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 μm mean weight diameter. The increase of both C and nitrogen (N) stable isotope composition (δ13C; δ15N) 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.