The Gold Standard · Ecology
Make the decision path visible
One fitted decomposition constant compresses an entire mass-loss trajectory into one number. When soluble compounds disappear early and resistant material dominates later, that number changes with the observation window.
This guide starts from the familiar exponential model, works interval-specific apparent constants from an original series, compares them with one overall constant, and keeps leaching, fragment loss, climate and substrate pools distinct.
Choose the route closest to your decision
Chapter contents
Read the constant · Chapters 1–3
Work the trajectory · Chapters 4–6
Test the mechanism · Chapters 7–9
Report the limit · Chapters 10–12
CHAPTER 1 OF 12 · READ THE CONSTANT
1. The single-k model makes a strong claim
The model M(t) = M₀e⁻ᵏᵗ says the instantaneous fractional loss rate k is constant. Taking logs gives ln(M/M₀) = −kt, a straight line through time.
The model can summarise a chosen interval. It does not guarantee that the same material, decomposer community or environment controls every stage.
CHAPTER 2 OF 12 · READ THE CONSTANT
2. State the measurement boundary before fitting
Litterbag mass loss includes biological transformation, soluble leaching, fragmentation and material that escapes the mesh. It is not identical to mineralisation.
Correct for handling and ash where appropriate, record mesh and retrieval methods, and define whether the outcome is dry mass or ash-free dry mass.
Evidence check
Inspect early handling controls and fragment retention before assigning rapid initial loss to microbial decomposition.
CHAPTER 3 OF 12 · READ THE CONSTANT
3. An interval-specific apparent k is a local summary
Between times t₁ and t₂, an apparent exponential constant is k = ln(M₁/M₂)/(t₂−t₁). Its units are inverse time and depend on the time unit used.
Calculate the interval from positive masses on the same basis. The value is descriptive unless the constant-rate assumptions are supported.
CHAPTER 4 OF 12 · WORK THE TRAJECTORY
4. Work an original mass-loss series
A litterbag begins at 10.0 g and retains 7.4 g at month 3, 6.0 g at month 6 and 4.8 g at month 12.
The trajectory slows visibly: 2.6 g disappears in the first three months, 1.4 g in the next three, and 1.2 g in the following six.
| Month | Mass remaining |
|---|---|
| 0 | 10.0 g |
| 3 | 7.4 g |
| 6 | 6.0 g |
| 12 | 4.8 g |
CHAPTER 5 OF 12 · WORK THE TRAJECTORY
5. Calculate three apparent rates
From 0–3 months, k = ln(10.0/7.4)/3 = 0.1004 month⁻¹. From 3–6, k = ln(7.4/6.0)/3 = 0.0699 month⁻¹. From 6–12, k = ln(6.0/4.8)/6 = 0.0372 month⁻¹.
The decreasing sequence shows that one constant cannot describe every interval equally well.
| Interval | Apparent k |
|---|---|
| 0–3 months | 0.1004 month⁻¹ |
| 3–6 months | 0.0699 month⁻¹ |
| 6–12 months | 0.0372 month⁻¹ |
CHAPTER 6 OF 12 · WORK THE TRAJECTORY
6. The overall k hides the slowing
Across all 12 months, k = ln(10.0/4.8)/12 = 0.0611 month⁻¹. That number is useful for the start-to-end contrast, but it is lower than the early rate and higher than the late rate.
Two studies can reverse rankings when they fit different windows. Always pair k with its observation period and harvest schedule.
Transfer check
Recalculate k for a shared interval before comparing experiments with different durations.
CHAPTER 7 OF 12 · TEST THE MECHANISM
7. Changing substrate quality is one explanation
Soluble and readily metabolised compounds can be lost early, leaving chemically or physically protected material. A mixture of pools can produce a declining apparent rate even if each pool has its own constant rate.
A two-pool model is plausible only when the data contain enough time points and the parameters are identifiable. A flexible curve is not proof of two literal compounds.
CHAPTER 8 OF 12 · TEST THE MECHANISM
8. Changed climate can also change the rate
Temperature, moisture, inundation and freeze–thaw conditions vary through time. Seasonal forcing can make apparent k change even if substrate quality were stable.
Pair mass measurements with environmental records and use common-garden or reciprocal designs when separating substrate and site effects.
CHAPTER 9 OF 12 · TEST THE MECHANISM
9. Changed context: cohort and calendar time
A litter cohort ages as seasons change, so age and calendar conditions are confounded in one deployment. Staggered cohorts can help separate them.
The same litter placed in wet and dry seasons may show different early slopes. Do not transfer an interval-specific k without transferring its exposure history.
CHAPTER 10 OF 12 · REPORT THE LIMIT
10. Model selection needs prediction and residuals
Compare a single exponential, phase-specific summaries or justified multi-pool models using residual structure, uncertainty and out-of-sample prediction where feasible.
Do not choose a complex model solely because it fits more closely. Additional parameters can be weakly determined, especially with few harvests.
CHAPTER 11 OF 12 · REPORT THE LIMIT
11. Mesh, fauna and fragmentation remain alternatives
Late-stage brittleness can increase fragment escape, while fine mesh can exclude fauna and change moisture. Both alter the apparent trajectory.
Use paired mesh treatments only when their physical differences are acknowledged, and connect mass loss to chemistry, respiration or nutrient measurements when the mechanism matters.
| Observation | Useful companion evidence |
|---|---|
| Early pulse | Leachate or handling control |
| Seasonal change | Moisture and temperature |
| Late fragmentation | Recovered fragments and mesh |
| Pool claim | Chemistry or spectroscopy |
CHAPTER 12 OF 12 · REPORT THE LIMIT
12. Report an inspectable decomposition comparison
Report starting material, deployment date, site, mesh, initial and remaining mass basis, harvest times, equation, fitted window, uncertainty and residuals.
Methodological studies show that apparent decomposition rates often decline through time and that conclusions depend on model and duration. Preserve the full trajectory rather than publishing k alone.
| Field | Keep visible |
|---|---|
| Boundary | Mass basis and mesh |
| Time | Dates and intervals |
| Model | Equation and residuals |
| Transfer | Climate and substrate context |
Frequently asked questions
Is k a property of the litter alone?
No. It also reflects environment, decomposers, method and time window.
Can I compare k values with different units?
Convert units and align intervals first.
Does declining k prove two pools?
No. Environment and method can also change.
When is one k useful?
As a transparent summary over a stated interval when its limitations are acceptable.
Continue through verified routes
Core routes
Ecology subject root · Gold Standard of Education · Gold Standard directory · How X Works hub
Specialist routes
- Litter Mixtures, Additive Expectations and Non-Additive Decomposition
- Litterbag Mesh Size, Fragment Escape and What Mass Loss Measures
- Litter Nitrogen Concentration, Nitrogen Stocks and Immobilisation