No abstract

Physicochemical protection of soil carbon provided by soil aggregates is critical to carbon (C) sequestration in terrestrial ecosystems. However, the stability of soil organic matter (SOM) in terrestrial ecosystems in response to atmospheric nitrogen (N) deposition is unclear. In this study, N was added to a forest soil dominated by deciduous trees on Changbai Mountain, China, at three different rates (0, 25 and 50 kg N ha−1 year−1) from 2007 to 2012. Its effect on C content and stabilization was evaluated by soil fractionation and stable isotope (δ13C) analyses. The results showed that large macroaggregates (2–8 mm) decreased and small macroaggregates (0.25–2 mm) increased with increasing rates of N addition, whereas soil C content remained unchanged. Irrespective of the N treatments, the C content of soil organic matter (SOM) fractions differed significantly between large and small macroaggregates, which suggests that the size of aggregate classes regulates C content in the SOM fractions. A slight increase in the C content of microaggregates within macroaggregates (Mm) and that of silt and clay fractions was recorded with the addition of N at 50 kg N ha−1 year−1. This increase also occurred in the silt and clay fraction within microaggregates (Intra‐SC). Unprotected C (comprising the free light fraction (Free‐LF) and coarse particulate organic matter (CPOM)) accounted for 18.9% only of the total C and decreased in response to the addition of N. The δ13C signature and C/N ratios obtained for SOM fractions showed that newly formed C was transferred from POM to Intra‐SC. Overall, our results suggested that long‐term addition of N might promote stabilization of C by increasing small macro‐ and micro‐aggregation within macroaggregates in temperate forest soil. Highlights Investigated stability of soil organic matter in response to long‐term N addition in a deciduous forest. Physicochemical protection of soil organic matter by aggregates changed after N addition. Soil organic carbon in microaggregates within macroaggregates increased with N addition. Long‐term N addition might promote C stabilization in temperate forest soil.