You can find SDI-12 libraries for the Arduino just google Arduino SDI-12 library
This addresses the interfacing your node to the SDI-12 sensors part of the puzzle. With this you can build the data logger piece.
The LoRa piece really has nothing to do with SDI-12. The logger code sends its output data to TTN, this is not an SDI-12 protocol output, instead is a series of bytes to be transferred in a LoRaWAN payload.