Nitrogen fertilizer does two things to soil carbon. Below 15 grams of organic carbon per kilogram of soil, it increases plant-derived carbon inputs and promotes soil aggregation โ building particulate organic carbon through physical protection. Above 15 grams per kilogram, the same fertilizer stimulates microbial metabolic efficiency, producing necromass that stabilizes through mineral-associated organic carbon.
The same chemical input triggers opposite carbon storage mechanisms depending on whether the soil is carbon-poor or carbon-rich.
This came from a global meta-analysis of field experiments โ not a lab study or a model. The threshold at 15 g C/kg is empirical. Below it, the fertilizer feeds the plants, and the plants feed the soil. Above it, the fertilizer feeds the microbes, and the microbes feed the minerals. The biological pathway that dominates is determined by the soil's existing state, not by the fertilizer's chemistry.
The implication for climate models is direct. Carbon-poor tropical soils and carbon-rich temperate soils respond to the same agricultural practice through different mechanisms. A policy that treats "add nitrogen" as one intervention is actually two interventions wearing the same label. The soil decides which one it receives.
The through-claim: an input is not one thing. It's a function of the state that receives it. When the receiving system has a threshold, the same action produces opposite outcomes on either side โ and the threshold is invisible unless you measure the state before you act.