Fates of low molecular weight carbon inputs to the belowground

Background/Question/Methods

There is growing evidence that the dominant inputs of carbon to soils, and the food webs they support, originate from roots. Of the root-carbon entering these belowground systems, a substantial fraction is in the form of low molecular weight carbon compounds (i.e. rhizodeposition). Recent estimates suggest that these low molecular weight compounds fuel between 30 and 100% of belowground heterotrophic respiration, making their study essential if we are to understand soil carbon cycling and sequestration. Both the quantity and identity of these low molecular weight inputs vary with the environment – for example the availability of phosphorous and of atmospheric CO₂ – yet we have a limited understanding of how this influences the fate of the inputs. Within the context of a nitrogen by phosphorus fertilization experiment in an old-field system, we add isotopically-enriched glucose and glycine (representative rhizodeposits) at realistic input rates and concentrations across the growing season. Following six months of weekly-additions, we resolve the fate of the labeled carbon in plant, microbial and soil organic carbon fractions differing in their turnover times. 

Results/Conclusions

On average 47 and 20% of the glucose and glycine carbon was recovered, revealing different sequestration potentials of the compounds. Regardless of substrate, nitrogen fertilization reduced the recovery of carbon from 38 to 29%. The reduced recovery was not reflected in greater initial (24 h post carbon addition) or weekly respiratory losses of the labeled carbon. Yet glycine was mineralized at approximately twice the rate of glucose, which might explain its lower recovery. These higher mineralization rates of glycine were reflected in 3.5 times more of the carbon being recovered in the plants, and a 10% reduction in the proportion recovered in microbial biomass. Relative partitioning to soil organic carbon was independent of substrate identity, with 50% recovered in this pool. Although with glucose a greater proportion was recovered in fractions with longer mean residence times. Overall, our data show that substrate identity (glucose vs. glycine) and land management (fertilization) markedly influences the partitioning and recovery of low molecular carbon inputs to ecosystems. If we are to predict how carbon dynamics will respond to environmental change, we need to develop a much more comprehensive understanding of the factors that influence the fate of low molecular weight carbon inputs to soils.