Please see the papers published on this aspect. If you know the initial C loaded through litter and analyse the remaining C content left in the pouch after decay at different time intervals, the data on natural logarithm of  C content could be plotted against the time (t) to get the decay constant as slope of the line.

The single exponential model assumes that all plant

tissue decomposes at the same rate where Wt is the percent of residue dry weight remaining at time t days and k is the decomposition rate constant.

Wt100 e-kt

you can figure this out by using a statistical program eg. non-linear

regression function to determine k-values

Dear Dr Maren

Please send me some papers for calculation on k values using non-linear regression. Is it possible to do the non-linear regression in Ms-Excel. Please suggest...

Is there any link for online calculation of non-linear regression function for exponential decay of leaf litter decomposition

The leaf litter mass loss and decomposition rate constant (k) computed using a single negative exponential decay function first of Olson (1963). The decay constant (k) estimate by Regressing - ln (Xt/Xo) vs t, where Xt is weight remained at time t, X0 is the initial weight of leaf litter, and t is time in days, months or years (it depends on the experimental length or period). Therefore, the slope of - ln (Xt/Xo) vs t is decay constant (k).