Transformation and Transport Mechanism of Nitrogenous Compounds in a Biochar “Preparation–Returning to the Field” Process Studied by Employing an Isotope Tracer Method

Abstract

Due to biochar’s excellent physical and chemical properties, such as rich void ration and large specific surface area, it could improve soil by conserving water and fertilizer and providing breeding grounds for soil microorganisms. Therefore, it has been attracting researchers’ interests for its potential as a soil amendment. In this work, elemental analysis-stable isotope ratio mass spectrometry (EA-IRMS) and X-ray photoelectron spectroscopy (XPS) were conjointly employed to investigate the migration and transformation mechanism of biochar nitrogenous compounds in the “preparation–returning” process. EA-IRMS data indicated that during the preparation process the nitrogen retention rate in biochar first dropped sharply (300–400 °C), then became stable (400–500 °C), and finally decreased slowly (500–800 °C) with increasing pyrolysis temperature. After returning biochar to soil, the measurable total nitrogen in biochar that migrated to soil and plants displayed a nitrogen mass distribution rate in the order of biochar after returning (88.40–90.42%) > soil (8.81–10.07%) > plants (0.77–1.53%). In addition, the pyrolysis temperature was negatively related to the nitrogen mass distribution rate in biochar, soil, and wheat. On the other hand, the pyrolysis atmosphere had little effect on the nitrogen retention rate in biochar before returning and the nitrogen mass distribution rate in biochar after returning to the field. XPS results suggested that alkaloid-N, free amino acid-N, protein-N, and NH4+-N in wheat straw were gradually transformed into pyridine-N, amino-N, pyrrole-N, quaternary-N, NH4+-N, NO2–-N, and NO3–-N in biochar during the biomass pyrolysis process. Biochar produced at 300 °C was in a transition stage that included all nitrogenous compounds present in wheat straw as well as biochar produced at the lower temperatures (≤500 °C). At higher temperatures, inorganic nitrogen species were more abundant and displayed higher contents. For pyrolysis temperatures ≤500 °C, biochars prepared under both N2 and CO2 atmospheres comprised similar nitrogenous compounds and contents. Moreover, the changes in nitrogenous compounds and nitrogen release patterns during the process of returning biochar to the field were not significantly different.