C-14-labelled ryegrass turnover and residence times in soils varying in clay content and mineralogy

The influence of soil texture and clay mineral composition on the decomposition of uniformly-labelled (C-14) ryegrass (Lolium hybridum Hausskn) has been investigated. Two clay and two silt loam soils amended with C-14-labelled ryegrass were allowed to decompose in micro-lysimeters under field conditions for 5 years. Periodically the micro-lysimeters were destructively harvested and the total amount of C-14 remaining in the soil and the fraction of the C-14 incorporated into microbial biomass measured. The influence of the amount and surface area of clay was assessed by calculating the mean and variances of residence times of biomass-C-14 and residual-C-14. Surface areas were estimated from the mineralogical composition of the soil, and also measured by adsorption of p-nitrophenol. After 5 years' of incubation, 12-25% of the labelled-C-14 was retained by the soils. Decomposition was initially very rapid, between one-third to one-half of the labelled C-14 being lost after 9 weeks. Thereafter, the rate of decomposition was much reduced. During the initial phase of decomposition (9 weeks), a greater portion of the C-14 was retained by the two clay soils than by silt loam soils. In amorphic and smectitic soils the residence times for C-14 were increased because of the protection of microbial metabolites by clay surfaces. The amount of C-14 remaining in the soil did not relate to the amount of clay but correlated with the surface area, as measured by p-nitrophenol adsorption. Apparently, the type of clay, expressed in terms of surface area, controlled carbon turnover rates and residence times. This parameter may provide a reasonable predictor of organic matter decomposition rates and stabilization in different soils. (C) 1997 Elsevier Science Ltd.