Oct 27, 1987 - (i) Measurements of standing crop and nitrogenase activity of blue-green algal communities were made during the flood season at two ...
Ecology of deepwater rice-fields in Bangladesh 4. Nitrogen fixation by blue-green algal communities J. A. Rother1, Abdul A ~ i z ' , ~N., Hye Karim3 & B. A. Whitton' Department of Botany, University of Durham, Durham DHl 3LE, England; * Department of Botany, University of Dhaka, Dhaka-2, Bangladesh; Bangladesh Rice Research Institute, Joydebpur, Bangladesh Received 25 August; accepted 27 October 1987
Key words: Bangladesh, nitrogen fixation, blue-green algae, Gloeotrichia Abstract (i) Measurements of standing crop and nitrogenase activity of blue-green algal communities were made during the flood season at two Bangladesh deepwater rice (DWR) locations, Manikganj and near Sonargaon. Most data were collected during 1983, but some also in 1981, 1982, 1985 and 1986 (ii) Floating colonies were quantitatively important only at Manikganj, but epiphytic algae were frequent at both sites during part of each flood season. The maximum blue-green algal crops recorded for any field for floating colonies and epiphytes were 186 and 54.8 mg rn-, chl a, respectively. Mean values were much lower. For instance, at Sonargaon in 1983, the mean for the total crop was probably less than 0.5 mg m - chl a during July and October, while the mean (excluding Azollapinnata) for August and September was only 18.4 mg m P 2 chl a. Azolla was present in the majority of 0.25 m-2 quadrats sampled, but seldom exceeded 10 mg dry wt m- ,. (iii) Mean rates of nitrogenase activity (AM), expressed per unit chlorophyll, were quite similar for all the free-living species except Scytonema mirabile. The sequence of maximum values for individual species (incubated under light flux similar to that in which the organisms had been growing) came in the order (from highest to lowest): Aulosirafertillissima, Anabaena sp. (non-planktonic), Gloeotrichia natans, G. pisum, Cylindrospermum majus, Tolypothrixpenicillata,Anabaena sp. (planktonic), Scytonema mirabile. (iv) There were marked die1 changes in ARA, with the maximum in late morning or early afternoon and the minimum during the late part of the night. ARA at night, as% of the 24-h total, ranged from 0.6 to 19.0% for free-living blue-green algae and 24.6% for Azolla. The use of results combined from one hour in late morning and one hour in early afternoon provides a reasonable prediction of the daily total. This 2-h total in nine experiments representing seven species ranged from 14.8 to 22.7 % of the 24-h total. (v) ARA of representative DWR tillers at Sonargaon near the period of peak flood was very low in 1982, but higher in August -September 1983, with totals ranging from 39.9 to 249 nmol C2H4mintiller- '. ARA was easily detectable down to node 6 and sometimes further. (vi) Estimates were made of N, fixed inside DWR fields during the 1983 flood season. These involved many approximations, so the values obtained at Manikganj and Sonargaon of 7 and 2 kg ha- ',respectively, must be treated cautiously. They indicate that blue-green algal nitrogen fixation is less important during the flood season than on moist soils in the period immediately prior fo flooding. (vii) It is suggested that the high crops of heterocystous blue-green algae present in many fallow areas provide an important source of fried nitrogen in the region; studies are required on lateral movement of water to establish whether this nitrogen reaches the deepwater rice-fields.
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Introduction Blue-green algae are sometimes frequent in Bangladesh deepwater rice (DWR) fields (Catling et al., 1981; Martinez & Catling, 1982) and usually abundant in fallow areas between the fields (Whitton et al., 1988b). The importance of their contribution to the nitrogen economy of these fields was stressed by Brammer (1983) in a discussion of the sources of nutrients for Bangladesh's floodplain soils. Brammer argued that sediments deposited by river floodwaters play only a minor role in maintaining fertility and that nitrogen fertility is provided by biological activity in the floodwater, especially that of bluegreen algae. He stated (without experimental evidence) that it is probable that such organisms can provide up to 30 kg ha- yr - N and perhaps even more on the deeply flooded areas where DWR is grown. In view of possible changes in the management of the Ganges in India and their likely impact on sediment loads in those Bangladesh fields flooded by Ganges water, this topic is of considerable political. Nitrogen fmation by blue-green algae is known to be quantitatively important in many rice-field ecosystems (Roger & Kulasooriya, 1980), so a study was planned to quantify nitrogen fmation at the two main locations, Manikganj and Sonargaon, used in this series of papers on DWR fields. These locations are flooded by waters from the Jamuna (-Brahmaputra) and Meghna systems, respectively. It was concluded in the previous paper (Whitton et al., 1988b) that blue-green algae are widespread in DWR areas at Manikganj and Sonargaon, although more taxonomically diverse at the former. They are much more abundant in fallow areas than inside DWR fields. The present account deals specifically with the latter, although many observations apply also to fallow areas. Kulasooriya etal. (1981) showed that it is feasible to use l5NZto measure epiphytic nitrogen fixation on DWR plants in an experimental plot. However a compromise was chosen for the present study because the diversity of types of field and heterogeneity within individual fields makes quantitative assessment very difficult. The
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approach adopted was to use the acetylene reduction assay (ARA) technique to establish the important aspects of nitrogenase activity for the more frequently occurring species, to relate nitrogen fmation to nitrogenase activity by laboratory studies on isolates of these species and to estimate standing crops of these species at various times during the season.
Materials and methods Field experiments All measurements of standing crop and most field experiments were carried out at Manikganj or Sonargaon, but a few of the results in Figs 3 and 4 are based on studies at other locations. Details of locations and environmental variables are given in paper 1 of this series (Whitton et al., 1988a); all PAR (photosynthetically active radiation) measurements included were made with the Biospherical Instruments quantum solar irradiance system and the 2 n reference and 4 n underwater sensors. Studies were carried out during the flood season, with the exception that data from moist soils during the period shortly before the floodwaters are included in Figs 2,3 and 4. Methodology and background for these soil data have been given by Rother & Whitton (submitted b). The majority of studies were made during 1983, but results from 1981,1982,1985 and 1986 are also included. Observations on frequency of blue-green algae and relative abundance of the main taxa were made at frequent intervals (18 visits to Manikganj and 25 visits to Sonargaon) during the 1983 flood season. Quantitative estimates of standing crop as chl a were made during the period when epiphytic colonies were obvious; these involved 372 samples, although 111 were below the detection limit (Table 2). It was assumed that epiphytic growths did not make an important contribution to nitrogen fxation when they were not visually obvious. Fields were selected as representative of the location and samples taken from random 0.25 mZquadrats within these. The number of tillers whose emergent leaves came within
a quadrat was counted, one tiller detached and epiphytic algae removed with forceps. Floating algae (mats, flocs, gelatinous colonies) and Azolla were also harvested from each quadrat, but additional quadrats were also included for these because of high variability. Estimates of amounts of floating colonies in the early and late parts of the flood season, which are incorporated into the approximate estimates of total nitrogen fmed, are based on observations on comparative cover. Routine experiments were in some cases carried out in the microhabitat from which the alga had been removed, but there were too many practical difficulties to do this when a large number of samples had to be incubated at the same time. For the latter the incubation vessels were held in wire racks immersed in a DWR channel and neutral density filters used to reduce light flux to values similar to those from which the alga had been taken. Filters or aluminium foil were wrapped around vessels for experiments on the influence of reduction in light flux. Some 24-h and other long-term experiments were done away from the site; for these, material was collected in the late afternoon, kept under conditions as similar to the field site as possible and the experiments commenced at dawn, where necessary using filters to reduce the percentage incident light. All the populations tested came either from the surface of the water or (Gloeotrichia pisum) near the surface and from relatively well illuminated rnicrohabitats. No attempt was made to simulate the changes in flux found inside DWR fields resulting from the greater shading from rice leaves early and late in the day due to the low angle of incident radiation. Material for experiments on various parts of DWR tillers was prepared as follows. Tillers were selected as representative for a field (and not near the edge of the field). They were cut into 20-30 cm long sections, each consisting of one node (including roots and leaf sheath + lamina, where still remaining) and one internode (stem section); the results for the various fractions are however grouped differently in Table 6. Branches were ignored for the studies reported below, because those sampled were either very short and so with
few roots or arose from a node not far above the sediments and hence would be treated as separate tillers during routine counts for agronomic purposes. Nitrogenase assay ARA methodology was adapted from Hardy et al. (1973) and Mague (1978). Assays were mostly done in 28-ml universal bottles, but some experiments on simple communities were done in 7-ml serum bottles and a depth profile of epiphytic algae on rice stems was done in 180-ml plastic pots. Five replicates were used with each treatment (including controls) in all experiments except the depth profile. C2H2was obtained from Bangladesh Oxygen Co. (now Ltd). C2H2 was injected into each container, which was then shaken and vented to restore atmospheric pressure; sufficient gas was added to provide about 10% of the volume of the container. Controls with and without both algae and C2H2were included. Most experiments were carried out over 60-min periods; where a single time is given in Results, this is the mid-point. At the end of the experiment each container was shaken vigorously to ensure C,H, equilibration between alga and gas phase. The final gas mixture was sampled using evacuated blood sampling 'tubes (Rocket, London; - 1984; Terumo, Belgium: 1986). Samples were analyzed in the U.K. on Varian aerograph gas chromatographs (1200 or 1400) using a column packing of Poropak R. Nitrogen fixation Comparisons with ARA results were made for six laboratory batch cultures of Bangladesh deepwater rice field isolates. Mean values were found for N,: C2H2 reduced of 1 mol: 5.62 mol in exponential phase cultures and 1: 4.66 at a later growth stage (Table 1). As it is impossible to simulate all possible field conditions, a value of 1: 5 was adopted when using nitrogenase data to estimate nitrogen fixation in the field. This may be compared with the theoretical value of 1 : 3 and the mean value of 1 : 4.4 reported by Peterson and Burris (1976).
fixation and C2H2nmol C,H, pg a - ' min- ' x Table 1. Comparison of 15N2(nmol N, pg chl a - ' x reduction by six bacteria-free Bangladesh strains sampled at two dates during batch culture. Cultures incubated at 32 "C, 160 pmol photon m-, s-I PAR. n = 4. Durham Organism culture
Isolated from
15N2
602 626 603 61 1 6 14 612
Gloeotichia sp. Gloeotrichia sp. Calothrix sp. Nostoc commune Nostoc sp. Frkherella muscicola
C2H2: N,
Day 5 (exponential phase)
Sonargaon Manikganj Agrakhola Sonargaon Sonargaon
C2H2
15N2
E
SD
E
SD
13.98 8.14 12.98 29.62 13.98
2.91 1.24 4.48 6.03 6.63
95.40 41.9 73.1 108.6 61.00
17.24 16.36 11.63 19.67 9.50
258.0
35.59 5.35
Sonargaon 48.23 7.48
Chlorophyll a Samples from standing crop surveys or experiments were stored in a cool-box and then deepfrozen on return to the laboratory. Chlorophyll was extracted using hot methanal (Marker et al., 1980a); a specific absorption coefficient of 77 was used (Marker et al., 1980b).
6.82 5.15 5.63 6.40 4.36
C2H2: N,
Day 10 (early stationary phase) C2H2
E
SD
14.87 15.33 17.09 12.04 9.32
4.73 3.37 3.38 1.81 2.31
31.8
7.64
E 52.74 69.9 96.32 52.82 37.92 183
SD 10.66 11.12 10.22 12.54 9.01
3.55 4.56 5.63 4.39 4.07
23.04 5.74
Results
Quantitative estimates of blue-green algal standing crop were made in August - September 1983 for floating and epiphytic communities (Table 2), which were typically dominated overwhelmingly by a single species. Nitrogenase assays were carried out on a range of these, including the Azolla - Anabaena association, with incubation done under conditions similar to those in which
Table 2. Estimates of blue-green algal (epiphytic and floating given separately) and Azollapinnata standing crops at Manikganj and Sonargaon during 1983 flood season; Azolla sampled on different days from the free-living blue-green algae. Date
Microhabitat
Fields samples
Samples per field
Standing crop (mg m-, chl a ) Range
Manikganj 29.8 29.8 12.9 12.9 26.9 Sonargaon 9,103 11.9 21.9
-
x
SD
DWR floating DWR floating floating DWR DWR DWR Azolla Azolla
0.24- 52.6 6.52 8.05 14.4 - 54.8 29.7 17.9 3.18- 4.72 3.95 standing crop (mg m-, d. wt) 0.22-292 76.2 180 0.34- 49.42 56.1 50.0
No. of samples < 0.0 1
the algae had been growing. The mean rates (per unit chlorophyll) were quite similar for all the free-living species except Scytonema mirabile (Table 3). The sequence of maximum values for individual species fell in the order (from highest to lowest): Aulosira fertilissima, Anabaena sp., Gloeotrichia natans, G. pisum, Cylindrospermum maim, Tolypothrixpenicillata, Scytonema mirabile. The first three are all species typically found under conditions of high light flux. Tolypothrixpenicillata and Scytonema mirabile also occur at the surface, but form dense clusters of filaments, so presumably the mean flux encountered by individual filaments is less. Measurements were included of population density and nitrogenase activity of a planktonic Anabaena, which formed a bloom at Sonargaon in mid-August 1986. This formed thin surface scums, but also found in samples from different vertical positions within the water column. Samples at - 10 cm had a mean of 128 pg 1- chl a (18.8.86: n = 31; range, 3.8-916 pg 1- '); there was a relatively low ARA per unit biomass (nmol C,H,pg chl a min-'X10-3): full light, 116 k 40; 44% light, 51 k 23; dark, 10.1 k 3.0. There were marked die1 changes in ARA (Figs 1,2), with the maximum somewhere between late morning and early afternoon and the minimum during the late part of the night. Nighttime ARA, expressed as a percentage of the 24-h total (Table 4), ranged form 0.6 (Gloeotrichia pisum) to 24.6 % (Azollapinnata). Table 4 suggests that the use of results combined from one hour in
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late morning and one hour in early afternoon provides a reasonable prediction of the daily total. This 2-h total in nine experiments representing seven species ranged from 14.8 to 22.7% of the 24-h total. There was also some indication (Fig. 3) of seasonal changes in ARA rates (per unit chlorophyll) for Aulosirafertilissima and Scytonema mirabile, but this effect was due largely to the daily maxima being lower during the flood season than on moist soil. Although ARA of individual species during the day was related broadly to light flux values, there was only a weak overall relationship between ARA and light flux when all data were pooled (Fig. 4). The data do however suggest that PAR values above 1600 pmol photon m-2 s- ' (measured with the 2-71 sensor) are sometimes inhibitory. In view of the fact that there were frequent changes in light flux during the monsoon season, with sometimes rapid decreases to very low values, the results for Aulosira fertilissima were separated according to how PAR values had changed in the hour preceding the experiment; however there was no detectable effect (Fig. 4). The influence of transfer from light to dark was also tested; there was in all cases a marked decrease in ARA, the effect being least in Azolla pinnata (Table 5). ARA was also measured for various fractions of representative DWR tillers (see Methods) at Sonargaon at about the period of peak flood in 1982 and 1983. ARA was very low in 1982, the total associated with all aquatic roots from a
Table 3. Compariosn of ARA per unit biomass of various species incubated at PAR from which population taken. Data based on experiments made at some period during 1000-1400; no more than two experiments included from any one site on any one day.
Organism Anabaena sp. Aulosira fertilirsima Cylindrospermum majus Gloeotrichia natans G. pbum Scytonema mirabile Tolypothriv penicillata Azolla pinnata
n
-
Unit
nmol C2H, pg chl a1 x 10-~
Range
'
n
n n
nmol C2H, mg d wt - '
-
x
--
C yl i n d r o s p e r m u m
Gloeotrichia
pisum
r
1
I
1
I
I
I
I
1
I
I
I
18
24
06
12
18
18
24
06
12
18
time ( h )
I dark
(or d a r k incubations d u r i n g day )
full PAR A
P A R a t ~ n c u b a t i o nm ~ c r o h a b ~ t a t
o temp ("C )
part night, part day
0
day
Fig. I. Die1 changes in environmental variables and ARA per unit biomass (nmol C,H, pg chl a - min- x 10- 3, at Sonargaon on 20-21 August 1983. Environmental variables are temperature, incident PAR, PAR at incubation microhabitat and 0, (mg 1- I ) ; PAR values each mean of six. Study carried out in field.
single tiller ranging from 0.24 to 1.0 nmol C,H, min-' (compare with 1983 in Table 6). Most tillers in August-September 1983 (Table 2) had a scattered cover of Gloeotrichia pisum and ARA was much higher (Table 5); examples of profiles (with data expressed per unit dry weight) are illustrated in Fig. 5. ARA was easily detectable down to node 6 and sometimes further. When the roots were washed thoroughly to remove macro-
scopically obvious algae, ARA decreased, but there was usually still a significant difference between results in the light and the dark; (Table 7), suggesting the importance of algae very closely associated with the root surface. Macroscopically visible algal colonies were seldom noted in the plankton (excluding floating), other than small detached pieces of otherwise typically loosely attached mucilaginous species
G. n a t a n s
7 S e p 81
[L
4 1
1
1
^20 01
1
1
1
1
,
1
1
,
-
4
.. ,
1
a
0
ooooo
0
Aulosira
,
'&&
.Has
a 1000
1
00 00
t
%wu?oA
1 1 Oc t 83
G. p i s u m
23 J u n 8 3
-
-
Scytonema
11
r
m
6 u!a~,@,.L
O c t 83
drm 1
m
Azolia
I
1
06
l
12
1
1
18
1
1
24
1
1
06
1
1
1
1
12
time
(h)
d a r k ( o r d a r k ~ n c u b a t i o n sd u r ~ n gd a y )
PAR a t ~ n c u b a t i o nm ~ c r o h a b ~ t a t
o tempf°C)
'
24
f u l l PAR A
1
18
p a r t night, part day
fl
day
'
'
Fig. 2. Die1 changes in environmental variables and ARA per unit biomass (nmol C,H, pg chl a - rnin- x 10 - ') with material taken from Manikganj and Sonargaon, but incubated away from the sites. One example (Nostoc sp.) from moist soil at Manikganj included for comparison.
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Manikganj Sonargaon Manikganj Manikganj Manikganj Sonargaon Sonargaon Manikganj Manikganj
Aulosira ferliirsima Cylindrospennum maju. Gloeorrichia natans
Nostoc Scytonema mirabile Azolla pinnata
Gloeotrichia pkum
Location
Organism Date
1000-1 100 1300-1400
17540 3830 21370 183800 34730 215530 51730 3650 55380 311300 1899 313199 9559 788 10347 48070 1825 49895 72300 9765 82065 7180 1688 8868 650000 212200 862200
Total (24h)
At night
Night
Day Night Day
Total in period
% of total ARA
Total hours ARA
--
1000-1 100 l3OO-14OO
% of total ARA
Table 4. Comulative totals of ARA (nmol C,H, pg chl a - ' rnin- x 10-3, calculated from individual 1-h incubations, including extrapolations for a few night-time periods. Totals for 1000-1 100 and 1300-1400 were selected to reflect the period when most routine ARA assays were made.
Sc y t o n e m a
Aulosira
k ey A
a
M a y J u n Jul Aug S e p O c t
a
soil water
M a y J u n J u I Aug S e p O c t N o v
Fig. 3. Relationship between ARA ofAulosirafertilCsima and Scytonema mirabile and month of study; results for studies on moist soil are separated from those in floodwaters.
-
.
7
m
a
Aulosira
a l l algae
3
,0°1
0
1000
2000
0
1000
P A R ( p m o l rn-'s-'l Fig. 4. Relationship between ARA and PAR for all blue-green algal taxa and for AulosirafirtilCsima only. For the latter symbols are used to indicate whether incubation conditions reflect a decrease or increase in incident light flux from the period immediately before incubation.
such as Aulosira fertilissima. However colonies of a gas-vacuolateAnabaena reached a high density at Sonargaon in mid-August 1986. They formed local thin surface scums, but were also present in samples taken elsewhere in the water column. Samples taken at - 10 cm gave a mean value of 128 pg 1-' chl a (18.8.86: n = 32; range, 3.8-916 pg I-'). There was no obvious environ-
mental change coinciding with this bloom. Plankton not macroscopically visible was not followed in detail, but samples taken at intervals suggested that this was not important quantitatively inside the DWR fields. However a small-celled Synechococcus sp. (demonstrated by passage through a GF/C filter and subsequent growth in medium) was frequent at Sonargaon in September 1985.
PAR 1 cultivar 2 date 3 time (mid point)
temp
( t i m o l m-2i') 1000 200025
i°C
ARA
0,
)
( m g 1.')
35 0
10
20
0,
i
1pO
0
100 ,
200 ,
nrnol C2 H4 mg (d. wt.1-I min-l x
I Duli Aman 2 10.8.83 3 1030 4 2255
1 2 3 4
Duli Aman 11.9.83 1030 2384
1 2 3 4
Duli Aman 11.9.83 1430 2102
1 2 3 4
Duli Aman 21.9.83 1125 2206
1 2 3 4
Gabura 21.9.83 1225 2102
Fig.5. Distribution of ARA per unit tiller biomass (nmol C,H, mg d. wt - min - x 10 - 3, at Sonargaon on various sections of DWR tillers, together with epiphytic algae (mostly Gloeotrichia pisum). Environmental data included are mean incident PAR during experiment and PAR, temperature and 0, at incubation microhabitat. Each rice cultivar represents a different field; four different tillers studied on 11.9.83 (two at 1030 and two at 1430).
Table 5 . Effect of transfer during daytime from light to dark on ARA per unit biomass during the first 1-1.5 h incubation period.
ARA (nmol C2H4pg chl a min-' x n
Anabaena sp. Aulosira fertilissima Gloeotrichia natans G. pisum Scytonema mirabile Tolypothrixpenicillata Azolla pinnata
1 6 1 5 4 2 4
Dark range
1.8-32.1 6.2-26.1 5.8-20.4 7.8- 8.8 6.8-40.2
As % light
-
Table 6. Total ARA (nmol C2H4 min- ') on tillers whose depth profiles of ARA unit biomass are shown in Fig. 5. Internode includes stem, sheath and lamina; node consists largely of roots. Assays carried out in two fractions: (i) stem + sheath and lamina, where remaining; (ii) aquatic roots.
Cultivar
Date
x
6.9 10.1 6.2 16.9 12.5 8.3 21.0
Time ARA (nmol C2H4min- ') Stem + leaf Roots Total (tiller)
Duli Aman Duli Aman Duli Aman Duli Aman Duli Aman Duli Aman Gabura
10.8.83 11.9.83 11.9.83 11.9.83 11.9.83 21.9.83 21.9.83
1030 46.0 1030 80.1 1030 116 1430 20.1 1430 30.5 1125 33.7 1125 13.0
30.8 76.8 45.9 126 132 249 22.9 53.0 28.8 59.3 17.9 51.6 26.9 39.9
Table 7. Comparison of light versus dark on ARA (nmol C2H4mg d. wt- ' min-' x lo-') shown by aquatic roots of DWR washed thoroughly to remove all macroscopically visible algae.
Location
Cultivar
Date
PAR of microhabitats (approx)
ARA (nmol C2H, mg d. wt-' x Light
Manikganj Manikganj Manikganj Manikganj Manikganj Manikganj Sonargaon Sonargaon
Dark
Bawalia Bawalia Digha Bawalia Digha Bawalia Digha Gabura Duli Aman
Discussion Blue-green algae are sometimes conspicuous during the flood season in the DWR fields at Manikganj, Sonargaon and elsewhere in Bangladesh (Whitton et al., 1988b), but differences during the season and from field to field within any one location make quantitative estimates difficult. Nevertheless, in view of Brammer's (1983) claim for their importance as a source of combined nitrogen, an attempt will be made to use the present data to make very approximate estimates
of nitrogen futed. The following assumptions are based on comparative observations during visits. Manikganj (i) Floating: that the mean of values in Table 2 applies during August - September, 50% mean for 30 June-3 1 July and the same value as the last sample for October. (ii) Epiphytes: that the mean of values in Table 2 applies for 29 August - 30 September, a mean of 0.5 mg m - chl a applies for 30 June - 3 1July and the crop was neghgible during October. (iii) Azolla : crop negligible.
Sonargaon (iii) Floating: that the mean value of 2 mg m-2 applies for 20 June-5 August and the crop was neghgible during October. (iv) Epiphytes : that the mean of values in Table 2 applies during August-September, a mean of 0.5 mg m P 2chl a applies for 20 June-3 1 July and the crop was neghgible during October. (v) Azolla: that the mean of values in Table 2 applies during September, but the crop was neghgible at other times. general (vi) That data in Table 3 of Whitton et al. (1988b) indicate the relative abundance of the important blue-green algal species. These data are combined with ARA values in Tables 3 and 4. (vii)That the value for N, : C,H, reduced be taken as 1 : 5 (see Methods). Estimates of nitrogen fixed during the flood season may be compared with values for the preand post-flood periods of the DWR crop (Table 8). More than twice the amount is fixed in the pre-flood than the flood phase at Manikganj, but contributions from the two are similar at Sonargaon. It was suggested (Rother & Whitton, submitted b) that much of the nitrogen fixed in the pre-flood phase is not recycled until the flood phase; however some of the flood phase algal crop persists until the post-flood phase. It seems probable that about 10 and 6 kg ha- N became available at Manikganj and Sonargaon, respectively, during the flood season. These values may be compared with N present in the water column at any one time. If the time of maximum water depth (say, 1.75 m) is considered and the mean seasonal values for inorganic aqueous N at Table 8. Estimates of nitrogen fixed inside DWR fields at Manikganj and Sonargaon in 1983. Data for soil based on Rother & Whitton (submitted) Manikganj
Sonargaon
Nitrogen fixed (kg ha-') pre-flood flood post-flood total
10.20 4.39 2.15
16.7
3.80 4.42
