Interactive effects of salinity and drought stresses on soil respiration and microbial activities

Increasing frequency of droughts and problems related to salinization in irrigated areas may directly affect soil biological functions and soil respiration. An incubation study was designed to elucidate the effects of salinity and water intercross stresses on soil respiration and microbial activities. Three different moisture levels: optimum moisture conditions (80% water holding capacity (WHC)), moderate drought (50% WHC) and severe drought stress (30% WHC) were maintained in normal (ECe=1.48 dS m(-1)) and saline (ECe=8 dS m(-1)) soils. Our results showed that drought stress resulted in significant increase (p < 0.05) in microbial biomass carbon (MBC) (601 +/- 79 mg C kg(-1) soil) and cumulative C-CO2 emissions (1153 +/- 96 mg C kg(-1) soil). While under drought stress, extracellular enzyme activities showed no significant difference except leucine aminopeptidase with significant increase in its activity. Similarly, under saline conditions, MBC and soil respiration were significantly higher compared with normal soil. Extracellular enzyme activities were also significantly increased under salinity stress. In contrast, when combined stresses of drought and salinity were applied, cumulative C-CO2 emissions were significantly decreased (968 +/- 91 mg C kg(-1) soil) as evident by decreased beta-glucosidase activity, involved in C cycling. In contrast, chitinase activity increased under combined abiotic stresses, confirming the availability of chitin from microbial dead biomass degradation. It was concluded that microbes adopted drought and salinity stresses when applied individually but combined salinity with drought stress limited microbial biomass and microbial activities in soil.