Carbon residence times obtained from labelled ryegrass decomposition in soils under contrasting environmental conditions A. Parshotam a, *, S. Saggar a , P.L. Searle b , B.K. Daly a , G.P. Sparling c , R.L. Par®tt a a Landcare Research, Private Bag 11-052, Palmerston North, New Zealand b Spectra Chem Ltd., Taita, Lower Hutt, New Zealand c Landcare Research, Private Bag 3127, Hamilton, New Zealand Accepted 15 July 1999 Abstract The rate of decomposition of ryegrass in New Zealand soils was studied. Six soils from contrasting New Zealand environments were amended with 14 C-labelled ryegrass (Lolium hybridum Hausskn), which was allowed to decompose in micro- lysimeters under ®eld conditions for 2 yr. Periodically, the micro-lysimeters were destructively sampled, and the amount of 14 C remaining in the soil and the fraction of 14 C incorporated into microbial biomass measured. After 2 yr of exposure, 18 to 32% of the labelled 14 C was retained by the soils. Decomposition was initially rapid, with almost one-half of the labelled 14 C 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 14 C was retained by one soil which had major water de®cits. The in¯uence 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- 14 C and residual- 14 C. Analytical solutions are presented to the model equations. The 14 C 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 14 C 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. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbon residence times; Decomposition; Environmental conditions 1. Introduction There have been considerable advances in under- standing soil organic matter (SOM) dynamics with the formulation and development of mathematical models that simulate changes in SOM (Parton et al., 1987; Jenkinson, 1990; Smith et al., 1997). These simulation models require climatic variables and soil process in- formation. Although a great deal of information is available on the independent eects of climatic and soil factors on SOM turnover from controlled studies, little is known about the quantitative aspects of the combined eects under ®eld conditions. Inadequate data on carbon turnover rates, for a wide range of cli- mates, soil types and mineralogies, as found in New Zealand, presently limit the use of such models (Saggar et al., 1996). Quantitative information on SOM turnover is needed, together with a more rigor- ous theory on the incorporation of interdependent, abiotic eects into SOM turnover models. In many SOM turnover models, the SOM `system' is divided into a number of conceptual compartments or `pools'. The transfer of material from one compart- ment to another is usually assumed to obey ®rst-order kinetics. A range of methods has been used to incor- porate abiotic functions in SOM models to adjust the decomposition rate of SOM at standard conditions to Soil Biology & Biochemistry 32 (2000) 75±83 0038-0717/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0038-0717(99)00131-5 www.elsevier.com/locate/soilbio * Corresponding author. Tel.: +64-6-356-7154; fax: +64-6-355- 9230. E-mail address: parshotama@landcare.cri.nz (A. Parshotam).