Black carbon (BC) from incomplete combustion of biomass and fossil fuel is widespread in sediments and soils because of its high stability in nature and is considered an important component of the global carbon sink. However, knowledge of BC stocks and influencing factors in forest ecosystems is currently limited. We investigated soil BC contents in burned boreal forests of the Great Khingan Mountains, northeast China. We collected soil samples from 14 sites with different fire severities, slope positions and aspects. The samples were analyzed by the chemo-thermal oxidation method to obtain their BC concentrations. The BC concentrations of the studied soils ranged from 0.03 to 36.91 mg C g⁻¹, with a mean of 1.44 ± 0.11 mg C g⁻¹. BC concentrations gradually decline with depth, and that was significantly less in the 20–30 cm layer compared to all shallower layers. Forests burned by moderate-severity fires had the highest soil BC, the shady aspect had higher soil BC than the sunny aspect. Our results provide some basic data for evaluating the soil BC sink in boreal forests, which is a useful amendment to current carbon budget and carbon cycle in boreal forest ecosystems.

Measuring the nitrogen isotope compositions (δ¹⁵N) of nitric oxide (NO) from different sources helps to quantify the relative contributions of atmospheric NO_x. Soil is one of the most important sources of atmospheric NO_x, but only limited measurements on the δ¹⁵N of soil‐emitted NO exist, hampering our ability to partition sources to air pollution. Here we conducted soil incubations to measure the δ¹⁵N‐NO under defined aerobic or anaerobic conditions, favoring either nitrification or denitrification. Soils were collected from seven sites spanning three ecosystems in northern China (two agricultural, two forest, and three grassland sites). We found that the δ¹⁵N‐NO and their associated N isotope fractionations were significantly different between anaerobic and aerobic conditions in seven soils. Under aerobic condition, the δ¹⁵N‐NO ranged from −62‰ to −50‰ (averaged −56 ± 4‰), being significantly more negative (by 23‰) than those under anaerobic condition (−45‰ to −23‰, averaged −33 ± 7‰). The apparent N isotope fractionation for NO production under aerobic condition (¹⁵ε_aerobic = 61 ± 3‰) was significantly higher (by 26‰) than under anaerobic condition (¹⁵ε_anaerobic = 35 ± 6‰), with a small variability among ecosystem types. Our study demonstrates that the δ¹⁵N‐NO from different soils are very different from fuel combustions (mainly from 0‰ to +20‰), supporting that measuring ¹⁵N is a useful tool to partition the contributions of soil NO to atmospheric NO_x. Our results also imply δ¹⁵N‐NO produced by nitrification and denitrification distinctly different, as these two processes are dominant processes producing NO under aerobic and anaerobic conditions, respectively.