Universality of priming effect: An analysis using thirty five soils with contrasted properties sampled from five continents

A general occurrence of the phenomenon of priming effect (PE) across varying land use and soil types has not been established so far, particularly on a large geographical scale. Moreover, the impacts of soil properties and soil organic matter (SOM) distribution among physical fractions on the magnitude of PE are still unclear. We addressed these knowledge gaps by incubating thirty five soils with or without addition of C-13 labeled cellulose for 262 days. The soils were sampled from different land uses and depths of contrasting soil types from five continents (Asia, Europe, America, Australia and Africa). Results showed positive PE in all soils including grassland, cropland, forest, savannah and orchard. On average, the cumulative PE represented 27.0 +/- 28.7% of the CO2 efflux in control soils and 28.48 +/- 21.08% of the remaining/unrespired cellulose-C. The PE was 72.1% higher in surface than deep soils suggesting that surface soils are more prone to PE induced by cellulose addition. Variations in PE were mainly explained by soil characteristics and not by land use. We found that the PE increased with the relative abundance of SOM not associated with minerals and rich in nitrogen (N). The observation of systematic positive PE in all soils suggested that microbial co-metabolism to decompose SOM is a widespread microbial strategy. Our results also support the idea that microorganisms use co-metabolism to mine nutrients in SOM since they target N-rich fractions. However, other mechanisms are also at play since positive PE was maintained despite the high availability of mineral nutrients. Overall, PE is a worldwide process playing a major role for soil C dynamics, especially in N-rich soils.