Carbon residence times obtained from labelled ryegrass decomposition in soils under contrasting environmental conditions

The rate of decomposition of ryegrass in New Zealand soils was studied. Six soils from contrasting New Zealand environments were amended with 14C-labelled ryegrass (Lolium hybridum Hausskn), which was allowed to decompose in micro-lysimeters under field conditions for 2 yr. Periodically, the micro-lysimeters were destructively sampled, and the amount of 14C remaining in the soil and the fraction of 14C incorporated into microbial biomass measured. After 2 yr of exposure, 18 to 32% of the labelled 14C was retained by the soils. Decomposition was initially rapid, with almost one-half of the labelled 14C being lost after 2 months. Thereafter, the rate of decomposition was much reduced. During the initial phase of decomposition (2 months), a larger portion of the 14C was retained by one soil which had major water deficits. The influence of environmental factors on decomposition rate was assessed by assuming a three compartment model and calculating the mean and variances of residence times of biomass-14C and residual-14C. Analytical solutions are presented to the model equations. The 14C residence times, adjusted for soil surface area, were related to rainfall. The residence times were compared with times obtained in our earlier study on soils varying in clay content and mineralogy. This provides a range of 14C residence times for decomposition of ryegrass in New Zealand soils. These results may be used to validate rate-reduction factors in soil organic matter models.