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 understand the role of RPE on ecosystem functions, such as C production. Here, we study the sensitivity to RPE of bulk soils and soil fractions (light, LF 26 250-2000 μm, intermediate, IF 53-250 μm and mineral, MF < 53 μm) isolated from soils 27 with contrasting mineralogy (illitic-kaolinitic versus allophanic). The sensitivity to RPE is 28 defined by the magnitude of RPE when the bulk soil or soil fractions are exposed to root 29 colonization. For this measurement, bulk soils and soil fractions were packed in small PVC 30 tubes provided with a mesh permeable to roots. These tubes were placed into pots filled 31 with inert substrate, in presence or absence of maize (Zea mays L) plants. After 131 days of 32 maize development, the tubes containing bulk soils or soil fractions were harvested and incubated to quantify the RPE. The difference in 13 33 C isotopic composition between maize 34 (C4) plant material and soil carbon originated from C3 plants was used to separate the two 35 sources of C (soil and plant) in CO2 emissions. The results showed that plant root 36 accelerated SOM-mineralization of bulk soils and soil fractions, that is, a RPE occurred in 37 all soil fractions. However, the magnitude of RPE greatly varied among soil fractions and 38 soil types. This magnitude decreased while soil fraction size decreased in allophanic soil, 39 whereas it increased in illitic-kaolinitic soil. The sensitivity of soil fractions to RPE was 40 determined by the content of reactive minerals in allophanic soil, and by the quality of 41 SOM characterized by the C:N ratio and aromaticity in illitic-kaolinitic soil. These finding 42 indicate that RPE controls the mineralisation of SOM associated to each soil fraction size and related ecosystem functions in a way that can be predicted by measuring simple physicochemical 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.