Long-term changes in dissolved inorganic carbon (DIC) across boreal streams caused by altered hydrology

A major challenge for predicting future landscape carbon (C) balances is to understand how environmental changes affect the transfer of C from soils to surface waters. Here we evaluated 14 years (2006-2019) of data on stream dissolved inorganic carbon (DIC) concentrations and export rates for 14 nested boreal catchments that are subject to climatic changes, and compared long-term patterns in DIC with patterns in dissolved organic carbon (DOC). Few streams displayed significant concentration or export trends at annual time scales. However, a clear majority of streams showed decreasing DIC concentrations during spring flood, and about half showing declines during summer. Although annual runoff has generally not changed during the studied period, intra-annual redistribution in runoff explained much of the seasonal changes in stream DIC concentration. We observed negative DIC-discharge relationships in most streams, suggesting supply limitation of DIC with increasing discharge. This was in contrast to DOC, which mostly showed a chemostatic behaviour. The distinct trend patterns observed for DIC and DOC underpin intra-annual changes in the total C pool (DIC/DOC ratio) in most streams and reflect differences in how these C forms are produced and stored, are mobilized by hydrological events, and are responding to long-term environmental changes. Collectively, our results indicate that future changes in hydro-meteorological conditions will affect the transfer of DIC from soils to water, and that these changes contrast to those of DOC. Such information is critical for future projections on how total C transfer from boreal system will respond on a changing climate.