Effects of stabilisation on soil organic matter porosity in cemented podzolic horizons Catoni M. 1 ,D’Amico M. 1 , Mittelmeijer-Hazeleger M.C. 2 , Rothenberg G. 2 , Bonifacio E. 1 1 DISAFA, Università di Torino, Via L. da Vinci 44, 10095 Grugliasco, Italy. 2 Van ’t Hoff Institute of Molecular Sciences, University of Amsterdam, Science Park 904, 1098XH Amsterdam, The Netherlands. e-mail: marcella.catoni@unito.it Organic matter (OM) occurs in soils in various pools showing different resistance degrees against degradation as a consequence of the mechanisms governing their stabilization. In Podzols, OM is mainly stabilised by association with the mineral phase, and with the ongoing of podzolisation an enhancement of metal-organic interactions (Fe- and Al-OM) occurs. Although OM typically has a very complex structure, in which the abundant microporosity may reach dimensions even <2 nm, when stabilized by mineral associations OM structure can change. A larger ultramicroporosity (<0.7 nm) than the labile pool is expected, reflecting the higher rigidity/condensation of OM structure given by association with metals. In well- developed Podzols, a metal-rich cemented horizon, called ortstein (Bsm or Bhsm), may form in addition to non-cemented Bs or Bhs ones. In these cemented horizons OM ultramicroporosity may be enhanced. To investigate OM structure the gas adsorption technique is particularly suitable: thanks to the different accessibility of N 2 (77K) and CO 2 (273K), the pores between ~<2 and 50 nm and down to less than 0.5 nm respectively can be characterized. In this work we evaluated the features of metal-organic associations in podzolic cemented horizons in term of surface properties and compared them with those of the more labile OM pools. Three Podzols were selected in NW Italy, and both the Bs or/and Bhs and cemented horizons were sampled. The samples were treated with 6% NaClO at pH 8 to eliminate labile OM, and the specific surface area (SSA) was evaluated before and after oxidation (UT and T samples, respectively) with both N 2 and CO 2 . The N 2 detectable SSA seemed to be strongly affected by the porosity of the mineral phase, but the variation of SSA upon oxidation was linked to the horizon type. Only the Bs/Bhs horizons showed the typical increase in SSA after the removal of labile OM, while in the cemented horizon the SSA decreased. This probably reflects the exposure of highly stabilised OM, richer in ultramicroporosity, in the ortsteins. Conversely, the SSA measured by CO 2 was mainly affected by the presence of OM in both UT and T samples (r 2 =0.973 and 0.918, respectively). The two functions showed equivalent intercept values (4.35 and 4.36 m 2 g -1 , respectively), indicating that oxidation had no effect on the contribution of the mineral phase on micropore surface. The slopes instead were different, and indicated that stabilised OM increased the SSA twice as much as the UT sample. Our results show a sharp variation in OM structure upon stabilisation, with a larger portion of CO 2 accessible surface in stabilised OM.