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Shifts in Above‐ Versus Below‐Ground Carbon Gains to Terrestrial Ecosystems Carbon Sinks Under Excess Nitrogen Inputs
Journal article   Peer reviewed

Shifts in Above‐ Versus Below‐Ground Carbon Gains to Terrestrial Ecosystems Carbon Sinks Under Excess Nitrogen Inputs

Xiaofei Lu, Frank S. Gilliam, Xu Yue, Bin Wang and Yuanwen Kuang
Global biogeochemical cycles, Vol.37(3), e2022GB007638
03/2023
DOI: https://doi.org/10.1029/2022GB007638
Web of Science ID: WOS:000947221800001

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Abstract

Although elevated atmospheric nitrogen (N) deposition can increase terrestrial carbon (C) sinks, the persistence of this C acquisition depends partly on the responses of plant biomass and soil organic carbon (SOC) to long‐term exogenous N input. Experiments with N‐additions show increases in above‐ground biomass (AGB), although C dynamics between above‐ and below‐ground under N enrichment are unconnected in many studies. Here, we meta‐analyzed data from 362 N‐addition experiments globally and found that both AGB (+34%) and below‐ground biomass (BGB, +13%) positively but root‐to‐shoot ratios negatively (−15%) responded to N addition. These responses were reversed, however, when the experimental duration was over 20 years, wherein BGB increased and AGB was unchanged or even declined, during which time plants invested more C into roots. The effects of increased AGB were weak and negative, whereas those of increased BGB were robust and positive on SOC. Under chronic N deposition, the significant increase in SOC (5%) was associated with the increased root C inputs and decreased soil C losses rather than with increased above‐ground C inputs. The results suggest that the sequestration of below‐ground C exceeds that of above‐ground C to sustain terrestrial C gain during long‐term N deposition. Plain Language Summary Elevated nitrogen (N) inputs caused by atmospheric N deposition globally affect plant growth and soil carbon (C) accumulation. The mechanisms of plant and soil C dynamics responding to long‐term N deposition remain poorly understood. In this study, we collected data from 362 N‐addition experiments with different durations to assess the consequences of plant and soil C budgets. Our results showed that above‐ground biomass responded more than below‐ground biomass to N addition but that the responses can be reversed when the excess N inputs prolonged, for example, the experimental duration was more than 20 years. The effects of above‐ and below‐ground C inputs on soil C accumulation differed, with increased above‐ground biomass having weak and negative effects, whereas increased below‐ground biomass having robust and positive effects on soil organic C. These findings suggest the greater capacity of below‐ than above‐ground C sequestration for terrestrial C sinks under long‐term N deposition. Key Points Below‐ground biomass (BGB) increases more than above‐ground biomass (AGB) under long‐term N addition Effects of BGB and AGB on soil carbon accumulation can be reversed with prolonged N‐addition duration Below‐ground carbon gains exceed above‐ground carbon gains under chronic nitrogen deposition

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