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.