Impact of 40 years poplar cultivation on soil carbon stocks and greenhouse gas fluxes

Abstract. Within the JRC Kyoto Experiment in the Regional Park and UN-Biosphere Reserve "Parco Ticino" (North-Italy, near Pavia), the soil carbon stocks and fluxes of CO₂, N₂O, and CH₄ were measured in a poplar plantation in comparison with a natural mesohygrophilous deciduous forest nearby, which represents the pristine land cover of the area. Soil fluxes were measured using the static and dynamic closed chamber techniques for CH₄ N₂O, and CO₂, respectively. We made further a pedological study to relate the spatial variability found with soil parameters.

Annual emission fluxes of N₂O and CO₂ and deposition fluxes of CH₄ were calculated for the year 2003 for the poplar plantation and compared to those measured at the natural forest site. N₂O emissions at the poplar plantation were 0.15$plusmn;0.1 g N₂O m^-2 y^-1 and the difference to the emissions at the natural forest of 0.07±0.06 g N₂O m^-2 y^-1 are partly due to a period of high emissions after the flooding of the site at the end of 2002. CH₄ consumption at the natural forest was twice as large as at the poplar plantation. In comparison to the relict forest, carbon stocks in the soil under the poplar plantation were depleted by 61% of surface (10 cm) carbon and by 25% down the profile under tillage (45 cm). Soil respiration rates were not significant different at both sites with 1608±1053 and 2200±791 g CO₂ m^-2 y^-1 at the poplar plantation and natural forest, respectively, indicating that soil organic carbon is much more stable in the natural forest. In terms of the greenhouse gas budget, the non-CO₂ gases contributed minor to the overall soil balance with only 0.9% (N₂O) and -0.3% (CH₄ of CO₂-eq emissions in the natural forest, and 2.7% (N₂O) and -0.2% of CO₂-eq. emissions in the poplar plantation.

The very high spatial variability of soil fluxes within the two sites was related to the morphology of the floodplain area, which was formed by the historic course of the Ticino river and led to a small-scale (tenth of meters) variability in soil texture and to small-scale differences in elevation. Differences of site conditions are reflected by differences of inundation patterns, ecosystem productivity, CO₂ and N₂O emission rates, and soil contents of carbon and nitrogen. Additional variability was observed during a flooding event and after fertilisation at the poplar site. Despite of this variability, the two sites are comparable as both originate from alluvial deposits.

The study shows that changes in soil carbon stocks and related fertility are the most visible phenomena after 40 years of land use change from a pristine forest to a fast growing poplar plantation. Therefore, the conservation and careful management of existing carbon stocks deserves highest priority in the context of the Kyoto Protocol.