Organic carbon, nitrogen pool and microbial abundance indifferent rice soils of Manipur valley, India

A study was undertaken in three blocks of Manipur viz. Haorang, Wangoi and Sawombung after rice harvest covering upland, midland and lowland areas. The oxidizable organic carbon (OC) of these soils was found to be fairly good and ranged from 5.0 to 18.1 g/kg. The average pH values of 5.72, 5.45 and 5.33 were noticed in the soils of Haorang, Wangoi and Sawombung blocks respectively. Significant relationships were observed between the organic carbon and mineralized nitrogen pools. The NO3-N content was found to be positively correlated with oxidizable OC (r = 0.335*), total N (r = 0.480**) and NH4-N (r = 0.818**) of these soils. The colony forming unit (CFU) of total viable bacteria (TVB), aerobic Non-Symbiotic nitrogen fixing bacteria (ANFB), phosphate solubilizing bacteria(PSB), actinomycetes and fungi varied widely. There was fair increase of microbial populations at pH=5.7 and effect of pH was more for PSB and ANFB.The abundance of microbes in the three land types was as follows: upland–ANFB>TVB>PSB >actinomycetes> fungi; Midland- TVB> ANFB>PSB>actinomycetes> fungi; and lowland –PSB>TVB>ANFB>actinomycetes> fungi. Key words: Organic carbon (OC), nitrogen pool, microbial abundance, rice soil.