Carbon dynamics in agricultural soils: Insights into methane fluxes under changing hydrological conditions

Soils serve as critical carbon sinks, playing a vital role in mitigating global warming and ensuring global food security. However, rapidly changing climatic and environmental conditions, such as extreme weather events, threaten the soil’s capacity to act as carbon sinks. Land use changes, particularly those driven by agricultural intensification, can alter hydrological regimes and carbon dynamics in landscapes. Understanding these dynamics by measuring GHG fluxes and investigating soil properties is crucial for designing sustainable land management practices that promote both environmental stability and climate resilience, ensuring soils continue to play a critical role in combating climate change.

Currently, little is known of how soil carbon responds to extreme events such as floods and droughts and how their repeated impacts shape carbon storage and loss and ultimately affect the carbon balance.

This study examined the impact of altered hydrological conditions, driven by the conversion of native vegetation to cropland, on carbon dynamics and carbon loss pathways. The aim is to identify patterns of methane and carbon dioxide emissions from naturally and recently inundated soils and their key driving factors.

We conducted in situ gas measurements using mobile trace gas analysers and a mobile smart chamber in the heavily agricultural Argentinian Pampas and Espinal ecoregions. Additionally, we collected soil samples from 0-30 cm depth for chemical analysis (including total and dissolved organic carbon) and measured soil temperature, electrical conductivity, soil moisture, and pH.

The results show complex interactions and dependencies between methane emissions and environmental variables. Methane fluxes are more than 5 times higher in saturated areas (median = 49.98 μg/m²) compared to dry areas (median =-8.34 μg/m²), primarily influenced by water table depth and soil moisture. Contrary to expectations, soil salinity, measured as electrical conductivity, exhibited a positive effect on methane production, reaching a threshold around 55 mS/m, suggesting possible tolerance or adaptation mechanisms of methanogens. Carbon dioxide emissions showed a reduction of almost 50 % in drier areas, primarily driven by soil moisture, highlighting the strong impact of moisture on carbon dynamics.

The findings highlight the critical role of hydrological conditions, particularly flooding, in driving methane fluxes from soils, emphasizing the need for targeted management practices to mitigate carbon loss and adapt to changing climatic conditions. Additionally, the effect of soil salinity on methane production underscores the importance of considering salinity in future research and management strategies.