The effect of clay minerals, litter quality, and earthworms on soil organic matter storage; clay macro-porosity is more important than surface area

Consensus holds that clay minerals play an important role in accumulation of soil organic matter, especially mineral associated organic matter (MAOM). However, little is known about how clay minerals interact with litter and soil fauna. This study used a series of year long microcosms experiments to observe the carbon (C) storage trajectories of three soil forming substrates dominated by the individual clay minerals of kaolinite, illite, and montmorillonite, supplied by litter from two tree species with contrasting carbon (C) to nitrogen (N) ratios (alder and oak), in the presence or absence of earthworms. We tested the hypothesis that MAOM would become a major source of soil organic matter. We expected storage to increase with increasing surface area of clay minerals (kaolinite < illite < montmorillonite), litter decomposability (oak < alder), and earthworm processing.

Experiment showed that illite provided the greatest C storage in mineral layer (in average 0.53 g microcosms⁻¹) followed by montmorillonite (0.25 g) and kaolinite (0.10 g), despite highest surface area of montmorillonite. C storage also appeared higher in soils supplied by alder litter relative to those supplied by oak litter. Soils with earthworms showed higher C storage relative to those evolving in the absence of earthworms. Most of the C stored in soils resided in the particulate organic matter fraction (POM), most pores of montmorillonite were nm size, too small to accommodate POM, while illite showed the highest proportion of micrometer size pores. Statistics detected a positive correlation between C storage, microbial biomass, respiration, and hot water extractable carbon. Results also showed that incorporation of POM from litter into mineral soil is a crucial transfer. This process becomes more pronounced in the presence of earthworms and easily decomposable litter.