The benefits of increasing soil carbon

One potential unintended consequence of the current debate around whether the 4 per Mille initiative is that it acts as a distraction from the development of a practicable approach to involving soils in society’s response to the climate crises, and the central role that organic carbon plays in soil.  Soil plays an important role in the carbon cycle and is one of the most complicated reservoirs of carbon. More carbon is stored in the world’s soils than is currently present in the atmosphere. However, the issue is broader that just how much carbon can be stored in soil, the attendant co-benefits that carbon brings to soil influence the water cycle and nutrient use efficiency.  Much of this is known and accepted, but the fundamental processes supporting soil function are understood less well. This paper describes the diverse benefits that soil organic matter (SOM) provides for soils and agricultural production.

Soil organic matter binds mineral particles and colloids together and is the fundamental causative agent generating structural complexity. Plant and animal residues are processed by microbes before joining the SOM pool, a continuum of progressively more extensively oxidized compounds. Much of this SOM is associated with 30–100 μm diameter pores. As a result, the effect of microbial processes—metabolism, extracellular degradation of compounds, polymer secretion and cell lysis—on soil structure is particularly evident at the scales less than 80 μm responsible for regulating convective and diffusive flow rates, as well as the balance of air and water at any given matric potential. These hierarchical processes exhibit characteristic properties of self-organizing and emergent systems.

We observe soil to be a self-organizing adaptive system, modulated by texture.  In high organic input systems such as pastures and manure-amended arable soils, total porosity and connected porosity are increased providing a greater capacity to store water and soluble nutrients.  This improves soil system resilience during periods of low rainfall or nutritional inputs. In addition, the extensive and highly connected pore network selects for assimilatory, and against dissimilatory, processes by permitting greater flux of O₂ through the system: it thus improves the efficiency of metabolic processes and C_org conversion into biomass while reducing potential losses of nutrients arising from leaching or emission of methane and nitrous oxide to the atmosphere—both potent greenhouse gasses.  Soils which experience low inputs of organic carbon develop less extensive and less connected pore networks through which O₂ and nutrients flow at reduced rates.  Consequently, anaerobic respiration becomes more prevalent and the efficiency of metabolic processes is reduced.  Dissimilatory processes such as denitrification and methanogenesis become more influential, increasing the loss of nitrogenous plant nutrients from the soil as nitrous oxide.

Microbial metabolism of organic carbon acts as the determining factor in adaptive soil systems since it creates the simple molecules which act to glue mineral particles together to create structure.  This indicates that continual inputs and flux, rather than soil organic carbon content, acts as the critical factor in soil systems.