Responses of Soil Microbial Activity and Biomass to Salinity After Repeated Additions of Plant Residues

Microbial adaptation to salinity can be achieved through synthesis of organic osmolytes, which requires high amounts of energy; however, a single addition of plant residues can only temporarily improve energy supply to soil microbes. Therefore, a laboratory incubation experiment was conducted to evaluate the responses of soil microbes to increasing salinity with repeated additions of plant residues using a loamy sand soil with an electrical conductivity in saturated paste extract (ECe) of 0.6 dS m−1. The soil was kept non-saline or salinized by adding different amounts of NaCl to achieve ECe of 12.5, 25.0 and 50.0 dS m−1. The non-saline soil and the saline soils were amended with finely ground pea residues at two rates equivalent to 3.9 and 7.8 g C kg−1 soil on days 0, 15 and 29. The soils receiving no residues were included as a control. Cumulative respiration per g C added over 2 weeks after each residue addition was always greater at 3.9 than 7.8 g C kg−1 soil and higher in the non-saline soil than in the saline soils. In the saline soils, the cumulative respiration per g C added was higher after the second and third additions than after the first addition except with 3.9 g C kg−1 at ECe of 50 dS m−1. Though with the same amount of C added (7.8 g C kg−1), salinity reduced soil respiration to a lesser extent when 3.9 g C kg−1 was added twice compared to a single addition of 7.8 g C kg−1. After the third residue addition, the microbial biomass C concentration was significantly lower in the soils with ECe of 25 and 50 dS m−1 than in the non-saline soil at 3.9 g C kg−1, but only in the soil with ECe of 50 dS m−1 at 7.8 g C kg−1. We concluded that repeated residue additions increased the adaptation of soil microbial community to salinity, which was likely due to high C availability providing microbes with the energy needed for synthesis of organic osmolytes.