Cascading effects of different root C ages in forests and grasslands
Roots are important sources of soil organic C and soil-atmosphere C fluxes. Different tools used to study fine roots lead to very different pictures of their dynamics. These have broad implications ranging from estimating root productivity and the degree to which root C persists in soil organic matter or is lost as CO2 or soluble organic carbon. In particular, radiocarbon-based approaches allow estimation of the ‘age’ of C used to construct fine roots – that is, the average time elapsed since structural C was originally fixed from the atmosphere. These measures have generally yielded results that confirm relatively short (<1 year to several years) ages for C in grassland roots. However, in forests the average age of C in <2mm diameter roots usually averages around a decade. Exept in cases of disturbance, newly-grown forest roots contain C fixed up to 1-2 years previously, implying that some fine roots – the ones that make up the majority of fine root biomass in soils – live for nearly a decade. These ages of live roots influence the age of C in the dead root pool.
What are the consequences for carbon cycling and especially for time lags found in forests versus grasslands? In forests, heterotrophically respired CO2 is often several years up to a decade old, while in grasslands it is <several years old – thus there are time lags between photosynthesis and respiration are longer in forests than grasslands with a similar soil type and climate. Our investigations demonstrate how time lags originating in woody roots carry over into low-density organic matter and heterotrophic respiration. Dead fine roots in forests decompose more slowly than those in grasslands in similar soils/climates, which can further add to the age difference However, in forests surface litter can also influence the age structure of respired C. In addition, we might expect that if the majority of roots live fast and die young, this would be reflected in the age of decomposing roots and respired C. Simultaneously balancing below ground C and 14C budgets in forests requires that we reexamine assumptions about fine roots and rhizosphere dynamics as well as the age and use of storage reserves in root growth and respiration.