Relationship between aggregate pore size distribution and organic-humic carbon in contrasting soils

The soil pore size distribuion controlling gas diffusivity, water potential and microbial accessibility, strongly influences the quantity, the repartition and the quality of soil organic carbon (SOC). Further knowledge of these interactions is thus fundamental to promote effective C sequestration practices in terrestrial sinks, as recognized by Article 3.4 of the Kyoto Protocol. In a long-term experiment established in the early 1960s in north-eastern Italy, we studied the pore size distribution of large aggregates (2 cm3) in widely different soil types (sand, clay and peaty) and fertilisers (mineral and organic), with the aim of assessing how the porosity could drive the organic carbon turnover/protection and its evolution into three humic fractions (HF) with different molecular weights. Quantity and quality of SOC were strongly affected by the type of soil, evidencing a clear relationship with the aggregate pore size distribution. The SOC was negatively correlated (r = 0.65) with the pore class 30–75 mm (meso-pores), dominant in the sand soil, and positively correlated (r = 0.97) with the pore class 0.1–5 mm (ultramicro-pores), more represented in the other two soils. In particular in the clay and peaty soils, characterized by a large porosity volume in the range 0.007–5 mm, a lower humic/organic carbon ratio (HC/OC) and a dominant presence of HF with lower molecular weight were found. These findings might be attributed to the presence of protection mechanisms that possibly slowed decomposition or preserved relatively decomposable C. On the contrary sand soil, due to the lack of small size pore volume (and hence of mechanisms of protection), evidenced SOC supposed to be more recalcitrant, due to the higher relative presence of HC with a high degree of polycondensation.