The effects of Fe- and Mn-oxides and imogolite in the presence of kaolinite on organic nitrogen mineralization and soil enzyme activities

Clay minerals and metal oxides play an important role in controlling the decomposition of soil organic matter and the mineralization of organic nitrogen. To study the effects of clay content and composition on mineralization of organic nitrogen, an incubation experiment was carried out. In the experiment, mineralization of organic nitrogen from alfalfa residues, activities of acid and alkaline phosphatase and CM-cellulase, and microbial biomass nitrogen were measured in mixtures of sand, kaolinite (0, 15, 30 or 45%) and non-layered colloids (NLCs, being goethite, manganese oxide or imogolite at a kaolinite:NLC ratio of 1:0, 5:1 or 10:1), with pure sand as the control, which were inoculated with microbes from natural soil and incubated at 23°C in the dark at 60% of water holding capacity for 180 days. Mineralization of organic nitrogen and enzyme activity decreased, but microbial biomass nitrogen increased as the contents of kaolinite and NLCs increased. There was an inverse relationship between the specific surface area of the mixtures and the enzyme activity and mineralization of organic nitrogen. Thus, the decreased mineralization of organic nitrogen, despite an increase in microbial biomass, appears to have been due to inactivation of extracellular enzymes by adsorption to the surfaces of kaolinite and NLCs, and perhaps also decreased accessibility of organic nitrogen substrate molecules due to adsorption. The microbial biomass nitrogen increased as the water holding capacity of the mixtures increased, indicating the importance of water-filled pores for accommodating an active microbial biomass and protecting it from desiccation and predators.