Historical trends and drivers of the laterally transported terrestrial dissolved organic carbon to river systems

Dissolved organic carbon (DOC) represents a critical component of terrestrial carbon (C) cycling and is a key contributor to the carbon flux between land and aquatic systems. Historically, the quantification of environmental factors influencing DOC leaching has been underexplored, with a predominant focus on land use changes as the main driver. In this study, the process-based terrestrial ecosystem model JULES-DOCM was utilized to simulate the spatiotemporal patterns of DOC leaching into the global river network from 1860 to 2010. This study reveals a 17 % increment in DOC leaching to rivers, reaching 292 Tg C yr^-1 by 2010, with atmospheric CO₂ fertilization identified as the primary controlling factor, significantly enhancing DOC production and leaching following increased vegetation productivity and soil carbon stocks. To specifically quantify the contribution of CO₂ fertilization, a factorial simulation approach was employed that isolated the effects of CO₂ from other potential drivers of change. The research highlights distinct regional responses. While globally CO2 fertilization is the dominant factor, in boreal regions, climate change markedly influences DOC dynamics, at times exceeding the impact of CO₂. Temperate and sub-tropical areas exhibit similar trends in DOC leaching, largely controlled by CO₂ fertilization, while climate change showed an indirect effect through modifications in runoff patterns. In contrast, the tropics show a relatively low increase in DOC leaching, which can be related to alterations in soil moisture and temperature. Additionally, the study re-evaluates the role of land use change in DOC leaching, finding its effect to be considerably smaller than previously assumed. These insights emphasize the dominant roles of CO₂ fertilization and climate change in modulating DOC leaching, thereby refining our understanding of terrestrial carbon dynamics and their broader implications on the global C budget.