Isotopic Tracers and Biochar in Soil Fertility and Plant Nutrition: Mechanistic Insights into Nutrient Cycling, Carbon Dynamics and Agroecosystem Functioning – A Review

Biochar has emerged as a multifunctional soil amendment with the potential to improve soil fertility, enhance nutrient use efficiency, mitigate contaminant mobility and contribute to long-term carbon sequestration. Produced through the pyrolysis of biomass under oxygen-limited conditions, biochar is characterized by a highly aromatic carbon structure, variable mineral composition and a chemically heterogeneous surface rich in oxygen-containing functional groups. These properties underpin its strong interactions with soil minerals, nutrients, microorganisms, and organic compounds. The mechanistic understanding of how biochar modifies biogeochemical processes in soil-plant systems remains incomplete. A major limitation in biochar research is the difficulty in disentangling direct physicochemical mechanisms, such as sorption and surface complexation, from indirect biological processes mediated by changes in microbial activity, nutrient cycling and rhizosphere dynamics. Conventional soil analyses largely rely on static measurements of nutrient pools and provide limited insight into fluxes, turnover rates and source-sink relationships. In this context, isotopic techniques employing stable and radioactive isotopes offer a powerful framework to resolve carbon, nutrient and contaminant dynamics in biochar-amended soils with high mechanistic resolution. This chapter synthesizes the principles, methodologies and applications of isotopic tracers in studies of soil fertility and plant nutrition under biochar amendment. Emphasis is placed on the use of 13C and 14C to elucidate carbon cycling and priming effects, 15N to quantify nitrogen transformations and biological nitrogen fixation and radioisotopes such as 32P, 33P and 35S to assess nutrient availability and turnover. Isotopic approaches are also discussed in the context of micronutrient dynamics and contaminant fate.