Distribution and diversity of algal communities in rice terrace agroecosystem of a hilly district of southern Assam, North-East India

Algal diversity in rice cultivation terraces at the foothills of Borail range in Dima Hasao district of Assam, North-East India has been studied. The rice cultivated terrace habitats maintained by different ethnic communities harbour plentiful algae. Physicochemical parameters of water and soil of the terraces were ascertained and seasonal algal distribution monitored. Water in the agroecosystem was assessed to be acidic to neutral with low nitrate-N and soluble reactive phosphorus (SRP) content. Dissolved silicon (d-Si) content ranged from 8.85 to 18.92 mg L-1. The soil pH was acidic with organic carbon (OC) in the range 1.02-1.42%, and water holding capacity (WHC) in the range 42.71-53.43%. Overall, 98 algal species under 50 genera of cyanophyceae and chlorophyceae with 29 species each have been documented. The bacillariophyceae was represented with highest number of species (40). Diversity was assessed to be highest in autumn. The species Anabaena torulosa, Cylindrospermum minimum, Nostoc spongiaeforme, Phormidium ambiguum, Cladophora glomerata, Oedogonium pusillum, Pleurotaenium ehrenbergii, Spirogyra collinsii, S. crassa, Zygnema cruciatum, Cymbella tumida, Gomphonema gracile were observed to be most dominant in the rice terraces. Algal distribution in the agro-ecosystems in relation to physico-chemical characteristics of water and soil was analysed by Principal component analysis (PCA) and Canonical correspondence analysis (CCA). Air temperature (AT), pH, total alkalinity (Talk), SRP, Free CO2 (F-CO2) of water were observed to be the most important factors for algal growth in the rice terraces. The most significant soil attributes included WHC, bulk density, porosity and soil organic carbon.