We investigated the role of whitesand ecosystems (WSEs) in blackwater formation to develop novel constraints for the terrestrial carbon export from land to ocean. We used Orbitrap mass spectrometry to identify markers in dissolved organic carbon (DOC) from ground- and surface waters of two contrasting WSEs feeding Rio Negro tributaries, and compared them with known Rio Negro markers. Tributaries were fed by a whitesand riparian valley connected to plateau, and a typical upland whitesand . WSE-DOC molecular composition differed by 80% from plateau DOC, which was characterized by reworked, highly unsaturated N- and S-containing molecules. WSE-DOC contained mainly condensed aromatics and polyphenols. WSE samples differed by 10% in molecular DOC composition and also by their isotopic content (C, O, H). Upland WSE-DOC was exported by fresh precipitation and had maximum age of 13 years, being five years older than riparian valley WSE-DOC. Unexpectedly, only markers from the upland WSE, which cover a small proportion of the landscape, were identical to Negro markers. Markers of the riparian valley WSE, which are widespread and known for high DOC export, surprisingly showed lower coverage with Negro markers. Our results suggest that terrestrial DOC from upland WSEs is a main source of specific blackwater molecules missing in the ecosystem C balance, whereas C export from the riparian valley and especially from plateaus represents mainly recycled and transformed carbon not directly affecting ecosystem C balance. Our study highlights the potential of high-resolution techniques to constrain carbon balances of ecosystems and landscapes.