Clay mineral surface area and pore size have contrasting effect on accumulation of soil organic matter fractions in the rhizosphere

Abstract

Abstract Clay minerals play an important role in soil organic matter (SOM) accumulation. This study used a series of pot experiments to observe carbon (C) storage in rhizosphere of two plant species Festuca rubra and Lotus corniculatus growing in three soil forming substrates dominated by various clay minerals (kaolinite, illite, and montmorillonite). Plants were either grown in separate pots filled with different substrates or plants were grown in larger pots filled with sand where clay substrates were buried in mesh bags. We tested the hypothesis that mineral associated organic matter (MAOM) storage will increase with increasing surface area of clay minerals (kaolinite < illite < montmorillonite), while the volume of larger pores (Illite > kaolinite and montmorillonite) will promote the storage of particulate organic matter (POM). Overall, the largest C storage was in montmorillonite dominated substrates followed by illite and kaolinite. These trends were the same across both types of clay exposure and for both plant species used. Accumulation of MAOM followed a similar pattern. On the contrary, accumulation of free POM was not affected by clay mineral identity and accumulation of occluded POM was significantly higher in illite compared to the other two minerals. Our results suggest that total surface area of clay minerals is crucial for accumulation of MAOM while occluded POM is more affected by the availability of larger pores.