The data accompanies the manuscript in review that evaluates salinity-associated shifts in organic C thermodynamics, biochemical transformations, and heteroatom content in a first-order coastal watershed in the Olympic Peninsula of Washington state, USA. The files contain raw data including soil chemical characteristics, data filtering steps, BNTI values, Operational Taxonomic Unit abundance, metadata corresponding to OTU table, FTICR-MS derived processed data from FREDA (water and chloroform fractions), community assembly process estimates, microbial abundance data and bar plot at the phylum level, and R codes. The files are related to the publication and have been used in the results and analyses section. This dataset contains 12T FTICR reports of water and chloroform fraction, processed data using FREDA, data filtering steps in FREDA, data used to generate van Krevelen plots, and code for analyzing FTICR data and performing statistics. FITCR data processing, sequence processing after quality filtering, community assembly analysis, and all statistical analyses were done by Aditi Sengupta, Jason Toyoda, and Rosalie Chu.