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Nitrogen deposition is slowing forest respiration

At Loobos, TER fell while GPP rose over 26 years; acid soils (pH 2.9) and N deposition appear to throttle decomposition.

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What unexplained trend did Loobos show?

A pronounced multi-decade decline in total ecosystem respiration despite a growing carbon sink.

How much did annual TER and GPP change from 1997 to 2021?

TER fell from 1350 to 1015 g C m−2 year−1; GPP rose from 1567 to 1904.

What share of TER is soil respiration?

About 69% soil versus 31% aboveground autotrophic respiration.

What happened to the organic layer between 2009 and 2024?

It compacted yet doubled in mass; implied carbon rose from 4.7 to 9.4 kg C m−2 if mixing ratio was constant.

Did temperature sensitivity (Ea) explain the TER decline?

No long-term Ea trend; R10 fell significantly (r²=0.54), implying a 36% TER drop before the 1.25°C soil warming, net −29%.

What was organic-layer pH in incubations without lime?

About 2.9–3.0 at the start and 2.9–3.1 at the end.

What did glucose addition show?

Respiration spiked for up to 5 days, so microbes are present but limited by accessible substrate.

How high was nitrogen deposition?

About 38 kg N ha−1 year−1 in 1990 and 21 kg N ha−1 year−1 in 2023.

Why do authors still worry about the growing NEE sink?

Declining TER may mean stalled decomposition and poorer ecosystem health, not a durable climate benefit.

Are other FLUXNET forests showing the same TER drop?

Several mature temperate/boreal sites (e.g. Hainich, Sorø, Morgan Monroe, Fyodorovskoye, Norunda, Lägern) show negative seasonal TER trends.

Could tower height or processing artefacts explain the whole trend?

Authors argue footprint, u*, and canopy-growth effects cannot account for its magnitude, but the post-2021 second tower still raised TER unexplained.

What biogeochemical story do they favor?

Sustained N deposition acidifies the organic layer, slowing microbial decomposition so carbon accumulates while TER falls.