Les Cyanophycées produisant des colonies mucilagineuses peuvent développer des fleurs d'eau volumineuses dans les rizières. En raison d'une teneur en eau et en cendres élevées l'apport d'azote est faible et n'a pas d'effet notable sur le rendement du riz. (Résumé d'auteur

Roger, Pierre-Armand

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Unicellular mucilaginous blue-green algae 
(BGA): impressive blooms but deceptive 
hisfertilizers 
P. A. RoLer, Maitre de Recherches 
ORSTOM, visiting scientist; S. Ardales, 
research assistant; and I. Watanabe, soil 
micro biologist, I R R  I 
Certain mucilaginous Nz-furing BGA 
develop impressive blooms in rice fields. 
Because of their high water and ash 
content, however, their N contribution 
may be low. The figure shows a bloom 
that has been observed annually since 
1980 in no-N plots of the IRRI farm in 
dry season. I t  comprises Aphanothece 
as dominant genus and Nostoc and 
Gloeotrichia as associated genera. Highest 
fresh weight biomass recorded was 58 
t/ha, but 98.6% water and 54% ash 
contents limited N content t o  13 kg/ha. 
In 1984 dry season, we studied the 
effects of such a bloom on rice yield. The 
experiment was in a Latin square of 16. 
4- x 4-m plots, with control and BGA 
treatments. After 2 plowings and 
harrowings, IR60 was transplanted at 
20- x 20.cm spacing. A basal application 
Aphanothece sp. (2 x 106/g), Nostoc 
(1.3 x 105/g), and CaZothrix sp. (4 x 
1041g). 
Twentyeight days after inoculation 
(30 days after transplanting [DTI), 
floating colonies were harvested and fresh 
weight, dry weight, and N content deter- 
mined. The same BGA developed in 
inoculated and noninoculated plots, 
Aphanothece sp. being dominant. BGA 
harvested from control plots and inoc- 
ulated plots were combined and redistri- 
buted evenly in the BGA-treated plots 
after these were drained. The following 
day (3 1 DT), the algal material was 
incorporated with ärotary weeder. Thirty 
days later (60 DT) the procedure was 
repeated. At maturity, grain and straw 
yield and grain protein content were 
measured. 
Thirty days after transplanting, the 
biomass of mucilaginous BGA in inocu- 
lated plots was about twice that in the 
control (see table). Aphanothece was 
dominant in all plots, giving the BGA 
biomass a high water content (99.4%). 
Fresh weight biomasses of 18.7 and 33.0 
tlha produced only 1.5 and 2.6 kg N/ha. 
Incorporating mucilaginous BGA in 
the surface soil negatively affected 
further development of BGA, as shown a t  
60 DT by a BGA biomass about 10 times 
lower in treated plots than in the control. 
At this time Nostoc sp. and Aphanothece 
sp. dominated. 
BGA bloom comprising Aphanothece, Nostoc, and Gloeotrichia genera. 
of 30 kg P/ha, 30 kg K/ha, and 0.5 kg a i  
furadan/ha was made in all the plots at 
transplanting. Two days later, half of the 
plots were inoculated with 20 g/m2 of a 
dry algal inoculum collected from the 
same plots in May 1983. At the time of 
application, the inoculum contained 
2.2 X lo6 colony-forming units of Nz - 
fixing BGA per g dry weight. The 
dominant strains in the inoculum were 
BGA biomass produced in the plots, BGA biomass incorporated in treated plots, and rice 
yie1d.a 
Produced Produced Level of Incorporated 
in in signficance of in BGA-treated 
control BGA-treated the difference plots 
(A) (BI (A - B) (A + B) 
BGA biomass 30 DT fw (t/ha) 18.7 33.0 10% 51.7 
dw (kglha) 74.8 132.0 10% 206.8 
N(kg/ha) 1.5 2.64 10% 4.8 
60 DT fw (t/ha) 7.5 0.6 1% 8.1 
dw (kg/ha) 75.2 6.2 1% 81.5 
N (kg/ha) 1.72 O. 14 1% 1.86 00 
30 DT fw(26.2) 26.2 33.6 ns 59.8 B 
e-: 
Rice yield Grain t/ha 3.24 f 0.29 3.31 i 0.20 ns o 
+ dw (kg/ha) 149.9 138.2 ns 288.1 
60 DT N(kg]ha) 3.22 2.78 ns 6.0 
straw t/ha 2.41 j :  0.21 2.38 f 0.14 ns 
protein % dw 1.45 f 0.06 1.41 f 0.06 ns 
o 
Cu 
c 
93 
Q 
aEach value is the average of eight replications. i% = fresh weight, dw = dry weight. 
Productivity in the control, in terms of 
dry weight and N, was similar during the 
1st and 2d months of growth. Total pro- 
ductivity (sum of biomasse; harvested at 
30 and 60 DT) was similar h control and 
BGA-treated plots, the higher productivi- 
t y  in treated plots during the 1st month 
of growth being balanced by lower pro- 
ductivity during the 2d. From 60 DT to 
harvest, biomass of mucilaginous BGA 
was very low. 
Total BGA bioniass incorporated in 
treated plots was equivalent to 60 t fresh 
weightlha (52 t a t  30 DT and 8 at  60 DT), 
corresponding to only 6 kg N (4.14 at 30 
DT and 1.86 at  60 DT). Previous experi- 
ments had shown that about 20-30% of 2 
the N of incorporated BGA is available to  3 
the plant. This suggests that in the pre- 3 
sent experiment about 2 kg N was avail- 5 3 
able to  the plant, which was not enough a 
to  significantly increase the yield in 
treated plots. 
Correlation between the different 
variables measured in the 8 replications 
f#- 
Z d  
2 jp 
@*c 
of the control and BGA-treated plots 
were significant only for grain and straw & * *  2 
.. 
yield. The absence of correlation b e t w e e n g  2 9 
BGA biomass produced in control plots 
and rice yieldindicates that N and growth- 
promoting substances exudated by the 
algae, if any, did not affect rice growth. 0 


Unicellular mucilaginous blue-green algae (BGA) : impressive blooms but deceptive biofertilizers

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