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Contribution of above ground litter to C sequestration in soil

Litter decomposition is one of the major processes determining C fluxes between the terrestrial biosphere and the atmosphere and contributing to soil organic matter (SOM) formation. Nonetheless, factors influencing the amount of C lost by decomposing litter and its partitioning into C emission to the atmosphere and to C input belowground are still not well known. Positive mutual feedbacks were found between leaf litter and rhizosphere respiration. On the other side, generally, mass loss rates for easily degradable litter tend to increase with N availability, particularly in the early stages of decomposition, while rates for lignified litter and humified organic matter tend to decline. With the aims to quantify the contribution of aboveground litter to C sequestration in soil and to test the hypothesis that elevated atmospheric CO2 and soil N availability can influence the amount and partitioning of C lost by decomposing litter, a 13C-labelled litter decomposition experiment was performed at the experimental EUROFACE site. In this site the rhizosphere activity was found to be stimulated by the enrichment in atmospheric CO2 concentration, as well as at the early stage, litter decomposition proceeded faster in CO2 enriched plots where, however, it showed sign of N limitation in the soil.

A standard P. nigra leaf litter, strongly enriched in 13C (?13C -160 0), was incubated from September 2004 to August 2005 under P. nigra trees, exposed to FACE and ambient atmosphere, and in natural and N fertilized conditions, in 3 replicates for each of the four treatment combinations. During the ten months of field incubation soil CO2 efflux and its isotopic composition was measured monthly. At harvest, litter was carefully collected from within collars, dried and weighted for determination of mass loss. Leaf litter mass loss ranged between, 62 and 97 % of the initial mass. Soil profiles under litter and control collars were sampled and divided into 4 depth intervals: 0-2, 2-5, 5-10, 10-20 cm. Soil profiles under litter show a typical trend for all treatments: going from more 13C enriched towards more 13C depleted values with depth (? in figure 8.2.1 a-d). Compared to control soil (? in figures 8.2.1 a-d), only the first two (0-2 and 2-5 cm) upper soil layers under litter were significantly enriched in 13C.

A two source mixing model was applied to quantify the litter contribution to the litter-derived C input to SOM, by depth. A significant fraction of litter derived C was only found for the first two upper soil layers. For the upper soil layer (0-2 cm) incubated with litter, the fraction of litter derived C was, on average, around 5 % of the total C in this soil layer. No significant difference was observed in the fraction of litter derived C between the four different treatments.

Percentages of C and N, as well as C/N ratios, decreased along the profile for both soil under litter and control soil. Concentrations of C in the first layer of soil incubated with litter, for all combination of treatments, are higher than the one in the respective control soil, whereas a decrease of N concentration is measured in comparison to control soil. However, differences between depths and treatments were not significant.

The fraction of litter C lost as an input into the soil were higher than the fraction released as CO2 for all the different treatments. This result, together with a very high litter mass loss, was interpreted hypothesising that litter derived C entered the soil matrix mainly as fragmented litter pieces, going to enrich the coarse particulate organic matter fraction in soils. Moreover, comparison with a previous decomposition experiment at the same site, where litter samples lost between 15 and 18 % of their initial mass, suggests how litter bags can limit decomposition by avoiding litter fragments to enter the soil matrix.

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