Physically protected organic matter drives soil carbon sequestration potential of a managed grassland ecosystem in Italian Alps

Mountain managed grasslands are characterized by a high biodiversity and are capable to store a considerable
amount of carbon (C) in the soil. In this study, we evaluated the chemical characteristics of physically protected
organic matter and its contribution to the soil C stock potential in a subalpine managed grassland ecosystem,
combining low-temperature ashing (LTA), stable isotope ratio mass spectrometry (IRMS) and infrared photoacoustic spectroscopy (PAS). The physically protected C fraction, evaluated through LTA by oxygen plasma,
amounted ~74% of the whole soil organic C (SOC) in the first 15 cm depth. This C fraction was closely linked to
mineral components in stable aggregates of ~587 ?m mean weight diameter. The increase of both C and nitrogen
(N) stable isotope composition (?13C; ?15N) in the physically protected organic matter, concomitant with a
decrease of its C/N ratio, suggests higher microbial transformation degree, age and stability against decomposition of protected organic fraction compared to unprotected one. However, also the younger and less protected C
fraction acted as cementing organic labile agent, creating larger but less stable soil aggregates. The infrared
photoacoustic spectroscopy showed that the physically protected C fraction was characterized by a more marked
presence of phenolic compounds and a reduced amount of proteinaceous and polysaccharides-like materials
compared to the more labile and younger organic fraction. These results highlight that managed grasslands can
effectively sequester large amount of long-residence C into the soil, promoting aggregate stability and soil
fertility, and playing a key role for mitigation strategies against climate change.