What happened
- Seven researchers, with Xing, Zhao, Liu and D. Broadbent among the authors, published on 4 November 2026 in PNAS an analysis of how soil respiration responds to prolonged nitrogen input.
- They used a time-series dataset built from 38 long-term experiments in forests and grasslands.
- Soil respiration is the CO₂ output produced by roots and microorganisms, and one of the main routes by which soil carbon returns to the atmosphere.
- The pattern they find shifts over time: a transient increase lasting about two years and then a sustained suppression.
Why it matters
- Soil holds the largest pool of actively cycling carbon on land, so its behaviour conditions climate projections.
- Human-driven nitrogen deposition has more than doubled since the 1860s, but its long-term effect on soil emissions was unclear.
- If models capture only the initial response, they will get the sign wrong when projecting decades.
The number
-6%. That is the mean suppression effect on soil respiration they find, with a 95% confidence interval between -10% and -3%.
Context
Forests and grasslands followed consistent nonlinear trajectories: a rapid initial decline in the magnitude of the response that then levelled off. Suppression emerged when cumulative nitrogen inputs exceeded a threshold of about 100 kilos per hectare, and it was stronger and faster under high inputs, above 50 kilos per hectare a year.
The authors associate these temporal dynamics with nitrogen-induced soil acidification and the changes that process causes in microbial properties. Their conclusion for modelling is direct: predicting land-atmosphere carbon fluxes well requires incorporating this evolution over time, rather than a fixed effect.
At Mamífero we follow soil carbon. We reported that nitrogen falling on grasslands destabilizes their production and that ground olivine stores more carbon only if microbes have food. Also that Cerrado soils hold carbon that markets ignore and that Baltic sediments are losing their capacity to retain sulphide.
What’s next
- Earth system models can incorporate these nonlinear trajectories instead of constant responses.
- The threshold of 100 cumulative kilos per hectare gives a reference for interpreting short experiments.
- How this effect interacts with warming and shifting rainfall has yet to be seen.
Bottom line
A two-year experiment and a twenty-year one can give opposite results under the same treatment. That, at bottom, is the warning this work leaves for those who model soil carbon.
Sources
- Xing, Zhao, Liu, D. Broadbent et al., “Progressive shifts in soil respiration response to long-term nitrogen enrichment across global forests and grasslands”, PNAS, 4 November 2026, DOI 10.1073/pnas.2622400123. This note is based on the paper’s abstract.
- Cover image: Horst J. Meuter, CC BY-SA 4.0, via Wikimedia Commons
Edited by Rodrigo Cornejo. How we select and verify: who writes these notes.
