Effect of decomposition products produced in the presence or absence of epigeic earthworms and minerals on soil carbon stabilization

International audienceMicrobial use efficiency is thought to greatly influence organic carbon storage in soils through the formation of decomposition products and their stabilization as organo-mineral complexes. Earthworm activity may play a significant role in these processes. Of the three ecological earthworm groups only two (endogeic and anecic species) are thought to be involved in soil organic carbon (SOC) stabilization as they are living in mineral soil. The aim of this study was to investigate the role of decomposition products produced, with and without minerals, by epigeic earthworms living at the litter-soil interphase for carbon stabilization. We investigated the impact of eight different types of decomposition products on CO2 emissions, microbial biomass, watersoluble C, mineral N and SOC pool allocation of an arenic cambisol during 79 days under laboratory conditions. Our results indicated that the nature of decomposition products affected their impact on SOC mineralization and soil physico-chemical parameters. In general, the presence of epigeic earthworms during OM decomposition decreased CO2 emissions after soil addition of decomposition products and increased microbial carbon use efficiency when compared to those produced without earthworms. Mineral-containing decomposition products increased CO2 emissions after their addition to soil when compared to their mineral free counterparts. They also changed carbon allocation to physico-chemically protected pools, decreasing the contribution of particulate organic matter when compared to mineral free decomposition products. Extrapolation of the data showed that these short-term effects were not necessarily long-term in nature, but they also indicated that microbial products produced in the presence of epigeic earthworms and minerals may increase SOC sequestration in the amended soil. We therefore conclude that the nature of decomposition products is crucial for their fate in soil and that in contrast to the general paradigm, epigeic earthworms could have an important role to play in SOC sequestration through the formation of material susceptible to be protected, when incorporated into mineral soil

Les techniques de séparation des colloïdes par ultrafiltration et les interactions matière organique - pesticides : limites et développement

The quantities of atrazine, simazine, terbutylazine and prométryne adsorbed on colloids from Seine river water are positively correlated to their octanol-water coefficient (log Kow). However, chlorophenylureas such as the linuron, diuron and chlortoluron show relative enrichment factor (retentat/permeat) 2-10 times higher than triazines. Pyrolysis-GC-MS analyses of permeat and retentat indicates a clear enrichment of the retentat with sugars and permeat with proteinaceous material . This preliminary data suggest several types of pesticide-organic matters interaction (p and hydrogen bonds). Nevertheless, systematic investigations have to be done to evaluate the influence of ultrafiltration membranes and/or the residual organic matter on pesticide sorption processes. / Les quantités d'atrazine, de simazine, de terbutylazine et de prométryne adsorbées par les fractions colloïdales d'eau de Seine sont corrélées positivement au logarithme du coefficient de partage octanol-eau (log Kow) de ces herbicides. En revanche, les phenylurées possédant un cycle benzénique chloré telles que le diuron, le linuron et le néburon présentent des coefficients d'enrichissement relatifs dans le rétentat enrichi en colloïdes 2 à 10 fois plus élevés que les triazines. L'analyse conjointe de la matière organique sur ces échantillons par pyrolyse-chromatographie en phase gazeuse-spectrométrie de masse montre un net enrichissement du rétentat en sucres et du perméat en matière protéinique. Ces premiers résultats suggèrent que plusieurs types d'interactions physiques et chimiques (liaisons p, liaisons "hydrogène", etc) sont susceptibles d'intervenir entre la matrice organique et certaines fonctions et structures chimiques des pesticides. Par ailleurs, pour des solutions dopées à 100 ppb, 50 % des pesticides restent adsorbés sur la membrane de polysulfone (10 KD) ce qui rend la technique de filtration tangentielle impropre à l'analyse des traces