Dynamics of soil carbon in a beechwood chronosequence forest

Abstract

Accurate estimates of forest soil organic matter (OM) are now crucial to predictions of global C cycling. This work addresses soil C stocks and dynamics throughout a managed beechwood chronosequence (28–197 years old, Normandy, France). Throughout this rotation, we investigated the variation patterns of (i) C stocks in soil and humic epipedon, (ii) macro-morphological characteristics of humic epipedon, and (iii) mass, C content and C-to-N ratio in physical fractions of humic epipedon. The fractions isolated were large debris (>2000 μm), coarse particular OM (cPOM, 200–2000 μm), fine particular OM (fPOM, 50–200 μm) and the mineral associated OM (MaOM, <50 μm). Soil C stocks remained unchanged between silvicultural phases, indicating a weak impact of this even-aged forest rotation on soil C sequestration. While humic epipedon mass and depth only slightly varied with beech development, C stocks in the holorganic layers were modified and the use of physical fractionation allowed us to discuss different aspect of quantitative and qualitative changes that occurred throughout the silvicultural rotation. Hence, changes in humic epipedon composition may be attributed to the modification of beech life-history traits with its maturation (growth vs. reproduction). Our results showed that C-POM can reached very high values (68%) in organo-mineral layers of older managed forest and that C-MaOM did not significantly change revealing the resistance of humified fractions of humic epipedon to logging and regeneration practices. C-to-N results indicated that N was probably not a limiting factor to litter degradation and explained our findings that OM did not accumulate in O horizons. This work confirms that forest harvesting and regeneration practices may have few effects on soil and humic epipedon C stocks, and that short- and long-term effects can be complex and may imply mechanisms with opposite effects.