Rhizosphere Functioning and Structural Development as Complex Interplay between Plants, Microorganisms and Soil Minerals

The rhizosphere, the volume of soil directly affected by root activity, comprises an important hot spot for a multitude of biotic and abiotic processes in soils. While enabling plant growth by the uptake of water and nutrients, roots also shape a unique physical and biogeochemical environment for microbiota and drive soil structure formation. Mycorrhiza as symbiotic partners of plants function as vectors for plant derived organic matter into the root free bulk soil and for soil organic matter-derived nutrients into the plant. At the same time, they influence and structure the microbial community comprised of other fungi, oomycetes and bacteria. Growing roots and mycorrhizal hyphae release tremendous amounts of organic carbon derived from photosynthesis into soil compartments which might not be reached by draining dissolved organic matter solely. The growing root itself fosters the spatial rearrangement of mineral soil particles in the rhizosphere due to the physical pressure of its growth. The physical properties of the rhizosphere microenvironment is also altered by the secretion from the root tips of mucilage, a gel that modifies the mechanical and hydraulic properties of the liquid solution in a very dynamic way. The interplay between physical and biological processes drives the formation of complex three dimensional soil structures namely micro- and macroaggregates. Especially the formation of microaggregates rich in organic carbon is known as an important long term stabilization mechanism for soil organic matter. But by exuding organic substrates into the rhizosphere, roots can also foster the priming of the decay of inherited soil organic matter leading to the mineralization of previously stabilized organic matter.

As the rhizosphere is hidden in the soil matrix it was previously difficult to extensively explore its complex interactions. Common qualitative and quantitative approaches mostly include destructive sampling and / or focus only on one component, either plant or microorganisms or soil. Thus the complex nature of the active rhizosphere asks for a combined approach making use of chemical, biological and physical concepts and methods. This includes work seeking to quantify the effects of the rhizosphere on soil processes and plant growth at the bulk soil or ecosystem scale. It is, however, also vital to also get down to micro and nanoscale approaches at the relevant process scale for water and nutrient supply and microbial activity. Over the last years a growing number of interdisciplinary work between soil physics, microbiology, plant physiology and biogeochemistry focused on the deeper understanding of the rhizosphere as a driving force in ecosystem functionality.

The main objective of this Research Topic is to provide a forum for a highly interdisciplinary group of scientists who are focusing their research on the multitude of possible interactions in the rhizosphere. This reaches from possible work regarding the molecular scale of symbiotic interactions to organic carbon allocation from roots to soil minerals and the impact of rhizodeposition on physical soil properties. Of special interest for the Research Topic are also contributions that seek to model the complex microbial, chemical and physical relationships in the rhizosphere.