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THE IMPORTANCE OF SOIL ORGANIC CARBON (SOC):

(Emma Askew, 2019)


THE ROLE OF SOIL


Since the 20th century, the topic of soils has gained significant importance in global policy. It is recognised that soils are a vital component of earth, critical for the function of many services, including food productivity, climate regulation, maintaining biodiversity, water and energy.

Importantly, global soils store three times the amount of carbon than in vegetation and twice the amount than the atmosphere. With this, soil organic carbon (SOC) is a particularly vital component to research and monitor.  However, with increasing pressures on soils from land-use change (LUC) and climate change, global SOC stocks are at high risk from long-term decline. This in turn could lead to subsequent implications of enhancing global warming, degrading ecosystems and declining food production.


SOIL CARBON CYCLE


Carbon is fixed by terrestrial photosynthesis and enters the soil through the rhizodeposition of root biomass and exudation, and the decomposition of organic matter. Soil organic matter (SOM) is the organic fraction of soil mainly derived from plant litter, and SOC makes up 55-60% of its total mass. Due to the difficultly in directly measuring SOM, research predominately quantifies the SOC component.

After carbon enters the soils, the topsoil horizon acts as the ‘active’ carbon pool, where carbon is formed and cycled with a short turnover time of weeks to years. In contrast, the subsoil (>1m) acts as a ‘passive’ carbon pool, where carbon is more stable and can be stored for centuries to millennia.

Carbon is then released from soils is through microbial mineralization and as respiratory products from roots and soil biota, as carbon dioxide (CO2). Carbon is also lost as Methane (CH4) under anaerobic conditions, via soil erosion and can be leached as dissolved organic carbon (DOC).


SOIL CARBON CYCLE


Carbon is fixed by terrestrial photosynthesis and enters the soil through the rhizodeposition of root biomass and exudation, and the decomposition of organic matter. Soil organic matter (SOM) is the organic fraction of soil mainly derived from plant litter, and SOC makes up 55-60% of its total mass. Due to the difficultly in directly measuring SOM, research predominately quantifies the SOC component.

After carbon enters the soils, the topsoil horizon acts as the ‘active’ carbon pool, where carbon is formed and cycled with a short turnover time of weeks to years. In contrast, the subsoil (>1m) acts as a ‘passive’ carbon pool, where carbon is more stable and can be stored for centuries to millennia.

Carbon is then released from soils is through microbial mineralization and as respiratory products from roots and soil biota, as carbon dioxide (CO2). Carbon is also lost as Methane (CH4) under anaerobic conditions, via soil erosion and can be leached as dissolved organic carbon (DOC).


SOILS AND CLIMATE CHANGE?


SOC sequestration is the process of storing carbon as a stable form in the soil through the direct or indirect fixation of atmosphere CO2, in which sequestration is primarily increased through the enhancement of carbon inputs into the soil, such as through increases in Net Primary Production (NPP). Indeed, research mainly addresses the main benefit of SOC sequestration as a cost-effective strategy to mitigate against climate change, such as through the enhancement of sustainable LUCs and land management practices. Critically, it has been suggested that SOC sequestration could be overestimated as a climate change solution with many limitations. Yet, research largely agrees that SOC sequestration is an essential development strategy that is vital for efficient ecosystem function, and should be one of the many initiatives taken to mitigate against climate change.


OVERVIEW:

It is crucial to make the protection and restoration of SOC a global priority, in which there has been large emphasis on SOC sequestration having considerable potential as an important solution to prevent negative long-term and large-scale impacts, including ecosystem devastation and climate change exacerbation, over the next century. In fact, it is possible for 80-100PgC to be sequestered in soils globally, equating to a reduction in atmospheric CO2 by 40-50ppm over 25-50 years. To optimise this opportunity, the understanding behind the controls and responses of SOC to different pressures requires urgent development as it currently lacks research efforts.



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