«（落葉樹の根圏動態に対する高CO2とO3及び高窒素負荷の影響に関する研究） Wang Xiaona 王 晓娜 Division of ...»
As I mentioned before, N was vital for keeping needle P content. Since P in leaves is an essential macro-element for ATP and NADPH, which are related to light reactions in photosynthesis (e.g., Reich et al. 2009), therefore, photolysis activity was inhibited without N loading (P50N0) which leading to a reduced aboveground biomass and fewer allocation to belowground. On the other hand, the high P condition may enhance root length of F1 without a proportional increase in root biomass as reported by Steingrobe (2001), as another possibility, the root biomass was reduced. There was no effect on biomass of JL, perhaps because a benefited flexible shift of ECM community structure under different nutrient regimes.
4.5 SUMMARY I have studied the distribution of different ECM types colonizing three larch species in high N condition with and without P loading. Species-specific diversity was found in the relationship between ECM and the host larches planted in volcanic ash soil. High N loading increased ECM colonization rate for JL and F1, not for DL. Contrary, ECM diversity was affected by nutrient treatment for DL neither JL nor F1. In general, the
JL was more sensitive to N and P treatment, with marked shift in ECM community structure to different nutrient regimes. More importantly, F1 kept efficient P uptake in high N condition than its parents due to the stable ECM diversity, particularly the contribution of Suillus grevillei. Or because of the different plasticity of ECM short root, further efforts are required to detect this prediction. The present results proved P was not a limited element for F1. It was tolerant to N deposition with stable ECM community in early age stage, promising for plantations in P efficiency region.
However, this is a short term treatment with larch samplings, further studies are required to detect response of long-term or with specific ECM species under several combined changing environments.
Figure 4.2 ECM colonization rate and species diversity of three larch seedlings under different N and P treatment.
Each value is the average of six replications, and the error bar denotes the SE. ANOVA: *** P 0.001, ** P 0.01, ns: not significant.
1 1 1
-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3
3 3 3
2 2 2
1 1 1
0 0 0
-3 -2 -1 0 1 2 3
-3 -2 -1 0 1 2 3
-3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3
Figure 4.4 Community structure of ECM species with the three Larix species as factors among the different nutrient regimes.
Results of distance-based redundancy analysis, with symbols indicating the results of ANOVA: *, P ≤ 0.05; ns, not significant.
1.2 0.8 0.4
Figure 4.5 Concentration of P in needles of three larch species under the different nutrient treatments.
Each value is the mean of six replications, and the error bar shows SE. ANOVA: *, P 0.05; ns, not significant. D, J and H are the abbreviations for three larch species.
Each photo is the different types of ECM identified during two-year treatment with N and P loadings. by microscope and molecular skill comprehensively. (black bar=1mm) Note: A Suillus laricinus, B Suillus grevillei, C Russula sp., D Inocybe sp., E Hebeloma sp., F Thelephora sp., G Tomentella sp.
5. General discussion Root is hidden half of the plant (Eshel and Beeckman, 2013) which suggests us to know more about plant function from viewpoints of shoot-root communication (e.g.
Koike et al. 2003). Many trials for root research have been developed and examined their own characteristics in many instruments (e.g. Satomura et al. 2007). Among them, I employed the mini-Rhizotron method modified by Nakaji et al. (2008). During monitoring experiments for birth and death of the fine roots of white birch grown under elevated CO2 in FACE, I also recognized real essential role of symbiotic micro-organisms in the rhizosphere, especially ectomycorrhizae (ECM). I further studied the different types of root tips colonized with several ECM of larches grown under different environmental conditions, such as elevated CO2, O3 and combined loading with N and P (as shown in Fig. 1.3 and Fig. 1.4).
Fortunately, number of ECM species colonized with larch have revealed to be limited, even though number of species of ECM increased with increasing of age (or size) of the host larch (Yamakawa 2012). Based on DNA analysis of the colonized ECM on larch seedlings and saplings grown under several environmental conditions, I examined essential role of ECM symbiosis with larch species. Here, I discussed first birth and death of the fine roots, and considered changes in nutrient condition of the rhizosphere caused by dead fine roots as resources (such as carbon [C] and nitrogen [N]). Second, I discussed abundance and composition of the ECM colonized with larch species, and consider about critical roles of the ECM on larch species.
5.1 Root dynamic under elevated CO2 Fine roots were operationally classified as ≤ 2 mm in diameter based on the definition proposed by Pregitzer et al. (2002) in Aspen FACE, and an assessment of roots order and function in longleaf pine (Pinus palustris) forests by Guo et al. (2004). Their
rapid birth and death significantly influence the C and N cycling in a forest ecosystem.
About 33% of global net primary production (NPP) is used in fine roots production and their functions (Jackson et al. 1997).
Previous studies have shown that NPP allocated to belowground is often greater than aboveground parts, and annual C and nutrients inputs to soil from fine roots frequently equal or exceed those from foliage (Norby and Jackson 2000). Because fine roots have a much shorter lifespan than coarse roots, as a consequence, their biomass varies both seasonality and environmental conditions. Therefore, effect of CO2 enrichment on roots survivorship is important because of the implications to potential increases in carbon inputs to the soil from dead roots.
Elevated CO2 usually increase production and turnover of fine roots (Wang et al.
2015). However, this is not consistent with the results of former studies (e.g. Pregizter et al. 2002; Norby and Zak 2011; Pregizter and Talhelm 2013). In this study, therefore, I investigated the fine roots dynamics under elevated CO2 for three years. Length of live fine root was suppressed by elevated CO2 in VA soil. It indicated that a lower soil nutrient level restricted the roots growth or prior supported for aboveground growth, which was found in evergreen (Larrea tridentata), drought-deciduous (Ambrosia dumosa), and winter-deciduous shrubs (Krameria erecta) (Housman et al. 2005).
Elevated CO2 increased length of live fine root in BF soil the first year, which was also found in previous study that root growth or biomass was increased under elevated CO2 (Lukac et al. 2003). However, a higher root production does not necessarily result in a high root turnover.
To the contrary, FRP and its turnover rate were decreased by elevated CO2 (Table 2.2), due to the positive correlation of ALRM and ALRP, I concluded a longer root lifespan occurred under elevated CO2. This indication was proved by results of the root longevity (Table 2.3). One possibility is that the nutrient limitation resulted in a lower turnover of FRP and FRM, because the root longevity might be inversely related to the duration of resource supply (Fitter and Hay 1999; Pregitzer et al. 1993;
Pregizter and Talhelm 2013). Another reason is that plants are preferentially to enhance the growth of aboveground as I discussed above, which may readily occur in nutrient limited condition, because root lifespan would be increased if construction costs would be higher than maintenance costs, or if the nutrient availability is low (Eissenstat et al. 2000).
Roots (including coarse roots) of different diameter class response to the treatments were different (see Table 2.4). This difference may be caused by the mycorrhizal symbiosis. It has been widely demonstrated that plants usually increases mycorrhizal colonization and/or decreases root N concentration under elevated CO2 (Pritchard and Rogers 2000; Tingey et al. 2000). Bidartondo et al. (2001) also found ECM colonizing the roots (D = 0.3~0.6mm) of Bishop pine (Pinus muricata) prolonged the root life. In my case, the shorter longevity of fine root was likely derived from the decreased ECM colonization during the third year.
Overall, fine root longevity can be influenced by several abiotic and biotic factors, such as climate, seasonality, soil conditions, diameter, root age and importantly the mycorrhizal colonization (see Eissenstat et al. 2000; Ruess et al.
2003). Therefore, to understand the fine root dynamics, I should deeply understand the response of mycorrhiza with various environmental factors.
5.2 ECM symbiosis under changing environment The majority of belowground root systems in boreal forests are symbiotically colonized by ectomycorrhizal fungi (ECMF) (Taylor et al. 2000). Particularly, larch (Larix sp.) is a typical ectomycorrhizal (ECM) species (Smith and Read, 1997; Qu et al. 2004, 2010). Hybrid larch F1 was developed as a promising afforestation species in northeastern part of Asia (Kuromaru 2008; Koike 2008; Ryu et al. 2009). Its character has been reported by Ryu et al. (2009) and the ECM symbiosis capacity was estimated by Qu et al. (2004; 2009). However, studies of the relationship between F1 and ECM
symbiosis are still limited, especially with the effect of changing environment, such as elevated CO2/O3 and N deposition on it (Watanabe et al. 2013: Mao et al. 2014).
In this study with elevated CO2 and O3 treatments, a significant increase for total number of species of ECM colonization in F1 under elevated CO2was observed.
However, the diversity did not follow the ECM colonization rate with F1 under elevated CO2. This result demonstrated that the ECM composition for F1 did not change with an increased total colonization rate. Additionally, the vital support of host photosynthates for ECM survival was reduced under O3 due to limited carbon allocation to belowground (e.g., Grantz and Farrar 2000), and this resulted in a decrease of ECM colonization rate. Importantly, lower ECM diversity caused by O3 indicated a shift in the ECM abundance. Species of Peziza spp. in this study could not have a symbiotic relationship with F1 under high O3, but Suillus grevillei took great proportion than other species. This proved there may be specific symbiotic relationship of ECM and F1 under O3 stress.
On the other hand, for hybrid larch F1, increased P content in needles, and K, Mg in roots were also proved that specific ECM was infected under this condition. Under external stress, the internal allocation of nutrients is liable to be altered by various ECM species (Weigt et al. 2011). According to the unaffected photosynthesis of F1 under elevated O3, the species of Suillus grevillei contributed significantly.
Furthermore, elevated CO2 diminished the harmful effects of O3 by stimulating higher ECM colonization. Then, how about other environmental factors?
Nitrogen (N) deposition is increasing sharply in northeastern Asia which has become a severe environmental issue (e.g. Galloway et al. 2008). Even N is often one of the most limiting nutrients in many terrestrial ecosystems (e.g. Schulze et al. 2005;
LeBauer et al. 2008), chronic N deposition can alter ecosystem functions, especially soil nutrient conditions. This changes influence the mycorrhizal symbiosis with the host plant (e.g. Choi 2008). Commonly, mycorrhizal fungi improve the survival in the harsh environmental conditions in forests. Several studies reported ECM-associated
Japanese larch and F1 have more efficient uptake of organic P and N than non-ECM ones (Qu et al. 2004; 2010).
To detect the influence of N deposition on ECM symbiosis with typical ECM species, I conducted the potted experiment (Wang et al. 2013). In this study, three larch species, F1 and its parents were planted in simulated volcanic ash soil with different N and P loading. The ECM species and colonization changed from their initial status after two-year treatment. This greater shift indicates a higher community structure of ECM among different treatments (Fig.4.2 b). The extent of colonization by ECM was enhanced by increasing N loading with elevated P (for Japanese larch and F1). This opposite result with previous studies (Parrent and Vilgalys, 2007; Sun et al. 2010) indicates high N and P treatment lead to nutrient imbalance, ECM assistance was required for nutrients uptake. Distinctly, ECM colonization rate or EMM production will increase with high P availability (Jentschke et al. 2001; Bakker et al.
2009). In addition, several similarities were found in ECM colonization rate of Japanese larch and F1 under different nutrient conditions. This may be due to their same pattern of root growth (Sato, 1995), which resulted in the same trend of colonization rate as well. In the deep part of rhizosphere, especially beyond the inorganic phosphate (Pi)-depletion zone, their shallow root system requires greater colonization by ECM for efficiently absorbing nutrients.