Role of 15N in tracing biologically driven nitrogen dynamics in soils amended with biochar: A review

Extensive reports on research into the effects of biochar additions on soil-plant systems show many inconsistencies in the response of various parameters including nitrogen (N) transformations. This review focusses on the use of the stable isotope 15N to provide unique information about the N dynamics in biochar-amended systems. The properties of biochars and their impacts on N cycling depends on the feedstock and pyrolysis parameters, as well as the soil type, plant species and form of 15N fertilizer. A review of the results of 15N tracer studies of the effects of biochar addition on a variety of N transformations shows the following: depending on the soil type, biochar addition can reduce N leaching; ammonia volatilization may be increased but more research is needed on this loss mechanism; biochar may catalyse the reduction of N2O to N2, thus reducing greenhouse gas emissions; biochar effects on biological N2 fixation are inconsistent and especially depend on the biochar feedstock as well as soil type; the mechanisms for the variability in the effects of biochar on N mineralization-immobilization remain obscure; pot studies show that plant recovery of 15N-labelled urea is generally less than 50% and inconsistently affected by biochar addition. The lack of consistency evident when studies are compared reflects the diversity of biochar properties and frustrates efforts to synthesise overarching hypotheses that could confidently predict the effects of biochar addition on any specified soil N transformation.