Addition of nitrogen enhances stability of soil organic matter in a temperate forest

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 C hangbai M ountain, C hina, 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 ( δ 13 C ) 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 ( M m) 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 ( I ntra‐ SC ). Unprotected C (comprising the free light fraction ( F ree‐ 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 δ 13 C signature and C/N ratios obtained for SOM fractions showed that newly formed C was transferred from POM to I ntra‐ 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.