Decomposition Rate Calculator

Litter decay is modelled as a first-order process, M(t) = M0 e^(-kt), where k is the fractional mass loss per unit time. That single constant summarises three separable controls. Substrate quality comes first, indexed by the carbon to nitrogen ratio and by lignin content, because decomposer microbes build their own biomass near 8:1 C:N and need roughly 24:1 in the substrate to avoid immobilising soil nitrogen. Manure near 15:1 and green grass near 20:1 break down quickly, while conifer needles near 80:1 and woody material above 300:1 break down slowly. Temperature is the second control, applied here as a Q10 of about 2, meaning the rate doubles for every 10 degrees C rise, an approximation that holds reasonably between about 5 and 35 degrees C and fails above roughly 40 degrees C. Moisture is the third control and is not monotonic. Decay peaks at intermediate water-filled pore space, falls sharply in dry litter where microbial water films cannot form, and falls again under waterlogging because oxygen diffusion collapses and anaerobic metabolism is far less efficient, which is why peat accumulates in bogs. From k follow the derived quantities: half-life is ln(2)/k, mean residence time is 1/k, and 95 percent loss takes ln(20)/k. The single-exponential form is a deliberate simplification. Real litter sheds soluble sugars and amino acids within the first weeks, then slows as lignified and mineral-associated fractions come to dominate, so two-pool or asymptotic models fit long runs better and a k fitted to a one-year litterbag trial will overpredict loss in later years.

The litterbag, still the workhorse method, was introduced by K. L. Bocock and O. J. W. Gilbert in 1957: a known mass of litter is sealed in mesh, returned to the field, and retrieved in sequence so that mass loss can be tracked against time. The exponential formalism came from Jerry Olson, whose 1963 paper in Ecology on energy storage and the balance of producers and decomposers defined the decay constant k and showed that at steady state a forest floor accumulates litter until annual input equals k times the standing crop, giving the litter accumulation ratio that is still used to infer k from a single survey. Broad-scale controls were mapped next. Vernon Meentemeyer showed in 1978 that actual evapotranspiration, a combined heat and moisture term, predicted litter decay rates across climates better than either temperature or precipitation alone. The Long-term Intersite Decomposition Experiment, run across North American sites from about 1990, placed identical substrates in many climates for a decade and separated the climate signal from the substrate quality signal. More recently the Tea Bag Index, proposed by Joost Keuskamp and colleagues in 2013, used commercially standardised green and rooibos tea as a cheap uniform substrate, allowing schools and citizen scientists to contribute comparable k values and making global decomposition maps feasible for the first time.