Root exudate and litter impacts on microbial turnover and soil carbon stabilization are species specific

Differences in root exudate and litter quality are known to regulate microbial activity and net soil carbon (C) accumulation. For example, legume root exudates are known to have high amino acid content, and their root litters are characterized by low lignin and high C/N ratio.  Therefore, root-derived C from legumes can potentially enhance microbial activity and turnover rates, and facilitate microbial necromass production – the precursor for mineral-associated organic matter (MAOM) in soils.

Stable isotope probing has been widely applied to trace C fluxes into microbial respiration and soil C pools such as particulate organic matter (POM) and MAOM. However, most existing studies rely on artificial substrates or single compounds as proxies for root exudates, thereby neglecting the chemical complexity of root exudates and potential interactions between root exudates and litters on microbial C processing.

Here, we address this gap by isolating ¹³C-labelled species-specific root exudates and litter derived from three grassland species (L. perenne, P. lanceolata, and T.pratense) with contrasting root traits for an incubation experiment. Matured plants were pulse-labelled with ¹³CO₂ for three days, after which ¹³C-labelled root exudates and litter were collected and amended to bare soil either individually or in combined in a 75-day incubation experiment. We expect high quality root exudate and litter from T.pratense to induce higher microbial respiration and priming effect, and overtime,  elevated necromass production and C stabilization in MAOM.

Root exudates and litter derived from P. lanceolata, and T. pratense induced higher cumulative priming effects than those from L. perenne. Thereafter, microbial respiration rates declined over time. By the end of the incubation, the highest microbial turnover rate was observed in the T. pratense litter treatment, suggesting rapid microbial mortality and substantial necromass production over the incubation period.

Consistent with this pattern, mean residence times of P. lanceolata, and T. pratense root exudates in MAOM (1973 and 1754 yrs) were higher than those of L. perenne root exudates (493 yrs). In addition, T. pratense root litter exhibited the longest mean residence time in POM (149 yrs). Together, these results suggest that legumes can increase soil C accumulation through the positive impacts of their root exudate and litter on microbial turnover.