Potential nitrogen constraints on soil carbon sequestration under low and elevated atmospheric CO2

Area of Science:

• Ecology
• Biogeochemistry
• Plant Science

Background:

• Anthropogenic increases in atmospheric CO2 influence ecosystem carbon and nitrogen dynamics.
• The progressive N limitation (PNL) theory posits that rising CO2 may be constrained by nitrogen availability for carbon sequestration.
• Understanding C-N interactions is crucial for predicting ecosystem responses to elevated CO2.

Purpose of the Study:

• To investigate the interaction between carbon (C) and nitrogen (N) dynamics under elevated atmospheric CO2 (Ca).
• To test the predictions of the progressive N limitation theory in a C3/C4 grassland ecosystem.
• To determine the impact of elevated Ca on soil organic matter, plant biomass, and net carbon sequestration.

Main Methods:

• A four-year field experiment exposing an intact C3/C4 grassland to a gradient of atmospheric CO2 concentrations (200–560 µmol/mol).
• Monitoring of decomposition rates, N mineralization, soil microbial biomass, soil respiration, and C:N ratios in soil and plant tissues.
• Analysis of C and N dynamics across different plant species (C3 forb Solanum dimidiatum and C4 grass Bothriochloa ischaemum).

Main Results:

• Decomposition and N mineralization varied significantly with plant species and Ca levels.
• Soil microbial biomass and respiration showed a nonlinear response to Ca, peaking around 440 µmol/mol.
• Elevated Ca induced a shift of N from soil organic matter to aboveground plant biomass, increasing plant production.
• No net soil carbon sequestration was observed at elevated Ca; stable soil C was decomposed to meet N demands.

Conclusions:

• The PNL theory was partially supported, as N was reallocated from soil to plants, initially overcoming N limitation.
• Elevated Ca did not result in net soil carbon sequestration due to the decomposition of stable soil C fractions.
• The transfer of N from soil organic matter to plant biomass may limit long-term carbon sequestration if plant-derived C is rapidly lost through decomposition.