Response of soil respiration and microbial biomass to changing EC in saline soils

The effect of salinity on soil microbes has been studied extensively, but usually with a constant salinity level throughout the experiment. In the field, soil salinity changes over time, but little is known about the effects of these changes on microbial activity and biomass. In this study, two experiments were conducted in which every 5 days, soil cores were dipped into a salt solution to increase or maintain the EC or salinity was decreased by leaching. The solution also contained glucose to ensure that C was not limiting the response of the microbes to salinity. One non-saline soil and two saline soils (ECe 1, 11 and 31 dS m−1) from the field were used. In Experiment 1, soil salinity was increased or reduced between EC 1, 11, and 31 dS m−1 repeatedly over six 5-day cycles. In Experiment 2, soil salinity was increased over four 5-day cycles from 1 or 11 to 31 dS m−1 either abruptly (within one cycle) or gradually (over at least 2 cycles). Soil respiration was measured daily in both experiments; in Experiment 1, microbial biomass C (MBC) was determined at the start of the experiment and at the end of cycles 1 (day 5) and 2 (day 10) and the last cycle (day 30). In Experiment 2, MBC was measured on day 0 and at the end of experiment (day 20). The results showed that soil microbes can respond quickly to changes in EC with respect to activity and growth when they are supplied with easily available C. A previous exposure to high EC did not limit the ability of the soil microbes to respond to a subsequent decrease in salinity. Compared to the originally saline soils, microbial activity and biomass in the originally non-saline soil were higher, less affected by EC increases and recovered more quickly after the EC was decreased. This suggests that a large biomass can better adapt to changes in EC than a small biomass which was already stressed by salinity in the field. Furthermore, a gradual increase of the EC did not result in greater respiration or microbial biomass compared to an abrupt increase.