Can nitrification inhibitors reduce greenhouse gas emissions and nitrogen leaching?
A one-year lysimeter study at the Department of Agroecology at Aarhus University found that the nitrification inhibitor DMPP significantly reduced nitrous oxide emissions from manure applied to maize on a sandy soil.
Nitrogen (N) being an essential plant nutrient, fertilisers, manures and crop residues are often used as sources of N in cropping systems around the world. However, the crops in the field utilize only part of the added nitrogen, and the rest are part of the earth's nitrogen cycle, where it risks being lost to the surrounding environment by emissions of nitrous oxide (N2O) to the atmosphere or leaching of nitrate to the aquatic environment.
“Nitrogen added in manure or crop residues undergoes transformations in soil through the microbially mediated processes of nitrification and denitrification (Ed: see fact box below), yielding nitrous oxide as a by-product. Nitrous oxide is a very powerful greenhouse gas (GHG) with a global warming potential that is 298 times higher than that of carbon dioxide (CO2). In addition to GHG emissions, the use of manures and fertilizers also increase the risk of nitrate leaching into water bodies,” says PhD student Drishya Nair from Department of Agroecology.
Mitigating nitrous oxide emissions
Denmark has an ambitious target of reducing 70% of its total GHG emissions by 2050, and agriculture plays an important role in achieving this goal since it is one of the major contributors to the total GHG emissions of Denmark. The country has considerably reduced agricultural GHG emissions by implementing policies that limit the use of nitrogen containing fertilizers and manures.
“The amount of nitrogenous fertiliser use was greatly reduced in Denmark during the 1990’s, and further reductions may require implementation of new and innovative strategies. A new strategy could be the use of nitrification inhibitors. These are compounds that delay the conversion of ammonia (NH4+) to nitrate. A delayed accumulation of nitrate can reduce nitrous oxide emissions as well as the risk for nitrate leaching. We therefore chose to investigate the potential of a nitrification inhibitor (DMPP) in reducing nitrous oxide emissions and nitrate leaching from silage maize,” says Drishya Nair.
“Maize is a high yielding crop. It is cultivated on dairy farms as an important silage crop for cattle feed. It is mostly cultivated in the Western part of Denmark, where a majority of the dairy farms are located. These regions are characterised by coarse sandy soil and high rainfall, conditions that increase the risk of nitrate leaching. In addition, delayed N uptake by maize increases the likelihood of N to be lost from the system,” says Drishya Nair.
The researchers used free drainage lysimeters (1x1x1.4 meters) to measure nitrous oxide emissions and nitrate leaching from maize grown in two rows within the lysimeters with manure and grass-clover residues as sources of N.
“In Denmark maize is often grown in rotation with grass-clover. When the grass-clover residue is ploughed into the soil it becomes a source of nitrogen for the maize, in addition to manure ,” says Drishya Nair.
Nitrification inhibitor effects tested in many ways
The research team chose to test the effect of the nitrification inhibitor DMPP in several different ways.
“In addition to testing the efficiency of DMPP added with manure, we included treatments where the grass-clover residues were also amended with DMPP,” says Drishya Nair.
The experiment ran for a year during which the researchers measured both the leaching of nitrate and the emission of nitrous oxide under normal rainfall conditions as well as under extra simulated rainfall during spring. Selected leachate samples were also analysed for the presence of DMPP.
Mitigating nitrous oxide emissions and nitrate leaching from maize: efficiency of DMPP
Strong seasonal patterns were observed in N2O emissions, with 49% to 86% of nitrous oxide emissions happening during spring after manure and grass-clover application. On the other hand, nitrate leaching at 1.4 m depth was low during spring and peaked in the late summer.
“We expected maximum leaching from this coarse sandy soil to happen during spring, however the highest leaching was observed during summer. This may be due to nitrogen released from livestock manure and clover grass residues during the spring has been transported to deeper layers in the soil, and the delay in nitrate leaching from 1.4 m depth may be due to the fact that sufficient water did not flow until the late summer where the nitrate is washed out,” says Drishya Nair.
Results of the measurements show that DMPP reduced nitrous oxide emissions by 46 to 67% under normal rainfall and by 44 to 48% under high rainfall conditions. The highest reduction was observed when DMPP was added with manure as well as grass-clover residues. In addition, trends indicated an increase in crop yield and N uptake with DMPP especially in treatments with extra simulated rainfall.
In contrast, DMPP had no effect on the nitrate leached, although trends in three of four treatments indicated this could be the case.
“This was in contrast to our hypothesis. A reduction in nitrate leaching was expected since slowing down of the nitrification process by DMPP would also reduce the amount of leachable nitrogen. The reason could be that, apart from nitrogen mineralised from manure and grass-clover residue, nitrate leaching could have originated from many other nitrogen sources, especially from the reaction of the soil's pool of organic matter, which was not affected by DMPP,” says Drishya Nair.
“We conclude that DMPP and probably also other nitrification inhibitors can be used as a strategy to lower the nitrous oxide emissions from farming. By contrast, DMPP had only a minor effect as a means of reducing nitrate leaching,” says Drishya Nair.
Nitrification is a process whereby bacteria in the soil break down dead, nitrogenous plant material. The bacteria convert or oxygenate ammonia and ammonium from fertilizer, manure or crop residues to nitrite and from nitrite to nitrate which can be leached into water bodies.
Denitrification is the opposite process in which nitrate is removed. This process is also carried out by bacteria in soil and water. Nitrate (NO3-) from, for example, fertilizer or nitrified bacteria, is converted by denitrifying bacteria under oxygen-poor conditions to N2O - nitrous oxide, which is released into the atmosphere.
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Collaboration partners: Department of Agroecology at the University of Aarhus and Department of Plant and Environmental Sciences at the University of Copenhagen.
Conflicts of interest: No conflict of interest
Funding: BASF, Germany. The project was financially supported by BASF, however; the company was neither involved in the planning and execution of the experiment, nor in the interpretation and publication of the results
Read more: You can read the scientific article here: Nitrate leaching and nitrous oxide emissions from maize after grass-clover on a coarse sandy soil: Mitigation potentials of 3,4-dimethylpyrazole phosphate (DMPP). It was written by Drishya Nair, Khagendra R. Baral, Diego Abalos, Bjarne W. Strobel and Søren O. Petersen.
PhD student Drishya Nair, Department of Agroecology at Aarhus University. Email: email@example.com
Professor Søren O. Petersen, Department of Agroecology at Aarhus University. Email: firstname.lastname@example.org