Agricultural Nitrogen Cycle and Mechanism of N Loss: Need for Nitrogen Use Efficient Plants
Subodh Kumar Sinha
National Research Centre on Plant Biotechnology, New Delhi

During last forty years between 1961 and 2001, the world human population has increased almost doubled, however the grain production, meat production and fertilizer consumption increased by 140, 230, and 600 percent. The respective increase during this period in grain production and fertilizer consumption on per capita basis was 21 and 254 percent respectively. As we know nitrogen (N) is a key limiting nutrient for the growth of any crops, the primary goal of using nitrogenous fertilizer has been to increase the yield output per unit of land area. However, we need to keep another fact in mind that the applied fertilizer nitrogen is only partly taken up and used by the crops and rest is lost to the environment. More than 60% of the soil N is lost through a combination of processes like leaching, surface run-off, denitrification, volatilization, and microbial consumption which further causes considerable impacts on environments throughout the world. The environment concerns together with increasing N fertilizer costs prompting the plant biologists throughout the world to look for more nitrogen use efficient (NUE) crops, that is, crops that are better able to uptake, utilize and remobilize the nitrogen available to them. Nitrogen Use Efficiency (NUE) is a term used to indicate the relative balance between the amount of fertilizer N taken up and used by the crop versus the amount of fertilizer N unused by plants. Nitrogen use efficiency is primarily influenced by two factors. The first one is the health of the photosynthetic factory i.e. the plant itself. The second factor that influences NUE is the combination of the frequency and severity of different nitrogen loss processes within the nitrogen cycle. The following are the major reasons of nitrogen losses in the environment.

Leaching of Nitrate-N
The ultimate fate of all applied nitrogenous fertilizers in agricultural field is their complete conversion into the nitrate-N form. And unfortunately, this form of nitrogen does not have much affinity with soil particles and hence they are loosely bound and not retained for too long in soil. These loose interactions make them a vulnerable candidate for leaching out from the soil profile especially in the condition of excessive rains. The situation gets even worse in lighter-textured soils. Nitrate-containing fertilizers, including Urea Ammonium Nitrate solutions and ammonium nitrate, are susceptible to leaching as soon as they are applied. Leaching, runoff, and erosion account for ∼37 Tg of the annual N losses.

Denitrification of Nitrate-N

The denitrifying bacteria can convert nitrate-N to nitrogen and oxygen gases. Such types of situation frequently found in saturated (anaerobic) soil conditions where these denitrifiers grow well. It has been observed in some the cases that the loss of nitrogen in such types of process i.e. volatilization, may reach up to as much as 5% of the available nitrate-N per day. Heavy soils with significant level of compaction which does not allow natural drainage are at greatest risk to denitrification N loss. Dentrification losses as gaseous dinitrogen (N2) amount to ∼14 Tg yr-1, and N2O and NO from nitrification/dentrification contribute another ∼ 8Tg to the total loss.

Volatilization of Urea-Based Products
The urease enzyme found in the soil and plant residues has ability to convert urea to free ammonia gas. Therefore, the urea-based nitrogen fertilizer products are susceptible to volatilization losses of nitrogen. This process has been found to be more enhance in cases when urea is surface-applied and not incorporated. As much as 15 ~ 20% loss through this process of volatilization within a week after application has been observed in urea-based nitrogen especially in warm sunny days. Ammonia volatilization from soil and vegetation contributes ∼ 21 Tg yr-1.

Nitrogen Immobilization
There are certain microbes found in soil that decompose high carbon-content plant residues to organic matter which use soil N during the decomposition process. Consequently, the nitrogen from the surface-applied fertilizer is used in the resulting organic matter and is temporarily unavailable for plant uptake until mineralization of the organic matter occurs at a later date.

Further, leaching and surface run-off of inorganic nitrogen in freshwater can cause algal blooms that in turn result in eutrophication of aquatic ecosystems. On the other hand denitrification of previously fixed N carried out by soil microbes results into the emission of nitrous oxide ( N2O, the greenhouse gas ∼300 times more potent than CO2) which has increased 5-7% per decade since 1979. In addition the energy required to produce much of the N in commercial fertilizers, through the Haber-Bosch process, is estimated to require approximately 1% of the worlds' annual energy supply, adding to food production costs. Apart from energy cost and environmental issues there has been considerable amount of reports which suggests incidence of stomach cancer in humans, particularly infants, and of non-Hodgkin's lymphoma due to intake of nitrate contaminated water. Furthermore, our strategy to boost yields using more and more nitrogen fertilizers has plateaued as there is a limit for every crop to metabolize nitrogen and that limit we probably has crossed. Naturally, there is growing interest in reducing fertilizer-N inputs by improving plant N-use efficiency. There have been several approaches adopted to develop nitrogen use efficient crop plants. Some of them are transgenic approach using candidate gene of nitrogen metabolism. As nitrogen use efficiency is widely believed to be a complex trait including various subcomponent in it, regulatory factors involved in this metabolic pathway has always been considered as one of the favorable target gene for manipulation. One of the key regulatory factors is small RNAs, specifically the micro RNAs, which play significant role in gene regulation in almost all aspects of plant life. Breeding for new varieties after extensive screening of large number of germplasms and then utilizing those parents which shows genetic variability for different component traits of NUE could be the alternative approach that should be able to take up more organic or inorganic N from the soil N and utilize the absorbed N more efficiently. Additional breeding approach may include more efficient symbioses with Rhizobia and arbuscular micorrhizal (AM) fungi for increasing plant productivity. However, there are several agronomic approaches are in practice to improve fertilizer use efficiency of crops at field level to minimize N loss.

References:
Smil, V. 1999. Nitrogen in crop production: An account of global flows. Global Biogeochemical Cycles 13:647-662.

Hirel, B., T. Tétu, Peter J. Lea, F. Dubois. 2011. Improving Nitrogen Use Efficiency in Crops for Sustainable Agriculture. Sustainability 3:1452-1485.

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