Measures of pyrogenic carbon in native forest and plantations of northern Australia

Pyrogenic carbon (PyC) is a highly stable fraction of soil organic carbon that plays an important role in the global carbon budget and biogeochemical processes. However, the stocks and dynamics of PyC in soils remain insufficiently characterized, largely due to longstanding challenges in obtaining accurate and reliable assessments. On Melville Island, northern Australia, fire is a common land management tool likely to influence both soil total organic carbon (TOC) and PyC. This study compared PyC to 50 cm depth and TOC to 100 cm depth in soils under three land uses (native Eucalyptus forests, Eucalyptus pellita plantations and Acacia mangium plantations) with differing recent fire frequencies. This study also compared the performance of nitric acid‑hydrogen peroxide digestion (PyC KMD ) and mid-infrared spectroscopy (PyC MIRS ) in estimating PyC, using hydrogen pyrolysis (PyC Hypy ) as the reference technique. The results showed strong evidence that TOC stocks (0–10 cm soil) were lower under E. pellita plantation (18 Mg ha −1 ) than either A. mangium plantations or native forests (each about 25 Mg ha −1 ) ( p < 0.01), and in all cases declined with increasing soil depth. PyC hypy stock was distributed relatively evenly to 50 cm depth in each land use, at about 4–5 Mg ha −1 per 10 cm depth interval. Although native forests were subject to prescribed burns approximately every two years (compared with fire exclusion over 15+ years in the plantations), the ratio of PyC hypy to TOC (PyC hypy /TOC) under native forests was lower than under A. mangium and E. pellita , suggesting that repeated fire events may lead to re-combustion of PyC. PyC hypy /TOC increased with increasing soil depth, strongly suggesting the preferential translocation of PyC into deeper soil layers relative to TOC in the sandy and highly porous soils of Melville Island. Among the three methods used to estimate PyC, PyC KMD was not well correlated with PyC Hypy , while PyC MIRS , based on the available calibration database, explained 23 % of the variation in PyC Hypy . Compared to direct measurements by Hypy, MIRS tended to underestimate PyC in soil with TOC less than 45 g kg −1 and overestimate PyC in soil with TOC greater than 45 g kg −1 . Expanded calibration datasets are required to improve the performance of MIRS for estimating PyC in the sandy forest soils of tropical Australia. Further work is required to quantify PyC beyond 50 cm deep soil, to track temporal changes in soil PyC across different land uses and fire histories, and to evaluate the performance of different methods in quantifying PyC across diverse soil types. • Topsoil total organic C stocks were lower in E. pellita than A. mangium or native forest. • Pyrogenic to total organic C ratio was lower in native forest than in plantations. • MIRS underestimated pyrogenic C below 45 g kg −1 TOC and overestimated above it.