Impact of elevated CO2 on soil organic matter dynamics as related to changes in aggregate turnover and residue quality

Increasing global atmospheric CO2 concentration can potentially affect C cycling in terrestrial ecosystems. This study was conducted to assess the impact of elevated CO2 concentration on soil organic matter and aggregate dynamics in Lolium perenne and Trifolium repens pastures. Soil samples from a 6 year old `free air CO2 enrichment' (FACE) experiment were separated in four aggregate size classes ( 2000 μm). Free light fraction (i.e. particulate organic matter (POM) outside of aggregates; free LF) and intra-aggregate-POM (i.e. POM occluded within the aggregate structure; iPOM) were isolated. The distinct 13C-signature of the CO2 used to raise the ambient CO2 concentration in FACE allowed us to calculate proportions of recently incorporated C (< 6 yr) in the physically defined soil fractions. The proportion of new C increased with increasing aggregate size class, except the two largest aggregate size classes had a similar proportion of new C; this indicates a faster turnover of macroaggregates compared to microaggregates. In addition, higher proportions of new C in macroaggregates under T. repens compared to L. perenne indicate a faster macroaggregate turnover under T. repens. This faster macroaggregate turnover is hypothesized to be a result of the higher residue quality (C:N ratio) of T. repens compared to L. perenne and reduces the potential of sequestering C under elevated CO2. In the L. perenne soil, elevated CO2 did not significantly increase total C, but led to: (1) a 54% increase in aggregation and (2) a 40% increase in total iPOM-C. It is hypothesized that the sequestration of iPOM-C induced by elevated CO2 in the low residue quality, L. perenne treatment, resulted from an increase in the proportion of large macroaggregates with a slow turnover.