Response of microbial activity and biomass in rhizosphere and bulk soils to increasing salinity

Abstract

BACKGROUND AND AIMS The impact of salinity on microbes has been studied extensively but little is known about the response of soil microbial activity and biomass to increasing salinity in rhizosphere compared to bulk (non-rhizosphere) soil. METHODS Barley was grown for 5 weeks in non-saline loamy sand to which salt (NaCl) was added. The electrical conductivity in the saturated extract (ECe) was 1, 13 and 19 dS m⁻¹ for non-saline and two saline soils. Pots without plants were prepared in the same manner and placed next to those with plants. The water content in all pots was maintained at 75% of water-holding capacity by weight. After 5 weeks the planted and unplanted pots were harvested to collect rhizosphere and bulk soil, respectively. The collected soil was then used for an incubation experiment. The EC levels in the pot experiment (EC1, EC13 and EC19, referred to as original) were either maintained or increased by adding NaCl to adjust the EC to 13, 19, 31 and 44 dS m⁻¹. CO₂ release was measured continuously for 20 days, microbial biomass C (MBC) was measured at the start and the end of the incubation experiment. RESULTS In general, cumulative respiration and microbial biomass C concentration in rhizosphere and bulk soil decreased to a similar extent with increasing adjusted EC. However, compared to the treatments where the EC was maintained, the percentage decrease in cumulative respiration when the EC was increased to EC44 was smaller in rhizosphere than in bulk soil. CONCLUSION Overall, the reduction of cumulative respiration with increasing salinity did not differ between rhizophere and bulk soil. But microbes in rhizosphere soil were more tolerant to high EC than those in bulk soil which could be due to the greater substrate availability in the rhizosphere even after the soil was removed from the roots.