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 ofBrassica 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.
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.
Figure1. 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.
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.
Figure2. 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.
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.
Figure3. 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.
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.
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.