Abstract of KTSOM2 project

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KTSOM 2 Abstract

Project Description Presentation

Impact of elevated CO₂ on soil organic matter dynamics as related to changes in aggregate turnover and residue quality. J. Six, A. Carpentier, C. van Kessel, R. Merckx, D. Harris, W.R. Horwath, and A. Lüscher. Global Change Biology. Submitted (2000)

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 (< 53, 53-250, 250-2000, and > 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. Large and small macroaggregates (> 2000 µm and 250-2000 µm) were found to be 23%-60% more enriched in new C compared to microaggregates (53-250 µm), confirming a hierarchical aggregate structure. In addition, higher proportions of new C in macroaggregates under T. repens compared to L. perenne indicate a faster macroaggregate turnover in 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 which would reduce the potential of sequestering C under elevated CO₂. In the L. perenne soil, elevated CO₂ did not increase total C, but led to: 1) a doubling of the amount of large macroaggregates, 2) a 40% increase in total iPOM-C, and 3) a sequestration of C in microaggregates. These observations indicate that the sequestration of iPOM-C and microaggregate-C induced by elevated CO₂ in the low residue quality, L. perenne treatment, resulted from an increase in macroaggregation without a change in macroaggregate turnover.