The Dark Site Of Nitrogen Fertilization

Published Date: 02 Nov 2017

Nitrogen(N) is the most important nutrient of a plant and plays a limiting factor in the plant growth and development. Thanks to the green revolution, occurred between 1940 and the late 1970's, the agriculture production around the world increased spectacularly. As a consequence the amount of synthetic nitrogen, applied to crops, has risen dramatically from 12 to 104 Tg⁄ year in the last 40 years. Increased fertilization resulted in a significant increases in yield but with considerable impacts on the environment worldwide.

The dark site of Nitrogen fertilization

The beneficial effect of nitrogen fertilization also come with a environmental cost. A balance should be found to decrease environmental pollution and maintain, or increase, N availability to crops.

The energy that is required to produce commercial N fertilizers, through the energy consuming Haber–Bosch process, is estimated to require approximately 1% of the world's annual energy supply. Much of the N added to the soil is lost into the environment. Only 30-50% is took up by the plant. The remaining 50-70% is lost to: i) Surface run-off, ii) Nitrate leaching, iii)Ammonia (NH3)volatilization,iv)Bacterial competition.

Inefficient use of N has an environmental impact

Excessive N pollution results in hypoxic areas, dead zones. All over the world these zones are more common since the 1e green revolution. The nitrogen , available, in the water makes algae bloom and they will deplete the oxygen out of the water.The oxygen is normally required for most marine life. The production and excessive usage of N fertilizers also plays a large role in stratospheric ozone(O3) depletion and global warming. Nitrous oxide (N2O) is the third most abundant greenhouse gas (GHG). Only carbon dioxide (CO2) and methane (CH4) are more prevalent. When comparing N2O to CO2 it has 310 times more the ability to trap heat in the atmosphere. The production of N2O will enhance global warming more than CO2. Ozone depletion start with NOx catalysis. This chemical reaction starts with NO and O3 resulting in O2 and NO2. The latter reacts with a oxygen atom resulting in NO and O2. The netto result is O+O3 -> 2O2. As you can see ozone depletion occurred.

Bacteria plays a crucial role in many natural processes. In general are microbial nitrification and denitrification natural processes. They convert Nitrate (NO3-) to Nitrite(no2-) to NO.It then gets converted to N2O and the finale conversion results in molecular nitrogen(n2). Addition of N fertilizers to soils significantly increase N20 production, giving it an (big) anthropogenic component. Could we as humans get rid of these problems all by one solution ? Definitely not. But different solution combined are necessary to maintain yield and more important life on the planet.

The environmental impact, along with increasing N fertilizer costs, has created a need for more 'nitrogen use efficient' (NUE) crops. Those crops are better able to uptake, utilize and (re)mobilize the nitrogen available to them. They could indirectly lower the environmental pollution. It has been published that experts are calling for a second Green Revolution which would allow increased productivity using sustainable agricultural methods. Nitrogen use efficiency (NUE) could play a crucial role in this story. Maintaining or even better, increasing the yield with less N could be the solution.

How NUE crops can be achieved ?

Alot of anti GMO people would definitely recommend traditional breeding. Traditional breeding could have been a strategy, used to improve NUE in crops. But they have experienced a plateau in yield. Adding more N doesn't result in a yield improvement anymore. New (genetics) solutions are needed to increase yields while maintaining or preferably decreasing the amount of N. Why are crops so bad at using nitrogen? Could it be that we have been breeding for crops that have high yields when there is plenty of nitrogen available. We may have created/selected inefficient plants? Starting over, and search for plants that can live in areas where nitrogen is limited, could give us more insight in which mechanism are responsible for N-uptake.

The most drastic variation will be obtained using genetics. A summary based on the literature of different approaches will follow below

As a first possible approach to increase NUE, the N- metabolism was scrutinized. The primary N metabolism consist of different stages: Uptake->Assimilation->Mobilization ->Utilization

->Remobilization . In theory each step could been modified to increase NUE. The first important question we should ask our self is where is the rate limiting step ? Is there a bottle neck somewhere ? There is (not yet) a clear answer on this matter. Further research on this topic should bring more clarity.

Everything start with the uptake of nitrogen into the plant.

The most abundant form of nitrogen found is NO3.Nitrate is water soluble which has an increased chance for leaching. Other forms like ammonium, and in smaller amounts : proteins, amino acids (AA),...can also been taken up. Nitrate is taken up from the environment by two main families of transporters: (Nitrate Transporter) NRT1 and NRT2. A logical outcome is to up regulate NRT1/2. Literature has showed that NRT mutants perform less good in N deficient soil. This confirm the role for OverExpressing(OE) NRTx. In yeast scientist have found a YNT1(Yeast Nitrate Transporter). When they did alternate the AA sequence, improved enzymatic activity was observed. The, in-field(!), grown maize had a improved nitrate uptake under low N conditions. As all good inventions, this gene is patented(US 20110047647). The next step is to reduce the nitrate in the cell to nitrite. Arabidopsis plants deficient for NR show increased expression of NRT1.1, and in times of low N, up-regulation of NRT2.2. These result imply that NR may play a regulatory role in NRT expression. And indeed, In Arabidopsis a protein called L31 interacts with the ubiquitin ligase. ATL31 in vivo resulting in degradation of 14-3-3 by ATL31. 14-3-3 seems to enhance NR. Double knockout mutants of two ubiquitin ligase enzymes (Atl31 and Atl6) in Arabidopsis showed an increase in 14-3-3 protein. Over-expression of 14-3-3 in Arabidopsis resulted in hypersensitivity to C and N stress conditions. Plants over-expressing 14-3-3 protein grown under C and N stress conditions experienced growth

arrest, in comparison to those lines with increased ATL31 activity, which grew even under high C and low N stress .This study and the work of others suggest that 14-3-3

has a role in regulating N assimilation

Ectopic increases in NR expression did not increase cereal crop NUE under low-N condition. But, suprisingly, utilizing the NR gene from red algae: Porphyra perforata (ppnr) and Porphyra yezoensis (pynr) result in an increased yield under limiting N conditions .I think we shouldn't focus on to 1 species but look for other candidates in others. Combining the knowledge of different species should make us been able the create NUE plants.