Chapitre 4 : Impact de cinq plantes exotiques envahissantes sur les propriétés chimiques du sol :
approche expérimentale
Dans les trois premiers chapitres de cette thèse, on a vu que les plantes exotiques envahissantes étaient capables de modifier les propriétés chimiques du sol. Le choix des sites envahis avait fait l'objet de critères de sélection sévères. En effet, nous n'avons travaillé que dans des sites dont les sols étaient suffisamment homogènes sur le plan de la texture (pas de différence de granulométrie entre placeaux envahis et non envahis), et où la plante envahissante était encore en phase d'expansion. Ces précautions avaient été prises afin d'éviter de confondre les causes et les conséquences de l'installation de l'espèce envahissante. En effet, deux hypothèses peuvent expliquer les différences de composition du sol entre placeaux envahis et non envahis : dans des sites hétérogènes, les espèces envahissantes sélectionnent les plages de sol avec une composition particulière ou les sites sont homogènes au départ et elles modifient les propriétés du sol qu'elles envahissent. Malgré les précautions prises, nos observations restent empiriques et on ne peut écarter de manière irréfutable l'hypothèse de la sélection de plage de sol particulière dans un site hétérogène au départ. Une vérification expérimentale apparaît, donc, comme nécessaire.
L’approche diachronique (comparaison des propriétés du sol avant et plusieurs années après le début de l’invasion) est la plus rigoureuse mais a été très peu utilisée. En effet, la toute grande majorité des études parues jusqu'à présent sur le thème de l'impact des plantes envahissantes sur les propriétés du sol s'appuient sur une approche empirique semblable à celle que nous avons utilisée dans les précédents chapitres (comparaison des propriétés des sols envahis et non envahis à un moment donné). Une approche expérimentale en pot a été utilisée par Ehrenfeld et al. (2001). Ces auteurs ont retrouvé les mêmes modifications du sol en pot que sur le terrain (augmentation du pH et du taux de nitrification dans le sol sous l'arbuste exotique Berberis thunbergii par rapport au sol sous l'espèce indigène Vaccinium pallidum). Blank & Young (2004) ont également observé des modifications de la disponibilité des nutriments dans le sol après trois ans de culture en pot de trois espèces envahissantes (Lepidium latifolium, Bromus tectorum et Centaurea solstitialis).
En octobre 2002, j'ai mis en culture cinq espèces envahissantes (Fallopia japonica, Heracleum mantegazzianum, Solidago gigantea, Senecio inaequidens, Impatiens glandulifera) et trois espèces indigènes appartenant aux mêmes genres (Heracleum sphondylium, Senecio jacobea, Impatiens noli-tangere) dans un sol de composition connue. Après quatre ans, les sols ont été récoltés et analysés afin d’évaluer dans quelle mesure ils ont divergé par rapport aux propriétés initiales.
63
Alien plant species impact on soil: a pot experiment
Introduction
Invasion by exotic plant species is one of the global environmental problems linked to increasing human activity and has large ecological and economic consequences (Mack et al., 2000). The impact of invasive plant species on soil properties has been widely studied (Ehrenfeld et al., 2001, Blank & Young, 2002, Vanderhoeven et al., 2006; Dassonville et al., 2007 and many others). A lot of studies document increased nutrient availability, increased N concentration or increased N mineralization rate under the invasive plant compared to indigenous vegetation (Ehrenfeld, 2003).
However, our results suggest that plant impact on soil properties was more dependant on initial soil conditions than on species identity. Invasion increases the fertility of initially poor soil but decreases fertility of initially rich soil (Dassonville et al., 2007, Dassonville et al., submitted).
Most of the work that has been done so far is based on empiric studies on the field.
Such studies compare soil properties of invaded and adjacent uninvaded vegetation.
Uninvaded vegetation is then considered as the initial state of the ecosystem prior to invasion. However, the existence of preexisting differences between the two situations can not be ruled out despite all precautions that can be taken in the sampling procedure (Dassonville et al., 2006). Diachronic studies (comparison of pre- and post-invasion state of a parameter) are in principle to be preferred. However, such studies are rare because they imply long term studies in sites during invasion.
Such diachronic approach has been used in only few studies (Petillon et al., 2005).
No study with such approach, documenting soil properties modifications caused by plant invasion has been found in the literature.
Another way to address this issue is to work in experimental conditions. The classic work of Wedin & Tilman (1990) is a remarkable example. In experimental plots with homogenous soil, monocultures of five different grass species (though not invasive ones) were established and their impact on N dynamics in soil was examined after 3 years. They found that initially identical soils under different species had diverged up to 10-fold in annual N mineralization rate.
Up to now, very few experimental studies have been done on invasive species to confirm empiric data. Ehrenfeld et al. (2001) used a pot experiment to assess the impact of the invasive Berberis thunbergii and Microstegium vimineum on soil N.
They found increased pH and increased nitrification rate in pots planted with the invasive species. These results were qualitatively the same as those observed in the field (Ehrenfeld et al., 2001). In another pot experiment, Blank and Young (2004) found increased enzyme activity, cations, P and N availability in soil planted with Lepidium latifolium, Bromus tectorum or Centaurea solstitialis. Increased fertility of
soil was paralleled by an increased biomass production, indicating that soil modifications resulted in a positive feedback favoring growth of the invasive plant.
More recently, Zou et al. (2006) used a pot experiment to compare the effect of native and invasive ecotypes of Sapium sebiferum on soil total and mineral N concentrations. They found that the invasive ecotype decreased total N concentration but increased mineral N concentration compared to the native ecotype.
We used a pot experiment to assess the effect of five of the most invasive plants in Western Europe on chemical properties of a soil with known initial characteristics. We compared the impacts of invasive species with those of three indigenous congeneric species and with the modifications in pots with bare soil after four years of cultivation.
Material and methods
In October 2002, 5 species chosen among the most successfully invasive alien plant species in Belgium (Fallopia japonica, Heracleum mantegazzianum, Solidago gigantea, Senecio inaequidens, Impatiens glandulifera) and 3 congeneric native species (Heracleum sphondylium, Senecio jacobea, Impatiens noli-tangere) were cultivated in 5 l pots (diameter: 21 cm, height 16 cm). Both Impatiens species were introduced by seeds, Fallopia and Solidago were introduced by 1 node rhizome fragments and the other species were introduced as seedlings (3-4 leaves stage) collected in the field. I. glandulifera was kept at a density of 4 plants /pot, I. noli- tangere: 6 plants /pot (natural regeneration by seeds followed by thin out of plants in excess). For the other species (perennial species), 3 plants were introduced at the beginning of the experiment and subsequent density increase (by rhizome extension or by seed) was not controlled. Dead plants were replaced (mortality was only observed in two pots of Senecio inaequidens). The pots were left outdoor during all the course of the experiment in a parcel of the experimental garden of Brussels Free University (Jardin Experimental Jean Massart, Belgium). The pots were placed in a completely randomized design and were moved annually. They were punctually watered with rain water in case of long dry period. Regularly, dead aboveground material was put back in the pots to mimic litter fall. One control treatment without plant was added. Each treatment was repeated 6 times. The substrate was an agricultural soil (Ap horizon) from the silty region around Brussels. It has been homogenized by sieving at 4 mm. At t=0, 8 subsamples (500 cm3 each) were collected. Soil subsamples were air dried and sieved (<2 mm). Then, they were analyzed for soil pH, exchangeable Ca, Mg, K, P, Mn, Zn (10 g dry soil: 50 ml CH3COONH4 pH 4.65 extraction followed by ICP-AES determination, (Vista MPX, Varian, USA)), total N and organic C with a CN analyzer (TruSpec analyzer CN Leco, USA). After 4 years of cultivation (October 2006), in each pot, soil cores were
pH ***
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Fal sol
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Magnitude of change
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Imp noli
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Magnitude of change
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-80 -70 -60 -50 -40 -30 -20 -10 0 10
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Figure 1: Species impact on all measured soil parameters expressed as the difference between value after 4 years of culture and value at the beginning of the experiment (t0). Whiskers are standard deviation.
Significant deviation from the value at t0 is marked by the significance level of the post-hoc HSD Tukey test: *
= p<0.05, ** = p<0.01, *** = p<0.001. (Fal = Fallopia japonica, Sol = Solidago gigantea, Her = Heracleum mantegazzianum, Imp = Impatiens glandulifera, Sen = Senecio inaequidens, spho = Heracleum sphondylium, noli = Impatiens noli-tangere, jac = Senecio jacobea, Tem = control treatment without plant cover). Means sharing the same letter are not significantly different (HSD test, p>0.05).
collected with a soil borer (2 cm in diameter, all the depth of the pot: 15 cm). These two cores were mixed up into a single bulk sample for each pot. Soil samples were air-dried until constant weight and sieved (< 2 mm). The same parameters as at t=o were analyzed.
Statistics
Data were analyzed with a one-way ANOVA with treatment (8 species+ control treatment + initial conditions) as fixed factor followed by Tukey HSD post-hoc tests.
Then, a principal component analysis was applied in order to detect eventual common impacts on several parameters. When necessary, data were log transformed to meet normality and homoscedasticity requirements (N content).
All statistics were performed with Statistica 7.1 software (Statsoft, 2007).
Results
The properties of the experimental soil at the beginning of the experiment are shown in table 1.
Table 1: Initial soil chemical properties (CH3COONH4 pH 4.65 extraction). Means and standard deviations (N=8)
pH Zn Mg K Ca Mn P N C
Mean 5,07 7,61 163,7 150,3 1615 34,0 14,7 0,37 3,23 s.d. 0,10 0,58 6,4 14,8 63 2,8 1,0 0,11 0,32 Values are in mg kg-1 except for N and C (in %)
In the one-way ANOVA (table 2), the treatment effect was significant for all variables except Ca. This significant effect was generally due to significant difference, in the same direction, between the value of the parameter at t0 and the value after 4 years of cultivation for all species and for the control treatment (figure 1).
Soil pH significantly increased in all treatments (including control treatment) for approximately a half unit. No species differed significantly in its impact on soil pH from the control treatment. However, while not significant, pH less decreased in Fallopia and Solidago (figure 1).
Ca availability did not vary significantly in any treatment (figure 1).
Extractable Mg, K, P, Mn and Zn always decreased (not significantly so for Mg in S.
inaequidens and Zn in I. glandulifera and the control treatment, figure 1).
pH Zn
Mg K
Ca Mn
P
Corg N
-1,0 -0,5 0,0 0,5 1,0
Factor 1 : 45,8%
-1,0 -0,5 0,0 0,5 1,0
Factor 2 : 19,1%
Figure 2: Factor coordinates of the variables in the PCA based on correlations.
-3 -2 -1 0 1 2 3 4 5
-6 -5 -4 -3 -2 -1 0 1 2 3 4
Factor 1: 45,8 %
Factor 2: 19,1 %
t0 Fal noli Her Tem spho
Sol Sen imp jac
Figure 3: Factor coordinates of cases in the PCA. Invasive species are in black (Fal = Fallopia japonica, Sol = Solidago gigantea, Her = Heracleum mantegazzianum, Imp = Impatiens glandulifera, Sen = Senecio inaequidens) and indigenous species in grey (spho = Heracleum sphondylium, noli = Impatiens noli-tangere, jac = Senecio jacobea). Tem = control treatment without plant cover.
Mn availability decreased from 27 mg.kg-1 in the control treatment. A similar decrease was observed in all other treatments except in pots with Solidago and Fallopia where Mn concentration less decreased than in the control treatment (significant for Fallopia) (figure 1).
Extractible Zn only decreased from 0.23 mg.kg-1 in the control treatment but decreased much more in planted pots (from 0.75 mg.kg-1 in Impatiens glandulifera until 2.3 mg/kg-1 in Heracleum sphondylium and H. mantegazzianum). However, a significant difference between the control treatment and planted pots was only found in H. sphondylium (figure 1).
P availability decreased from 6.5 mg.kg-1 in the control treatment and no planted treatment was significantly different to the control treatment. However, P availability decreased significantly less in pots with Solidago gigantea (5.0 mg.kg-1) than in any other planted pots (figure 1).
Mg and K decreased respectively from 28.7 and 53.6 mg.kg-1 in the control treatment. For both elements, decreased availability was significantly higher in Fallopia (58.2 and 88.2 mg.kg-1 for Mg and K respectively ) and H. sphondylium (54.1 and 91.0 mg.kg-1 for Mg and K respectively) than in the control treatment. In both Senecio species, while not significant, Mg concentration was higher than in any other treatment. Concerning K, the highest concentrations were found in pots with Solidago (figure 1).
C and N showed few significant deviation from t0 (only a significant decrease in C in H. sphondylium) and always in various directions. While not significant, the increased C and N concentration in pots with Fallopia and Solidago is remarkable (figure 1).
Table 2:One-way ANOVA testing treatment effect (8 species + control treatment + initial conditions) on soil chemical properties. F and significance level: * = p<0.05, **
= p<0.01, *** = p<0.001
d.f. pH Zn Mg K Ca Mn P N C
Treatment 9 13,7*** 5,85*** 39,4*** 28,0*** 1,66 74,7*** 50,8*** 2.89** 4.51***
F and significance level; * = p<0.05, ** = p<0.01, *** = p<0.001. d.f. = degree of freedom.
The first two axes of the PCA explained 45.8 and 19.1 % of the total variance respectively. Factor 1 was positively correlated with pH and negatively with extractable Mn, K, P, Zn and Mg. Factor 2 was positively correlated with C and N content and negatively with extractable Ca (figure 2). All t0 points were located on the left part of the factor 1 axis while nearly all the points after 4 years of cultivation were located on the right part. This is in line with the ANOVA results. The position of Solidago points on first axis were intermediate between t0 points and other treatments. Invasive species points were mostly located on the upper part of the factor 2 axis while the indigenous species were mostly located in the lower part of the
67 factor 2 axis suggesting that impact of the five invasive species on C and N is positive while that of the three indigenous species is negative (figure 3).
Discussion
Drift of soil properties in the control treatment
Since the control treatment had no vegetation cover and that no nutrient external inputs occurred (except atmospheric depositions which are considered negligible except for the N case which will be discussed after), the concentration of all nutrients had decreased after four years. These concentration decreases can tentatively be ascribed to nutrient leaching by rain which is not compensated for by atmospheric inputs. The control treatment lost as much as 3% of its initial Zn content, 5% of Ca, 17% of Mg, 36 % of K, 44 % of P and 80 % of Mn. Alternatively, the decreased quantity of extractible elements could be the result of decreased availability (transformation in less accessible forms) rather than a net loss. Indeed, the availability of some elements is controlled among other things by soil pH (Lambers et al., 1998). The strong Mn availability decrease was most probably due to increased soil pH in the control treatment (Sarkar & Wynjones, 1981; Sims, 1986; Lambers et al., 1998).
Similarly, C concentration also decreased during the course of the experiment in the control treatment. C loss occurred because soil respiration and soluble C-compounds leaching were not compensated for by new plant C inputs. The C loss in the control treatment was 20 % of the initial C content. Oxidation and leaching of organic acids may also explain the pH increase. C loss probably caused a decreased cation exchange capacity (CEC). The lower CEC certainly promoted nutrient loss by leaching.
Contrasting with other nutrients and C, N concentration did not decrease. This suggests that N loss were compensated for by N inputs. These are of two kinds:
atmospheric deposition and asymbiotic N2 fixation. It has been calculated that with an annual atmospheric deposition of 40 kg.ha-1.y-1 (Typical value for Western Europe according Goulding et al. (1998) and Fangmeier et al. (1994)) during 4 years, total N concentration in pots should increase by 0.05 %, which is not negligible compared to the initial value of 0.37 %. Asymbiotic N2 fixation is not often higher than 1-2 kg.ha-
1.y-1 (Clark & Woodmansee, 1992), which gives an increase of only 0.001 to 0.002 % in total soil N after 4 years.
Since all other treatments were also submitted to nutrient leaching and soil respiration, species effect on soil must be examined by comparison with the control treatment.
Differences between species in their effects on soil
Species identity can modify nutrient availability in four different ways:
- Nutrient leaching rate can be modified by the differences between species in when and how much nutrients are absorbed for plant growth and in overwintering form.
Annual species are theoretically less able to limit nutrient leaching than perennial species due to the absence of active roots in winter. In California, elevated N leaching has been found under exotic annual grasses compared to perennial indigenous grasses due to differences in phenology and summertime activity (Maron
& Jefferies, 1999).
- A non-negligible proportion of nutrients present in the system can have been stored in biomass, especially in storage belowground organs. This strategy is to be found in perennial species.
- Mineralization rate of organic forms of nutrients (especially N and P and to a lesser extent Ca and Mg) can be different between species. This can be explained by different enzyme (e.g. phosphatase) activity in the rhizosphere of the different species.
- Third, as already mentioned above, decreased extractible nutrient concentration does not necessarily reflect nutrient loss but can also reflect modified availability. For instance, the solubility of P and Mn are known to depend on soil pH which can also be influenced by plant species identity.
Compared to initial soil properties, all treatments varied in the same way. Most of the time, extractible nutrient concentration decreased after the four years of cultivation.
This result was well illustrated by the PCA analysis. Indeed, the separation of t0 points from all other treatments on the first axis reflected the loss of nutrients (leaching) with time. Indeed, like in the control treatment, there were no possible inputs.
Thus, differences in species effects on soil nutrient concentrations depend on their ability to limit nutrient leaching (positive effect), to immobilize nutrients in their living or dead biomass (negative effect) and/or to modify directly (mineralization) or indirectly (effect on soil pH) nutrient availability.
Soil pH increased in all treatments without significant difference between species and the control treatment. However, soil pH was lower, though not significantly so, in pots
69 with Fallopia and Solidago. In a previous field study, soil pH was lower under Fallopia than under the uninvaded vegetation in five of the six sites studied (Dassonville et al., 2007). In the study of Herr et al. (2007), Solidago has also been shown to decrease soil pH.
Mn availability decreased in all pots and this can reasonably be ascribed to increased soil pH, which reduced Mn solubility (Sarkar & Wynjones, 1981; Sims, 1986; Lambers et al., 1998). Indeed, Mn availability decreased less in pots with Fallopia and Solidago, two species for which increased pH was less pronounced than in all other treatments.
Soil exchangeable P in Solidago pots was slightly higher than in the control pots and than in other planted treatments. Thus, P availability in pots with Solidago decreased less than in any other treatment. This can be seen in the PCA analysis. Along the first axis, which is negatively correlated to soil P, Solidago points are less remote from T0 points than other treatments. This means that P losses were compensated for by remobilization of less available forms of P. In Herr et al. (2007), the increased P availability under Solidago gigantea compared to uninvaded vegetation was ascribed to the acidification of the rhizosphere. Lower pH was also found here. However, the lower pH observed in pots with Fallopia did not result in increased P availability under this species. Thus, lower pH does not completely explain higher P availability in pots with Solidago. In Chapuis-Lardy et al. (2006), the higher P availability under Solidago than in the uninvaded vegetation was ascribed to higher phosphatase activity. Thus, the field observation of higher P availability under Solidago (Chapuis-Lardy et al., 2006; Vanderhoeven et al., 2006; Herr et al., 2007) has been reproduced in experimental conditions.
Concerning Mg, Zn and K, some species reduced their concentrations at significantly lower levels than in the control treatment. Zn concentration was lower in all plant treatments than in control treatment and particularly in pots with perennial species (significant only for H. sphondylium). Indeed, pots with both Impatiens species (annuals) had slightly higher Zn concentration than other planted treatments (except Solidago). This is probably due to Zn immobilization in perennial tissues (roots, rhizomes). The same pattern was found for Mg and K concentrations which were significantly lower in pots with Fallopia and H. sphondylium. This could be the result of nutrient accumulation in the rhizome and in the thick taproot respectively.
Alternatively, it could also be the result of nutrient exportation by litterfall in Fallopia.
Indeed, the senescent leaves of Fallopia are very rich in K (See chapter 5). Despite the fact that dead material was put back in the pots, it is likely that some material have been lost, especially in Fallopia for which leaves do not remain attached to the stem after senescence.
The availability of Ca did not change during the course of the experiment. This is not readily explained. This element is not very labile (good adsorption on the cationic exchange capacity of the soil). This could have prevented Ca loss through leaching.
Concerning organic matter, the most striking result is the increased C content in pots with Fallopia and Solidago compared to initial concentration while it decreased in the control treatment and with other species (but less so with the invasive I. glandulifera, H. mantegazzianum and Senecio inaequidens). Similarly, total N concentration increased in pots with the invasive Fallopia, Solidago, H. mantegazzianum and I.
glandulifera while it decreased or was unchanged in other treatments. This result was well illustrated by the PCA analysis. Indeed, on the second axis, which was correlated to C and N concentration, invasive species (especially Fallopia) were mostly located on the upper part of the graph while indigenous species were located lower. This was particularly obvious for Fallopia and Solidago. This suggests that invasive species tend to increase C and N stock in the soil while the indigenous counterparts have the opposite effect. This is probably due to high differences in primary productivity and litter quality. Biomass and nutrient concentrations have not already been analyzed in this experiment. However, in the field, invasive species have been found to have much higher biomass and higher C/N ratio than the uninvaded vegetation (chapter 3). As already said, external N inputs are atmospheric deposition and asymbiotic N2 fixation. The amount of N brought by atmospheric N deposition is the same in all pots and therefore can not explain differences between species. Asymbiotic N2 fixation rate may have been influenced by plant species by modifying the microclimate (Ley & D'Antonio, 1998). However, the major driver of differences in C and N content in soil between species is the quality and quantity of above and below ground litter return. Concerning the quantity of litter, invasive species have been found to have a higher productivity than the resident vegetation they invade (chapter 3). For the quality of the litter, we only have data for Fallopia.
The litter of Fallopia has high C/N and lignin/N ratios (Chapter 5). These parameters often lead to dead organic matter accumulation and to decreased C and N mineralization rate (Vitousek, 2004). Higher C content tend also to increase N immobilization by soil microorganisms (Barett & Burke, 2000) which could lead to relatively stable organic N accumulation.
The major weakness of this study and other pot experiments is that root development is limited in area and depth. Limiting rooting depth could have artificially masked some species effect since nutrient uplift was not possible. Indeed, nutrient uplift has been hypothesized to explain the impact of Fallopia japonica on nutrient availability (Dassonville et al., 2007). The use of large lysimeters like that used by Ulery et al.
(1995) is much more realistic since depth is not limiting.
71
Conclusions and perspectives
Some effects of invasive plant species observed in the field have been confirmed in pots after four years of cultivation (increased P availability under Solidago, acidification of soil under Fallopia and Solidago…). On the other hand, we failed to reproduce our major finding of increased cations availability under the invasive species (chapter 1 to 3) most probably due to the limitation of rooting depth. This suggests that the nutrient uplift mechanism, as proposed in the first chapters, is effectively the mechanisms involved in increased cations availability under invasive species. Recurring differences in species impact between indigenous and invasive species have not been detected for cations and P. On the other hand, invasive species had a higher positive impact on soil C and N concentration compared to indigenous ones. Solidago and Fallopia stand out as the two species with the largest effects. They decrease soil pH and increase C and N concentrations. High productivity and permanent belowground organs may be the key traits explaining their impacts.
This experiment is not completely finished. At the next peak of biomass (summer 2008), above and belowground organs will be collected, weighted and analyzed for C and nutrient concentration. This will allow us to quantify the amount of nutrients stored in the biomass and to confirm higher biomass in pots with invasive species. In addition, the promotion of nutrient loss by decreased CEC should be verified by determining the CEC in pots and in the t0 samples. The utility of experimental work on species effect has been demonstrated but the necessity to work in conditions closer to field reality is obvious. In the future, this experiment should be repeated in experimental field.
