Nitrogen addition promotes the coupling of deep soil carbon and nitrogen under different vegetation restoration types in the Chinese Loess Plateau

Carbon-nitrogen coupling is important to maintain various functions in forest ecosystems and is thus, an important indicator of forest ecosystem health. However, the magnitude of this indicator's importance to environmental changes remains virtually unknown, especially for deep soils across vegetation types. In this study, four representative sites, namely Pinus tabulaeformis forest, Robinia pseudoacacia forest, Pinus tabulaeformis x Robinia pseudoacacia mixed forest, and Populus davidiana x Quercus wutaishanica natural secondary forest, were selected as representatives of typical artificial and natural forests. A one-year N addition experiment was conducted to analyze C-N coupling conditions of different vegetation restoration types, and soil properties in the 0 - 100 cm layer, litter traits, and rainfall distribution characteristics were measured and compared during the growing season. (1) Soil C and N in artificial forests decoupled with increasing soil depth, while soil C and N were highly coupled among all soil depths in natural forest. (2) N addition had a greater effect on deep soil nutrient accumulation compared to topsoil. (3) N addition decreases the rates of change of deep soil N, which, in turn, enhanced the C and N coupling. (4) Moreover, further analyses with a structural equation model showed that summer precipitation is the key regulator of soil C:N ratio in topsoil. Soil pH, litter C:N ratio, and N deposition were primarily responsible for controlling deep soil C and N coupling. These results indicate that N addition and vegetation restoration types affect soil C and N coupling and should be taken into consideration when assessing deep soil C and N biogeochemical cycles. The importance of deep soil should be considered as much as possible during afforestation on the Loess Plateau. Afforestation should be conducted in a way of natural restoration. This study provides novel insights into the regulatory mechanisms of C and N biogeochemistry and the afforestation patterns of vegetation restoration in arid and semi-arid regions.