Agriculture: greenhouse gas in soils is a promising solution

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.

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.

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.

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.

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.

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.