Physical protection of organic matter in minesoils assessed by low-temperature ashing (LTA)

Physical stabilization of soil organic matter (SOM) was assessed by a novel combination of methods in minesoils of Central Italy, cultivated or afforested with different species by 30 years. We studied this stabilization in: 1) a managed (thinned and mowed) English oak (Quercus robur L.) plantation; 2) a similarly managed 1:1 mixed plantation of Italian alder (Alnus cordata Loisel.) and English oak; 3) an unmanaged portion of the mixed plantation; 4) a yearly tilled and manured cropland; 5) an adjacent forest growing on a natural soil, which served as a term of reference. We focused on water-stable soil aggregates, in which SOM is physically protected from decay. Aggregates of 0.5 to 1.0 mm in diameter were subjected to Low-Temperature Ashing (LTA) by oxygen plasma, a technique able to progressively remove SOM with minimal or no damage to mineral constituents and soil fabric. All minesoils had behaved as C sinks, although to a different extent depending on land use, with cropland storing around half soil C than the afforested areas. Carbon enrichment improved soil structure promoting the formation of large and water-stable soil aggregates. The C contained in the core of these aggregates, measured here as the one able to resist 48 h of LTA treatment and confidently assumed as the C fraction best protected from decay, ranged from 43 to 70% of total C, depending on land use and horizon. The strict relationships found between the amount of “protected” SOM and the median diameter (d50) of water-stable soil aggregates (R2 = 0.88), on the one hand, and the larger class of pores in water-stable soil macroaggregates (R2 = 0.99), on the other hand, demonstrated the crucial role of this SOM pool in soil structure formation. This study showed the potential of LTA used in combination with laser diffraction analysis, FTIR-PAS spectroscopy, and mercury intrusion porosimetry to distinguish C stocks benefiting from different physical protection in aggregates and to assess their role in soil structure formation. Such a methodological approach could be highly useful to understand the mechanisms by which soils at the first stages of pedogenesis, such as reclaimed minesoils, sequester C and to reveal how much tillage and other types of soil disturbances expose the protected C to decay.