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.

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.