Interactive effects of salinity and SOM on the ecoenzymatic activities across coastal soils subjected to a saline gradient

Salinity and soil organic matter (SOM) are key edaphic factors that exert substantial influence on soil microbial activity. To understand the interactive effects of both factors on microbial activity, nine ecoenzymes involved in C, N, P and S cycling were assayed across a saline gradient (coastal areas of the Bohai Gulf, CN). The electrical conductivity (EC) of the soils ranged from 0.14 to 13.65 dS m−1. The response of the ecoenzymatic activities (EEAs) to salinity followed a nonlinear relationship due to the interaction of SOM and salinity on EEAs. The nonlinear response demonstrated divergent patterns below and above an EC of 2 dS m−1. Specifically, when EC < 2 dS m−1, no correlation between EEAs and conductivity was observed and EEAs had a positive correlation with SOM; when EC > 2 dS m−1, the EEAs were negatively correlated with conductivity and no correlation with SOM was observed. Substrate amendment at EC < 2 dS m−1 significantly improved EEAs that reached the levels similar to those of non-saline soils, while EEAs remained unrecovered when EC > 2 dS m−1. This EC value may stand as critical (threshold) salinity. Under higher EC conditions, EEAs are mainly affected by salinity while SOM has no critical role. This becomes reversed when salinity (as EC) drops below the critical value. This study provides with a numerical salinity reference for the potential of the land ‘to be reclaimed by organic matter addition’ and assists in defining strategies for soil reclamation based on the proposed salinity threshold.