Secondary Salinity Effects on Microbial Biomass in Irrigated Cotton Soils.
Dilfuza Egamberdiyeva, National University of Uzbekistan, of Biology and Soil Sciences, Tashkent, 700175, Uzbekistan and Khandakar Islam, Ohio State University - OARDC, OSU South Centers, 1864 Shyville Rd, Piketon, OH 45661-9749.
Secondary soil salinilization is a major problem in irrigated agriculture. To evalaute the effects of various degrees of irrigation-induced salinity on microbial biomass and associated biological activities in soils under long-term cotton (Gossypium hirsutum L.) monoculture, a study was conducted in the SyrDarya region of NW Uzbekistan. Composite soil samples were randomly collected at 0-30 cm depth from weakly saline (2.3 dS m-1), moderately saline (5.6 dS m-1), and strongly saline (7.1 dS m-1) replicated fields (n = 4), 2-mm sieved, processed, and analyzed for pH, EC, N and P, and exchnageable ions, C fractions, total organic C (COrg), extractable C (CExt), hot water-soluble C( CHWS), microbial biomass (Cmic), basal respiration (BR) and specific maintenance (qCO2) respiration rates. On average, Ca+2, Mg+2, CO3-2 and Cl- were the dominant salt associated cations and anions in all the soils. However, Na+ and Cl- concentration was 36 to 80% higher in strongly saline soil than in weakly saline soil. The COrg concentration decreased by 10, CHWS by 29, and CExt by > 40%, respectively with an increase in salinity, and the decrease in concentration of various C fractions was most pronounced in bulk soil than in rhizosphere especially in strongly saline soil. Increasing salinity quadratically accounted for significant decrease in Cmic content (18 to 42%) and Cmic-to-COrg ratios (8 to 26%) with an exponential increase (16 to 65%) in qCO2 i.e. greater loss of labile C through microbial catabolism. The adverse effects of salinity on Cmic, Cmic-to-COrg, and qCO2 were most pronounced in bulk soil than in rhizosphere of strongly saline soil. However, increasing COrg and CHWS concentration supported a significantly larger Cmic pool with an exponential decrease in qCO2 in saline soils. The response of qCO2 to CHWS was more contrasting between bulk soil and rhizosphere than COrg concentration.