Declining Ecosystem Respiration Linked to Nitrogen Deposition: Insights From a 26-Year FLUXNET Record.

Area of Science:

• Ecology
• Biogeochemistry
• Forest Science

Background:

• Long-term carbon flux data from the FLUXNET site Loobos (Netherlands) show a surprising decrease in total ecosystem respiration (TER).
• This decline, observed between 1997 and 2021, is not explained by temperature fluctuations or measurement methods.
• Increased soil organic matter stocks paradoxically correlate with reduced ecosystem respiration.

Purpose of the Study:

• To investigate the biogeochemical mechanisms behind the declining total ecosystem respiration (TER) at the Loobos pine forest site.
• To assess the impact of soil conditions, particularly acidity and nitrogen deposition, on microbial decomposition rates.
• To evaluate the long-term implications for forest ecosystem health and carbon sequestration capacity.

Main Methods:

• Analysis of long-term carbon flux measurements (1997-2021) from the FLUXNET Loobos site.
• Soil incubation experiments to assess microbial activity and substrate utilization.
• Glucose addition experiments to test microbial response under acidic conditions.
• Evaluation of soil pH and nitrogen deposition data.

Main Results:

• Total ecosystem respiration (TER) declined by tens of percent over the study period.
• Soil incubation revealed microbial activity is limited by poor substrate quality and highly acidic conditions (pH 2.9).
• These acidic conditions are linked to substantial nitrogen deposition, suppressing microbial function despite available organic matter.

Conclusions:

• Declining ecosystem respiration in the Loobos pine forest is driven by microbial limitations due to soil acidification, not substrate availability.
• High nitrogen deposition contributes to soil acidity, potentially compromising forest carbon sinks and ecosystem health.
• Measuring soil pH is crucial for understanding and modeling TER, especially in nitrogen-impacted regions, and Loobos may indicate wider ecosystem vulnerabilities.