Intelligent fertilisers reduce agricultural pollution

Since the 19^th 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.

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 21^st Century. 

Nitrates pol­lute run-off water and rivers, con­t­a­m­i­nate under­ground drink­ing water resources and pro­mote the growth of algae.

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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.