a polynomial curve of second degree. This indicates that the
DIFFERENTDECAYPROCESSESOFEICHHORNIACRASSIPES
271
decay during the initial phase of 4 days was solely due to physical and autolytic leachings. After the initial phase of decay, the contribution of microbial processes increased and that of non-microbial processes decreased exponentially at almost the same rates. After 98 days of decay in both green and brown litters, the contribution of microbial processes became 100%. On extrapolation, it was found that 31-33% of 0 day dry mass was lost solely because of abiotic decomposition. This is considerably higher than that reported ( 1825%) for other emergent and free-floating macrophytes (Polunin, 1982; Federie and Vestal, 1982; Brock, 1984). This anomaly may be attributed more to the basis of estimation used by the above workers than to the interspecific variation in the plants alone. As the aerial portion of water hyacinth remains afloat usually for 1-2 months after senescence in the inhabited water body, becoming dry, brown and refractive to microbes, it seems likely that its natural course of degradation is much more slow than has been reported in the literature on the basis of its green litter (DeBusk et al., 1983; DeBusk and Dierberg, 1984; Moorhead et al., 1988). For example, the green litter required only 93 days for a 50% turnover by microbial decay as compared with 288 days required by the brown litter. The brown litter will thus pass its energy to the organisms of detritus food webs at a much slower rate than the green litter, and the bulk of this transfer will occur through microbial assimilation of leached dissolved organic matter by abiotic decomposition rather than through microbial decomposition of particulate organic matter of the litter. ACKNOWLEDGMENTS
The authors are thankful to the referees for their constructive criticisms. The M.P. Council of Science and Technology, India, partly financed this study through a research grant No. ENV-35.
REFERENCES ~:ock, Th.C.M., 1984. Aspects of decomposition of Nymphoides peltata (Gmel.) O. Kuntze (Meilyan~haceae). Aquat. Bot., 19:131-156. Brock, Th.C.M., Huijbrechts, C.A.M., van de Steeg-Huberts, M.J.H.A. and Vlassak, M.A., 1982. In situ studies on the breakdown of Nymphoides peltata (Gruel.) O. Kuntze (Menyanthaceae); some methodological aspects of the litter bag technique. Hydrobiol. Bull. (Amsterdam), 16: 35-49. DeBusk, T.A. and Dierberg, I.E., 1984. Effect of nitrogen and fibre content on the decomposition of the water hyacinth [Eichhornia crassipes (Mart.) Solms]. Hydrobiologia, 118:199204. DeBusk, T.A., Williams, L.D. and Ryther, J.H., 1983. Removal of nitrogen and phosphorus from waste water in a water hyacinth based treatment system. J. Environ. Qual., 12: 257262.
272
P.g. SINGHALETAL.
Federle, T.W. and Vestal, J.R., 1982. Evidence of microbial succession on decaying leaf litter in an arctic lake. Can. J. Microbiol., 28: 686-695. Godshalk, G.L. and Wetzel, R.G., 1978. Decomposition ofa¢uatic angiosperms. II. Particulate components. Aquat. Bot., 5: 301-327. Larsen, V.J., 1982. The effect of pre-drying and fragmentation on the leaching of nutrient elements and organic matter from Phragmites australis (Cav.) Trin. litter. Aquat. Bot., 14: 2939. Mann, K.H., 1972. M~crophyte production and detritus food chains in coastal waters. Mere. Ist. ltal. Idrobiol., 29 (Suppl.): 353-383. Mason, C.F. and Bryant, R.J., 1975. Production, nutrient content and decomposition of Phragmites communis Trin. and Typha angustifolia L. J. Ecol., 63:71-95. Moorhead, K.K., Reddy, K.R. and Graezt, D.A., 1988. Water hyacinth productivity and detritus ~ccumulation. Hydrobiologia, 157:179-185. Neely, R~K. and Davis, C.B., 1985. Nitrogen and phes~horus f=~!!;zation ofSparganium eurycarpum Engelm. :,nd Typha glauca Godr. stands. II. Emergent plant decomposition. Aquat. Bot., 22: 363-375. Olah, J., 1972. Leaching, colonization and stabilization during detritus formation. Mem. Ist. ltal. ldrobiol., 29 (Suppl.): 105-127. Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology, 44:322-33 I. Padgett, D.E., Hackney, C.T. and Sizemore, R.E., 1985. A technique for distinguishing between bacterial and non-bacterial respiration in decomposing Spartina alterniflora. Hydrobiologia, 122:ll3-119. Polunin, N.V.C., 1982. Processes contributing to the decay of reed (Phragmites australis) litter in fresh water. Arch. Hydrobiol., 94:182-209. Rogers, K.H. and Breen, C.M., 1982. Decomposition ofPotamogeton crispus L.: the effect of drying on the pattern of mass and nutrient loss. Aquat. Bot., 12: 1-12. Sharma, A. and Singhal, P.K., 1988. Impact of floating and emergent vegetation on the trophic status of a tropical lake: I. The macrophytes and physicochemical status. J. Environ. Biol., 9: 303-311. Olehlo~,a, B., 1978. Decomposition processes in the fish pond littoral. In: D. Dykyjovfi and J. Kv6t (Editors), Pond Littoral Ecosystems. Springer, Berlin, pp. 341-353.
265
Relative contribution of different decay processes to the decomposition of Eichhornia crassipes (Mart.) Solms P.K. Singhal, S. G a u r and L. Talegaonkar Department of Biological Sciences, R.D. L'niversity, Jabalpur 482 OOl, India (Accepted 12 August 1991 )
ABSTRACT Singhal, P.K., Gaur, S. and Talegaonkar,L., 1992. Relativecontribution of different decay processes to the decompositionofEichhornia crassipes (Mart.) Solms.Aquat. Bot., 42: 265-272. The relative contribution of different decay processeswere detenained for Eichhornia crassipes (Man.) solms. The dry weight loss in the initial 4 days of decay was solely due t¢ non-raicrobial processesand thereafter, the contribution of microbialprocessesincreasedand that of non-microbial processesdecreasedexponentially.The greenlitter recordedsignificantlyhigherrate constantsof nonmicrobial and microbial decay than the brown litter. In 30 days of decay, the dry mass loss due to physical,autolytic and microbialleachingswas 27%, 5% and 20%, respectively,in the greenlitter, and 18%, 2% and 7%, respectively,in the brown litter. The non-~,icrobiai decomposition solelycaused 31-33% dry mass loss in the E. crassipes litter.
INTRODUCTION T h e d e c o m p o s i t i o n o f aquatic m a c r o p h y t e s involves three f u n d a m e n t a l steps which m a y o c c u r simultaneously o r after t i m e lags, i.e. physical, autolytic a n d microbial leachings. T h e leaching o f d;~ssolved inorganic a n d organic m a t t e r through abiotic d e c o m p o s i t i o n facilitates microbial colonization a n d catabolism o f the decaying litter ( M a n n , 1972; Olah, 1972; Mason a n d Bryant, 1975; G o d s h a l k a n d Wetzel, 1978; Olehlov~, 1978). As the m i c r o o r g a n i s m s d e p o l y m e r i z e structural materials (cellulose, hemicellulose a n d lignin), the rates a n d relative c o n t r i b u t i o n o f different decay processes m a y p r o v i d e a better insight into the flux o f energy b e t w e e n decaying lignocellulosic litter a n d organisms o f detritus food webs in aquatic e n v i r o n m e n t s . However, there is little i n f o r m a t i o n on this aspect o f d e c o m p o s i t i o n o f aquatic macrophytes. In the present paper, an a t t e m p t has been m a d e to quantify various decay processes o f E i c h h o r n i a crassipes ( M a r t . ) Solms. © 1992 Elsevier Science Publishers B.V. All fights reserved 0304-3770/92/$05.00
266
P.K. SINGHAL El" AL.
MATERIALS AND METHODS
The E. crassipes leaves, used in decomposition experiments, were harvested in Robertson Lake (described by Sharma and Singhal, 1988) in 1989. The leaf litter collected immediately after blooming was green with yellow patches and fresh, while that collected 45 days after blooming was brown and dry. Both green and brown litters were air dried at room temperature (28 _+2 °C) for 3 days, to achieve constant dry weight (the coefficient of variat;on (CV) of the mean was less than 2%), prior to their use in decomposition experiments. A portion of leaf litter ( 10 g) was placed in polythene bottles (approximately I 1) containing 500 ml lake water, filtered through a sieve of I mm mesh, for incubation in the laboratory (in vitro experiments) or in nylon bags of I mm mesh for incubation in the lake (in situ experiments). The following combinations of treatments were used to determine the relative contribution of various decay processes of coarse Eichhornia detritus. Treatment 1: air dried leaf litter and lake water without antibiotics (C). Treatment 2: air dried leaf litter and lake water with antifungal and antibacterial agents (CABF ). Treatment 3: oven dried leaf litter (dried at 105°C for 24 h) and lake water without antibiotics. Cyclohexamide and chloroamphenicol (both supplied by Hi media, Bombay) were used as antifungal and antibacterial agents, respectively. Antibiotics were added in each bottle to a final concentration of 60 mg 1- l, and were renewed weekly. This antibiotic dose was identical to that applied by Padgett et al. (1985). The bacterial and fungal suppression in the decaying li'~ter and the lake water was determined by using standard plate count procedures. A portion of the litter was macerated in an alc0hol-sterilized blender at low speed. The macerated litter and the lake water were passed through a 60/~m Nitex screen and divided into several portions of antibiotic treatments. The effectiveness of cyclohexamide for inhibition of fungi was tested by plvfing 0.1-1.0 ml portions of the control and treated samples ~n potato dextrose agar containing 100 mg 1- ~chloramphenicol, and that of chloramphenicol for inactivation of bacteria by plating the sample aliquots on nutrient agar containing 100 mg 1-~ cyclohexamide. The bacterial counts were made after 48 h and fungal counts after 120 h incubation at 28 _+2 ° C. The average suppression of bacteria and fungi was 91 _+7% and 94 _+4°/0, respectively (n = 9 ). The mouth of each bottle was covered by a coarse cotton cloth, and the bottles were then incubated in the dark at 28 + 2°C. Each bottle was shaken twice a day, and its water level was maintained by weekly addition of sterile glass distilled water. The residual dry weight of detritus was analyzed in triplicate after various decay periods. The detritus was dried at 60°C for 72 h in an oven for its dry weight, and was ashed at 550°C for 3 h in a Muffle furnace for its ash free dry weight (AFDW).
DIFFERENTDECAYPROCESSESOF EICHHORNIA CRASSIPES
267
It was assumed that the dry mass loss in the C treatment was due to physical, autolytic and microbial leaching, that in the C^BF treatment was due to physical and autolytic leachings, and that in the C~os treatment was due to physical and microbial leachings. The Ctos treatment was used to inactivate the enzymes which cause autolysis of plant litter after senescence (Brock, 1984). The relative contribution of different decay processes was calculated by the following methods.
Physical leaching. (i) Dry mass loss in Clos-dry mass loss due "to microbial leaching; or (ii) Dry mass loss in C-dry mass loss due to autolytic and microbial leachings.
Autolytic leaching. (i) Dry mass loss in C-dry mass loss in Clos; or (ii) Dry mass loss in CABF-dry mass loss due to physical leaching.
Microbial leaching. (i) Dry mass loss in C-dry mass loss in CABF; o r (ii) Dry mass loss in Cl05-dry mass loss due to physical leaching. The weight losses due to the above mentioned decay processes were quantified by using the first method, while the second method was used mainly to verify the results obtained from the first method. The results obtained from both the methods were significantly in congruence (Fp> 0.05). The decay rate constant (k) was calculated by fitting the values of the remaining dry weight into an exponential function of the type IV,= Woe-k' (cf. Olson, 1963 ), where IV, is the mass remaining after a time interval t (in days), and Wo is the initial mass. RESULTS The dry weight loss of green leaf litter was best explained by a simple exponential curve having a significantly pronounced rate profile for the first 50 days compared with that of the remaining decay period (Fig. 1 ). In the in vitro experiments, the dry weight loss was relatively higher than in the in situ experiments in the first 80 days, but thereafter it was relatively higher in the in situ experiments. Hence, for further decomposition experiments, the incubation period was restricted to 30-45 days. The green litter lost dry mass significantly faster than the brown litter (Fig. 2). The loss of dry mass was very similar in the initial 4 days of decay under different treatments (Fp> 0.05). After 4 days of decay, the differences in the extent of dry mass loss in both types of litter became significant between C and CABFtreatments (Fp<0.01), but remained insignificant between C and Cio5 treatments (Fp > 0.05). The rate constants of the overall decay (C treatment) and the decay due to physical and microbial leachings (C~o5 treat-
268
P.K. SINGHAL ET AL.
.~ 100 c
"5 E
~L 80
g ~ 60 "D
40
=
20
<
10
2b
6
4b
40
8'o
16o
~o
Days
Fig. 1. Pattern of in situ and in vitro decay of leaf litter o f E. crassipes (mean + SD ). Green litter
Brown litter
100
go, .tO
80
E ABF
T: 7o 60-
5040o
lb
2'0 Days
30 Days
Fig. 2. Pattern of decay in leaf litter o f E. crassipes under different treatments (mean + S D ) . C, air dried litter; CASE, air dried litter with antibacterial and antifungal agents; Cws, oven dried litter.
ment) were almost three times higher in the green litter than in the brown litter (Table 1 ). The rate constant of the non-microbial decay (CABF treatment) was only two times higher in the green litter than in the brown litter. The dry mass loss due to physical leaching was rapid in the initial 4 days of decay, but became slow later on (Fig. 3). It, the initial 4 days of decay, b(~th types of litter lost similar quantities of dry mass by physical leaching (Fp> 0.05). These results indicate that the predrying of the litter had no significant impact on the rate of physical leaching as suggested previously by some workers (Godshalk and Wetzel, 1978; Larsen, 1982; Rogers and Breen,
DIFFERENTDECAYPROCESSESOF EICHHORNIA CRASSIPES
269
TABLE 1 Decay rate constants of leaf litter of E. crassipes under different treatments
El/2
Treatment
Type of litter
n
k
SE
CD
C (air dried) Cios (oven dried)
Green Brown Green Brown
I0 34 10 34
- 0.023 -0.008 -0.02 - 0.007
0.004 0.0014 0.0028 0.0013
0.94 0.82 0.96 0.8;
31 86 34 99
G ~een Brown
10 34
- 0.0 ! 1 -0.0046
0.005 0.0017
0.72 0.55
63 150
CABF ( air dried with antibiotics)
n, number of observations; k, decay rate constant; SE, standard error of/c, CD, coefficient of determination; t=/2, time in days for 50% turnover of litter. The data fit the simple exponential curve ifCD is more than 0.80 for both the litters, and when the values of k/SE are more than 5.03 and 3.65 for green and brown litters, respectively. Green l i t t e r
Brown litter 0
50.
40.
0
_o 30,
P
p
t-
"~ 20'
r~ 10,
A
b
10
2'0 Days
3"0
b
1'5
:30
4'5
Days
Fig. 3. Pattern o f decay o f leaf litter o f E. crassipes due to different decay processes: O, overall decay; P, decay due to physical leaching; A, decay due to autolytic leaching; M, decay due to microbial leaching.
1982; Brock et al., 1982; Neely and Davis, 1985). This was further confirmed by almost similar decay rate constants in both C and Clos treatments (Table 1 ). The green litter lost dry mass at a faster rate owing to physical leaching than the brown litter between 4 and 30 days of decay (Fp<0.01). After 30 days of decay, the dry mass loss due to physical leaching was 27% in the green litter and only 18% in the brown litter. The dry mass loss dt~v to autolytic
270
P.K. SINGHALET AL.
TABLE 2 Decay rate constants of leaf litter ofE. crassipesby different decay processes Decay process
Type of litter
n
k
SE
CD
h/2
Physical leaching Non-microbial leaching Microbial leaching
Green Brown Green Brown Green Brown
10 34 l0 34 10 34
- 0.0087 - 0.0047 -0.0106 - 0.005 -0.0074 -0.0024
0.003 0.0016 0.005 0.0016 0.001 0.00026
0.78 0.64 0.69 0.60 0.97 0.95
79 147 65 138 93 288
Overall
Green Brown
l0 34
-0.022 - 0.008
0.004 0.0014
0.94 0.82
31 86
See footnotes to Table 1.
leaching was meagre: 5% in the green and 2% in the brown litter. These results suggest that the brown leaf litter lost a substantial portion of its leachable non cell wall fraction during its drying in the air in the lake, most possibly by downward translocation. The dry mass loss by microbial leaching was negligible in the initial 4 days of decay, and increased exponentivlly thereafter in both types of litter (Fig. 3). The green litter lost dry mass significantly faster than the brown litter owing to microbial leaching (Fp ~ 0.001 ). The rate constant of microbial decay of the green litter was three times higher than that of the brown litter (Table 2). This shows that the drying of litter in the lake for 45 days might have substantiaily increased its refractivity to the microbial decay. In our in vitro experiments, the fungi which could readily grow on the green leaves also failed to grow on the brown leaves. However, the insignificant differences between the decay rate constants of C and Clos litters indicate that the drying of litter did not change its lability to the microbial decay. As the non cell wall fraction of the litter, the labile fraction to microbes, does not leach out completely during the initial phase of decay, it is plausible that it plays a crucial role in the colonization and catabolism by microbes on the litter. Thus, the higher microbial decay of the green litter in comparison with the brown litter might have been due to the hig~ler quantities of leachable non cell wall fraction in the green litter even after the initial phase of decay (see Fig. 2 ). DISCUSSION
The patterns of overall decay and microbial decay were best explained by the simple exponential curve, which, however, did not explain the non-microbial decay (Tables 1 and 2). The pattern of non-microbial decay was best explained by a polynomial curve of second degree. This indicates that the
DIFFERENTDECAYPROCESSESOFEICHHORNIACRASSIPES
271
decay during the initial phase of 4 days was solely due to physical and autolytic leachings. After the initial phase of decay, the contribution of microbial processes increased and that of non-microbial processes decreased exponentially at almost the same rates. After 98 days of decay in both green and brown litters, the contribution of microbial processes became 100%. On extrapolation, it was found that 31-33% of 0 day dry mass was lost solely because of abiotic decomposition. This is considerably higher than that reported ( 1825%) for other emergent and free-floating macrophytes (Polunin, 1982; Federie and Vestal, 1982; Brock, 1984). This anomaly may be attributed more to the basis of estimation used by the above workers than to the interspecific variation in the plants alone. As the aerial portion of water hyacinth remains afloat usually for 1-2 months after senescence in the inhabited water body, becoming dry, brown and refractive to microbes, it seems likely that its natural course of degradation is much more slow than has been reported in the literature on the basis of its green litter (DeBusk et al., 1983; DeBusk and Dierberg, 1984; Moorhead et al., 1988). For example, the green litter required only 93 days for a 50% turnover by microbial decay as compared with 288 days required by the brown litter. The brown litter will thus pass its energy to the organisms of detritus food webs at a much slower rate than the green litter, and the bulk of this transfer will occur through microbial assimilation of leached dissolved organic matter by abiotic decomposition rather than through microbial decomposition of particulate organic matter of the litter. ACKNOWLEDGMENTS
The authors are thankful to the referees for their constructive criticisms. The M.P. Council of Science and Technology, India, partly financed this study through a research grant No. ENV-35.
REFERENCES ~:ock, Th.C.M., 1984. Aspects of decomposition of Nymphoides peltata (Gmel.) O. Kuntze (Meilyan~haceae). Aquat. Bot., 19:131-156. Brock, Th.C.M., Huijbrechts, C.A.M., van de Steeg-Huberts, M.J.H.A. and Vlassak, M.A., 1982. In situ studies on the breakdown of Nymphoides peltata (Gruel.) O. Kuntze (Menyanthaceae); some methodological aspects of the litter bag technique. Hydrobiol. Bull. (Amsterdam), 16: 35-49. DeBusk, T.A. and Dierberg, I.E., 1984. Effect of nitrogen and fibre content on the decomposition of the water hyacinth [Eichhornia crassipes (Mart.) Solms]. Hydrobiologia, 118:199204. DeBusk, T.A., Williams, L.D. and Ryther, J.H., 1983. Removal of nitrogen and phosphorus from waste water in a water hyacinth based treatment system. J. Environ. Qual., 12: 257262.
272
P.g. SINGHALETAL.
Federle, T.W. and Vestal, J.R., 1982. Evidence of microbial succession on decaying leaf litter in an arctic lake. Can. J. Microbiol., 28: 686-695. Godshalk, G.L. and Wetzel, R.G., 1978. Decomposition ofa¢uatic angiosperms. II. Particulate components. Aquat. Bot., 5: 301-327. Larsen, V.J., 1982. The effect of pre-drying and fragmentation on the leaching of nutrient elements and organic matter from Phragmites australis (Cav.) Trin. litter. Aquat. Bot., 14: 2939. Mann, K.H., 1972. M~crophyte production and detritus food chains in coastal waters. Mere. Ist. ltal. Idrobiol., 29 (Suppl.): 353-383. Mason, C.F. and Bryant, R.J., 1975. Production, nutrient content and decomposition of Phragmites communis Trin. and Typha angustifolia L. J. Ecol., 63:71-95. Moorhead, K.K., Reddy, K.R. and Graezt, D.A., 1988. Water hyacinth productivity and detritus ~ccumulation. Hydrobiologia, 157:179-185. Neely, R~K. and Davis, C.B., 1985. Nitrogen and phes~horus f=~!!;zation ofSparganium eurycarpum Engelm. :,nd Typha glauca Godr. stands. II. Emergent plant decomposition. Aquat. Bot., 22: 363-375. Olah, J., 1972. Leaching, colonization and stabilization during detritus formation. Mem. Ist. ltal. ldrobiol., 29 (Suppl.): 105-127. Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology, 44:322-33 I. Padgett, D.E., Hackney, C.T. and Sizemore, R.E., 1985. A technique for distinguishing between bacterial and non-bacterial respiration in decomposing Spartina alterniflora. Hydrobiologia, 122:ll3-119. Polunin, N.V.C., 1982. Processes contributing to the decay of reed (Phragmites australis) litter in fresh water. Arch. Hydrobiol., 94:182-209. Rogers, K.H. and Breen, C.M., 1982. Decomposition ofPotamogeton crispus L.: the effect of drying on the pattern of mass and nutrient loss. Aquat. Bot., 12: 1-12. Sharma, A. and Singhal, P.K., 1988. Impact of floating and emergent vegetation on the trophic status of a tropical lake: I. The macrophytes and physicochemical status. J. Environ. Biol., 9: 303-311. Olehlo~,a, B., 1978. Decomposition processes in the fish pond littoral. In: D. Dykyjovfi and J. Kv6t (Editors), Pond Littoral Ecosystems. Springer, Berlin, pp. 341-353.