«（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 ...»
Therefore, fine root production (FRP) and mortality (FRM) take the vital role of below-ground processes, however, they are less well understood (Norby and Jackson 2000; Aber and Melillo 2001; Fitter 2005). Since fine roots are increasingly recognized as a key to balancing nutrient cycling of trees and ecosystem, and the carbon (C) sequestration to soil (Norby and Jackson 2000; Matamala et al. 2003;
Norby et al. 2004), understanding about the effect of CO2 enrichment on root survivorship is highlighted. However, root longevity and turnover are reported discrepantly, this largely resulted in great uncertainty about terrestrial C cycles (Pritchard et al. 2001a, b; Lichter et al. 2005; Hogberg and Read 2006).
On the one hand, inconsistent findings are reported, as more C being allocated to the roots under elevated CO2, however negative responses of belowground were happened, because of the enhanced plant growth under elevated CO2 was reported convergent over time (Arnone et al. 2000; Higgins et al. 2002). The root turnover and longevity are getting mysterious according to this uncertain allocation and root production. On the other hand, even several studies have found production and mortality of fine roots produced by trees growing under CO2 enrichment are significantly increased (Matamala and Schlesinger 2000; Pregitzer et al. 2000; King et al. 2001; Pritchard et al. 2001), the results are still inconsistent. So far, the stimulation of NPP by CO2-enrichment at Duke FACE has persisted after more than 8 years amid speculation that nutrient limitations will eventually constrain a positive CO2 response (Luo et al. 2004a, b; Finzi et al. 2006; Johnson 2006). In particular, NPP is strongly affected by soil nutrient limitation (Oren et al. 2001). Due to the fine roots account for large degree of NPP, fine root dynamics must dramatically affected by soil condition.
Recently, as reported, elevated CO2 accelerated growth and increased plant nutrient
demand and uptake capacity (Bielenberg and Bassirirad 2005). In infertile soil condition or nutrient limitation stress, how the fine roots adjust their dynamics to balance the costs and benefits of the whole plant is rarely addressed.
White birch (Betula platyphylla var. japonica) is widely distributed and well acclimated itself in several environmental conditions, its distribution range covers from central Honshu to Far Eastern Asia (including Siberia) (Koike 1995, Shi et al.
2010). Moreover, this species exists under various conditions, having a strong tendency to form a pure birch forest. White birch is well used in several regions of Hokkaido (Terazawa 2005) as well as in Russia (Zyryanova et al. 2010) for promising species of green afforestation. To estimate the C cycling of boreal forest in east Aria under elevated CO2, root dynamics of birch plantation is emphasized as great forest component. Specific in northern Japan, the soil is widely covered by volcanic ash soil which usually has phosphorous (P) deficiency and relative low N concentration (Kayama et al. 2009). Furthermore, P availability is regarded to be a limited factor to tree growth due to several mechanisms, especially with N deposition (Vitousek et al.
2010). Therefore, assessment of future C sequestration should consider the limitations imposed by soil fertility.
In this study, I attempt to understand the root dynamics of Japanese white birch under elevated CO2 involving two soil types-volcanic ash (VA) soil and brown forest (BF) soil. I hypothesize that 1) In BF soil, elevated CO2 stimulated plants growths better than VA soil because of the nutrient limitation. Therefore, root length production is increased by elevated CO2 in BF soil not VA soil. 2) Fine root turnover may be increased with elevated CO2 due to the increased carbon allocation, and the value in BF soil is higher than VA soil over time. 3) Fine roots have a longer lifespan under elevated CO2 and also relative longer in VA soil than BF soil, because of a
longer lifespan may lower the cost for root production in nutrient limited soil.
2.2 MATERIALS AND METHODS 2.2.1 Study site and FACE system The experiment was conducted in Free Air CO2 Enhancement (FACE) system located in Sapporo Experimental Forest, Hokkaido University, Japan (43° 60′ N, 141°20′ E) (Eguchi et al. 2008; Watanabe et al. 2010). The FACE system was constructed with a size about 7.0 m width and 5.2 m height. The whole-plot treatment consisted of two levels of CO2 [ambient (380-390 μmol mol-1 CO2) and elevated CO2 (500 μmol mol-1 CO2)] with three site replications. The tanked CO2 was supplied mainly in daytime, coving the whole photosynthesis period. Totally I constructed six FACE rings giving a total of six sites for data analysis and including the variance among the sites location.
2.2.2 Plant materials and soil type The present experiment had a split-plot factorial design and employed the randomized block method. Three-year old Japanese white birches (Betula platyphylla var.
japonica) were planted randomly in each FACE site. There were two soil types
-brown forest (BF) soil and pumice included volcanic ash (VA) soil-in each FACE site.
The chemical and physical properties of these two types of soil were described by Eguchi et al. (2008). The N content in VA soil was (0.14 mg g-1) lower than BF soil (0.30 mg g-1). Much more significant was P content, P deficiency was severer in VA soil (0.58 mg g-1) than BF soil (4.48 mg g-1).
2.2.3 Mini-rhizotron system To detect fine root dynamics, Minirhizotrons (fine root observation tubes) and specialized camera or scanner equipment have been widely adopted for in situ observation. This technique is a non-destructive method that can be used to monitor the same roots over selected time intervals, which can vary from days to years (Andersson and Majdi 2005). Comparing with ingrowth core or sequential soil core, it takes several advanced points, such as to identify the same roots on successive dates (Hendrick and Pregitzer 1992; Majdi 1996), to quantify the data on root length production, root length mortality, longevity, root density and root diameter (Hendrick and Pregitzer 1996; Majdi and Andersson 2005).
In each FACE site, two birch seedlings were randomly selected as observed target in each soil type and mini-rhizotron tube was installed matching each observed seedling. Totally four birch seedlings were measured by four Minirhizotron (MR) tubes buried beside the seedlings in one FACE site. All the seedlings were planted together with tubes in June 2010. I installed transparent acrylic tubes (0.5 m long with a 5.08 cm inside diameter) at an angle of 45° to the soil surface. I captured digital image in the depth of 15-30cm using a scanner which was exactly right matched the tube size as the schematic was showed by Maeght et al. (2013) (Fig. 2.1) The measurement did not start in 2010, because after installing the tubes, root growth and death at the soil MR tube interface may not be representative of these processes in bulk soil since a lag period of up to a year is required to stabilize the density of fine roots (Joslin and Wolfe 1999). Thus, to avoid the potential gap between soil and tubes, I started the image scanning one year later from April 2011 to October 2013 for an
accurate measurement, and the CO2 fumigation started from early June each year. I collected three years images excluding the period of snow with a three weeks interval, the size of all the images I obtained is about 21.59 × 19.56 cm2. These images were used for detecting the fine root dynamics. Fig.2.2 showed the images of one tube rom the first until last session during one year observation.
2.2.4 Root image analysis I used the program WinRHIZOTron (Regent Instruments, Quebec, Canada) to analyze the root in the captured images. It was difficult to distinguish whether one root appeared from the time when I scanned the image. Therefore, roots that were unsuberized and white when observed for the first time were recorded as new, whereas those remaining white or changing to brownish in subsequent viewings were recorded as living. Roots were defined as dead when they turned black and wrinkled and produced no new roots in subsequent viewings. For each tube, I traced the length and diameter of each individual root appeared in the image area. The sum of the length of new roots and the increase in the length of existing roots during each observation interval was calculated as FRP. Parallelly FRM was evaluated as the length of root that disappeared (Tingey et al. 2000; Satomura et al. 2007; Nakaji et al.
2008). Each parameter was obtained from the 12 MR tubes.
Fine root turnover (y-1) can be estimated generally in two ways: (1) as the ratio of annual root length production to average live root length observed; (2) the inverse of median root longevity (Majdi et al. 2005). In this study, I calculated the turnover of FRP and FRM following the first method according to the annual length-based method (Gill et al. 2002),
Production rate (mm cm-2) =ALRP/LRLmax or ALRP/LRLmean Mortality rate mm cm-2)=ALRM/LRLmax or ALRM/LRLmean where ALRP is annual length-based root production and ALRM is annual length-based root mortality. LRL is live root length (standing crop). LRLmax and LRLmean denote for the maximum and mean value of LRL during the corresponding year.
I define fine lifespan (median root longevity) obtained from MR, as the time during which 50% of the fine roots die (Andersson and Majdi, 2005; Green et al.
2005). Additionally, the fine root diameters (D) were classified into five orders: D
0.2mm, 0.2-0.3mm, 0.3-0.4 mm, 0.4-0.5mm and 0.5mm. Roots of D 2 mm were not estimated for all parameters in this study.
Due to the plant canopy was closed since 2012 (Hara 2014), thus, I separated the first year data from next two years for calculating and plotting graphs, particularly for the live root length, fine root production and mortality.
2.2.5 Soil texture According to the report of Eguchi et al. (2008), nutrient concentration was relative lower in VA soil than BF soil, I mainly detected the C and N concentration in soil during 2011 and 2012, the measurement was conducted with NC analyzers (NC-900, Sumica-Shimadzu, Kyoto, Japan).
2.2.6 Statistical analysis I estimated the FRP and FRM of different treatment (ambient and elevated CO2) on
different soil types in each year by multiple liner models. The fine root median and mean longevity were analyzed using nonparametric Kaplan-Meier survival function with the factors of diameter class, and different soil and CO2 treatment. The statistical analysis unit was three FACE replications, all the data were undertaken by SPSS software.
2.3 RESULTS 2.3.1 Soil nutrient concentration I detected the C and N concentration of two soil types (Table 2.1), VA soil showed lower content than BF soil of both C and N, no significant effect of elevated CO2 was found on soil nutrient content.
2.3.2 Live fine root length During the three-year treatments, LRL showed higher amount in the period of early growing season (June to Aug) from 2011 to 2013 (Fig. 2.3). In 2011, LRL under elevated CO2 was significantly higher than ambient treatment in BF soil. However, the result was contrary in VA soil, LRL showed higher values in ambient than elevated CO2 condition (Fig. 2.3 a). From 2012 to 2013, there was no distinct effect of elevated CO2 on LRL in BF soil, and LRL was extreme higher in ambient VA soil than other three conditions (Fig. 2.3b). Elevated CO2 markedly reduced LRL in VA soil during the three observed growing year.
2.3.3 Fine root production and mortality In BF soil, fine root production rate did not affect by elevated CO2 during 2011 except July and August when it was increased by it (Fig. 2.4a). It was unaffected during 2012 and reduced by elevated CO2 in August of 2013(Fig. 2.4b). In VA soil, no significant difference was found between elevated CO2 and ambient treatment in 2011(Fig. 2.4a), however it was reduced during the early growing season in 2012 and 2013 (Fig. 2.4b).
No clear trend was found for mortality rate in BF soil, and elevated CO2 tend to reduced it in the late growing season in VA soil (Fig. 2.5).
Turnover of fine root production and mortality differed significantly among the treatments (Table. 2.2). Elevated CO2 did not affect the production and mortality turnover, but there was an interaction effect of year and CO2 on mortality turnover. It was reduced by elevated CO2 in two kinds of soil over time. The effect of soil significantly influenced production turnover, it showed lower trend in VA soil than BF soil. Time had significant effect on turnover and reduced it during the three-year treatment. The interaction effect of soil and year influenced the production turnover.
There was no interaction effect of CO2 and soil, nor CO2, soil and year. Additionally, the annual length-based root production (ALRP) and annual length-based root mortality (ALRM) of each tube with different treatments had positive correlation with each other (Fig. 2.6).
2.3.4 Fine root longevity The median fine root longevity was analyzed by different treatments and root diameters. Overall, median root longevity differed with different treatments, and it
was increased under elevated CO2 in 2011 for BF soil and VA soil (Table 2.3). From 2012 to 2013, median fine root longevity was gradually reduced under elevated CO2 in BF soil, and the increased effect on median fine root longevity in VA soil was weakened by elevated CO2.
Median root longevity of different diameter classes showed significant response to different treatments. The fine root (D 2mm) were not affected by elevated CO2 in all conditions in 2011 and 2012, but it was increased by elevated CO2 in BF soil and reduced in VA soil in 2013 (Table 2.4). Roots of diameter between 0.2 mm and 0.3 mm, their longevity were markedly increased by elevated CO2 except the year of 2013 in BF soil, when longevity was contrary reduced by elevated CO2. Roots of diameter between 0.3 and 0.4cm showed significantly increased effect on their longevity by elevated CO2, there was no effect on median longevity of fine root (D 0.5mm) in 2013.