Knowledge for better food systems

Sequestering soil organic carbon: a nitrogen dilemma

Photo: United Soybean Board, Soil, Flickr, Creative Commons License 2.0

In this short perspective piece, researchers from the Netherlands, USA and the UK critically assess the COP21 4 per 1000 initiative, which seeks to increase global yearly agricultural soil organic carbon sequestration by 4‰ (= 0.4%, or 1.2 billion tonnes). The authors argue that as soil organic matter (SOM) also contains nitrogen (N), with a C-to-N ratio always approaching 12, this will require the sequestration of an extra 100 million tonnes of N per year, and they question the feasibility of achieving this. 

The required extra N fertilisation would counterbalance the desired reduction in greenhouse gas emissions due to the energy-intensive Haber-Bosch process used to produce it, as well as produce associated N2O emissions.

Nitrogen surpluses exist in various agricultural soils across the world. Theoretically these could be incorporated into the increased SOM without the need for additional fertilisation. While this would have the added benefit of reducing N pollution there are two barriers to this: Firstly, these surpluses are highly localised into discrete regions and thus hard to redistribute. Secondly, in order to be incorporated into SOM rather than stay in the soil, the nitrogen would first have to be taken up by crop plants. Only on degradation of plant matter the nitrogen would then become part of the SOM. The rate at which this process would occur is far slower than would be required.

The authors applaud the global effort to increase the carbon content of soils, but call for the environmental science community to recognise the soil nutrient limitations on this endeavour, and to “redefine the 4 per 1000 goals within a spatially explicit action plan”.

Abstract

To slow down rising levels of atmospheric CO2, the “4 per 1000” (4p1000) initiative was launched at the COP21 conference in Paris (http://4p1000.org). This initiative aims at a yearly 4‰ (0.4%) increase in global agricultural soil organic carbon (SOC) stocks. If applied to all (also nonagricultural) soils, such a C sequestration rate could in theory fully compensate increases in atmospheric CO2−C levels of 4300 Tg yr−1. We question the feasibility of the 4p1000 goal, using basic stoichiometric arguments. Soil organic matter (SOM) contains nitrogen (N) as well as C, and it is unclear what will be the origin of this N.

Reference

van Groenigen, J.W., van Kessel, C., Hungate, B.A., Oenema, O., Powlson, D.S. and van Groenigen, K.J. (2017). Sequestering Soil Organic Carbon: A Nitrogen Dilemma. Environmental Science and Technology, 51, 4738−4739.

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Comments

Julian Water21's picture

 

There is no nitrogen dilemma - this is an incorrect conclusion that demonstrates stoichiometric arguments are inadequate for understanding the infinite complexity of agri/ecological systems.

It also demonstrates misunderstandings about the chemical kinetics associated with SOCS processes and how these are most cost effectively achieved - ie certainly not with artificial nitrates from Haber-Bosch process; this is a red herring that has no place in this discussion.  No wonder UK aquifers are awash with dangerous levels of nitrates if academia and presumably governments also work with such misunderstandings of best least cost plant nutrition.

Natural systems recieve ample nitrogen nutrient from the digestive function of ruminants; converting cellulose to nitrogen. The dispersal of this nutrient naturally to pastures, or significantly only after composting to arable land, can best and most cost effectively enable SOCS processes.

There are only a very few arid regions in the world where such agro-ecological practices cannot quickly restore social welfare, soil carbon, rainfall and climate.

1% annual SOCS is being consistently achieved here in Cotswolds with exclusively cattle sourced nutrient; the French “4 per 1000” (4p1000) initiative is reasonable and achievable. Some areas will struggle with 0.4% SOCS per annum but most will comfortably over-achieve this by returning to safely and slowly evolved traditional and indigenous farming practices.

Julian (Water21.org.uk)

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