Species and root traits impact macroaggregation in the rhizospheric soil of a Mediterranean common garden experiment

We evaluated the influence of plant species and life forms on soil aggregate distribution among size-classes, total macroaggregate mass and aggregate mean weight diameter (MWD), and examined how specific root traits were related to these aggregation variables. We analyzed the soil attached to the roots (i.e., rhizospheric soil) under 13 Mediterranean species grown in monocultures in a common garden experiment for four years, and compared it to a bare soil. The mass distribution of aggregates in six size-classes and aggregate MWD were calculated, both on a rhizospheric soil and root biomass basis. Compared to bare soil, macroaggregate mass increased by an average of 13% in the presence of plants, with a strong effect of species and life forms (both P < 0.0001); some species such as Sanguisorba minor showing increases of up to similar to 40%. Although the soil under graminoids had a greater macroaggregate mass, their MWD was lower than under non-woody dicots. Large (2000-1000 mu m) and intermediate (1000-500 mu m) macroaggregate mass increased with root mass and length density and decreased with root lignin concentration, while very large macroaggregate (6000-2000 mu m) mass and the MWD increased with root soluble compound concentration. Species and life forms differently influenced the distribution of macroaggregates among size-classes and aggregate MWD. Easily-decomposable roots with traits related to resource acquisition (i.e., high fine root length, high water-soluble compound concentration) are more favorable for the development of water-stable macroaggregates than roots traits related to resource conservation (high lignin concentration, thick roots).