Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Assessment of the long-term stability of the new sequestered C

Understanding how forest soils will respond to elevated [CO2], in terms of C stability and sequestration potential is a must for corrected prediction of future C cycling. The expected shift in C allocation from leaves to fine roots and the increased substrate availability for microorganisms, under elevated CO2, may lead to increased microbial activity and increased soil aggregation. Aggregates physically protect organic matter against microbial decomposition, and an increase in soil aggregation under elevated CO2 is expected to enhance carbon sequestration in soil. A growing number of studies are addressing the effects of elevated CO2 on soil organic C and SOM aggregation, however, for forest ecosystems results are still controversial. Despite the increased C input to the organic layer, no changes in stabilised SOM fraction in the mineral soil were observed at an aggrading Loblolly pine plantation after 6 years of FACE exposure. In contrast, in a ten-year old Liquidambar styraciflua plantation, a significant increase of C in the microaggregate fraction of SOM, after 5 years of FACE exposure, has been reported.

At the EUROFLUX site, we performed a study aimed to determine how SOM pools and dynamics are affected: 1) by changing land use from a wheat crop to a poplar plantation; 2) comparing poplar plantations, by exposure to FACE.

In particular, the study was conducted to test the hypotheses that: 1) poplar plantation increases the aggregation of a soil previously managed to grow wheat; 2) exposure to elevated [CO2] further stimulates soil aggregation, through enhancing C input belowground; 3) in both cases, the increased C input belowground and aggregate formation, promote C sequestration into the afforested soil and does it to a larger extent under elevated [CO2]. The additional C was mainly expected to be in the physically protected microaggregate fraction. SOM was fractionated by size and density, following recently established procedures.

Soil organic C dynamics were not significantly modified by afforestation: SOM aggregation, contrary to our hypotheses, was not enhanced in the afforested soil, as compared to the agricultural soil, and only an initial trend toward a stabilization of C was observed in physically protected fractions.

When afforested soils exposed to ambient and FACE atmospheres were compared, elevated CO2 appeared to promote soil aggregate formation, with an increase of 8.8% and 18.0% being observed in the macroaggregates fraction (> 250m) of soil, under P. nigra and P. x euramericana, respectively. In the afforested FACE soils, however, a significant decrease, on average of 60%, was observed in the physically protected C of microaggregates (53-250m) and silt and clay (< 53?m) fractions, as compared to afforested control soils.

This effect was likely induced by a priming of these pools.

Our results suggest that, in order to detect how SOM dynamics might be altered in the short term by environmental changes, it is indispensable to consider different SOM pools and not the soil as a unique entity. Only by applying this study approach it was possible to underline SOM dynamics determined by this type of fast growing woody plantation.

Afforestation of cropped soils induced only a trend of increasing C that was stabilized in physical protected fractions: this suggests that, in the long term, C sequestration can be promoted in these soils. Contrarily to our hypothesis, aggregation was not enhanced by afforestation, while it was by elevated CO2, possibly as a consequence of altered rhyzosphere processes and microbial biomass. FACE soils, however, showed a reduction in total C induced primarily by a priming effect of microaggregates and mineral-associated C, thus indicating those fractions as particularly important in determining SOM response to environmental changes.
It must be remarked, however that the higher C input is promoting the stabilization of C in microaggregates protected by macroaggregates (mM) especially in soils under the most reactive Populus species (P. nigra). Even if initial conditions promotes the decomposition of stable and protected pools, it is not to be excluded that, if increased C input is sustained in FACE soils, C accrual in protected microaggregate fractions may be seen, in the long term, in afforested soils exposed to elevated CO2.

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