Salinity affects the response of soil microbial activity and biomass to addition of carbon and nitrogen

Addition of carbon (C) and nitrogen (N) to soil can enhance microbial tolerance to salinity, but it is not known if salinity changes the response of microbial activity and biomass to addition of C and N, or how nutrient addition affects microbial tolerance to salinity. We prepared salinity treatments of non-saline soil [electrical conductivity (EC1 (:5)) 0.1 dS m(-1)] without salt addition or adjusted to four salinity levels (2.5, 5.0, 7.5, 10 dS m(-1)) using a combination of CaCl2 and NaCl. The soils were amended with 2.5 mgC g(-1) as glucose or as mature wheat straw (C/N ratio 47 : 1), with NH4Cl added to glucose to achieve a C/N ratio similar to that of wheat straw, or with NH4Cl added to glucose or wheat straw to achieve a C/N ratio of 20. Soil respiration was measured over 30 days. Microbial biomass C and N (MBC, MBN), dissolved organic C (DOC), and total dissolved N (TDN) were measured on day 30. Cumulative respiration and MBC concentration decreased with increasing EC, less so with glucose than with wheat straw. The MBC concentration was more sensitive to EC than was cumulative respiration, irrespective of C source. Addition of N to glucose and wheat straw to bring the C/N ratio to 20 significantly decreased cumulative respiration and MBC concentration at a given EC. This study showed that in the short term, addition of a readily available and easily decomposable source of energy improves the ability of microbes to tolerate salinity. The results also suggest that in saline soils, irrespective of the C substrate, N addition has no impact, or a negative impact, on microbial activity and growth.