DECOMPOSITION OF C-14-LABELED GLUCOSE AND LEGUME MATERIAL IN SOILS - PROPERTIES INFLUENCING THE ACCUMULATION OF ORGANIC RESIDUE-C AND MICROBIAL BIOMASS-C

Relationships were established between the properties of 23 soils. Particularly identified were those soil properties correlated with (i) native biomass C, with (ii) the extent of decomposition of added [C-14]glucose and [C-14]Medicago littoralis plant residues after 44 and 66 weeks incubation respectively, and with (iii) the concentrations of substrate-derived C found in the microbial biomass and non-biomass residues. The native biomass C and biomass C-14 from glucose and M. littoralis were highly correlated with each other and with soil clay content and other related soil properties, e.g. CEC and total soil pore space. For glucose-amended soils total residual C-14 was also correlated with soil clay content. Differences between soils in the concentration of total residual organic C-14 were due entirely to differences in the amounts of C-14 present in the microbial biomass. Thus, statistically non-biomass C-14 accounted for a constant proportion of input C-14. In contrast with glucose decomposition, total residual organic C-14 from M. littoralis decomposition was not significantly correlated with clay content and related properties except when the statistical analyses were confined to soils of neutral to alkaline pH. Soils of mildly acidic pH retained more residual non-biomass C-14 than did neutral to alkaline soils of similar clay contents. The close direct correlations between biomass C-12, and biomass C-14 from glucose and plant material metabolism, and soil properties indicated that soil charge or structure or both are important factors influencing microbial biomass accumulation in soils. These factors may override such influences as substrate type, concentration and efficiency of utilisation in determining biomass C concentration in soils after long (1 yr) incubation.