Influences of Exogenic Organic Materials Application on Soil Fertility Status and Paddy Growth under a Coastal Saline Soil Condition

Paddy cultivation in saline soil can rapidly reduce soil salinity, which is an important approach for managing, utilizing, and improving such soils. However, the high salinity of saline soil severely limits the sustainability of paddy production. Adding exogenic organic material to improve soil fertility in saline soil is a key measure for obtaining high-yield, efficient and sustainable cultivation of paddy. This study used a field experiment to explore the influences of different organic materials application on soil desalination and fertility improvement in saline paddy soil. The results showed that the application of dairy manure (DM), sludge vermicompost (SV), and vinegar residue (VR) reduced soil barrier factors, including electrical conductivity (EC) and pH, increased soil fertility, including soil organic carbon (SOC), nitrogen (N), and phosphorus (P), and promoted paddy growth in saline soil. Specifically, soil EC decreased by 29.0%, 32.9% and 49.4% and paddy biomass increased by 27.7%, 63.7% and 107.6% in DM, SV, and VR-treated soils with the highest application rates, respectively, compared to the control. At an equal carbon application rate, VR was more conducive to decreasing soil EC and pH and increasing paddy biomass. Compared to DM and SV, VR addition resulted in an average decrease of 20.7% and 19.1% in soil EC, respectively, and an average increase of 57.3% and 29.5% in paddy biomass. In addition, soil water-stable aggregates (WSA), SOC, N, and P contents in VR-treated soil were lower than those in DM and SV-treated soils. Correlation and path analysis revealed that there was a significant negative correlation between paddy biomass and soil barrier factors. However, EC in VR-treated soil had a direct negative effect on paddy biomass, while EC in DM and SV-treated soils had an indirect negative effect on paddy biomass. Additionally, the direct contribution of soil pH to paddy biomass was higher with VR (−1.49) than that with DM (−0.21) and SV (0.89). In contrast to DM and SV, the effect of soil WSA on paddy biomass in VR-treated soil was mainly an indirect positive effect, and the direct effect was negative. The corresponding results provided new options and ideas for the efficient utilization of saline soils and high-yield cultivation of paddy.