Microbial population and soil enzymatic activities under long term rice-fallow and uncultivated soils of Nalbari district, Assam, India

Microorganisms in soils and their enzymatic activities play a critical role in maintaining soil fertility and sustaining crop productivity by nutrient transformation and organic matter decomposition. The present investigation was carried out to assess the effect of long term continuous cultivation of rice on microbial population and soil enzymatic activities that was compared with adjacent uncultivated soils. For this Geo -referenced (N:26⁰31.882'to 26⁰ 18.224'and E:091⁰30.536'to091⁰15.750') soil samples (0-15cm depth) were collected after harvest of rice from rice-fallow system and adjacent uncultivated sites. A total of 120 numbers of soil samples were collected and microbial counts of general bacteria, fungi, Azotobacter, Azospirillum, Phosphate solubilizing bacteria (PSB) and soil enzymatic activities viz. Dehydrogenase (DHA), Fluroscence diacetate hydrolases (FDA), Phosphomonoesterase (PME) and Arylsulphatase (ARYL) were determined following standard procedures. The results revealed that population of bacteria (6.49 log₁₀ cfu g^-1), Azotobacter (3.77log₁₀ cfu g^-1), Azospirillum (3.75log₁₀ cfu g^-1), Phosphate solubilizing bacteria (3.71log₁₀ cfu g^-1), registered higher in rice-fallow cultivated soils compared to adjacent uncultivated soils (bacteria-6.36 log₁₀ cfu g^-1, Azotobacter-3.62 log cfu₁₀ g^-1, Azospirillum-3.54 log₁₀ cfu g^-1, PSB- 3.69 log₁₀ cfu g^-1 and differed significantly (p<0.05), whereas the population of fungi was found more in uncultivated (5.38log₁₀ cfu g^-1) soils compared to rice cultivated soils (5.30 log₁₀ cfu g^-1). The soil enzymatic activities viz. DHA, FDA, PME and ARYLwere recorded significantly (p<0.05) higher in rice fallow soils (DHA-79.01μg TPF g^-1 24 h^-1, FDA-12.16μg fluorescein g^-1 h^-1, PME-114.06μg p-nitro phenol g^-1 h^-1, ARYL-21.43 μg p -nitrophenol g^-1 h^-1) as compared to uncultivated soils (DHA-66.26μg TPF g^-1 24 h^-1, FDA-9.49 μg fluorescein g^-1 h^-1, PME-92.14μg p-nitro phenol g^-1 h^-1, ARYL-15.72μg p-nitrophenol g^-1 h^-1). The results indicated that continuous cultivation of rice crop followed by a fallow period enhanced the biological properties of soils including microbial population count and enzymatic activities in soils.