Effects of exchangeable cations, mineralogy and clay content on the mineralization of plant residue carbon

Interactions of clay minerals with organic materials and the consequences of these interactions on dynamics of organic carbon (OC) have been reviewed in detail. However, the effects of exchangeable cations and clay types on the turnover rate of OC have not been given much attention. Appropriate amounts of homoionic Na-, Ca- and Al-clays from Georgia kaolinite, Illinois illite and Wyoming bentonite were mixed with pure sand to prepare artificial soils with different clay contents, exchange cations and clay types. Alfalfa plant residues were incorporated into the artificial soils and the soils were inoculated with microbes from a natural soil and incubated for 184days to study the effects of clay contents, exchangeable cations and clay types on the mineralization of OC and plant residue derived microbial biomass. Mineralization of plant residue carbon was significantly (⁎⁎p<0.01) more rapid in pure sand than in soils containing 5 and 10% clay, indicating that clay contents influence the capacity of soils to protect and store OC. The amounts of biomass carbon were lowest in pure sand and highest in soils with 10% clay. There was a significant influence (⁎⁎p<0.01) of exchangeable cations on microbial biomass and hence the mineralization of OC over a period of 184days. At 5 and 10% clay contents, the amounts of microbial biomass carbon and the mineralization of plant residue carbon were highest in Ca-soils and lowest in Al-soils. Statistical analysis of data showed a significant effect (⁎⁎p<0.01) of clay type on mineralization of plant residue carbon and microbial biomass. However, the effects of clay types on the mineralization of OC were much less than those of clay contents and exchangeable cations. The mineralization of plant residue carbon was highest in soils with Georgia kaolinite clay and lowest in soils with Wyoming bentonite clay. The amounts of biomass carbon were slightly lower in soils with Wyoming bentonite clay than soils with Georgia kaolinite and Illinois illite clay minerals. The results of this study indicate that OC and microbial biomass are stabilized in soils through the interaction with clay minerals and a small amount of clay (5%) slows OC decomposition significantly and reduces carbon dioxide emission from soils. Exchangeable cations exert their influence on microbial biomass and hence carbon dynamics by controlling the size and activity of the microbial population through modifying the physicochemical characteristics of microbial habitats.