Abstract

Numerous experiments reported a strong acceleration (up to 300%) of soil organic matter (SOM) mineralization induced by plant presence, an effect called rhizosphere priming (RPE). However, SOM is composed of several fractions varying in chemical composition, binding types with minerals, turnover rate and functions in the ecosystems. A better knowledge on how these SOM fractions are affected by RPE is crucial to understand the role of RPE on ecosystem services, such as C sequestration and biomass production. Here, we study the sensitivity to RPE of bulk soils and soil fractions (light, LF 250-2000 µm, intermediate, IF 53-250 µm and mineral, MF < 53 µm) isolated from soils with contrasting mineralogy (illitic-kaolinitic versus allophanic). The sensitivity to RPE is defined by the size of RPE when the bulk soil or soil fractions are exposed to root colonization. For this measurement, bulk soils and soil fractions were packed in small PVC tubes provided with a mesh permeable to roots. These tubes were placed into pots filled with inert substrate, in presence or absence of maize (Zea mays L) plants. After 131 days of maize development, tubes containing bulk soils or soil fractions were harvested and incubated to quantify the RPE. The difference in 13C isotopic composition between maize (C4) plant material and soil carbon originated from C3 plants was used to separate the two sources of C (soil and plant) in CO2 emissions. The results showed that plant root accelerated mineralization of organic carbon of bulk soils and soil fractions, that is, a RPE occurred in all soil fractions. However, the magnitude of RPE greatly varied among soil fractions and soi types. This magnitude while soil fraction size decreased in allophanic soil, whereas it increased in illitic-kaolinitic soil. The sensitivity to RPE was determined by the content of reactive minerals in allophanic soil, and by the quality of SOM characterized by the C:N ratio and aromaticity in illitic-kaolinitic soil. These finding indicate that RPE controls the dynamics of SOM associated to each soilfraction size and related ecosystem functions in a way that can be predicted by measuring simple soil properties. Our findings also suggest that RPE depends on the strategy of microbes and plants to obtain nitrogen from SOM, with different energy costs depending on the type of organo-mineral association.