Discordance Between Surface Wetting and Soil Salinity Responses and Low Persistence of Soil Salinity Ranks After Autumn and Spring Irrigation

Irrigation is expected to increase soil water and promote salt leaching in salt-affected farmland, yet wetter soil does not necessarily mean lower salinity, and relative salinity positions may shift after irrigation. This observational before-and-after study compared soil conditions around two seasonal irrigation events in the Hetao Irrigation District, Inner Mongolia, using 344 field observations collected before and after autumn and spring irrigation. From these observations, 89 unambiguous co-located pre- and post-irrigation pairs were established, including 37 autumn and 52 spring pairs. Soil moisture (SM) was measured in the 0–10 cm surface layer, whereas EC was determined as EC1:5 by averaging measurements from the three soil layers (0–30 cm). Surface SM increased at all paired sites, but the 0–30 cm mean EC1:5 responses varied markedly. Under an operational classification based on the relative change in EC1:5, a decrease of at least 10% was defined as “effective desalination”; this response occurred at 54.1% and 53.8% of the autumn- and spring-irrigation sites, respectively, whereas EC1:5 increased by at least 10% at 37.8% and 26.9% of the sites. These inverse responses remained evident under alternative relative and absolute thresholds, and the magnitude of soil wetting was not significantly related to the net change in EC1:5. Salinity ranks were also poorly preserved across irrigation events. Same-quartile retention was only 32.4% after autumn irrigation and 23.1% after spring irrigation, while 55.6% and 84.6% of the post-irrigation Q4 sites had not belonged to Q4 before irrigation. These results show that surface wetting and the 0–30 cm mean EC response were not consistently aligned at the sampled locations, while relative salinity ranks showed limited short-term persistence. These point-level findings do not demonstrate reorganization of the continuous district-wide salinity pattern. Whether post-irrigation salinity verification or map updating improves monitoring performance requires validation using spatially representative mapped data.