The effect of canal blocking on aquatic carbon dynamics in a retired Acacia plantation on tropical peatland

Tropical peatlands are among the greatest terrestrial carbon stores on Earth, able to influence the contemporary global carbon budget. Despite the necessity to protect those large carbon stocks, tropical peatlands are increasingly being exposed to degradation as a result of climate and land use change. Canal blocking is a prerequisite to restoring degraded peatlands. This physical action has the potential to increase water table level in the upstream catchment and minimize carbon loss along the aquatic continuum and toward the atmosphere. However, the impact of canal blocking on aquatic carbon exchange has yet to be quantified – and particularly so in tropical settings. Here, we compare the hydrodynamics, fluvial carbon concentrations and fluxes between a flowing and a stagnant (blocked) canal in a retired Acacia plantation in South Sumatra, Indonesia. The two canals were hydrologically and biogeochemically monitored using a combination of automated long-term sensors and manual discrete samples over the year 2022. Our results show that one direct consequence of canal blocking is the decrease in water flow and dissolved oxygen which enhanced dissolved CH₄ concentrations and CH₄ evasion, including from ebullition. No significant difference in dissolved organic carbon (DOC) and dissolved carbon dioxide (CO₂) concentrations between the two canals was observed, which suggests that canal blocking does not alter the quantity of carbon available for mineralization. The evasion of CO₂ was higher from the flowing canal, particularly from the spillways that were narrower and generated higher water flow. The main benefit of canal blocking was related to the absence of surface downstream carbon export. However, undocumented subsurface flow may still be occurring. Considering the ambivalence of these findings, we argue that a mass balance model that integrates the exchange of water and carbon at the catchment scale is the only way to accurately determine by how much canal blocking minimizes aquatic carbon losses. We conclude that the benefit of canal blocking on aquatic carbon losses from peatlands may be more complex than conventionally assumed, and recommend management responses aimed at minimising aquatic carbon losses from degraded and restored tropical peatlands.