Effects of sampling density on interpolation accuracy for farmland soil organic matter concentration in a large region of complex topography

•Observations from 235,309 sites were used to interpolate SOM in complex topography.•Accuracy improvement rate was slower first and then faster as samples increased.•Change-points of sampling density seemed to vary in different landforms.•Valley-basin had the best performance in interpolation as opposed to plain-platform.•Interpolation accuracy was more sensitive to sampling density in simple topography.