The substrate-induced respiration (SIR) method utilizes the initial respiratory response of soil microbes to substrate amendment to provide an estimate of the amount of C held in living. In this work, SIR measurements were used to evaluate the effects of chronic nitrogen and saline water additions on the active soil microbial biomass pool in a 3-year sorghum crop established in a Eutric Fluvisol at Alentejo, Portugal. A triple emitter source irrigation system was installed to deliver different combinations of NaCl and NH4NO3. The main effect happens in the plot irrigated with saline water, where the SIR rates were c.a. 40% lower than in the control plot, denoting the expected detrimental effect of salt on the active fraction of soil microbial biomass. Smaller decreases in SIR rates were also observed in the plots amended with N-fertilizer, particularly in the highest concentration. The combined application of intermediate amounts of NH4NO3 with NaCl resulted in higher SIR rates than with saline water alone, thus mitigating the declining effect of salinity on soil microbial activity. By the end of the irrigation season, all SIR rates were increased, reaching similar levels as in the beginning of the crop cycle.
