Roots of non-woody perennials accelerated long-term soil organic matter decomposition through biological and physical mechanisms

The rhizosphere priming effect (RPE) is increasingly recognized as an important factor in mediating soil organic matter (SOM) decomposition, which influences the CO2 release from terrestrial systems to the atmosphere. However, little is known about the long-term RPE of non-woody perennial species and the physical mechanisms underlying the RPE. Here the RPEs of three non-woody perennials (Stipa grandis, Leymus chinensis, Medicago sativa) differing in root traits and exudation were quantified using a natural 13C tracer method (C3 plant - C4 soil system) during a 476-day experiment. The results indicated that all plant species showed positive RPEs, with M. sativa showing the largest RPE and S. grandis the smallest RPE. Differences in the RPEs between species are likely associated with differences in root biomass, length, and exudates. Furthermore, the positive RPEs of the three species increased continuously with sampling time, indicating the long-term nature of the RPE. Results from structural equation modeling indicated that plant roots accelerated SOM decomposition via three mechanisms. The first mechanism (biological) was through the positive effect on dissolved organic C and microbial biomass C by the presence of the rhizosphere, consistent with the microbial activation hypothesis. The second mechanism (biological) was where plant roots significantly reduced soil mineral N and in turn promoted SOM decomposition, providing support for the microbial N-mining hypothesis. The third mechanism (physical) was where plant roots caused a net positive effect on SOM decomposition through net destruction of macroaggregates possibly as a result of high root length density and the associated drying-rewetting cycles. Overall, this study provides the first experimental evidence for the aggregate destruction hypothesis as a physical mechanism causing the RPE and highlights the importance of multiple mechanisms of the RPE on long-term SOM decomposition.