Response of soil organic matter fractions to biochar application and climate change in a Mediterranean agroecosystem

The sustainability of Mediterranean croplands is threatened by climate change, including global warming and decreasing precipitation. In this context, the agronomic use of biochar as an amendment represents a tool to store organic carbon (C) in soil, thus removing carbon dioxide (CO2) from the atmosphere. However, little is known about the responses of soil organic carbon (SOC) to the joint effects of climate change and biochar in Mediterranean croplands. An in-depth evaluation of the vulnerability of SOC to climate change, and the consequent adoption of adequate strategies for C storage, can be better understood investigating its main fractions, i.e., the particulate organic matter (POM) and the mineral-associated organic matter (MAOM). To help fill this knowledge gap, we took advantage from a 5-years long field experiment and evaluated the effects of partial rain exclusion alone or combined with warming in biochar-amended (20 t ha-1) vs. unamended plots under a crop rotation. Thus, the main objective of this study was to determine the effects of both partial rain exclusion (~30%) and increased temperature (~2°C), combined with biochar amendment, on the distribution of soil organic matter (SOM) into POM and MAOM pools. In order to detect structural changes occurring in response to warming, Raman microspectroscopy (RM), a tool sensitive to alterations of graphitic-like structures, was exploited, while changes in SOM stability were estimated by thermal analysis. In addition, changes in wetting and drying cycles dictated by decreasing precipitation might alter iron (Fe) species involved in C storage mechanisms; those have been studied by Fe extended X-ray absorption spectroscopy (EXAFS). [...]