Soil C:N Ratio & Nitrogen Mineralization Lab

Amendment Presets

Manual Controls

Residue C:N ratio 20:1

Residue applied (g C/m²) 400

Soil temperature 20°C

Simulation speed ×1

Live Readouts

Net N flux

0.0 kg/ha/day

Available soil N

40.0 kg/ha

Residue C remaining

100%

Elapsed time

0 days

How it works

Soil microbes decompose organic residue by first-order kinetics and need nitrogen in proportion to their own biomass (C:N ≈ 8:1) to build new cells. Whether nitrogen is released to the soil (mineralization) or pulled out of it (immobilization) depends only on the residue's own C:N ratio versus a critical threshold:

dC/dt = −k·C            k = k₀·Q10^((T−20)/10)
N released per unit C decomposed = ΔC/CN_residue
N required by microbes  = ΔC·ε / CN_microbe   (ε ≈ assimilation efficiency 0.40)

Net flux = ΔC·( 1/CN_residue − ε/CN_microbe )
Critical C:N ≈ CN_microbe / ε ≈ 8 / 0.40 = 20:1

C:N below ~20 (manure, legumes, fresh grass) — microbes have surplus nitrogen and excrete the excess: net mineralization, available soil N rises.
C:N above ~20 (straw, sawdust, dry stalks) — microbes are nitrogen-starved and scavenge it from the soil pool: net immobilization, available N falls, sometimes below what a growing crop needs.
Temperature — decomposition rate follows a Q10 ≈ 2.2 response: every 10 °C roughly doubles microbial activity.
The blue points inside the soil block are plant-available mineral nitrogen (NH₄⁺/NO₃⁻); the brown points on the surface are the undecomposed residue carbon pool, shrinking as it is consumed.

This C:N threshold is the real reason agronomists mix low-C:N legume cover crops with high-C:N straw mulch, and why fresh sawdust applied alone can starve a crop of nitrogen for months.

Residue carbon Available soil N

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