Carbon and nitrogen mineralization at different salinity levels in Omani low organic matter soils

•Date palm straw strongly promoted saprotrophic fungi, even in highly saline soil.•Manure in combination with straw led to maximum microbial biomass C contents.•Negative salinity effects on C mineralization increased with recalcitrance.•Salinity strongly increased N immobilization after application of date palm straw.

A 56-d incubation experiment at 30 °C was carried out to study how salinity affects C and N mineralization of composted dairy manure and date palm straw. A low- and a high-saline soil were amended with (1) manure, (2) manure + low straw, (3) manure + straw, and (4) sole straw. The microbial and fungal biomass contents are very low in Omani soil abandoned for at least 6 years. Straw application revealed a highly significant increase in microbial biomass C, but especially in ergosterol in the low-saline soil. In contrast, straw led only to an increase in ergosterol in the high-saline soil, where only the combined application of manure with straw had significant positive effects on microbial biomass C. In the high-saline soil, the sum of C mineralized reached only 55% of SOC-derived CO2–C, 65% of manure-derived CO2–C, and 75% of straw-derived CO2–C in comparison with the respective treatments of the low-saline soil. The application of straw led always to a net N immobilization, which was markedly stronger in the high- than in the low-saline soil. The increase in salinity by composted cattle manure should be considered if this fertilizer is applied to soils sensitive to changes in salinity.