Addition of glucose increases the activity of microbes in saline soils

Abstract

Adaptation of soil microbes to salinity requires substantial amounts of energy. We hypothesised that addition of glucose would increase microbial activity and growth and alleviate the negative effect of salinity on microbes. An incubation experiment was conducted with four salinity levels by using one non-saline and three saline soils of similar texture (sandy clay loam), with electrical conductivities (EC1:5) of 0.1, 1.1, 3.1 and 5.2 dS m–1. Glucose was added to achieve five organic carbon concentrations (0, 0.5, 1, 2.5, 5 g C kg–1). Soluble nitrogen (N) and phosphorus (P) were added to achieve a carbon (C) : N ratio of 20 and a C : P ratio of 200 to ensure that these nutrients did not limit microbial growth. A water content of 50% of the water-holding capacity (optimal for microbial activity in soils of this texture) was maintained throughout the incubation. Soil respiration was measured continuously over 21 days; microbial biomass C and available N and P were determined on days 2, 5, 14 and 21. Cumulative respiration was increased by addition of glucose and was reduced by salinity. The percentage decrease in cumulative respiration in saline soils compared with non-saline soil was greatest in the unamended soil and lowest with addition of 5 g C kg–1. At this rate of C addition, the percentage decrease in cumulative respiration increased with increasing salinity level. Microbial biomass C (MBC) concentration on days 2 and 5 was strongly increased by ≥1 g C kg–1 but decreased over time with the strongest decrease at the highest C addition rate. The MBC concentration was negatively correlated with EC at all C rates at each sampling date. Addition of C resulted in N and P immobilisation in the first 5 days. Biomass turnover as a result of depletion of readily available C released previously immobilised N and P after day 5, particularly in the soils with low salinity. This study showed that over a period of 3 weeks, addition of glucose increased microbial activity and growth in saline soils and alleviated the negative impact of salinity on microbes.