Nitrogen deficiency is a common problem of rice soils worldwide. Several species of blue-green-algae or cyanobacteria with nitrogen fixing capacity are currently used as bio-fertilizer, especially to wetland paddy, in many parts of the world [1] . Cyanobacteria as a fertilizer supplement are capable of replacing 30 - 40 percent of urea nitrogen requirement to paddy and they have other positive roles in soil management [2] . Role of cyanobacteria in paddy fields is a much explored theme of research [3] -[9] . Since most blue-greens are cosmopolitan species with wide ecological amplitude to occupy diverse environmental conditions, exploration of the diversity and ecology of them from specific paddy zones has global relevance towards the development of sustainable paddy farming everywhere in the world. Moreover, as they are photosynthetic species, quantitative assessment of them in such soils will be significant to the accounting of global carbon sequestration potentials of paddy soils in general. These algae are also suggested as solution to soil pollution [10] . Their roles in soil crust formation and surface stabilization [11] , soil genesis, soil conservation [12] , general soil fertility enhancement [13] are well known. Some algae are also known as agents of producing plant growth promoting substances [14] . Very many specific reports of cyanobacteria from paddy fields of Kerala region are available [15] -[18] , but those of specific zones of Kuttanadu fields are quite rare [19] [20] and pretty old.

In these contexts, a comprehensive evaluation of the ecology and diversity of blue-greens of Kuttanadu rice fields is carried out. Major objectives included isolation and identification of all the specific species of blue- greens in the paddy fields of Kuttanadu, Kerala, South India in relation to soil regions, seasons and crop- growth-stages. Quantitative ecological aspects of the entire cyanobacterial community in terms of population dynamics and relative abundance in relation to soil pH, Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and Magnesium (Mg) from three different soil regions of Kuttanadu in relation to two crop seasons (puncha and virippu) and two growth stages of paddy (seedling and panicle stage) are compared and analyzed. As such, this is the pioneer attempt to explore the ecology and diversity of blue-green-algae in Kuttanadu wetland- paddy soils.

Kuttanadu (90˚17'N to 90˚40'N; 760˚19'E to 760˚33'E) is a unique tropical flooded paddy land of global interest situated in the South India Figure 1. It is the traditionally well known rice bowl of Kerala, spread around the Vembanadu Ramsar zone with a size of approximately 25 km in east-west direction and 60 km in north-south, direction, spread over 54 revenue villages of the three districts of Kerala State. This inland backwater zone had an estuarine character; but the construction of the “Thannirmukkam-barrage” during 1970s to protect paddy cultivation in the area from saline incursion during the non-monsoon season has caused the entire zone to become an artificially maintained freshwater system. These paddy fields lie at an average of 2 to 2.5 m below the mean sea level and remain freshwater-flooded all throughout the year except during the crop seasons when water is pumped out from the fields.

Major paddy crop seasons of the zone are Puncha (the summer crop―December to March) and Virippu (the monsoon crop―May to October). In general, the soil of Kuttanadu is highly compressible dark brown “alluvial” clay of high organic and specific mineral constituents [21] -[23] with slight variations over soil regions. Upper Kuttanadu, Lower Kuttanadu (Kari soils) and Kayal-lands are the three major soil regions of Kuttanadu [22] .

On the basis of geological features and biochemical characteristics, soils of Kuttanadu are grouped into three categories such as Karappadom (found mostly in the upper Kuttanadu), Kari (mostly lower Kuttanadu) and Kayal (mostly in Kayal lands) soils. Karappadom soils spreads over a large part in the upper Kuttanadu covering an area of about 33,000 hectares. They are river-borne alluvial soils. They are located more towards the interior and exhibit salinity less than that of the Kari and Kayal soils. They are deep and poorly drained soils. Soil colour varies from dark grey to deep brown or black. The surface soils are generally clay loams and characterized by high acidity and a fair amount of decomposing organic matter and nitrogen. They are generally deficient in available plant nutrients, phosphorus and lime.

Kari soils are characterized by acidity and salinity, which are deep black in colour due to relatively higher proportion of organic matter. They are seen in both the upper and lower Kuttanadu regions. They are peat soils found in large isolation patches in Alappuzha and Kottayam districts covering an area of about 9000 hectares. They exhibit characteristics of submerged and burned mangrove forest area, but are not silted up. Soils are characterized by heavy texture, poor aeration, bad drainage and low content of available plant nutrients. The natural formation of sulphuric acid is reported in Kari soils. Free sulphuric acid is formed by the oxidation of sulphur compounds present in the wood fossils under the soil. Large amounts of woody matter at various stages of decomposition occur embedded in this soil.

Kayal soils are found in rice fields reclaimed from the Vembanadu lake beds at the Kottayam and Alappuzha

districts and occupy an area of about 13,000 hectares. They are deep and poorly drained soils with a dark brown colour. The texture varies from silt loam to silty clay loam. They are slightly acidic to neutral in reaction. They are low in inorganic matter content, poor in total and available plant nutrients, but are fairly rich in calcium.

In all tables R1 represents the Upper Kuttanadu, R2 the Lower Kuttanadu and R3 the Kayal Lands; S1 represents the Pre-monsoon (Puncha) crop and S2 the Monsoon crop (Virippu); G1 represents the seedling growth stage and G2 the panicle stage.

Separate soil samples were collected simultaneously for algal studies and physico-chemical analyses of soils. All the analyses were carried out using standard Methods. Soil sampling for blue-green-algae was the same as that for soil and green algal studies of the zone already reported [23] . Soil sampling was carried out from five different paddy fields located about- 2 km intervals for each of the three soil regions. Two different composite soil samples of 1 - 5 cm depth were collected from each paddy field. At each field, samples were collected from several random plots of 10 m² size and mixed them together to a composite soil sample. Altogether 40 different composite samples (5 fields × 2 plots × 2 crop seasons-Puncha and Virippu × 2 crop-growth stages-seedling stage and panicle stage) were collected from a soil region. Parallel to soil sample collection for physico-chemi- cal analyses, surface samples for algal studies were also collected in the same pattern from all the fields.

Overall, 120 composite soil samples (40 × 3 soil regions) were collected from the three different areas of Kuttanadu for both the soil and algal studies. Algae were identified according to the keys of Desikachary, 1959 [24] and the classification of Guiry and Guiry, 2012 [25] . The morphological parameters such as nature of the filaments, the shape and size of vegetative cells, heterocysts, akinetes, width and length of intercalary cells, presence or absence of constriction at the cross wall and of the sheath; colour of the sheath; nature of trichomes and filaments; presence or absence of heterocyst; width and length of heterocyst were taken into consideration during the identification of the taxa. Statistical analyses such as ANOVA and Pearson’s correlation coefficient were carried out using the SAS 9.2 (2010) version software.

Physico-chemical characteristics of the soil, such as pH, total organic carbon (C), total Kjeldal N, plant available P, K, Ca and Mg studied in relation to crop seasons and different stages of paddy growth in the three different regions of Kuttanadu are already reported [23] .

Altogether 64 species of blue-greens belonging to 22 genera and 6 orders have been found out from Kuttanadu paddy fields during the study period (2009-11). Overall distribution of them in the three soil regions in different crop-seasons and growth stages of paddy are given in Table 1. The genus Oscillatoria with 12 species dominated the soils, followed by Anabaena (9 species) and Notoc (7 species). Phormidium and Leptolyngbya with 5 species each were also found out. Out of the sixty four species, thirty species were observed in culture only, thirty three were found out from fresh soil samples and Leptolyngbya cf. battersi was the only species appeared in both culture and fresh soil samples. Relative abundance of all species is given in Figure 2. The six orders of algae observed were Chroococcales (5 genera, 9 species; 14%), Synechocystales (3 genera, 4 species; 6%), Nostocales (4 genera, 18 species; 28%), Stigonematales (3 genera, 4 species; 6%), Pseudoanabenales (1 genus and 5 species; 8%) and Oscillatoriales (5 genera, 24 species; 38%). Population characteristics of all species are given in Table 1. Species richness and diversity index of all groups are given in Table 2 and soil chemical characteristics in Table 3. Plates 1-4 show photographs of all the 64 species found out from Kuttanadu.