Tracing Soil CO2 Fluxes under Drying-Rewetting Cycles: Isotopic Insights from an Automatic Soil Incubation System

TL;DR

The study uses a new system to track soil CO2 emissions during drying and rewetting cycles, revealing how different conditions affect carbon release.

Contribution

The novel Online Automatic Soil Incubation System (OASIS) enables isotopic tracking of CO2 pulses under controlled drying-rewetting cycles.

Findings

Extreme drying-rewetting cycles caused rapid CO2 release peaks but reduced emissions during dry phases.

Isotopic data showed CO2 pulses originated from recent plant carbon and long-persisting soil organic carbon.

Abstract

Climate change is expected to increase the intensity
and frequency
of droughts and heavy rainfall events globally, with significant consequences
on the terrestrial carbon cycle. One of the most critical yet highly
variable components of the carbon cycle is the soil CO2 pulse triggered by precipitation events in dryland ecosystems. To
examine the processes underlying the soil CO2 pulse under
changing precipitation patterns, we developed a unique Online Automatic
Soil Incubation System (OASIS) that allows (1) accurate manipulation
of drying and rewetting regimes; (2) continuous monitoring of soil
CO2 fluxes; and (3) identification of their isotopic sources
(13C and 14C). Using OASIS, we investigated
how normal and extreme drying-rewetting cycles (NDWC vs EDWC) influence
CO2 pulse emissions and their isotopic signatures from
soils of the Loess Plateau, while controlling for total water input.
Our results showed that EDWC induced a rapid peak in the CO2 release rate within minutes, but this was offset by reduced emissions
during the dry phase compared to NDWC. In addition, total CO2 release was strongly influenced by CO2 influx through
dissolution, which was limited during the prolonged dry phase under
EDWC. Isotopic data indicated that the CO2 pulse that originated
from substrates was derived from recent plant carbon input within
minutes of rewetting and was potentially influenced by exchange with
the inorganic carbon pool, followed by contributions from bulk SOC
that may have persisted for hundreds to thousands of years. These
findings underscore the importance of accounting for CO2 pulses driven by substrate availability and different carbon sources,
which is crucial for improving model predictions of soil CO2 flux and carbon storage in drylands under changing precipitation
patterns. We highlight the applications of OASIS in revealing how
interacting climatic, biological, and physicochemical factors drive
soil greenhouse gas emissions.

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