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 cy cling in terrestrial ecosystems. This study was conducted to assess the imp act of elevated CO2 concentration on soil organic matter and aggregate dyna mics in Lolium perenne and Trifolium repens pastures. Soil samples from a 6 year old `free air CO2 enrichment' (FACE) experiment were separated in fou r aggregate size classes (< 53, 53-250, 250-2000, and > 2000 mum). Free lig ht fraction (i.e. particulate organic matter (POM) outside of aggregates; f ree LF) and intra-aggregate-POM (i.e. POM occluded within the aggregate str ucture; iPOM) were isolated. The distinct C-13-signature of the CO2 used to raise the ambient CO2 concentration in FACE allowed us to calculate propor tions of recently incorporated C (< 6 yr) in the physically defined soil fr actions. The proportion of new C increased with increasing aggregate size c lass, except the two largest aggregate size classes had a similar proportio n of new C; this indicates a faster turnover of macroaggregates compared to microaggregates. In addition, higher proportions of new C in macroaggregat es under T. repens compared to L. perenne indicate a faster macroaggregate turnover under T. repens. This faster macroaggregate turnover is hypothesiz ed to be a result of the higher residue quality (C:N ratio) of T. repens co mpared to L. perenne and reduces the potential of sequestering C under elev ated CO2. In the L. perenne soil, elevated CO2 did not significantly increa se total C, but led to: (1) a 54% increase in aggregation and (2) a 40% inc rease 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, r esulted from an increase in the proportion of large macroaggregates with a slow turnover.