Effect of litter quality and soil variables on the stabilization of organic matter

Marcella Catoni1, Michele E. D'Amico1, Eleonora Bonifacio1
1 DISAFA, University of Turin

O 4.3 in Below ground turnover of C and nutrients in forest soils

14.07.2014, 11:55-12:15, H17

Organic matter (OM) is the most important factor affecting soil fertility and nutrient recycling in forest soils. However, the amounts of organic C stored in soils depend on the dynamic equilibrium between emissions and retentions, which is specific of the different OM pools. A key element for assessing C dynamics in forest soils is the identification of the links between soil OM pools and stabilization mechanisms. OM may in fact be stabilized against decomposition thanks to its inherent recalcitrance and to organo-mineral associations, including coulombic (H-bonding, anion and cation exchange, ligand exchange, cation bridges) and non-coulombic interactions.

The purpose of this study was the assessment of the main mechanisms of SOM stabilization in forest soils as a function of the characteristics of the mineral phase and of litter quality. In a mountain area of the Ligurian Alps (NW Italy), we selected 21 soil profiles taking into account both lithology and forest cover. Soil chemical analyses were performed and both organic and mineral horizons were oxidized with 6% NaClO at pH 8 to remove the most labile OM pool. Subsequently the mineral fraction was dissolved with 10% HF to split the NaClO-resistant pool into recalcitrant and mineral-associated OM.

The profiles mostly showed a low degree of evolution (Regosols and Cambisols), although in some cases the amount of OM was enough to form a mollic epipedon. Podzols and Alisols/Luvisols were found where the slope was less steep and pedogenic factors more favourable.

The parent material sharply affected the characteristics of mineral horizons: fine texture prevailed in the presence of calcareous or dolomitic rocks and non-calcareous slates, resulting in a higher cation exchange capacity, while coarser fractions dominated in soils developed from gneiss and quartzite. The presence of carbonates induced a larger amount of crystalline Fe phases and influenced pH and base saturation along the whole profile, as a consequence of the low pedogenic development.

Although the total amount of organic C and the C/N ratio did differ in relation to forest cover, litter quality was the key variable in determining OM stabilization by chemical recalcitrance. Soils developed under beech or chestnut had the largest amounts of recalcitrant OM (around 40% of OC), while the soils of fir and open larch stands, as well as those found in hornbeam-ash associations, showed values around 10% of OC. Litter quality and characteristics of the parent material interacted in determining organo-mineral associations. Poorly crystalline Fe oxides played a key role in stabilising OM, emphasizing the importance of ligand exchange reactions in non-calcareous soils where, however, the final effect depended also on litter characteristics. On carbonate substrates, a complex suite of chemical properties affected organo-mineral interactions that varied according to the horizon type and to the pedogenic processes that have occurred.

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last modified 2014-04-04