Soil salinity, fertility and carbon content, and rice yield of salt-affected paddy with different cultivation period in southwestern coastal area of South Korea

Salt-affected reclaimed tidelands (RTLs) in coastal areas of South Korea are commonly used for rice (Oryza sativa L.) cultivation as excessive salts are removed through leaching by irrigation during rice growth. However, changes in soil properties and rice growth in RTLs by continuous rice cultivation are not well understood. In the present study, we investigated soil salinity indices (EC_e, electrical conductivity of soil extracts; SAR, sodium adsorption ratio; ESP, exchangeable sodium percentage), fertility (mineral nitrogen (N), and available phosphorus (P) concentrations), and carbon (C) content as well as rice biomass and yield in 72 paddy fields within 10 RTL areas, which have been cultivated for different time periods (13–35 years). Continuous rice cultivation decreased EC_e by 0.36 dS m⁻¹ per year, indicating desalinization, but ESP and SAR were not changed. Available P concentration increased but mineral N concentration did not change with rice cultivation years. There was also an indication of increased soil organic C (SOC) concentration with rice cultivation. However, rice biomass and yield were not different among the RTLs. Instead, rice biomass and yield as well as SOC concentration were negatively correlated with EC_e when all individual measurements from 72 sampling sites within the 10 RTLs were pooled, suggesting that salinity is a constraint for both rice production and SOC sequestration at field scales. Interestingly, soil mineral N concentration was positively correlated with EC_e but negatively with rice biomass and yield. This result implies that heavy application of N fertilizer in highly saline soils may not benefit rice growth due to salinity stress. Our results show that long-term rice cultivation with fertilization and irrigation decreases salinity while increasing P availability. However, SOC content and rice biomass and yield were more responsive to the salinity status of each paddy field than during cultivation years, highlighting the need for field-specific soil management for improved rice production and enhanced SOC sequestration.