Tradeoffs between soil and plant carbon sink after forestation along water and nitrogen gradients

Forestation is regarded as an effective strategy for increasing terrestrial carbon sequestration. However, its carbon sink potential remains uncertain due to the scarcity of large-scale sampling data and limited knowledge of the linkage between plant and soil C dynamics. Here, we conduct a large-scale survey of 163 control plots and 614 forested plots involving 25304 trees and 11700 soil samples in northern China to fill this knowledge gap. We find that forestation in northern China contributes a significant carbon sink (913.19±47.58 Tg C), 74% of which is stored in biomass and 26% in soil organic carbon, while soil inorganic carbon contributes minimally. Further analysis reveals that the biomass carbon sink increases initially but then decreases as soil nitrogen increases, while soil organic carbon significantly decreases in nitrogen-rich soils. A tradeoff between organic carbon (biomass + soil organic carbon) and inorganic carbon dynamics is also observed along water gradient. These results highlight the importance of incorporating plant and soil interactions, modulated by nitrogen and water supply in the calculation and modelling of current and future carbon sink potential.