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Agriculture: can we lower emissions whilst feeding the world?

Agriculture: greenhouse gas in soils is a promising solution

Anaïs Marechal, science journalist
On February 23rd, 2022 |
3 mins reading time
3
Agriculture: greenhouse gas in soils is a promising solution
Claire Chenu 2
Claire Chenu
Professor at AgroParisTech and Member of the scientific and technical committee of the 4 for 1000 initiative
Key takeaways
  • Increasing the carbon stock in soils reduces atmospheric carbon in the form of CO2, an abundant GHG. Thanks to organic matter, soils are one of the planet's main carbon reservoirs.
  • Several agricultural practices help to increase carbon input into soil: maintaining a vegetation cover between crops, extending the life of temporary grasslands, grassing between the rows of vines and fruit trees, etc.
  • Carbon losses are linked to soil erosion and, above all, mineralisation, a process during which carbon reverts to its gaseous CO2 form.
  • Modelling on a European scale estimates that the increase in carbon stocks could offset 5 to 12% of agricultural CO2 emissions.
  • In addition, these agricultural practices have other benefits. Reducing ploughing has little effect on GHG emissions, but the practice is very beneficial to biodiversity and soil health.

By the end of 2022, the Euro­pean Union wants to adopt a cer­ti­fi­ca­tion frame­work for “car­bon farm­ing”. This con­cept cov­ers agri­cul­tur­al prac­tices that man­age car­bon stocks and flows, and green­house gas­es (GHGs) on the scale of farms with the aim of mit­i­gat­ing cli­mate change.

It is a sub­ject has been on the table since COP21, when the inter­na­tion­al “4 by 1,000” ini­tia­tive1 was launched. Its aim? To pre­serve soil car­bon stores and increase them, as soon as pos­si­ble, to con­tribute to food secu­ri­ty, adap­ta­tion, and mit­i­ga­tion of cli­mate change. Increas­ing soil car­bon stocks reduces the amount of car­bon in the form of CO2 in the atmos­phere, a well-known green­house gas. Thanks to organ­ic mat­ter, soil is one of the main car­bon reser­voirs on the planet.

How can farmers contribute to soil carbon storage?

The amount of organ­ic car­bon in soil is the result of the bal­ance between how much car­bon is put into the soil and how much is tak­en out. Sev­er­al agri­cul­tur­al prac­tices result in a net increase in car­bon enter­ing soil whilst increas­ing crop yields ten­fold: these include keep­ing a plant cov­er between crops, extend­ing the lifes­pans of tem­po­rary grass­lands, grass­ing between rows of vines and fruit trees, plant­i­ng hedges and agro­forestry. More organ­ic mat­ter can also be added in the form of compost.

A study by INRAE in 2019 showed that these mea­sures are effec­tive and tech­ni­cal­ly fea­si­ble in France. Today, dif­fer­ent lever­ages could help devel­op the use of these prac­tices: train­ing and sup­port for farm­ers – par­tic­u­lar­ly with regards to the ben­e­fits – and finan­cial incen­tives, for exam­ple through the Com­mon Agri­cul­tur­al Pol­i­cy, to com­pen­sate for their addi­tion­al cost.

Around the world, the prin­ci­ples are the same, but not all prac­tices are rel­e­vant. For exam­ple, inter­me­di­ate crops can be very water-inten­sive in some regions. Soil con­ser­va­tion agri­cul­ture is very often iden­ti­fied as a lever for improv­ing soils. In France, we do not have enough stud­ies to eval­u­ate its effects.

Could the adoption of these practices result in other spin-offs?

Var­i­ous col­lat­er­al effects are receiv­ing atten­tion from the sci­en­tif­ic com­mu­ni­ty. For exam­ple, soil cov­er changes the albe­do – the reflec­tiv­i­ty of the soil – and influ­ences glob­al sur­face tem­per­a­ture. Per­ma­nent cul­ti­va­tion of light-coloured soils can con­tribute to high­er tem­per­a­tures, thus coun­ter­act­ing the pos­i­tive effects of car­bon stor­age. These albe­do effects were under­es­ti­mat­ed until recently.

Anoth­er exam­ple is the man­age­ment of per­ma­nent grass­lands. Their mod­er­ate inten­si­fi­ca­tion through fer­til­i­sa­tion allows more car­bon to be stored in the soil, but it also gen­er­ates more nitrous oxide emis­sions, anoth­er GHG. Hence, a full account of GHG bal­ance needs to be done.

Final­ly, it should be not­ed that there are con­flicts of use around plant bio­mass. Its direct return to the soil in the form of crop residues is an impor­tant source of car­bon. But what is the best form to do this: plant residues, manure, com­post, or by-prod­ucts from methanis­ers? Depend­ing on the mate­r­i­al returned, the per­sis­tence of its car­bon in the soil is not the same. We lack car­bon and nitro­gen bal­ances for plant bio­mass recov­ery sys­tems. More­over, methani­sa­tion is a source of income for farm­ers: thus, a sec­to­r­i­al approach to this issue is needed.

Soil carbon stock depends on inputs, but also on losses: how important are changes in land use?

Car­bon loss­es are linked to soil ero­sion and, above all, min­er­al­i­sa­tion, a process dur­ing which car­bon reverts to its gaseous form of CO2. Car­bon stocks decrease when loss­es are greater than inputs. This is the case when there is a change in land use, when a for­est or per­ma­nent grass­land is con­vert­ed into a crop. The loss of forests and per­ma­nent grass­lands is the most impor­tant fac­tor in the decrease of soil car­bon stocks on a glob­al scale. In France, forests are gain­ing ground, but the con­ver­sion of per­ma­nent grass­land is con­tin­u­ing and con­tribut­ing to the loss of carbon.

How are soil carbon stocks evolving today?

Var­i­ous projects have recent­ly estab­lished ini­tial assess­ments at the French2, Euro­pean3 and glob­al4 lev­els. Their evo­lu­tion over time is not known on a large scale, but local long-term tri­als pro­vide esti­mates. In France, the evo­lu­tion of car­bon stocks in agri­cul­tur­al and for­est soils is cur­rent­ly between ‑0.2 and +3.2 per thou­sand per year5, with great spa­tial het­ero­gene­ity. Some regions show loss­es, oth­ers enrichment.

Cli­mate change is also hav­ing an impact on stocks. Increas­ing tem­per­a­ture great­ly increas­es the rate of min­er­al­i­sa­tion and thus car­bon loss­es in soils.

What are the benefits for the climate of all these agricultural practices?

The INRAE study shows us that the imple­men­ta­tion of stock­ing prac­tices would allow an addi­tion­al stor­age of about 30 mil­lion tonnes of CO2 equiv­a­lent per year, main­ly in field crops where cur­rent stocks are low. This rep­re­sents 41% of agri­cul­tur­al car­bon emis­sions and 7% of total nation­al emis­sions. Euro­pean-wide mod­el­ling6 esti­mates that the increase in car­bon stocks could off­set 5–12% of agri­cul­tur­al CO2 emis­sions. There is no equiv­a­lent esti­mate on a glob­al scale. Imple­ment­ing agri­cul­tur­al prac­tices that allow addi­tion­al car­bon stor­age in soils would there­fore con­tribute to the mit­i­ga­tion of green­house gas emissions.

But a glob­al assess­ment of agri­cul­tur­al prac­tices is still nec­es­sary. For exam­ple, reduc­ing plough­ing has lit­tle effect on soil car­bon stocks, but this prac­tice is very ben­e­fi­cial to soil bio­di­ver­si­ty and its abil­i­ty to resist ero­sion. How­ev­er, cli­mate change mit­i­ga­tion should not be the main pur­pose of these agri­cul­tur­al prac­tices. The pri­ma­ry goal is obvi­ous­ly sus­tain­able agri­cul­tur­al pro­duc­tion, in which soils con­tribute to mul­ti­ple ecosys­tem ser­vices and biodiversity.

1https://​www​.4p1000​.org/fr
2Voir www​.gis​sol​.fr
3Voir pro­jet LUCAS Soil : https://​ec​.europa​.eu/​e​u​r​o​s​t​a​t​/​w​e​b​/​l​u​c​a​s​/​d​a​t​a​/​d​a​t​abase
4Voir pro­jet Glob­al Soil Organ­ic Car­bon map : https://www.fao.org/global-soil-partnership/pillars-action/4‑information-and-data-new/global-soil-organic-carbon-gsoc-map/en/
5Stock­er du car­bone dans les sols français, quel poten­tiel au regard de l’objectif 4 pour 1000 et à quel coût ? INRA, juil­let 2019
6Luga­to, E., Bam­pa, F., Pana­gos, P., Mon­tanarel­la, L., Jones, A., 2014. Poten­tial car­bon seques­tra­tion of Euro­pean arable soils esti­mat­ed by mod­el­ling a com­pre­hen­sive set of man­age­ment prac­tices. Glob­al Change Biol­o­gy 20 (11), 3557–3567