Review Article

Agri Res & Tech: Open Access J

Volume 8 Issue 2 - July 2017

Copyright © All rights are reserved by Maria Jose Ibarrola Rivas

DOI: 10.19080/ARTOAJ.2017.08.555635

Estimating Future Global Needs for Nitrogen
Based on Regional Changes of Food Demand
MJ Ibarrola Rivas* and S Nonhebel
Center for Energy and Environmental Sciences, University of Groningen, Netherlands

Submission: June 12, 2017; Published: July 17, 2017

Corresponding author: Maria Jose Ibarrola Rivas, Center for Energy and Environmental Sciences, University of Groningen, Nijenborgh 4, 9747 AG
Groningen, Netherlands, Tel:
; Fax: +31 503637168; Email:*

Abstract
The use of synthetic nitrogen fertilizer is of great importance for global food supply, but it is also causing large environmental problems.
The amount of fertilizer needed on a global scale is affected by the size of the population (more people more food), the type of their diets(luxurious diets require more resources) and the agricultural production system (intensive systems use more fertilizer than extensive systems).
In the coming decades, global population will increase, diets will change and a strong intensification of agriculture is expected. In this paper, we
assess how the changes in all these factors will affect future needs of fertilizers on both a global and a national scale. We developed a model that
describes the various relations among population density, diets and production systems, and validated the outcomes with historical country level
data. The variations over the globe are large. The model shows that in the regions with low population densities not much change in nitrogen
applications is to be expected. On the other hand, in densely populated regions, with high population growth rates and expected changes in diets,
the needs for synthetic nitrogen show an exponential increase. By using existing projections with respect to population growth and dietarychanges, by 2050 over 70% of the global population will live in countries where an exponential increase of the nitrogen application is needed tofeed the people. This may result on large impact for the global environment.
Keywords: Synthetic nitrogen fertilizer; Dietary changes; Population density; Historical trends; Global analysis

Introduction
The use of synthetic nitrogen fertilizer has had enormous
global environmental impacts [1]. First, it is highly energy intensive.
Globally, half of the energy use in agriculture is related to the
production and application of synthetic nitrogen fertilizer and it isa major greenhouse gas emitter [2]. Secondly, its use has strongly
affected the global nitrogen cycle [3-5] by doubling the emissionsof reactive nitrogen into the biosphere [6] which has affected theglobal climate [4]. Third, large application of nitrogen has local
effects on the environment such as acidification, eutrophication andloss of biodiversity [3,6,7]. However, the food for the present global
population cannot be produced without it, and nowadays half of theworld food consumption is based on its use [8,9].
The world population will reach nine billion people by 2050
[10], and food consumption per capita will increase as well [11].
Synthetic nitrogen fertilizer will play an important role in theincrease of global food supply as well as in the environmental
impacts related to the increase of food production [1,12-14]
estimates that if the future global consumption of nitrogen fertilizer
follows a linear extension of the past 50 years’ trend, it will lead
Agri Res & Tech: Open Access J 8(2): ARTOAJ.MS.ID.555635 (2017)

to serious global environmental consequences. So, achieving global
food security while reducing environmental problems is one of thebiggest challenges humanity will face in the next decades [15-17].

The use of nitrogen for food production has been widely
studied. We have identified three lines of research addressing
different drivers of its use.
First, population density drives the amount of nitrogen applied
for a certain population. Smil [8] and Arizpe et al. [18] show thatfor several countries the increase of population density, due topopulation growth, has led to an increase in the nitrogen application
per unit of area.

Second, food consumption patterns drive nitrogenrequirements. Different types of diets require different amountsof nitrogen [19,20]. Food products of animal origin require more
nitrogen than those of vegetal origin because of the inefficiency
of producing animal proteins from vegetal proteins [21,22]. Also,
differences exist among animal products [23], for instance theproduction of one kilogram of beef requires more nitrogen than the
production of one kilogram of chicken [22].
0039Agricultural Research & Technology: Open Access Journal
Third, the type of agricultural production system drives theuse of nitrogen fertilizer depending on the application rate level(kilograms of Nitrogen per hectare). Large regional variability exists
in its use and in the corresponding local environmental impacts
because of different application rates [24,25]. Some regions, likeindustrial countries and Eastern Asia, have an excess of reactive
nitrogen in their environment due to high application rates [26-29].
Other regions, like most African countries, have a lack of nitrogenin their soil because of the low application rates which limits theiragricultural production and affects their food supply.

So, changes in population, diets and agricultural systems affectthe need for nitrogen fertilizer. The expected changes differ largely
over the globe. In Western Europe, population growth has come toa standstill, their average diets already contain large amounts ofanimal products and agriculture is very intensive. So, no changes
in nitrogen needs are expected in this region. In other parts of
the world, population growth rates are still high (Africa) or largechanges in meat consumption are taken place (China) and nearly
everywhere agricultural production systems are intensifying. Basedon these expected changes, we can expect an increase in global
needs for nitrogen fertilizers. In this paper, we assessed the futureneeds of nitrogen fertilizer on global scale based on the existingregional variations of population, diets and agricultural systems,
and the expected changes. First, we studied the historical changes inthe drivers for different global regions from 1960-2010 (section 2).
Then, we developed a model to integrate the impact of the various
drivers of the nitrogen demand (section 3). The results of the model
were validated with country level data (section 4), and finally weused the model to assess the global nitrogen demands in 2050
(section 5).

Historical Analysis of Regional Changes in Population,
Diets and Nitrogen Use

In this section, we obtain insights on the trends of the main
drivers for the global demand of nitrogen fertilizer. In order to do
this, we look back in time. The main drivers are population density,
the type of diet and the type of agricultural technology. These
drivers differ throughout the world; therefore, we split the global
population into six different regions with different characteristics of
population dynamics, dietary patterns and agricultural technology.
For these regions, we studied the changes on these parameters overthe last 5 decades.

Methods and data

We obtained country level data of the main drivers for 19602010 from existing databases: population [11], food supply [30]arable land [31] and nitrogen fertilizer use. Population density wascalculated by dividing the size of the population by the area of thearable land because we are interested in the availability of arable
land per person which is, in general, where the nitrogen fertilizer
is being used. The amount of animal products consumption in thediet of a person is the main factor in the requirement of nitrogen to
produce the food of that person [21,32]. Therefore, we determined
0040

the total caloric intake of animal products per person per day.
Finally, we used the application of nitrogen fertilizer as the indicatorof the type of agricultural technology. This factor was calculatedby dividing the total national consumption of synthetic nitrogen
fertilizer by the amount of arable land of each country.

The countries were clustered in seven regions: A: Africa (with950 million people in 2010), B: India (1210 million people),
C: Southeast Asia (950 million people), D: China (1370 million
people), E: Latin America (570 million people), F: North America(340 million people) and G: Western Europe (400 million people).
These regions accounted for 85% of world population in 2010.

Results

In nearly all regions, population density, consumption of animal
products and nitrogen application has been increasing over the last
decades (Figure 1a). However, the absolute values differ per region.
In 2010, Asia was the region with the largest population density,
followed by Western Europe, Africa, Latin America and North
America, see black bars of Figure 1b which indicate the values in2010. So, Asia was the region with the smallest amount of arable
land per person and North America with the largest. The population
density in China was twice that of Western Europe, three times
larger than that in both Latin America and in Africa, and as much as
ten times larger than that in North America. During the last decades,
global population density increased because global population
doubled [10] while arable land only increased 10% [31]. Population
density has changed differently among the regions due to differentchanges in both population numbers and arable land. Africa had
the largest population growth and strongly increased its population
density. But India, with a smaller population growth, ended up witha larger growth in population density because it did not increase
their arable land as much as Africa. Western Europe increased its
population density not because of a large population growth but
because its arable land decreased.

The consumption of animal products differs amongst regions.
Western Europe and North America consumed the largest amountof animal products per capita, followed by Latin America, China,
Southeast Asia, India and Africa; see the values of 2010 of Figure
1c. During the last fifty years, the global consumption of animal
products strongly increased, but the increase was not the same inall regions. North America, Western Europe and Africa were moreor less stable, but Africa consumed much less than the former two.
North America and Western Europe were large consumers and theyshow a levelling off in contrast with Latin America and parts of Asia
which had the largest increase in consumption of animal products
and they did not show a levelling off. Some of these regions stillconsume half the amount of animal products compared to that ofWestern Europe and North America.
Nowadays, the region with the highest application of nitrogen
per hectare is Asia, and Africa has the lowest (Figure 1c). During
the last five decades, mainly Asia but also Latin America strongly
increased the nitrogen application rate. Africa did not increase

How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.

Agricultural Research & Technology: Open Access Journal
substantially and by 2010 its average application rate was still
very small compared to the rest of the regions. North America andWestern Europe showed a leveling off from 1960 to 2010. North
America increased its application rate until 1980, and then, it

remained stable. Western Europe had by far the largest application
rate in 1960. It increased until 1980, and then, it began to decrease(Figure 1).

Figure 1: Drivers for the use of nitrogen fertilizer for the seven regions. The regions are: A: Africa, B: India, C: Southeast Asia, D: China,
E: Latin America, F: North America and G: Western Europe.

Discussion
When we compare our regional results with relations mentioned
in literature some striking differences can be seen. Figure 1 showsthat large consumption of animal products is not always relatedwith large nitrogen application as suggested by Smil [21,22] andLeach et al. [19]. The regions with the largest consumption ofanimal products did not have the largest nitrogen application rate.
North America and Western Europe had the largest consumptionof animal products but Asia had the largest application rate. In
addition, Smil [8] and Arizpe et al. [18] show that countries with ahigh population density have high nitrogen application rates. Figure
1 shows that some regions follow this pattern like Asia (B, C and
D). These three regions had relatively high population density andhigh nitrogen application rate. But Africa (A) and North America (F)do not follow this pattern. Africa has a larger population density
than North America, but a smaller application rate. In addition,in Western Europe, population density increases over time whilein the last decades a decline in fertilizer use is measured. So, therelations mentioned in the literature are not always found onregional scales. Therefore, in the next sections, we look in detail into
the relationships among population density, dietary patterns andagricultural technology, the main trends of the last decades, and thedifferences throughout the world.

Integration of the drivers: model development

In section 2, we have shown that the general relationships
between two drivers described in the introduction are not always
valid for all regions (e.g. the highest nitrogen application is usedin the region with the highest population density). Due to theclose interrelationship of the three drivers (population, diets andagricultural technology), we believe that the solution to understand0041

these deviations is by studying the three drivers at the same time.
Therefore, we integrated the three drivers into a model. The model
aims to understand patterns rather than calculate exact data. The
three parameters are population density (by means of the inverse:
ha of arable land cap-1), consumption of animal products (kcal cap-1
day1), and nitrogen application rate (kg N ha-1). Population density
and consumption of animal products are independent variables,
and nitrogen application rate is the dependent variable of the othertwo parameters. The model illustrates their interrelationships on acrop field scale.

Model description

We took the food consumption as starting point and wedetermined the amount of nitrogen needed to produce a certain
amount of food. Under natural conditions (no fertilizer use)
the natural nutrients available in the soil for uptake by the crop
determine the amount of food that can be produced. If we want toproduce more food two strategies exist: either more land shouldbe put into production (land expansion) or the crop yield should
be increased (the amount of food produced in a certain area:
intensification). In this paper, we focused on the increase of foodproduction by increasing crop yields, as intensification of the
production is expected to play the major role in the global increaseof food production in the coming decades Alexandratos & Bruinsma[11]. Nutrients have to be added to the soil in order to increase crop
yields. The number of people we can feed from a certain amount of
nutrients depends on their diets, and the amount of animal products
plays an important role. We distinguished three hypothetical diets
based on the Nutrition Transition Theory Chesnais [33] which
describes the main trends and global differences in diets: a vegetarian
diet Dv requiring N amount of nitrogen for its production, a more
affluent diet Da, with a low amount of animal products, requiring

How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.

Agricultural Research & Technology: Open Access Journal
Results

50% more nitrogen than Dv, and a diet rich in animal products Da
ext requiring twice the amount of nitrogen than Dv.

Figure 2 shows the results of the model. The application of
external nitrogen increases exponentially with the decrease ofavailability of land per capita due to population growth. So, theincrease of population density increases the application of nitrogen
fertilizer, as shown by Arizpe et al. [18]. Also, diets rich in animal
products require a larger nitrogen application rate. Our aim withthis
model is only to show the relationships between the parameters,
therefore we do not assign units of numerical values to the model,so (Table 1 & Figure 2) do not have units.

Our starting point is that in area A we can produce the food for
one person with a vegetarian diet Dv using the natural nitrogen in
the soil which is N. If the population doubles, we need 2N to produce
the food for both people. Since the natural nitrogen in area A is N,
then we need to apply another amount of nitrogen N. Column Dvof Table 1 shows the amount of external nitrogen that we need toapply if the population grows from one to ten people. The following
columns show the amount of nitrogen application required for diets
Da and Da ext.

Table 1: Application of external nitrogen for different population numbers and diets on a crop field scale.
NaturalNitrogenin the Crop
Field [N]Population

Number ofpeople1

Availabilityof landExternal
nitrogenrequired[A cap-1]

[N]

1

2

0.5

…

…

3

10

0.3

Discussion

Diet Dv (Vegetarian)

0.1

1
1

1

…
1

Applicationof Nitrogen[N A-1]

1
2

3

…

10

Diet Da (Low Animal
Products)
External
nitrogenrequired[N]

0

Applicationof Nitrogen[N A-1]

1.5

0.5

2

4.5

3.5

9

15

14

1

3

…

…

2

…

Diet Da Ext (Large Animal
Products)
External
nitrogenrequired
[N]
2

4
6

…

20

Applicationof Nitrogen[N A-1]
1
3

5

…

19

Figure 2: Model integrating the drivers of external nitrogen fertilizer use.

Figure 2 shows two things. First, the increase in consumptionof animal products increases the application of external nitrogenmuch more in densely populated areas than in lightly populatedareas. Second, the reduction of availability of arable land per capita
increases exponentially the application rate of external nitrogen in
densely populated areas. Thus, the nitrogen application rate tends
to be much higher in densely populated areas than in low populatedareas for a similar consumption of animal products per capita.
This explains the deviations discussed in section 2 (Figure 1). For
example, China and Latin America have a similar consumption

0042

of animal products in 2010 but China has a much larger nitrogen
application due to a much higher population density. The samedeviations are shown for Africa and India. This model explains these
deviations, so we use it to analyse the global trends in section 4.

It is important to point out that in the model we determined thenitrogen needed by the crop to produce the food. This deviates fromwhat a farmer should apply to obtain a certain yield. In agricultural
practice, only a part of the fertilizer applied is taken up by the crop,
the remainder is lost to the environment. In our analysis, we exclude
nitrogen losses.

How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.

Agricultural Research & Technology: Open Access Journal
Validation model outcomes with country data
In section 3, a model without values was developed showing therelations between the drivers and the nitrogen needed for food. In
this section, we validate this model with country level data in order
to use it to assess the future global use of nitrogen fertilizer (section
5).We used country level data from several databases of population
numbers [10], arable land, food supply of animal products andnitrogen fertilizer use. With these data, we calculated population
density, animal products consumption per capita and fertilizer
application for each country. With this, we made the same graphs as
the one in Figure 2, but now with values: Figure 3. Since drivers changeover time (Figure 1) we did this for the years 1960, 1980 and 2010.
In the model, three different consumption patterns are recognized.
In this section, we distinguished four food patterns depending on
the amount of animal products in the diet: 0-300, 300-600, 600-900
and 900-1200kcal of animal products per day. We referred to “poordiets” if the animal product’s consumption is between 0-300kcal

cap-1 day-1, to “transition diets” is the consumption is 300-600 or
600-900 kcal cap-1 day-1, and to “luxurious diets” if it is higher than
900kcal cap -1 day -1. We were able to do this for 110 countries. We
also determined the global averages on the consumption of animal
products, population density and fertilizer’s application.

Results

The global availability of land decreased from 0.4ha cap-1 in
1960 to 0.3ha cap-1 in 1980 and to 0.2ha cap-1 in 2010. At the sametime, the global nitrogen application rate and the consumption ofanimal products increased; the former from 10kg N ha-1 in 1960 to
44kg N ha-1 in 1980 and to 75kg N ha-1 in 2010; and the latter from350kcal cap-1 day-1 in 1960 to 390kcal cap-1day-1 and to 500kcal
cap-1day-1 in 2010. This shows a linear increase in the nitrogen
application rate. However, large variations exist between the global
average and the country level data. Some countries have higher orlower consumption of animal products, nitrogen application andpopulation density than the global average.

Figure 3: Country level data validating the model. The colors indicate the consumption of animal products per capita. The black diamondindicates the global average for each year. The average caloric intake of animal product for each year is indicated in the legend.

In Figure 3, the trend lines for the three diets in 1960 showa polynomial relation between availability of land and nitrogen
application similar to the trend lines of our model, and the type of
diet parallels this relation. So, the data of 1960 of Figure 3 fit with
our model: countries with high amount of animal products in the
0043

diets used more nitrogen per ha than countries with both small
amounts of animal products in their diets and a population densityof less than 0.2ha/capita. Also, for the vegetarian diets fertilizationis needed (green dots).The results for 1980 do not fit our model
results. Especially the countries with high meat consumption values

How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.

Agricultural Research & Technology: Open Access Journal(red and brown dots) use far more nitrogen than the expectations
based on the model. The data for 2010, however, fit with the model
again.

Discussion

The nitrogen application rates in 1960 were generally lower
for a certain population density than in 1980 and in 2010. Oneof the reasons is that during this period global population morethan doubled: from three billion to seven billion people [10] andglobal arable land only increased 10%. In contrast, global use of
nitrogen fertilizer increased eight times (Table S2 in the Electronic
Supplementary Material), and crop yields strongly increased whichlead to a doubling of the global food supply. The three billion people
in 1960 were already using the natural nitrogen available in thesoil, so for the new four billion people extra nitrogen in the formof synthetic nitrogen fertilizer had to be applied in the crop fields
in order to produce their food by increasing the crop yields. This
enormous population growth increased the population density of
all countries (reducing the availability of arable land per capita).
This pattern can be seen in Figure 3 where from 1960 to 2010 all
countries moved to the left side of the graphs and also upwards
showing that the increase of population density increased thenitrogen application rates, in accordance with our model. The
population density increased the most for the countries with thelowest consumption of animal products (green dots in Figure 3).
These countries are mainly in Africa.

During the 1960-2010 period studied, countries increased
their animal products consumption and changed colour from green
towards brown. In 1960, large differences in diets existed among
countries showing large inequalities. More than two thirds of the
global population had diets with only up to 300kcal of animal
products (green group), one fifth had luxurious diets (up to 1200kcal
animal products, brown group), and few countries had transition
diets (orange group 330-600kcal). In 1980, many countries of thegreen group had moved towards the orange group and a new group
emerged: the red group. By 2010, the inequality of diets decreased
because of the large change towards luxurious diets in the transition
countries. More than half of the world population had transition
diets (red and orange diets), only one third had poor diets (greengroup) and one tenth had luxurious diets (brown group).
As mentioned earlier, in 1980 the red and brown countries
do not fit in our model results. The nitrogen application rates ofthese countries were much higher for their values of availability ofland, and so the data points exceed the trend lines of our model.
This indicates that these countries were using more nitrogen perhectare than what we expected based on our model, which resultsin overuse of nitrogen fertilizer. In 2010, the country level data fitswith our model again. The countries that did not fit with our modelin 1980 reduced the nitrogen application rates by 2010. (See thevalues of the Netherlands, Greece, UK, Denmark and Bulgaria in the
Table S1 in the Electronic Supplementary Material). In some of thesecountries, strict environmental regulations with respect to nitrogen
0044

applications were put in practice to reduce the environmental
problems related to fertilizer use in agriculture (eutrofication) [3436]. This phenomenon is also shown in Figure 1c where Western
Europe reduced their application of nitrogen after the 1990’s.
Our results showed that during the last fifty years the increaseof both population density and consumption of animal products
has increased the nitrogen application rates. These numbers areaverage country level data which should not be considered as strictvalues, but they help us to identify the different drivers to the use ofsynthetic nitrogen fertilizer and their interrelationships.

The combination of the population density and the diets makes
it possible to explain the unexpected results in Figure 1. The factthat North America has high meat consumption levels but low
fertilizer application rates can be explained from the fact that the
population density is low. The fact that population density in Africa
is much higher than in North America, while nitrogen application
rates in Africa are much lower can be explained from the very lowmeat consumption levels in Africa.

Discussion of our approach and the consequences of ourfindings for the future needs for nitrogen

In this paper, we studied the use of nitrogen fertilizer from ademand perspective. We analysed the impact of the global differencesin diets, population and agricultural systems on the use of nitrogen
fertilizer. To do this, we linked data of food patterns and population
densities with data of production systems. We used the averagevalues of 110 countries as examples in order to have an overview ofthe global variations. By doing this, we have not calculated the exact
use of nitrogen in each country. If we wanted to do this, we would
have needed to consider the food imports and exports of eachcountry; and to combine the data of one food pattern with the dataof two or more production systems (the production system of thecountry itself and the country or countries where the food imports
were produced). So, by including the food imports it is not possible
to illustrate the relations of one diet with one production system,
which is the aim of this paper. Therefore, including food imports
and exports is outside the boundaries of our research. Thus, with
our analysis it is possible to understand the relations of the global
differences in diets, population and production systems to assess
the future use of nitrogen.
An important finding of our analysis is that when population
densities are below the 5 persons per ha arable land (p/ha) (0.2
ha of arable land per person) the relation population density andnitrogen demand is more or less linear, and it becomes exponential
with higher values of population density. The present global
population density is 5p/ha, and at global level we observed alinear increase of the nitrogen applications over the last decades.
By 2050, the global population is expected to reach 9 billion people.
If no major increase in arable land area is expected, 9 billion people
will imply a population density of 7p/ha. (0.15ha/cap). This value
is already in the exponential part of the curves in Figure 3. And so,

How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.

Agricultural Research & Technology: Open Access Journal
the need for nitrogen can no longer be derived from the historicallinear trend.

Next to this, the increase of the global population will not beevenly distributed over the globe. If we divide the projected 2050
population numbers for each country by the present arable land ofeach country, then by 2050 six billion people (or 70% of the global
population) will live in countries with less than 0.1ha/cap arable
land.

To make things even worse: the countries with the expected
population increase are also the countries with the present low
consumption of animal products in their diets. It is rather likelythat meat consumption will increase in these regions which will
increase the need for nitrogen even more. This double effect can berecognized in Figure 1 for China. In this country, both population
density and meat consumption increased and the increase innitrogen application rate was huge.
The existing estimates with respect to future needs for nitrogen
fertilizer involve the doubling of the present use [37] which will
result in major impacts for the global environment. Using ourmodel with global average values, we could reproduce the estimatesof Tilman et al. [37]. However, our model also shows that a large
share of the global population will deviate from this global average
to much higher nitrogen application due to both higher population
density and higher consumption of animal products. Due to the nonlinearity in this part of the graph (Figure 3), the future estimates ofnitrogen fertilizer use could be larger with higher environmental
impacts in the regions where 70% of the global population will live
[38-41].

Acknowledgment

This project was partly funded with the CONACYT scholarship“Posgrados en el extranjero” No 209358. Also, we thank Ton SchootUiterkamp for his insightful comments.

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How to cite this article: M Ibarrola R, S Nonhebel. Estimating Future Global Needs for Nitrogen Based on Regional Changes of Food Demand. Agri Res& Tech: Open Access J. 2017; 8(2): 555635. DOI: 10.19080/ARTOAJ.2017.08.555635.