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.

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Alpine dwarf pine populations are dwelling in a climate-sensitive habitat, where detection of the carbon (C) cycle is still valued for sustainability. Foliar and soil parameters are key factors that combine to jointly affect aboveground C storage in alpine ecosystems, but how they generate combined contributions to aboveground C in alp dwellers still needs more research. In this study, Pinus pumila, a typical alp dwarf pine species in a canyon of the Great Khingan Mountain, was focused on. Their natural populations were investigated for individual growth and needle and soil parameters in plots across six categorized elevations from 800 m to 1200 m. Aboveground C storage was estimated by three allometric models which were all found to increase against increases in elevation. Along the increasing elevational gradient, needle concentrations of nitrogen (N) and phosphorus (P) both showed decreasing trends, but activities of N and P assimilation enzymes and chlorophyl contents, as well as the soil contents of ammonium N and organic matter, all showed increasing trends. Multiple linear regression models indicated that elevation (parameter estimate, PE: +0.01), needle P (PE: +0.66) and chlorophyl contents (PE: +0.60) made jointly positive contributions to estimated C storage while soil pH had a negative contribution (PE: −1.80). For the purpose of sustainable C fixation by alp P. pumila populations, strategies should be considered to increase P availability and control high soil pH. Our results fill the gap about C storage and driving forces in alpine ecosystems, and their applications are not limited to being referenced by other alpine plants.