Interactions among root inputs, microbes, and clay on soil organic matter stabilisation and decomposition

Abstract

Soil organic matter (SOM) plays a vital role in providing plant nutrients and stabilising soil structure. The interplay of soil physicochemical properties, microbial activity, and plant inputs significantly impact carbon (C) stabilisation in soils. I collected 18 soil samples at two different soil depths which are varied in clay content, soil organic carbon (SOC), mineral-associated organic matter (MAOM-C), and cation exchange capacity (CEC). Notably, the Vertosol soil, with the highest clay fraction, demonstrated higher SOC and CEC levels than Chromosol and Dermosol. A meta-analysis was conducted, which revealed that carbon use efficiency (CUE), especially with glucose as a substrate, significantly increased with decreasing substrate application, increasing clay content, increased soil pH, and decreased SOC content, resulting in greater C stabilisation. I integrated existing frameworks of SOM stabilisation, highlighting the significant role of clay in determining the efficiency of organic matter stabilisation into MAOM, particularly for substrates with a high adsorption affinity. Further investigations were conducted by adding 13C labelled plant substrates (glucose, oxalic acid, wheat root biomass) showed that microbial growth, decomposition, and CUE were higher for glucose and oxalic acid than wheat root biomass across all soil samples. An increase in clay content decreased microbial growth and CUE for glucose and oxalic acid but had no effect on microbial growth and CUE for wheat root biomass. Lastly, I conducted a glasshouse experiment using a 13CO2 pulse labelling method on ryegrass grown and positive relationships observed between clay content and rhizodeposit-derived C in microbial biomass and the MAOM fraction. More microbial products from rhizodeposition were incorporated into MAOM with increased clay content, indicating a significant role of rhizodeposition in soil C stabilisation.