Radiolabelled
Material containing one or more types of radioactive atoms.  Molecules that are radiolabelled are sometimes called tracers, as they allow one to track the various biochemical pathways that a molecule follows during metabolism.

Raptors
Birds of prey that live on other birds and animals.

rbcL
The gene that encodes for the large subunit of rubisco.

rbcS
The gene that encodes for the small subunit of rubisco.

Refractory
Resistant to decomposition.

Respiration
The cellular process by which sugars and other organic compounds are broken down to release stored energy and to obtain carbon skeletons used in the growth and maintenance of the cell.

Rhizodeposition
The exudation of carbon and other organic nutreints from plant roots into the soil volume (rhizosphere) closely associated with them.

Rhizome
A rootlike stem that grows horizontally under or along the ground that sends out roots from its lower surface and leafy shoots from its upper surface.

Rhizosphere
An area of soil typically within 1.5 mm of plant roots characterized by heightened biological activity and chemical weathering of minerals.

Roman Warm Period
The period between the 5th century B.C. and the 1st century A.D. when global temperatures were warmer than present.

Root exudate
Small molecules that are released from plant roots, which include sugars, amino acids, organic acids and amides.  These molecules influence soil nutrient availability both directly and indirectly by stimulating the activities of certain microbial and fungal components of the soil biota.

Rosette leaves
Leaves arranged in a circular cluster resembling or suggesting the arrangement of the petals of a rose.

Rubisco
A bifunctional enzyme that can fix carbon dioxide or molecular oxygen, which leads to photosynthesis or photorespiration, respectively.  Rubisco is the most abundant enzyme on earth.