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Increased atmospheric nitrogen (N) deposition has greatly influenced soil organic carbon (SOC) dynamics. Currently, the response of SOC to atmospheric N deposition is generally detected through understory N addition, while canopy processes have been largely ignored. In the present study, canopy N addition (CN) and understory N addition (UN, 50 and 100 kg N ha−1 year−1) were performed in a Moso bamboo forest to compare whether CN and UN addition have consistent effects on SOC and SOC turnover times (τsoil: defined as the ratio of SOC stock and soil heterotrophic respiration) with a local NHx:NOy ratio of 2.08:1. The experimental results showed that after five years, the SOC content of canopy water addition without N addition (CN0) was 82.9 g C kg−1, while it was 79.3, 70.7, 79.5 and 74.5 g C kg−1 for CN50, CN100, UN50 and UN100, respectively, and no significant difference was found for the SOC content between CN and UN. Five-year N addition did not significantly change τsoil, which was 34.5 ± 7.4 (mean ± standard error) for CN0, and it was 24.9 ± 4.8, 22.4 ± 4.9, 30.5 ± 4.0 and 22.1 ± 6.5 years for CN0, CN50, CN100, UN50 and UN100, respectively. Partial least squares structural equation modeling explained 93% of the variance in τsoil, and the results showed that soil enzyme activity was the most important positive factor controlling τsoil. These findings contradicted the previous assumption that UN may overestimate the impacts of N deposition on SOC. Our findings were mainly related to the high N deposition background in the study area, the special forest type of Moso bamboo and the short duration of the experiment. Therefore, our study had significant implications for modeling SOC dynamics to N deposition for high N deposition areas. [ABSTRACT FROM AUTHOR]

Increased atmospheric nitrogen (N) deposition has greatly influenced soil organic carbon (SOC) dynamics. Currently, the response of SOC to atmospheric N deposition is generally detected through understory N addition, while canopy processes have been largely ignored. In the present study, canopy N addition (CN) and understory N addition (UN, 50 and 100 kg N ha−1 year−1) were performed in a Moso bamboo forest to compare whether CN and UN addition have consistent effects on SOC and SOC turnover times (τsoil: defined as the ratio of SOC stock and soil heterotrophic respiration) with a local NHx:NOy ratio of 2.08:1. The experimental results showed that after five years, the SOC content of canopy water addition without N addition (CN0) was 82.9 g C kg−1, while it was 79.3, 70.7, 79.5 and 74.5 g C kg−1 for CN50, CN100, UN50 and UN100, respectively, and no significant difference was found for the SOC content between CN and UN. Five-year N addition did not significantly change τsoil, which was 34.5 ± 7.4 (mean ± standard error) for CN0, and it was 24.9 ± 4.8, 22.4 ± 4.9, 30.5 ± 4.0 and 22.1 ± 6.5 years for CN0, CN50, CN100, UN50 and UN100, respectively. Partial least squares structural equation modeling explained 93% of the variance in τsoil, and the results showed that soil enzyme activity was the most important positive factor controlling τsoil. These findings contradicted the previous assumption that UN may overestimate the impacts of N deposition on SOC. Our findings were mainly related to the high N deposition background in the study area, the special forest type of Moso bamboo and the short duration of the experiment. Therefore, our study had significant implications for modeling SOC dynamics to N deposition for high N deposition areas.

The high uncertainty associated with the response of terrestrial carbon (C) cycle to climate is dominated by ecosystem C turnover time (τeco). Although the relationship between τeco and climate has been extensively studied, significant knowledge gaps remain regarding the differential climate sensitivities of turnover time in major biomass (τveg) and soil (τsoil) pools, and their effects on vegetation and soil C sequestration under climate change are poorly understood. Here, we collected multiple time series observations on soil and vegetation C from permanent plots in 10 Chinese forests and used model‐data fusion to retrieve key C cycle process parameters that regulate τsoil and τveg. Our analysis showed that τveg and τsoil both decreased with increasing temperature and precipitation, and τsoil was more than twice as sensitive (1.27 years/°C, 1.70 years/100 mm) than τveg (0.53 years/°C, 0.40 years/100 mm). The higher climate sensitivity of τsoil caused a more rapid decrease in τsoil than in τveg with increasing temperature and precipitation, thereby significantly reducing the difference between τsoil and τveg (τdiff) under warm and humid conditions. τdiff, an indicator of the balance between the soil C input and exit rate, was strongly responsible for the variation (more than 50%) in soil C sequestration. Therefore, a smaller τdiff under warm and humid conditions suggests a relatively lower contribution from soil C sequestration. This information has strong implications for understanding forest C‐climate feedback, predicting forest C sink distributions in soil and vegetation under climate change, and implementing C mitigation policies in forest plantations or soil conservation. Plain Language Summary: Carbon turnover time is the average time that a carbon atom stays in an ecosystem from entrance to exit. Together, ecosystem carbon input via photosynthesis (i.e., productivity) and carbon turnover time determine ecosystem carbon sequestration. However, in contrast to the well‐studied ecosystem productivity, carbon turnover time was found to dominate the uncertainty in terrestrial carbon sequestration and its response to climate. However, the climate sensitivities of carbon turnover times in various plant and soil pools and their effects on carbon storage have not been well‐studied. Here, we quantified that carbon turnover time in soil (τsoil) was more sensitive to climate than that of vegetation (τveg). This finding indicated the difference between τveg and τsoil (τdiff) being shortened in warm and humid regions. We further found that τdiff, as an indicator of the balance between soil carbon input and the carbon exit rate, is closely associated with the capacity for soil carbon sequestration. Therefore, a decreasing τdiff with increasing temperature/precipitation indicates a smaller proportion of carbon sequestered by soil than vegetation. Our findings facilitate understanding of carbon‐climate feedback and the prediction of carbon sink distributions under climate change and could guide the implementation of carbon mitigation policies for vegetation/soil conservation. Key Points: The carbon turnover time in soil (τsoil) has a higher climate sensitivity to temperature and precipitation than that of biomass (τveg)The strong climate responses of woody allocation and soil decomposition in combination contribute to the higher climate sensitivity of τsoil than τvegThe higher climate sensitivity of τsoil than τveg led to a decreased soil carbon sequestration capacity under warm and humid conditions [ABSTRACT FROM AUTHOR]