Quantifying the effects of clay minerals on composted plant materials

Abstract

The effects of clay minerals on the decomposition of plant materials was investigated by differential scanning calorimetry with isotope mass spectrometry evolved gas analysis (DSC-IRMS). Different types of yard waste compost material, enriched in ¹³C, allowed for C source tracing within different chemical compound classes and microorganisms in laboratory incubation studies. Mineral treatments included kaolinite and montmorillonite, in combination with Ca²⁺, and a control. The DSC-IRMS method proved useful to assess the organic matter status in organo-mineral systems. The addition of clay minerals was shown to reduce the microbial utilization of low temperature stability C compounds, thereby challenging the previous assumption that mineral-associated organics are represented by the highest temperature region of thermograms. Energy release and mass loss dynamics were unique between clay treatments, highest in the mineral-free treatments assessed calorimetrically, and ultimately demonstrating that composted organic matter quality decreases over the course of typical municipal windrow composting regimes (here, 157 days). This shows that amending soil with compost is not a straightforward method for increasing stocks of organic matter in soils. Evolved gas analysis showed that the microbial biomass discriminated between different plant materials (e.g. wood, leaves, etc) and specific compounds (e.g., lignin, cellulose, sugars, lipids) and that organo-mineral chemistry affected substrate consumption and quality. It is apparent that organic matter in soil systems is not treated equally by the microbial population and that the long-held assumption that thermal stability correlates with biological stability is untenable. Microbial community structure and size was also examined through phospholipid fatty acid analysis. Clay minerals reduced the microbial biomass and, consistent with calorimetric data, the community structure was affected uniquely by mineral presence and chemistry. The carbon isotope enrichment of each microbial group showed preferential utilization of plant material substrates and mineral influence. The studies presented here offer new insights into the complexity of the organo-mineral-microbe relationship and underpin a paradigm shift in the thermochemical analysis of soil materials.