Study

The global stoichiometry of litter nitrogen mineralization

Manzoni, Stefano; Jackson, Robert B.; Trofymow, John A.; Porporato, Amilcare

Science, 2008

Peer-reviewedmeta analysismixed

What they did

Analyses compiled litter decomposition datasets to determine when decomposing litter releases nitrogen to the soil and when it instead takes nitrogen up, relating the switch to the carbon-to-nitrogen ratio of the material and to decomposer requirements.

Studied in: Plantae, Bacteria, Fungi
Sample size: compiled litter decomposition datasets across biomes

What they found

Decomposing litter initially immobilises nitrogen — taking it from the surrounding soil — when its carbon-to-nitrogen ratio is high, and only begins releasing nitrogen once decomposition has narrowed that ratio sufficiently. The threshold varies with decomposer efficiency rather than being a single universal value.

What it means

The authors' reading, and ours. Where they differ, that difference is the point.

How the authors put it

Whether decomposition supplies or consumes available nitrogen depends on litter stoichiometry relative to decomposer requirements.

How NatureHQ reads it

The record behind one of the least intuitive facts in this whole subject: adding dead plant material to soil can make less nitrogen available to plants, not more. The microbes doing the decomposing need nitrogen to build themselves, and if the material is carbon-rich they take it from the soil around them. The threshold is not a fixed number, which is why gardening rules of thumb about carbon-to-nitrogen ratios are approximations rather than constants.

Limitations

A compilation across studies with differing methods and litter types.
The threshold depends on decomposer efficiency, which is itself variable and hard to measure.