Wildfire effects on ecosystem nitrogen cycling in a Chinese boreal larch forest, revealed by 15 N natural abundance

Abstract

Wildfire is reported to exert strong influences on N cycling, particularly during the early succession period immediately after burning (i.e., < 1 year). Previous studies have mainly focused on wildfires influences on inorganic N concentrations and N mineralization rates; but plant and soil 15N natural abundance (expressed by δ 15 N), as a spatial-temporal integrator of ecosystem N cycling, could provide a more comprehensive understanding of wildfire on various N cycling processes at a relatively broader time scale. In this study, we attempted to evaluate legacy effects of wildfire on nitrogen cycling using δ 15 N in a boreal forest of northeastern China, which is an important yet understudied ecosystem. We measured inorganic N concentrations (NH 4 + and NO 3 −) and net N transformation rates (net ammonification, net nitrification, and net mineralization) of organic and mineral soil 4 years after a wildfire and compared with unburned area. We also measured δ 15 N of plant and soil samples in 4 and 5 years after the fire. We found that even 4 years after burning, wildfire still increased net mineralization and net ammonification in the organic soil and increased NH 4 + and total inorganic N (TIN) concentrations in the mineral soil. Organic soil and foliar δ 15 N were significantly higher (by 2.2 ‰ and 7.4 ‰, respectively) in the burned area than the unburned area. Five years after fire, plant tissues such as foliar, branch, fine roots and moss in the burned area were increased significantly (by 1.7 ‰ to 6.4 ‰) greater than that in unburned area. The wildfire also significantly increased the δ 15 N of Oi, Oa+e and 0–10 cm mineral soil, but had no significant effects on deeper layer of mineral soil. These results indicate the wildfire had a strong legacy effect on N cycling. We suggest that the change of abiotic environment was the primary mechanism determining inorganic nitrogen transformation rates, and the NH 3 volatilization might play a key role in severe N losses and thereby affect soil and plant 15 N in this ecosystem.