The Affects of Nitrogen Source on the Interspecific Competition of
Two Varieties of Brassica Rapa (Rapeseed)
Introduction
Soil nitrogen (N) availability is a major determinant of plant biomass production in many
ecosystems, natural as well as agricultural. Decades of reaserch have been dedicated to
understanding soil N turnover and plant N uptake but today some of the basic biogeochemical
principals of N cycling are still not understood (Nasholm et al. 2000). There has been a focus on
inorganic forms of N as the only forms used by plants, growing from two parallel and
complimentary theories pertaining to our understanding of plant-soil interactions. The first
thread is the mineral-nutrition theory of plant nutrition, developed in 1842 it became widely
accepted that plants only used inorganic forms of N. This theory went unchallenged for over 150
years. The second thread in developing the current view of N cycling was the recognition that
NH4 is a byproduct of microbial processes. These ideas have been the basis of the N cycle for a
century (Schimel and Bennett, 2004). Recently there has been a paradigm shift in the thoughts
on N uptake by plants and in 2000 a paper by Nasholm et al. showed that not only do plants have
the ability to uptake organic forms of N but it is also a useable resource.
This paradigm shift, from an entirely inorganic N cycling system to one that involves
both organic and inorganic N sources has large implications on the industrial fixation of N and
the use of fertilizers in agriculture (Nasholm et al. 2000). These new paradigm shifts also have
interesting implications for species interactions. Studies have suggested that various plant
species respond differently to various forms of organic and inorganic N and have speculated as
to how plants compete for organic and inorganic N under field conditions (Nasholm et al. 2000).
As a result of the 2000 study by Nasholm et al. we questioned what affects N source has on
inter/intra specific competition. The current study was undertaken to test the effects of three
forms of N (NH4, NO3 and Glycine) on competition between two variations of Brassica rapa
(tall and dwarf rapeseed). Brassica rapa was chosen because it is quick growing, it grows in a
very low productivity environment, it is a good representative of agricultural plants, and there is
a prevailing idea is that inorganic N (specifically NO3), is the only source of importance for
agricultural plants although they have been shown to absorb simple organic N sources in
laboratory studies (Nasholm et al. 2000).
Methods
Sand was obtained from a home supply store and autoclaved it on a dry cycle for thirty
minutes above 200ºF to remove any organisms that could affect the growth of our plants. 0.74L
pots were sterilized using a 5% bleach solution and they were filled with autoclaved sand. Seeds
of two varieties of Brassica rapa (tall and dwarf rapeseed) were obtained from Carolina
Biological Supply. There were three different nitrogen source treatments (Glycine, NO3 and,
NH4) and two mixture types, monoculture and mixed culture pots. The concentrations of each
nitrogen source insured that each treatment received the same moles of nitrogen. The
monocultures received thirty seeds of the respective variety of rapeseed and the mixes received
fifteen of each variety. A control of no N was established to account for background levels of N
and all N-source treatments had higher biomass than these control treatments (p<0.05).
The plants were grown in a greenhouse for twenty-two days and over the course of the
experiment three doses of the appropriate N-source were administered to each pot. The plants
were watered as necessary. At the end of the experiment we counted the surviving plants and
number of flowers. We clipped the above ground biomass and dried the plants for forty-eight
hours at above 80ºC and then we weighed all of the biomass from each pot. We expressed
biomass and flowering on a per-stem basis as plant survival varied between replicates. We then
compiled and analyzed the data using a two-way ANOVA for biomass and flower number. We
then used Fisher’s LSD to test for differences among the groups.
Results
The two-way ANOVA test shows that for tall rapeseed mixture versus mono culture had
no affect on biomass per stem, and mixture by N source also had no effect. N source alone
however does have a significant (p=0.004396) effect on the biomass per stem (as seen in Table
1). The N sources Glycine and NH4 had no significant effects on the biomass of tall rapeseed
individuals. The N source that had the greatest affect on the biomass of tall rapeseed was NO3
(see figure 1). The NO3 fertilized treatment shows approximately a 30% increase in biomass per
stem over Glycine fertilized treatments and nearly a 50% increase over NH4 fertilized treatments.
The two-way ANOVA test shows that inter/intraspecific competition is a significant
factor in the flowering response per stem (p= 0.02644) for tall rapeseed individual stems
generally had between two and three stems for all treatments. The analysis of variance also
shows that N source and mixture by N source had no effects on the flowering response of the
same variation of rapeseed (see table 2). Inter/intraspecific competition had the largest effect on
the flowering of tall rapeseed. There was a significant difference in the flowering response per
stem between mixed cultures and monocultures, the flowering response increases approximately
25% in monocultures compared to mixed cultures (See figure 2).
The two-way ANOVA test shows that for dwarf rapeseed inter/intraspecific competition
was a significant factor affecting the biomass per stem (see table 3). The same analysis shows
that dwarf rapeseed biomass was not significantly affected by N source or mixture by N source.
Inter/intra specific competition had the largest effect on the biomass of dwarf rapeseed, the graph
shows a significant difference in biomass per stem between mixed cultures and monocultures
(See figure 3).
The two-way ANOVA test shows that for dwarf rapeseed flowering was significantly
affected by N-source but only in mixtures (P=0.03017). The flowering response for dwarf
rapeseed was smaller in general than that of tall rapeseed, for all treatments flowers per stem
averaged between zero and one. Although the p value suggests that N source is also significant,
it can not be analyzed as such because of the existence of significant interacting factors. N
source and mixture alone have no effect on flowering of monocultures (see table 4). Dwarf
rapeseed fertilized with NO3 shows a significant increase in the flowers per stem but only in
mixtures. Treatments fertilized with the other N sources (Glycine and NH4) show no significant
changes in flowering response. Monocultures and mixtures showed no changes based on N
source.
Discussion
This study shows that organic N can be used by two variations of Brassica Rapa grown
in the laboratory. Organic or inorganic N source did not seem to affect tall or dwarf rapeseed;
however of the three N source treatments NO3 did significantly increase the biomass of tall
rapeseed compared to the other two treatments. This supports the belief that NO3 is of primary
importance to agricultural plants (Nasholm et al. 2000). I do not suggest however that this can
be used to make conclusions about the affects of organic versus inorganic N sources because
there was no significant difference shown between Glycine an organic source and NH4 an
inorganic source.
Interspecific competition did affect both varieties of rapeseed but in different ways. Tall
rapeseed showed a decreased flowering response when grown in competition to dwarf rapeseed,
and dwarf rapeseed showed a decrease in biomass when grown with tall rapeseed. This could be
a result of the structure of each variety and its resource allocation and life history strategy. The
tall rapeseed had in all treatments between two and three flowers per stem, allowing this variety
to decrease its flowering response without inhibiting reproduction. The dwarf rapeseed however;
generally had no more than one flower per stem. As a result it was biomass that decreased,
because if the flowering response would have been affected reproduction would have been
inhibited. The data suggests that the varieties did have competitive effects on one another,
however, how these effects are related to N source and uptake is unclear.
Dwarf rapeseed showed a significantly increased flowering response when fertilized with
NO3 but only in a mixture treatment, suggesting that dwarf rapeseed may increase its uptake of
NO3 when competing with another species. It is believed that the N source of primary
importance for agricultural plants is NO3 (Nasholm et al. 2000). However our data shows that
dwarf rapeseed was not affected by N source when grown in a mono culture; this suggests that
the importance of NO3 could be more connected to interspecific competition than the species its
self. Further research would include experiments comparing dwarf rapeseed when in
competition with other species, and a measurement of the NO3 uptake as performed in Nasholm
et al. 2000.
While this study does address the research questions, it is also limited in its scope and our
by our abilities. This study was performed in an extremely low productivity environment, in the
absence of any nitrogen fixing bacteria or other environmental factors. The study is very much a
laboratory experiment, and its lack of applicability in the field is problematic. As in previous
studies, we have shown that in an environment that lacks inorganic nitrogen plants can utilize
organic N. However it would be useful to know to what extent plants utilize organic N when
inorganic is also available. A second limitation of this study deals with our inter/intraspecific
competition component. Because of an error with the seed provider, we were forced to look at
two variations of the same species rather than entirely different species. Because we knew little
about the morphology of each Brassica Rapa variety it was difficult to compare the two closely
related varieties. The use of an entirely different species could have greatly altered the results
for the portion of the study dealing with inter/intraspecific competition.
The results of this study support the findings of previous studies. Nitrogen source affects
growth of plants, although it is now known that it is not solely inorganic nitrogen that is the basis
of the nitrogen cycle. There is a preference by agricultural plants for NO3 but they can utilize
organic nitrogen (specifically glycine) if it is available. While the study did not result in any
conclusive arguments for the affects of N source on inter/intra specific competition between
species, it suggests that species may utilize available N differently when in competition with
other species, and leaves room for more research on these affects. The implications of this study
connect in many ways to the large scale agriculture industry and the fertilization of agricultural
fields as well as relating directly to the new paradigm in nitrogen cycling ideas.
References
Nasholm, T., K. Huss-Danell, and P. Hogberg. 2000. Uptake of organic nitrogen in the field by
four agriculturally important plant species. Ecology 81(4): 1155-1161.
Schimel, J.P. and J. Bennet. 2004. Nitrogen mineralization: challenges of a changing paradigm.
Ecology 85:591-602.
NSOURCE$ (3
GLY, NH4,
levels)
MIX$
NO3 (2
MIX,
levels)
MONO
Dep Var: BMASS_ST N: 36 Multiple R: 0.582635 Squared
multiple R: 0.339463
Analysis of Variance
SS
df
MS
FP
TableSourc
NSOURC
0.00001
2
0.00000 6.53884
0.00439
e
ratio
MIX
0.00000
1
0.00000
1.72461
0.19905
E$
8
9
8
6
NSOURCE$*MI
0.00000
2
0.00000
0.30764
0.73747
$
2
2
3
7
Erro
0.00004
30
0.00000
X$
1
0
1
1
r
1
1
Table 1: Two-way analysis of variance for tall rapeseed and Nitrogen source, culture type, and
the combined affects. Only the nitrogen source significantly affected (p<0.05) the biomass
response.
BIOMASS/STEM
0.009
B
0.008
0.007
A
A
0.006
0.005
0.004
GLY
NH4
NO3
N-SOURCE
Figure 1. Effect of N source on biomass of tall rapeseed. Different letters indicate significant
differences (p<0.05). Error bars represent +1standard error of the mean.
NSOURCE$ (3 levels)
GLY, NH4, NO3
MIX$ (2 levels)
MIX, MONO
Dep Var: FLOW_STE N: 36 Multiple R: 0.58141 Squared multiple R: 0.33804
Analysis of Variance Table
Source
SS
NSOURCE$
0.77496
MIX$
0.90567
NSOURCE$*MIX$ 0.86482
Error
4.98458
df
2
1
2
30
MS
0.38748
0.90567
0.43241
0.16615
F-ratio
P
2.33206 0.11445
5.45082 0.02644
2.60249 0.09073
Table 2: Two-way analysis of variance for tall rapeseed and Nitrogen source, culture type, and
the combined affects based on flowering/stem. Only the mixture effects significantly affected
(p<0.05) the flowering response.
3
FLOWERS/STEM
2
B
A
1
0
MIX
MONO
Figure 2. Effect of inter/intraspecific competition on flowering response of tall rapeseed.
Different letters indicate significant differences (p<0.05). Error bars represent +1standard error
of the mean.
NSOURCE$ (3
GLY, NH4,
levels)
MIX$ (2
NO3
MIX,
levels)
MONO
Dep Var: BMASS_ST N: 36 Multiple R: 0.48287 Squared
multiple R: 0.23316
Analysis of Variance
SS
df
MS
FP
TableSourc
NSOURC
0.0000
2
0.0000
1.1271
0.3372
e
ratio
MIX
0.0000
1 0.0000
5.0173
0.0326
E$
2
1
4
9
NSOURCE$*MI
0.0000
2 0.0000
0.9250
0.4075
$
4
4
2
6
Erro
0.0002
30 0.0000
X$
2
1
6
2
r
6
1
Table 3: Two-way analysis of variance for dwarf rapeseed and Nitrogen source, culture type,
and the combined affects based on biomass. Only the mixture effects significantly affected
(p<0.05) the biomass.
BIOMASS/STEM
0.03
0.02
B
0.01
0.00
A
MIX
MONO
Figure 3. Effect of inter/intraspecific competition on biomass of dwarf rapeseed. Different
letters indicate significant differences (p<0.05). Error bars represent +1standard error of the
mean.
MIX$ (2 levels)
MIX, MONO
NSOURCE$ (3 levels)
GLY, NH4, NO3
Dep Var: FLOW_STE N: 36 Multiple R: 0.58454 Squared multiple R: 0.34168
Analysis of Variance Table
Source
SS
MIX$
0.02402
NSOURCE$
0.89882
MIX$*NSOURCE$0.94715
Error
3.60288
df
1
2
2
30
MS
F-ratio
P
0.02402 0.20005 0.65789
0.44941 3.74207 0.03541
0.47358 3.9433 0.03017
0.1201
Table 4: Two-way analysis of variance for dwarf rapeseed and Nitrogen source, culture type,
and the combined affects based on flowering response. Flowering increased when fertilized with
(p<0.05) but only in mixtures. N source had no effect on flowering in monocultures.
FLOWERS/STEM
2
B, a
1
A, a A, a
0
MIX
MIX
MONO
GLY
NH4
NSOURCE
NO3
Figure 4. Effect of N source and inter/intraspecific competition on flower number of dwarf
rapeseed. Different letters indicate significant differences (p<0.05). Upper case compares
differences among N sources; lower case compares differences between monoculture and
mixtures. Error bars represent +1standard error of the mean.