Litter decomposition rate constants across the global temperature gradient

The responses of organic matter decomposition to temperature rise is increasingly important topic. Most of the previous studies measured temperature dependence of soil respiration under laboratory incubation, while we analyzed the temperature dependence of organic matter decomposition under field conditions. 

To identify the factors regulating litter decomposition rate constants and their temperature sensitivity, we compiled the data of site information [Country, District, Vegetation or land use, Plant type, Mean annual air temperature (ºC), Mean annual precipitation (mm y-1), Soil type (Soil Taxonomy), Soil pH, and Decomposition rate constant (Table S1), Organic horizon C stock (Mg C ha-1), Litterfall C input (Mg C ha-1 y-1), Litter turnover (y) (Table S2).

These parameters allow us to compare decomposition rate constants and provide temperature dependence indicators of organic matter decomposition (activation energy and Q10) across the global temperature gradient.

Methods
We compiled the data of in-situ decomposition rates of cellulose filter paper, leaf litter (i.e., leaves of coniferous and broadleaved trees; leaf/straw of maize, rice, and wheat/barley), root litter, lichen litter, moss litter, and SOM and site information (climatic properties, plant type, soil type) from 68 studies (Tables S1 and S2, respectively). We extracted substrate decomposition data from in situ field incubation studies using litterbags buried (incorporated) in soil from the literature by searching for “litter bag” or “cellulose” or “organic matter”, “decomposition”, “turnover”, “mass loss”, and “soil” in Google Scholar. When rate constants of substrate decomposition in a single exponential decay function were not available in the published studies, original data, which were either actual substrate mass (g) or the remaining fraction of the initial substrate mass at each incubation time (y), were digitized from figures or extracted from tables. To ensure the comparability of published studies, we excluded the published studies of litter decomposition experiments conducted in the laboratory or litter on the surface. The method of the field incubation of substrates were concisely summarized in the following sections.

Measuring cellulose and litter decomposition

The methods to quantify the decomposition rates of organic matter in the data sources differed slightly between substrates. Cellulose filter papers (99% cellulose; e.g., 55 mm diameter) and root litter bags (root diameter < 2 mm) were packed in a nylon mesh bag (mesh pore size ≤ 100 μm) and buried in the surface mineral soil (A horizon, 5 cm depth), while leaf litter bags (leaf fragments [ca. 10 × 10 mm size] were buried at the boundary between the organic horizon and the mineral soil. The litter-bag method (Hayakawa et al., 2014) was used to quantify the decomposition rates of cellulose, leaf litter, root litter, lichen litter, and moss litter. 

The remaining weight of the substrate (70°C, 24 h) was calculated on an ash-free basis by subtracting the weight of the soil adhering to the substrate, which was estimated by dry combustion (600°C, 4 h). The proportion of the remaining substrate was calculated by dividing the remaining weight by the initial substrate weight. We fitted the data to a single exponential decay function (Eq. 1) below to obtain the decomposition rate constant (k).

Rr/ Ri = e–kt  (Eq. 1)

where Rr is the remaining proportion of the substrate (%), Ri is the initial proportion of the substrate (i.e., 100%), k is the decomposition rate constant (y–1), and t is time (y).

In case of forest ecosystems, we also provided an alternative estimation on the litter-C turnover rate (y–1) as a comparison to the decomposition rate constants obtained from the litter bag method. By assuming that the organic layer C stock reaches a steady-state in forests, the litter-C turnover rate (y–1) in the organic layer was calculated by dividing litterfall C input (Mg C ha–1 y–1) by the organic layer C stock (Mg C ha–1) (Olson, 1963).

Measuring SOM decomposition

Decomposition rates of SOM can be estimated using isotopes (13C isotopic signatures and bomb 14C). The turnover of SOM in warm temperate to tropical regions can be traced in vegetation change between C3 and C4 plants, using the difference in their litter 13C isotopic signatures (−24 to −34‰ for C3 plants and −6 to −19‰ for C4 plants) and soil C stocks (Fujii et al., 2020). The changes in C3-derived and C4-derived SOC stocks were plotted respectively against time after vegetation change (y). The data were fitted to a single exponential decay function (Eq. 1) to estimate the decomposition rate constant k (y-1). The mean residence times were estimated from 1/k, assuming a steady-state.

In polar or boreal ecosystems, where studies on vegetation shifts between C3 and C4 plants were not available, SOM turnover was estimated using bomb 14C (Trumbore, 2000). Radiocarbon produced by atmospheric nuclear weapons testing between 1955 and 1964 was assimilated by plants and transferred to the soil. The 14C measurements of the archive and present soil samples or measuring/modeling the soil CO2 flux provide the mean residence times of organic matter (Trumbore, 2000).