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Graeme Nicol
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Intelligent fertilisers reduce agricultural pollution

Graeme Nicol, CNRS Research director in soil microbial ecology at Ecole Centrale de Lyon

Since the 19th cen­tu­ry we know that cer­tain microor­gan­isms play a key role in the nitro­gen cycle. They are respon­si­ble for car­ry­ing out essen­tial steps in the process, pro­duc­ing dif­fer­ent forms of nitro­gen that can be used by plants. Nitro­gen-based fer­tilis­ers used in agri­cul­ture accel­er­ate this cycle with major envi­ron­men­tal and eco­nom­ic con­se­quences. Thus, an ‘intel­li­gent’, slow and pro­gres­sive dis­tri­b­u­tion of nitro­gen could be used to lim­it many of these neg­a­tive effects.

Fer­tilis­ers put nitro­gen into soil

Plants need nitro­gen to grow, using either ammo­ni­um or nitrate (nitro­gen-rich mol­e­cules) as a source. Ammo­ni­um finds its way nat­u­ral­ly into soil at a rate of 110 mil­lion tonnes per year. This hap­pens through depo­si­tion after light­ning and the death of organ­ic mate­r­i­al, but main­ly via the fix­a­tion of nitro­gen from the atmos­phere such as in the nod­ules of legu­mi­nous plants. Nitrate, how­ev­er, is the result of con­ver­sion from ammo­ni­um by microorganisms. 

Even though plants can take up both, it is the con­ver­sion of ammo­ni­um to nitrate, which has major con­se­quences on agri­cul­tur­al sys­tems. Vast quan­ti­ties of nitro­gen (an addi­tion­al 100 mil­lion tonnes per year) added to soil in the form of fer­tilis­ers accel­er­ates micro­bial activ­i­ty, result­ing in the over­pro­duc­tion of nitrate from ammonium.

This excess nitrate is respon­si­ble for the neg­a­tive envi­ron­men­tal impact because it makes the nitro­gen more mobile, increas­ing its pol­lu­tion poten­tial by allow­ing it to move out of the soil in water or even into the air. Nitrate pol­lu­tion in runoff water, ground­wa­ter and rivers encour­ages algal blooms and con­t­a­m­i­nates drink­ing water. It also leads to sub­stan­tial increas­es in the emis­sion of nitrous oxide (N2O), the third most impor­tant green­house gas, con­cen­tra­tions of which have increased by 20% since pre-indus­tri­al times. N2O is also tipped as the com­pound pri­mar­i­ly respon­si­ble for deple­tion of stratos­pher­ic ozone in the 21st Century. 

Intel­li­gent fer­tilis­ers: a smart solution

The main effect of ‘intel­li­gent’ fer­tilis­ers is to favour a slow, grad­ual release of nitro­gen over weeks into soil. In doing so, the pro­por­tion that is actu­al­ly used by the plants is increased, there­by increas­ing effi­cien­cy of the fer­tilis­er. Hence, the activ­i­ty of soil microor­gan­isms is reduced and there­fore less ammo­ni­um is con­vert­ed to nitrate. The result of such is less nitrate avail­able to leach into water­ways, reduc­ing pollution. 

Anoth­er ben­e­fit of pro­gres­sive-release fer­tilis­ers is to reduce haz­ardous N2O emis­sions. Stud­ies have shown that large-scale, rapid appli­ca­tion of ammo­ni­um fer­tilis­ers favours growth in microor­gan­ism pop­u­la­tions capa­ble of trans­form­ing them at high con­cen­tra­tions. This results in a dra­mat­ic increase in activ­i­ty of cer­tain micro­bial pop­u­la­tions, par­tic­u­lar­ly those which con­tribute the most to nitrous oxide emis­sions. Hence, by using a slow-release fer­tilis­er instead, the over­ac­tiv­i­ty of these microor­gan­isms is pre­vent­ed result­ing in low­er emissions. 

These pro­gres­sive-release fer­tilis­ers respond ade­quate­ly to some of the prob­lems posed by tra­di­tion­al fer­tilis­ers, at least. But this is not the only approach we can take. Anoth­er exam­ple is to use inhibitors to lim­it the growth of bac­te­r­i­al pop­u­la­tions that cause the neg­a­tive con­se­quences of nitro­gen trans­for­ma­tion. The idea is not to remove them, or ster­ilise them, but rather it starves them, to con­trol their activity. 

This allows for both bet­ter con­trol of the nitro­gen bal­ance in soil and ensures that crops will ben­e­fit from most of the added nitro­gen fer­tilis­er instead, mak­ing it pos­si­ble to reduce the quan­ti­ties added to soil. The two approach­es are not exclu­sive, of course. Any­thing that makes it pos­si­ble to reduce nitro­gen-asso­ci­at­ed pol­lu­tion in inten­sive agri­cul­ture is welcome.

Contributors

Graeme Nicol
Graeme Nicol
CNRS Research director in soil microbial ecology at Ecole Centrale de Lyon

Graeme Nicol was previously a university lecturer at the University of Aberdeen in Scotland. He joined Ecole Centrale de Lyon in 2015 as the AXA Research Fund Chair in Ecosystem Engineering and Microbial Ecology. His research interests are focused on understanding the contribution of microorganisms to the soil nitrogen cycle.