Microscale distribution and function of soil microorganisms in the interface between rhizosphere and detritusphere

Abstract

The rhizosphere and the detritusphere are hot spots of microbial activity, but little is known about the interface between rhizosphere and detritusphere. We used a three-compartment pot design to study microbial community structure and enzyme activity in this interface. All three compartments were filled with soil from a long-term field trial. The two outer compartments were planted with maize (root compartment) or amended with mature wheat shoot residues from a free air CO2 enrichment experiment (residue compartment) and were separated by a 50 μm mesh from the inner compartment. Soil, residues and maize differed in 13C signature (δ13C soil −26.5‰, maize roots −14.1‰ and wheat residues −44.1‰) which allowed tracking of root- and residue-derived C into microbial phospholipid fatty acids (PLFA). The abundance of bacterial and fungal PLFAs showed clear gradients with highest abundance in the first 1–2 mm of the root and residue compartment, and generally higher values in the vicinity of the residue compartment. The δ13C of the PLFAs indicated that soil microorganisms incorporated more carbon from the residues than from the rhizodeposits and that the microbial use of wheat residue carbon was restricted to 1 mm from the residue compartment. Carbon incorporation into soil microorganisms in the interface was accompanied by strong microbial N immobilisation evident from the depletion of inorganic N in the rhizosphere and detritusphere. Extracellular enzyme activities involved in the degradation of organic C, N and P compounds (β-glucosidase, xylosidase, acid phosphatase and leucin peptidase) did not show distinct gradients in rhizosphere or detritusphere. Our microscale study showed that rhizosphere and detritusphere differentially influenced microbial C cycling and that the zone of influence depended on the parameter assessed. These results are highly relevant for defining the size of different microbial hot spots and understanding microbial ecology in soils.