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Rice farmers in Mwea Irrigation Scheme routinely apply P and N fertilizers which affect water nutrient levels. A study was conducted to establish the effects of nutrient N and P application on Azolla biomass accumulation. The study was conducted in a batch culture experiment, using 5g of fresh Azolla biomass samples from each of the six major paddy schemes namely: Mwea, Ahero, West Kano, Bunyala, TARDA, and Taveta.
Treatments consisted of 0 and 3mg P l-1 and 0 and 200mg N l-1, laid out in a randomized complete block design replicated three times. Azolla samples were grown in batch culture plastic pots of 8.4 x 10-3m3 for 10 days using canal water, which was replenished every three days. Fresh Azolla biomass weight was recorded at 0, 5 and 10 days after inoculation. Data was subjected to analysis of variance using SAS statistical package version 9.1 and means separated using the least significant difference test (p≤0.05). The pH levels in irrigation water averaged 7.2 while N, P and K levels were 20.2, 11.6 and 15ppm respectively. Tissue N and K for Azolla accession averaged 4.2% and 1.6% respectively. Biomass accumulation and doubling time of Azolla were significantly affected by exogenous P and N nutrient application. Doubling time ranged from 5.5 days to 6.7. Application N and P significantly reduced Azolla biomass accumulation and increased biomass doubling time.
Introduction
Azolla is a pteridophyte which forms a symbiotic
association with a cyanobacterium-Anabaena azollae
(Bocchi et al., 2010) and fixes nitrogen at a rate higher
than legumes (Wagner, 1997). A study on Azolla has
reported that Azolla contains 4.5% N, 0.4% P and 1.5-
3% K (Watanabe, 1989). In Mwea Irrigation Scheme,
the existing species was found to contain 3.9% N, 0.44%
P, and 1.08% K (unpublished). Azolla is capable of
providing 40kg N ha-1 to the rice crop due to its nitrogen
content which is released upon decomposition
(Kannaiyan et al., 1982) reported that. This can reduce
the cost of rice crop inorganic fertilizer, which
constitutes 20% of rice production cost in Mwea
Irrigation Scheme (Rice MAPP, 2012).
In Mwea Irrigation Scheme, Azolla coverage is
estimated at 30-50% during peak times and this is
majorly dependent upon water availability
(unpublished). Nutrient status and other
environmental factors are major factors affecting
Azolla biomass growth (Wagner, 1997).
Extensive and
intensive inorganic fertilizer use forms the primary
source of the water nutrient status and eutrophication
in water bodies (FAO/ECE, 1991). Depending on the
levels, this can cause atmospheric, aquatic and
ground water system pollution (Choudhury et al.,
2005). Farmers in Mwea irrigation scheme apply
estimated P and N fertilizer amounts of 58kg of P2O5
and 56kg of N per hectare respectively, based on
recommendations by Wanjogu et al. (1997). These
fertilizers applied contribute to the water nutrient
status being conducive for Azolla growth.
The nitrogen fixing ability of Azolla makes it able to
grow in nitrogen deficient waters (Watanabe, 1979;
Hussner, 2010). However, its growth is limited by the
nutrient element P (Kitoh et al., 1993). Kondo et al.
(1989) reported a maximum growth rate for Azolla
under application of 3.1ppm P with a threshold limit of
0.5-0.6% P.
The level of phosphorus in water bodies is
varied and can be high due to fertilizer use and runoff.
This can lead to Azolla blooms in paddies or flood
waters. Depending on the water nutrient status, Azolla
multiplies fast; doubling its biomass in less than 10
days and readily colonizes new areas (Campbell, 2011).
Hussner (2010) reported a doubling rate of 3-10 days,
while Kitoh (1993) reported a doubling rate of 2-3 days
under laboratory conditions.
The nutrient P is important for Azolla growth and
reproduction (Sadeghi, 2012). Its deficiency has been
shown to hinder the acetylene reduction activity (Tung
et al., 1989). However, excess levels of nutrient P has
been reported to have a negative effect on Azolla
growth (Pitt et al., 2014).
According to Rains et al.
(1979), a P level of 0.34ppm is the lower threshold limit
below which there is deficiency. Subudhi et al., (1981)
reported that external P level of 5ppm is the higher
threshold limit beyond which Azolla tissue N content is
affected negatively. The nutrient N is important for
Azolla growth but because of its N fixing ability, it is
capable of growing in N free media (Hussner, 2010).
External N has been shown to inhibit the activity of
acetylene reductase activity (Yatazawa et al., 1980).
Kitoh, 1991 showed that external ammonia N
negatively affects Azolla growth and N fixation activity.
The objective of this study was to determine the effects
of P and N on Azolla biomass growth and tissue N and
P uptake, in Mwea Irrigation Scheme. Get the full articles and pdf at: Int. J. Agron. Agri. Res. 14(2), 1-9, February 2019.
International Journal of Agronomy and Agricultural Research - IJAAR is an open-access scholarly research journal, published by International Network for Natural Sciences. IJAAR publishes original scientific research articles in the field of Agronomy and Agricultural Sciences. IJAAR published 2 Volume and 12 issue per the calendar year.
Effects of inorganic nutrient P and N application on Azolla biomass by International Network For N... on Scribd
Citation Sample
WA Oyange, GN Chemining’wa, JI Kanya, PN Njiruh.
Effects of inorganic nutrient P and N application on Azolla biomass growth and nutrient uptake. Int. J. Agron. Agri. Res. 14(2), 1-9, February 2019.
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