Lily,
land surface module deals with soil physics. It predicts soil temperature and soil moisture.
The option "pxlsm_soil_nudge" turns on soil nudging when you run with PX surface scheme. WIth this option being activated, soil moisture and soil temperature produced by PX surface scheme will be indirectly 'corrected' based on observations. It is expected that the model results will be ore consistent with observations.
For more details, Please read the paper "An Indirect Data Assimilation Scheme for Deep Soil Temperature in the Pleim–Xiu Land Surface Model" (DOI: https://doi.org/10.1175/2009JAMC2053.1)