Increased physical protection of soil carbon in the mineral soil of a poplar plantation after five years of free atmospheric CO 2 enrichment (FACE)

Abstract

Free airCO2 enrichment (FACE) experiments in aggrading forests and plantations have demonstrated significant increases in net primary production (NPP) and C storage in forest vegetation. The extra C uptake may also be stored in forest floor litter and in 5 forest soil. After five years of FACE treatment at the EuroFACE short rotation poplar plantation, the increase of total soil C% was larger under elevated than under ambient CO2. However, the fate of this additional C allocated belowground remains unclear. The stability of soil organic matter is controlled by the chemical structure of the or- ganic matter and the existence of protection offered by the soil matrix and minerals. 1 Fresh litter entering the soil enhances microbial activity which induces the binding of 0 15 organic matter and soil particles into macro-aggregates. As the enclosed organic mat- ter is decomposed, microbial and decomposition products become associated with mineral particles. This association results in the formation of micro-aggregates (within macro-aggregates) in which organic matter is stabilized and protected. FACE and N- fertilization treatment did not affect the micro- and macro-aggregate weight, C or N frac- tions obtained by wet sieving. However, Populus euramericana increased the micrcro- and small macro-aggregates weight and C fractions. The obtained macro-aggregates were broken up in order to isolate recently formed micro-aggregates within macro- aggregates (iM-micro-aggregates). FACE increased the iM-micro-aggregate weight 2 and C fractions. This study reveals that: 1) Species has an effect on the formation of 0 macro-aggregates. The choice of species in a plantation or the effect of global change on species diversity, may therefore affect the stabilization and protection of soil C in aggregates. And 2) Increased atmospheric CO2 concentration increases the stabi- lization and protection of soil C in micro-aggregates formed within macro-aggregates. 2 This mechanism increases the C sink of forest soils under increasing atmospheric CO2 5 concentration. 872