Rampant Fertiliser Use Could Slow Down Nitrogen Cycle, Rajasthan University Scientists Say

The researchers have now set their sights on developing software to calculate optimum fertiliser amounts for crop fields given other cultivation parameters.

Bags of fertilisers stacked next to farmland. Credit: Unsplash/pixabay

Bags of a fertiliser stacked next to farmland. Credit: Unsplash/pixabay

An interdisciplinary, international study reveals a new way fertiliser overuse could affect the soil. The team, led by two researchers from the Central University of Rajasthan, predicts that overusing ammonium in crop fields could throw the soil’s natural nitrogen cycle off balance.

The demand for nitrogen, a key component of all proteins and so an element sought after by plants, is generally met by the nitrogen cycle. This is a natural intricate web of chemical reactions supplying nitrogen in various forms across different environments. Plants, bacteria and fungi work to keep the cycle effective by curating organic nitrogen in the soil system. Although other harmful effects of fertiliser overuse – like groundwater pollution – have been known, the study illustrates the consequences for the soil’s natural nitrogen circulation mechanism.

“Overuse [of fertilisers] could stop key reactions in the nitrogen cycle and make plants depend on an artificial source,” Amit Chakraborty, the corresponding author of the published study, told The Wire. He suggested that rampant fertiliser use was one of the main reasons urban and semi-urban areas struggled to maintain green cover. On the other hand, forest ecosystems do this with relative ease. “Trees that can extract atmospheric nitrogen with the help of soil bacteria thrive in forest landscapes, where nitrogen is already abundant in the soil. However these trees cannot grow in agricultural fields, where nitrogen circulates less freely,” he said.

The team wanted to know how anthropogenic forces affected the nitrogen cycle, so they modelled the cycle’s chemical networks using MATLAB, a programming language. “Through the simulation, we discovered new mechanisms maintaining the N-cycle and how it behaved based on nitrogen and oxygen availability,” Chakraborty explained.

He also revealed that the N-cycle maintained its balance at low fertiliser inputs. However, if the amount was increased by 10-15-times, the cycle stopped receiving ammonium altogether. “There was a new stable state with a new demand for ammonium to maintain the cycle’s flow of chemicals,” he said.

According to him, even if the crop yield in farms increases in the next two or three years, there will also be a parallel increase in the soil’s demand for fertilisers. “We may create a situation where the amounts of fertiliser we currently add may not be sufficient, years later. So if a farmer is using 10 kg in the first two years, this quantity may not be enough to grow the same crop few years later. The farmer may need 15-20 kg next, and soon the only source of nitrogen the farmer can provide the crops would be artificial.”

Moreover, such overuse can also lead nitrate accumulating in the soil. Nitrates are nitrogen-based chemicals in the N-cycle. Chakraborty says they nourish algae – this could lead to algal blooms in water bodies once the substance gets washed into them, and suffocate other aquatic creatures.

N.D. Shrinithi Vihahshini, at the department of environmental management at Bharathidasan University, Madurai, said, “This is an intriguing [study]. The next step forward should be to include other factors like soil temperature, moisture levels, pH changes and cation exchange capacity [which indicates the soil’s natural tendency to hold on to essential nutrients] in the simulation. This will offer a more refined view.”    

Chakraborty also emphasised on the growth of invasive plant species in places where the nitrogen cycle has become dysfunctional. “Exotic European annual grasses are heavily invading the regions of southern California and rapidly displacing the California natives. The region is undergoing transition from shrub land to grassland and this has a strong positive correlation with N-supply and maintenance changes,” he said.

This evolved understanding of the N-cycle has just as the Tamil Nadu government passed multiple instructions to district-level authorities to eliminate the invasive, poisonous and groundwater-depleting plant Prosopsis juliflora. On February 27, the Madurai bench of the Madras High Court directed the state government to bring an act to eradicate P. juliflora, also known as seemai karuvelam, from all 32 districts. The weed is known to make its own nitrogen and growing opportunistically in arid or semi-arid regions. Its deep root systems help outdo other plants while competing for groundwater. “It remains to be seen, however, if there is any link between fertiliser overuse and the spread of this weed,” Chakraborty added.

He thinks we may need a new approach to farming since, according to him, even traditional methods may have also become obsolete. Chakraborty’s stand was that human activities have impacted the soil over time, so there is an urgent need to map and network the factors affecting the soil’s ability to sustain the N-cycle in its current state. And subjecting the soil to simple tests to look for the presence of chemicals and their levels may not be effective anymore. We also need alternative methods to examine how the chemicals interact with each other. “We need to know the long term behaviour of these chemical networks and how they are maintained,” he suggested.

The hope is that, through this study, we can now exploit the N-cycle to enhance soil nitrogen levels. The Rajasthan researchers have now set their sights on developing software to calculate optimum fertiliser amounts for crop fields given the other parameters of cultivation. An “efficient and targeted delivery of fertilisers” is imperative.

In January 2017, a Sri Lankan research team tested a sustained release delivery system for fertilisers using pellets of fertilisers coated with hydroxyapatite (a mineral of calcium) nanoparticles. The team tested its novel product in the rice fields of in the island country’s eastern regions. The sections of the field fertilised by their mechanism yielded 10% more rice than those fertilised by common methods. The same team also found that, in laboratory conditions, the hybrid fertiliser pellet took nearly a week to get washed away by water while normally-applied fertilisers were washed away within five minutes. However, Chakraborty clarified that developing sustainable agriculture systems alone wouldn’t be enough. “An understanding of the deep secrets in natural balance processes of tropical forests is also necessary.”

According to several reports, the food-grain output in developed countries may already have plateaued. If this is true, it is unlikely that enhancing fertiliser inputs could offer any significant gains. On the other hand, though the scope for increasing yields in developing countries still exists, the key is to ensure that this happens by a sustainable use of fertilisers as well.

Arupathi Kalyanam, the general secretary of the Federation of Farmers’ Associations of Delta Districts, said, “Fertiliser usage happens in a very crude manner here. I have seen many farmers simply use one sack of urea per acre of land. There’s no real calculation involved.”

Srinithi concurred: “Some fertiliser products are suitable for crops growing in wetland condition (like paddy and wheat), some are for tropics. For best results, the fertilisers need to be chosen in the right combination, the way doctors prescribe iron tablets along with folic acid for preventing anaemia. Many farmers are unaware of the methods,” she said.

Kalyanam believes the best way to minimise fertiliser usage would be to make a transition from growing commercially important crops to region-specific farming. This, he argues, would not only minimise the use of fertilisers but also that of water and pesticides. “We have 127 agro-climatic zones in India and seven in Tamil Nadu. We should enforce customised farming to these regions to avoid the unnecessary use of resources,” he said. “We all dream of smart cities, but don’t we also need green, self-sufficient villages?”