Abstract
Rising atmospheric CO2 and temperatures are probably altering ecosystem carbon
cycling, causing both positive and negative feedbacks to climate. Below-ground
processes play a key role in the global carbon (C) cycle because they regulate storage
of large quantities of C, and are potentially very sensitive to direct and indirect effects
of elevated CO2 and temperature. Soil organic matter pools, roots and associated
rhizosphere organisms all have distinct responses to environmental change drivers,
although availability of C substrates will regulate all the responses. Elevated CO2
increases C supply below-ground, whereas warming is likely to increase respiration
and decomposition rates, leading to speculation that these effects will moderate one
another. However, indirect effects on soil moisture availability and nutrient supply
may alter processes in unexpected directions. Detailed, mechanistic understanding
and modelling of below-ground flux components, pool sizes and turnover rates is
needed to adequately predict long-term, net C storage in ecosystems. In this synthesis,
we discuss the current status of below-ground responses to elevated CO2 and temperature and potential feedback effects, methodological challenges, and approaches to integrating models and measurements.
