Abstract
The terrestrial carbon (C) cycle has received increasing
interest over the past few decades, however, there
is still a lack of understanding of the fate of newly assimilated
C allocated within plants and to the soil, stored within
ecosystems and lost to the atmosphere. Stable carbon isotope
studies can give novel insights into these issues. In
this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope
signatures. The first part of the review focuses on isotopic
fractionation processes within plants during and after
photosynthesis. The second major part elaborates on plantinternal
and plant-rhizosphere C allocation patterns at different
time scales (diel, seasonal, interannual), including the
speed of C transfer and time lags in the coupling of assimilation
and respiration, as well as the magnitude and controls
of plant-soil C allocation and respiratory fluxes. Plant
responses to changing environmental conditions, the functional
relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of
the C counterflow from the rhizosphere to the aboveground
parts of the plants, e.g. via CO2 dissolved in the xylem water
or as xylem-transported sugars, is highlighted. The third part
is centered around belowground C turnover, focusing especially
on above- and belowground litter inputs, soil organic
matter formation and turnover, production and loss of dissolved
organic C, soil respiration and CO2 fixation by soil
microbes. Furthermore, plant controls on microbial communities
and activity via exudates and litter production as well
as microbial community effects on C mineralization are reviewed.
A further part of the paper is dedicated to physical
interactions between soil CO2 and the soil matrix, such
as CO2 diffusion and dissolution processes within the soil
profile. Finally, we highlight state-of-the-art stable isotope
methodologies and their latest developments. From the presented
evidence we conclude that there exists a tight coupling
of physical, chemical and biological processes involved in C
cycling and C isotope fluxes in the plant-soil-atmosphere system.
Generally, research using information from C isotopes
allows an integrated view of the different processes involved.
However, complex interactions among the range of processes
complicate or currently impede the interpretation of isotopic
signals in CO2 or organic compounds at the plant and ecosystem
level. This review tries to identify present knowledge
gaps in correctly interpreting carbon stable isotope signals
in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps