Nutrient cycle: Carbon and nitrogen

Carbon and nitrogen cycles in soils are controlled by heterotrophic microbial communities (yeast, moulds and bacteria), feeding on incoming organic matter, like; plant residues, pruning waste, effluents and organic products, known as trophic resources._[1]

Organic matter is at the centre of many soil ecosystems._[2] Their degradation by soil organisms enables various functions to take place, like the supply function thanks to the recycling of nutrients (mainly nitrogen, phosphorus and sulphur), essential for plant growth and the life of ecosystems.

Organic matter stored in soils plays a role in maintaining soil structure and fertility (support function). The storage of organic matter in soils also helps to regulate greenhouse gas emissions, which can upset climate balance and also to limit nitrate leakage into the environment, which is responsible for water pollution (regulation service).

This is why their management is important in the evolution of cropping systems, in the reduction of environmental impacts linked to nitrogen, and in the potential for carbon sequestration by soils._[3,4]

Regenerative agriculture tends towards cropping systems which do not rely on human control (e.g. reduction and elimination of chemical inputs, crop diversification, reduction or abandonment of ploughing), and based more on organic resources and their recycling (crop residues, green waste, industrial, urban and livestock effluents).

Inventing or designing new agricultural systems that are based on increased plant diversity can provide many advantages (we will explore these advantages in week 2). This is at the heart of regenerative agriculture, an approach that aims to enhance ecological processes in order to reconcile economic and environmental performance within agricultural systems.

The relationship between cycles at the soil-plant system scale

The following diagram shows the processes involved in the coupling of the biogeochemical cycles of carbon (C) and nitrogen (N) during the transformation of soil organic matter.

• Atmospheric carbon and nitrogen initially enter the soil-plant system through the biological processes of photosynthesis from CO₂ released by primary producers (autotrophs) e.g. plants and organisms (such as cyanobacteria) and through nitrogen fixation by cyanobacteria and symbiotic N-fixing bacteria.

  In addition, carbon and nitrogen can enter the cycle through the chemical synthesis of ammonia, the Haber-Bosch process, which enables the production of synthetic mineral fertilisers._[5]

• The entry of organic matter into soils occurs by the replacement (restitution) of plant biomass (crop residues, decaying organs, roots and rhizodeposits) and by the input of livestock manure and organic waste products when they are spread on the soil.
• Heterotrophic soil microorganisms, also called “microbial biomass”, get their energy and the nutrients necessary for their growth from the decomposition of these organic materials. This results in the mineralisation of part of the carbon in the form of carbon dioxide emitted to the atmosphere (oxidative metabolism produces ATP* by consuming oxygen and releasing carbon dioxide).

  The remaining part is absorbed, or assimilated, by the microbial bodies. The partition of carbon between assimilation and mineralisation occurs during microbial recycling, leading ultimately to a large part of the carbon supplied to soil being mineralised.

  Dead microorganisms are in turn consumed by living microorganisms during “microbial recycling”. A small fraction of the organic matter brought to the soil (plant and animal residues, manure etc.) is directly humified or transformed into organic matter that is not accessible to biodegradation. The products of microbial recycling can also be included in aggregates and/or adsorbed on mineral surfaces and contribute to humification*.

  The partitioning of carbon entering the soil or being mineralised therefore varies according to time (short term or long term), which subsequently has different implications depending on the functions considered (e.g. short term: effects on soil biological life, long term: balance of carbon storage).

• The activity of heterotrophs, also known as consumers, has a direct impact on the nutrient cycle, of which the main component is nitrogen. However, the importance of the fluxes of nitrogen and other major elements (phosphorus and sulfur) associated with carbon degradation depends on the relative richness of these elements in the organic matter entering the soil and in the organisms.

Heterotrophic soil microbial communities are therefore at the heart of the coupling between the carbon and other nutrient cycles, especially nitrogen. The intensity of the processes that consume or, conversely, feed the mineral and organic forms, mineralisation, nitrification, microbial assimilation (organisation), depend on the nature of the resource and their activities.

These flows are themselves the “source” of other flows such as volatilisation, leaching and denitrification, which have an impact on water quality (e.g. nitrate content) and the atmosphere (ammonia and nitrous oxide emissions). Today, there are several solutions to make agro-ecological systems more efficient and autonomous. These are mainly based on the complementarity of species in relation to their eco-systemic services.

_{*Heterotrophic soil microorganism: an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter.}

_{*ATP: Adenosine Triphosphate (ATP) is the source of energy for use and storage at the cellular level. Animals store the energy obtained from the breakdown of food as ATP. Likewise, plants capture and store the energy they derive from light during photosynthesis in ATP molecules.}

_{*Humification: is the natural process of changing organic matter into humic substances (humus, humate, humic acid, fulvic acid, and humin) by geo-microbiological mechanisms.}

_{Photo by Алексей Вечерин on Pexels}

Diversification and mixing of species for better resource management

Different plant covers provide different functions to soil-plant interactions that depend on the characteristics above the ground (biomass produced, morphology, composition, etc.), the parts below-ground (root density, rooting depth, root length and diameter) and the kinetics of growth and nutrient accumulation._[6]

The amount and dynamics of nutrient requirements and their ability to intercept resources (light, water, nutrients) are not the same for all species. It has been shown that litter or crop residues have very different characteristics that regulate soil communities and their functions (respiration, carbon storage, mineralisation/organisation fluxes, enzyme production, etc.)._[7]

Research on the development of new cropping systems, enhancing soil-plant interactions and biogeochemical cycle looping, is focused on increased resource recovery and crop diversification._[8]

Some examples of recent research are:

• The principle of crop mixes is based on niche complementarity, for example where two species do not use the same resources (e.g. nitrate and atmospheric nitrogen), and do not have the same functional characteristics during growth (leaves, roots) and in the recycling phase (characteristics of litter and crop residues). This principle has been used to explore the potential of species mixtures (legumes and grasses) in intermediate crops._[9] One crop, the grass, would contribute more to the efficient capture of mineral nitrogen, and the other, the legume, is specifically grown as a “green manure” i.e. to add nitrogen to the soil for the following crops. Thanks to this complementarity, the mixture can fulfil two ecosystem functions.
• Another example is the increased use (or reintroduction) of leguminous crops in cropping systems,_[10] which makes it possible to stop using synthetic mineral fertilisers in certain phases of the rotation.

  Of course, the possibility of doing this on a large scale while ensuring the necessary level of agricultural production is subject to much controversy, but it is interesting to note that the solutions involve changing the length and composition of crop rotations and not just the practices applied to current cropping systems.

• The growing of trees with annual crops, within the framework of agroforestry, also appears to be a promising agroecosystem. Agroforestry systems could lead to increased resilience to climate change._[11]

  They are also described as being able to promote a wide range of ecosystem services, including those associated with below-ground biotic interactions such as C sequestration, maintenance of soil quality/fertility, protection against soil erosion and reduction of nitrogen leaching losses.

We will look at agroforestry in more detail in week 2 of this course._[12]