Evaluation of the surface affinity of water in three biochars using fast field cycling NMR relaxometry

Many soil functions depend on the interaction of water with soil. The affinity of water for soils can be altered by applying soilamendments like stone meal, manure, or biochar (a carbonaceous material obtained by pyrolysis of biomasses). In fact, the addi-tion of hydrophobic biochar to soil may increase soil repellency, reduce water-adsorbing capacity, inhibit microbial activity, altersoil filter, buffer, storage, and transformation functions. For this reason, it is of paramount importance to monitor water affinity forbiochar surface (also referred to as ‘wettability’) in order to better address its applications in soil systems. In this study, we proposethe use of fast field cycling NMR relaxometry technique with the application of a new mathematical model for data interpretation,as a valid alternative to the traditional contact angle (CA) measurements for biochar wettability evaluation. Either NMR or CA re-sults revealed the same wettability trend for the biochars studied here. The advantage of NMR relaxometry over CA measurementslies in the possibility to obtain at the microscopic level a variety of different information in only one shot. In fact, while CA providesonly wettability evaluation, NMR relaxometry also allows achievement of the mechanisms for water molecular dynamics onbiochar surface, thereby leading to the possibility to understand better, in future research, the role of biochar in increasing soilquality and plant nutrition.