14C-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 (14C) ryegrass (Lolium hybridum Hausskn) has been investigated. Two clay and two silt loam soils amended with 14C-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 14C remaining in the soil and the fraction of the 14C 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-14C and residual-14C. Surface areas were estimated from the mineralogical composition of the soil, and also measured by adsorption of p-nitrophenol. After 5 yearsʹ of incubation, 12–25% of the labelled-14C was retained by the soils. Decomposition was initially very rapid, between one-third to one-half of the labelled 14C 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 14C was retained by the two clay soils than by silt loam soils. In amorphic and smectitic soils the residence times for 14C were increased because of the protection of microbial metabolites by clay surfaces. The amount of 14C 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.