Micropore characteristics of organic matter in cemented and non-cemented spodic horizons

Soils store three times as much organic carbon (OC) as is found in living plants, and most forest soils are active CO 2 sinks at present. With climate change however, the capacity of soils to preserve OC against degradation will highly depend on the presence of stabilized organic matter (OM) pools. Podzols are among the most characteristic soils in forest ecosystems and their formation is linked to OM movements through the profile and the following stabilization by metal-organic interactions (Fe-and Al-OM) in deeper soil horizons. Recently, we found that the type of metal-organic association changes with increasing intensity of podzolisation and that the variations can be followed by evaluating specific surface area (SSA) by N 2 adsorption. We hypothesised that at later stages of podzolisation stable OM is more tightly bound to metals, hence more rigid and poorer in N 2-accessible pores. In well-developed Podzols, cemented metal-rich horizons (ortsteins i.e. Bsm or Bhsm) may form in addition to non-cemented Bs or Bhs ones. In orsteins the rigidity/condensation of stabilized OM should be further enhanced both because of component ageing and of high amounts of metals. Consequently, OM in cemented podzolic horizons should have a higher amount of N 2 non-accessible pores. In this work we evaluated the specific surface area and porosity of bulk and stabilized OM in cemented and non-cemented podzolic B horizons using gas-adsorption. Thanks to the different accessibility of the probe molecules, the pores between ~<2 and 50 nm and down to less than 0.5 nm can be characterized by, respectively, N 2 (77K) and CO 2 (273K). Four Podzols were selected in Northwestern Italy and both Bs or/and Bhs and cemented horizons were sampled. The samples were treated with 6% NaClO at pH 8, to eliminate the most labile OM, and surface properties were measured before and after oxidation (UT and T samples, respectively) with both N 2 and CO 2. The Bs/Bhs horizons had around 23% of oxidation-resistant OC while orsteins generally showed a slightly higher proportion (29%). The N 2 detectable specific surface area (SSA) seemed to be strongly affected by the porosity of mineral phase, but the variation of SSA upon oxidation was linked to the horizon type. Only the OC richest horizons (Bs/Bhs and Bhsm) showed the typical increase in SSA after removal of labile OM, while in the cemented horizons the SSA decreased. This probably reflects the exposure of highly stabilized OM, richer in small micropores, in the ortsteins. Plotting the SSA measured by CO 2 vs. the amount of OC revealed that the microporosity occurrence mainly depended on the presence of OM in both UT and T samples (r 2 =0.973 and 0.918, respectively), with the exception of Bhsm horizons that, in both cases, showed a lower SSA with respect to OC content than the other samples. The anomalous behavior of Bhsm could be related to the high presence of roots, hence to the presence of chemical recalcitrance as an additional mechanism in OM stabilization. The relationships between SSA and OM in UT and T samples had equivalent intercept values (4.35 and 4.36 m 2 g-1 , respectively), pointing out that oxidation had no effect on the contribution of the mineral phase on micropore surface. The slopes instead were different, and indicated that stabilized OM increased SSA twice as much as the UT sample. Our results show that stabilized OM structure in Podzols sharply differ from that of more labile OM pools thanks to a larger presence of small micropores. The high amount of <0.5 nm pores may contribute in preserving organic matter towards degradation, thus decreasing its turnover even in a climatic change perspective