Declining Ecosystem Respiration Linked to Nitrogen Deposition: Insights From a 26-Year FLUXNET Record.

Long-term carbon flux measurements at the FLUXNET site Loobos, a Pine forest in the Netherlands, reveal a counter-intuitive decline in total ecosystem respiration (TER) by tens of percents between 1997 and 2021. This trend cannot be explained by temperature variability or methodological changes alone. Instead, our findings point to a biogeochemical mechanism: despite a doubling of soil organic matter stocks, ecosystem respiration appears limited by decomposition rates rather than substrate availability. Soil incubation experiments indicate that microbial activity is limited by substrate quality and strongly acidic conditions (pH = 2.9), associated with large nitrogen deposition. Glucose addition experiments confirm the presence of an active microbiome, but its activity is suppressed under the present acidic soil conditions. These results raise concerns about ecosystem health under conditions of nitrogen deposition and the long-term sustainability of the observed carbon sink. Loobos may serve as an early indicator of broader ecosystem responses to environmental disturbances, as similar negative TER trends have been observed at other long-term FLUXNET sites. To advance understanding of the global carbon cycle, it is essential that observed flux trends are attributed and corroborated by changes in carbon and nitrogen stocks, and that models are continuously confronted with observational data. We therefore discuss the need of periodically measuring pH as soil acidification can be a limiting factor and suggest the need to introduce this variable in model representations of TER near regions sensitive to nitrification.