High soil salinity reduces straw decomposition but primes soil organic carbon loss

Abstract

Straw incorporation is a widely recommended agronomic practice to increase organic carbon (C) in saline soil. The mechanism of straw induced priming effect (PE) on soil organic matter (SOM) decomposition is likely to be influenced by salinity, which may stimulate microbial processes and enzyme activity because of osmotic stress and nutrient resource limitation. We incubated ¹³C-labeled straw in soil for 90 d under three salinity levels: low electrical conductivity (EC_1:5) of 0.31 dS m⁻¹, medium EC_1:5 of 0.97 dS m⁻¹, and high EC_1:5 of 1.6 dS m⁻¹). During the first 15 d, the low salinity soil had 31 % greater PE than the high salinity soil, apparently due to microbial preference for labile straw-derived C over SOM under negligible osmotic stress. This trend was reversed from day 30 onward, with medium-and high-salinity soil showing amplified PE (1.1-fold and 1.7-fold increase respectively versus low-salinity control), associated with microbial N limitation (inorganic N dropped more than 16 %) and dominance of copiotrophic taxa: Proteobacteria, Bacteroidota, Ascomycota. High salinity decreased microbial biomass and diversity, and slowed down straw decomposition, which lowered necromass by 13 % and increased plant-derived C by 6.9 % compared to low soil salinity. Quantitative modeling demonstrated linear salinity effects on C cycling - each 1 dS m⁻¹ increase in soil EC_1:5 amplified the annual PE by 930 mg C kg⁻¹ soil year⁻¹ and reduced the net C balance by 3.8 g C kg⁻¹ soil. Therefore, high soil salinity enhances SOM loss, while increase in straw-derived C primarily comes from plant-derived C rather necromass C. Our findings make the connection between soil salinity and C dynamics in straw-remediated saline soil, which is linked to the C sequestration potential of saline lands.