«（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 ...»
The majority of the researchers have found that the growth and root system of plants increases under elevated CO2 accompanied by higher water and nitrogen use efficiency of plants (e.g. Qu et al. 2004; Koike et al. 2010; Norby and Zak 2011). This is because, usually at elevated CO2, plants slightly close their stomata (Morison 1998).
To deeply understand the response, a number of studies have evaluated the effects of elevated CO2 on the plant’s physic-ecological process (Ceulemans and Mousseau
1994; Curtis and Wang 1998; Norby and Zak 2011). Their general conclusion is that usually, elevated CO2 leads to a temporal increase of photosynthesis and consequently accelerates plant and root biomass. Moreover, the carbon gain and allocation to the below-ground increased, and this indirectly changes the colonization rate of ectomycorrhizal (ECM) fungi and species composition according to the mutualistic relationship with host trees (Wang et al. 2015).
Tropospheric or ground-surface ozone (O3) is recognized as one of the phytotoxic air pollutants (Karnosky et al. 2003; Matyssek et al. 2012, 2013;
Agathokleous et al. 2015). The current concentrations of O3 have a significant adverse effect on yield of crops, forest growth and species composition compared to the effects of elevated CO2 (Ashmore 2005; Hoshika et al. 2013; Watanabe et al. 2013;
Yamaguchi et al. 2011, 2013). Most experiments of O3 effects on tree growth are based on observations of above-ground parts, but much little is known about below-ground processes (Andersen 2003; Karnosky et al. 2003; Matyssek et al. 2013).
Exposure of trees to O3 modifies the allocation of carbon to roots, which disrupts root metabolism and influences the activity of rhizosphere organisms (Scagel and Andersen 1997).
Hence, mycorrhizal fungi may reduce overall retention of carbon (C) in the plant-fungus symbiosis by increasing C in roots. On the other hand, by means of root production and mortality, C sink retention is also reduced through belowground respiration (e.g. Rygiewicz and Andersen 1994). Therefore, the fine roots (D 2.0 mm) production and their lifespan must be taken into account under changing environments. Since the colonization of ECM fungi usually occurs with only fine root tips (Smith and Read 1998), this rapid turnover of root growth and death plays an essential role in the function of C cycling (Fig. 1.1).
Figure 1.1 Schematic overview of the important role of ectomycorrhizal symbiosis and root dynamics in forests carbon cycling.
Carbon pools are represented with boxes and circles, carbon fluxes by arrows (Solid line denotes the increase and dash line denotes the reduction) and processes are marked with text only (illustrated partly based on the idea of Fransson 2012).
During this couple of decades, nitrogen (N) deposition has been dramatically increasing in East Asia (Galloway et al. 2004; 2008). N is an essential element for plant photosynthesis and growth (e.g. Evans 1989) and is also often a limiting element of production in forest ecosystems (e.g. Schulze et al. 2005). As an essential macro element in forest ecosystems, increased N deposition usually leads to enhancement of CO2 fixation in vegetation. Previous results showed that increased N deposition enhanced forest growth and increased carbon sequestration in soil (Hunter and Schuck
2002; Hungate et al. 2003； De Vries et al. 2006).
The effect of N deposition on belowground of trees was also detected. Since ectomycorrhizal fungi (EMF) exchange nutrients extracted from soil for carbohydrates delivered by plant roots, increased soil N supply may affect host plant-EMF interactions in any ecosystem by altering the abiotic soil environment (Lilleskov et al. 2002; Nakaji et al. 2001; 2004) or by changing the plant’s allocation of resources to roots (Treseder and Allen, 2000; Helmisaari et al. 2008). Many studies documented that N deposition changed relationship between host plants and mycorrhizal fungi. For example, a study on a spruce forest across a stand-scale N deposition gradient (from 27 to 43 kg N ha−1 yr−1) revealed that with increasing N deposition, ECM root tip abundance and mycelial production decreased five and 10-fold, the change of ECMF community and the species richness decreased (Kjoller et al. 2012). According to the study of Lilleskov et al. (2002), with N loading, composition of ECM species shifted from N-uptake to N-tolerant species, and given that under high-N, low-P and acidified conditions, the ECM species changed to favor types specialized for P uptake. However, there is limited report on N deposition with/without P loading effect on ECM symbiosis (e.g. Kalliokoski et al. 2009; Qu et al. 2010; Leppälammi-Kujansuu et al. 2012).
1.2 Root dynamics under changing environment Roots account for 50 % of the total biomass of forests (e.g. Kubiske and Godbold 2001). Simultaneously, about 50 % of global net primary productivity (NPP) of terrestrial ecosystems comes from forest ecosystems (Field et al. 1998). But is it going to be the same several years later? It seems, it will not be the same: the scenario is that
changes will occur under elevated CO2 and O3 conditions (Karnosky et al. 2005;
Norby et al. 2005; Grantz et al. 2006; Norby and Zak 2011) and the NPP of belowground may exceed 50 % of the total NPP (Kubiske and Godbold 2001).
Consequently, this may lead to unknown and unpredicted implications to the ecosystem’s sustainability. Therefore, deep understanding of interactive response of forest NPP to elevated CO2 and O3 will determine the terrestrial C balance (Karnosky et al. 2003a; Pregitzer and Talhelm 2013), and more important is the changes in belowground NPP, as they represent C inputs below the soil surface, where a relative amount of C may eventually be formed.
1.2.1 Fine root biomass under elevated CO2 In various forest types exposed to elevated CO2, NPP usually increases both aboveand belowground (e.g. King et al. 2005). However, plants prefer to allocate C to roots other than to shoots when grown under elevated atmospheric CO2, and therefore belowground function of forest ecosystems may change significantly, such as root production, respiration and longevity (Pritchard et al. 2001; Karnosky et al. 2003).
Some general patterns appear for the response to elevated CO2,including increase in C allocation to belowground, particularly to fine root production and biomass (Nowak et al. 2004; Jackson et al. 2009; Lukac et al. 2009), and increase in soil carbon inputs (Jastrow et al. 2005; Lichter et al. 2008; Hoosbeek and Scarascia-Mugnozza 2009). Concurrent with greater root production, these same experiments have shown increase in soil respiration as well (King et al. 2004;
Pregitzer et al. 2006).
Elevated CO2 increases root growth; especially fine root growth and this can be
said to a range of species and experimental conditions. For example, Lukac et al.
(2003) concluded that as a result of FACE treatment, standing root biomass of a Populus plantation increased by 47-76 % together with an increase of fine root biomass by 35-84 %. In another report, Pregitzer et al. (2008) found that belowground carbon allocation was positively affected by elevated CO2. However, we have few information on the effect of elevated O3 on belowground NPP of woody plants (Matyssek et al. 2012; 2013). Elevated CO2 increases not only root biomass but affects root diameter and specific root length (SRL). Non-destructive measures show that elevated CO2 increases fine root biomass through increase in the number and length of fine roots (Tingey et al. 1997; Thomas et al. 1999). However, influence on fine root growth was not positively consistent. Using mini-rhizotron tubes, Thomas et al. (1999) found that the effect of CO2 on fine root growth was delayed until the second growing season. Therefore, I followed previous experiments to set up the root systems.
1.2.2 Fine root biomass under elevated O3 The studies focusing on the effects of elevated O3 on individual fine root biomass are still limited (Grebenc et al. 2007; Wang et al. 2015). Almost neither sufficient information nor consistent results are given from studies concluded. Usually the experiments were conducted with elevated CO2 and O3 combined (e.g. Kasurinen et al.
1999; Karnoskey et al. 2003; Pregitzer et al. 2008).
One famous experiment result found at the aspen community of Aspen FACE in Michigan, U.A.S. is that, annual fine-root production and mortality positively correlated with elevated O3 (Karnosky et al. 2003; Pregitzer et al. 2008; Pregitzer and
Tlhelm 2013): soil respiration was the greatest at elevated CO2 and CO2+O3 mixed conditions, and soil respiration correlated with increase in fine root biomass. With combined fumigation of CO2 and O3, they found an increase in belowground carbon allocation after 10 years of exposure. They also found that fine root biomass is actually enhanced by elevated O3, and especially mixed CO2+O3 treatment.
Results of mixed conditions of elevated CO2 and O3 are rarely reported and there is not enough data to prove consistent results (Koike et al. 2012; Matyssek et al. 2013;
Wang et al. 2015). However, to comprehend the C cycling of belowground well, another important indicator that must be calculated is the fine root turnover (Nakaji et al. 2008; Leppalammi-Kujansuu et al. 2014).
1.2.3 Turnover of fine root A substantial amount of the carbon assimilated by plants is transported below ground to produce fine roots (Vogt et al. 1998). The network of tree root system support the fine roots lives and dies rapidly (Hendrick and Pregitzer 1992). The release of carbon fuels to the food web of belowground result in increasing the accumulation in soil organic matter, and may return to the atmosphere. This flux of carbon from vegetation to soil is called fine root “turnover” (Tierney and Fahey 2002). To estimate and to get accurate results of fine root turnover is not easy, thus the understanding of this process is limited and is the key constraint to quantifying terrestrial carbon cycling, vital for predicting the impacts of global environmental changes (Norby and Jackson 2000).
With our current knowledge it is “hard” to forecast the root lifespan of individuals, populations, or ecosystems, and one reason is, the wide variability of the
root turnover (Eissenstat and Yanai 1997). Several methods have been used to calculate rates of root production and mortality (Aerts et al. 1992; Hendrick and Pregitzer 1992; Andersson and Majd, 2005), but turnover rates of fine root obtained seem to vary according to the different methods used (Gill and Jackson 2000; Hertel and Leuschner 2002; Tierney and Fahey 2002). Currently no standard method exists for assessing fine root turnover (Lauenroth 2000; Norby and Jackson 2000).
Currently, mini-rhizotron provides a nondestructive, in situ method for viewing roots and is one of the ideal tools available for directly studying roots. By permitting the simultaneous measurement of fine root production and disappearance, mini-rhizotrons provide relatively accurate results which cannot be obtained through other means: by using sequential coring, in-growth cores or even excavation approaches (Majdi 1996; Johnson et al. 2001).
1.3 ECM symbiosis under changing environment Ectomycorrhizae play an essential role in boreal forest ecosystems: most tree species vitally create a symbiotic relationship with ECM fungi to survive in diverse harsh conditions, such as Siberia, through efficiently nutrient uptake by ECM fungi (e.g. Qu et al. 2010; Jung and Tamai 2012). In nutrient limited soil, ECM are recognized to preferentially supply phosphorus (P) and water to the above-ground parts of host plants rather than their roots (Wallander 2000; Alves et al. 2010). In return, the symbiotic fungi receive carbon from photosynthetic assimilation and about half of the CO2 efflux from soil originates from symbiotic microbes (Högberg et al. 2001). In comparison with non-mycorrhizal plants of the same species, ECM plants take up more organic P and N, since ECM produces ectoenzymes (Turnbull et al. 1996;
Vander and Sanders 2002; White and Hammond 2008). Is this relationship stable under changing environment? Apparently the answer is negative.
Under the circumstance of the changing environment, ECM richness and community will be altered to adapt and defend against these stresses. It will lead to the changes of symbiosis between ECM fungi and host trees (Wang et al. 2015).
Many tree species around the world rely on mutualistic ECM to fulfill their nutrient requirements (Smith and Read 1997). For instance, Landeweert et al. (2001) reported that a major function of ECM symbiosis is the contribution to tree nutrition update by means of mineral weathering and/or mobilization of nutrients from organic matter (e.g.
Read and Perez-Moreno 2003), making the host tree dependent on the fungal partner.
What kind of changes will happen with this functional partner under changing environment?
1.3.1 Effect of elevated CO2 on ECM symbiosis Relationships between ECM symbiosis and host trees can be altered according to the carbon gain and their allocation from the aboveground to belowground. The distinct reflection of the results of the effects are changes in ECM colonization rate and further on their diversity as well as community structure (e.g. Parrent et al. 2006).
Usually elevated CO2 plant increases carbon supply to ECM and the growth of host plants is accelerated (Qu et al. 2004); this results in a higher demand for mineral nutrients and finally to changes in abundance of ECM community.
Elevated CO2 increases the ECM mass, the infection and colonization, and the quantity of extra-matrical hyphae (Tingey et al. 2000; Langley et al. 2003). In a study of loblolly pine (Pinus taeda) forests, richness and diversity of ECM were not