Mycorrhiza DOI 10.1007/s00572-015-0654-3

ORIGINAL ARTICLE

Arbuscular mycorrhiza improve growth, nitrogen uptake, and nitrogen use efficiency in wheat grown under elevated CO2 Xiancan Zhu 1,2 & Fengbin Song 1 & Shengqun Liu 1 & Fulai Liu 2

Received: 11 May 2015 / Accepted: 25 June 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning was investigated in Triticum aestivum L. plants grown under elevated CO2 in a pot experiment. Wheat plants inoculated or not inoculated with the AM fungus were grown in two glasshouse cells with different CO2 concentrations (400 and 700 ppm) for 10 weeks. A 15 N isotope labeling technique was used to trace plant N uptake. Results showed that elevated CO2 increased AM fungal colonization. Under CO2 elevation, AM plants had higher C concentration and higher plant biomass than the non-AM plants. CO2 elevation did not affect C and N partitioning in plant organs, while AM symbiosis increased C and N allocation into the roots. In addition, plant C and N accumulation, 15 N recovery rate, and N use efficiency (NUE) were significantly higher in AM plants than in non-AM controls under CO2 enrichment. It is concluded that AM symbiosis favors C and N partitioning in roots, increases C accumulation and N uptake, and leads to greater NUE in wheat plants grown at elevated CO2.

Keywords Arbuscular mycorrhiza . C/N partitioning . C/N accumulation . 15N recovery rate . Nitrogen use efficiency . Triticum aestivum L

* Fulai Liu [email protected] 1

Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Changchun 130102, People’s Republic of China

2

Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Taastrup DK-2630, Denmark

Introduction Increase in atmospheric carbon dioxide (CO2) concentration is one of the most important environmental factors reflecting global climate change. The concentration of atmospheric CO2 has risen to 400 μmol l−1 at present (NOAA-ESRL 2015), and it is projected to reach ca. 1000 μmol l−1 by the end of this century (IPCC 2013). An elevated CO2 concentration has significant influence on plant growth and development, soil carbon (C) and nitrogen (N) dynamics as well as agricultural ecosystem function (He et al. 2014; Aljazairi et al. 2015). It is well known that C and N fixation, decomposition, metabolism, and dynamics in the ecosystem are mediated by soil microbes (Treseder et al. 2003; He et al. 2014). Therefore, it is essential to study the effects of CO2 elevation on C and N dynamics in plant-soil systems and their links with soil microbes. Arbuscular mycorrhizal (AM) symbioses, formed by over 80 % of plant families and fungi of the monophyletic phylum Glomeromycota (Smith and Read 2008), have been found to be beneficial for sustaining agroecosystem functioning, such as improving crop nutrient uptake and tolerance to abiotic stress (Gianinazzi et al. 2010; Zhu et al. 2015). AM fungi are known to receive photosynthetic C from the host plants and have a complex C metabolism including C accumulation, partitioning, and decomposition. On the other hand, the flux of C is accompanied by the acquisition and transport of N in the AM symbiosis (Fellbaum et al. 2012; Liu et al. 2013). Govindarajulu et al. (2005) have shown that AM fungi can take up and transfer significant amounts of N (accounting for 20–50 % of the total plant N uptake) to the host plant. Wheat (Triticum aestivum L.) is an annual C3 crop and is very important for the global agricultural production and food security. Many studies have documented that elevated CO2 has a positive effect on C3 crops due to stimulation of

Mycorrhiza

photosynthesis (Ainsworth and Long 2005; Pacholski et al. 2015) and may thus benefit AM fungal colonization (Rillig et al. 1999). In turn, AM symbiosis may further increase C assimilation thereby stimulating growth of the host plant under elevated CO2 (Gavito et al. 2000; Baslam et al. 2014). However, the combined effect of AM fungal inoculation and CO2 elevation on the partitioning of C among different organs in wheat plants and its implications for plant N balance remains largely unknown. It is known that N plays a key role in AM fungal mediated C metabolism and plant N uptake under elevated CO2 (Cheng et al. 2012). N is an important mineral nutrient controlling plant growth, and often there is a reduction in plant tissue N concentration under CO2 elevation (Mitsutoshi et al. 2005; Xu et al. 2013; Bloom et al. 2014). Evidence indicates that AM fungi have a potential to improve plant N acquisition at elevated CO2 (Gamper et al. 2005; Cavagnaro et al. 2007; Cheng et al. 2012). Chen et al. (2007) reported that AM fungal inoculation effects on plant N uptake at high CO2 concentration were species-specific, with enhancing N acquisition in Plantago lanceolata L. but not in Festuca arundinacea Schreb. To date, however, the efficiency of the acquired N for C accumulation as well as its partitioning in different organs in wheat plants as influenced by AM fungal colonization under elevated CO2 has not been investigated. Therefore, the objective of this study was to evaluate the effects of an AM fungus on biomass, C and N accumulation and partitioning, N use efficiency of wheat plants grown in ambient and elevated CO2 concentrations. The results obtained will be helpful in understanding the C and N dynamics in plants in future climate change scenarios.

Materials and methods Experimental setup The experiment was conducted from March to May 2014 in a climate-controlled glasshouse at the experimental farm of the Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen, Taastrup, Denmark. The experimental pots were 4 l (15.2 cm diameter and 25 cm deep), and the bottom of the pots was covered by mesh (1.5 mm). The pots were filled with 6.0 kg of soil. The texture of the soil was sandy loam with a pH of 6.8, total C 11.8 g kg−1, total N 1.15 g kg−1, and Olsen-P 19.2 mg kg−1. The soil had a volumetric soil water content of 30 % at 100 % water holding capacity and 5 % at permanent wilting point. The soil was sieved by passing through a 2-mm mesh and sterilized at 95 °C for 4 h each on three consecutive days.

Treatments The experiment involved two AM (inoculated with Rhizophagus irregularis and not inoculated as control) and two atmospheric CO2 concentrations (ambient, 400 ppm and elevated, 700 ppm CO2) treatments. The AM fungus was obtained from INOQ GmbH, Schnega, Germany. The AM inoculum was a mixture of vermiculite with spores, hyphae, and mycorrhizal root residues (210 spores per liter of substrate). Twenty milliters of inoculum was mixed into soil for the inoculation treatment, and 20 ml sterilized inoculum plus 10 ml AM fungus-free filtrate from the inoculum suspension for the non-inoculated treatment. Then, half of the pots were placed into a glasshouse cell with a CO2 concentration of 400 ppm, and the rest into another glasshouse cell with an elevated CO2 concentration at 700 ppm. The CO2 concentration in the greenhouse cells was monitored every six seconds by a CO2 Transmitter Series GMT220 (Vaisala Group, Helsinki, Finland). The experiment was a complete randomized block design with four replicates for each treatment. Eight seeds of wheat (var. Tuareg) were directly sown into each pot, and tap water was used for irrigation to maintain optimal soil water content, i.e., 90 % water holding capacity. The conditions in the glasshouse were set at 25/16±2 °C day/ night air temperature, 60 % relative humidity, 16 h photoperiod, and >500 μmol m−2 s−1 photosynthetic active radiation (PAR) supplied by sunlight plus meta-halide lamps. However, the actual mean air temperature, relative humidity, PAR, and CO2 concentration in the two cells varied from the set points during the experimental period (Fig. 1). Nevertheless, except for the air CO2 concentration, the climatic variables were identical in the two cells (Fig. 1). It should be noted that the set point of the CO2 concentration (i.e., 700 ppm) in the CO2 elevated cell (cell 2) was maintained only during the first three weeks of the experimental period, and it fluctuated between 500 and 650 ppm thereafter. This was most probably attributable to the increased CO2 consumption during the day due to a larger leaf area in the later plant growth stages. Three weeks after emergence, the eight seedlings were thinned to four per pot, and 1.0 g N and 0.8 g K per pot were applied to meet the nutrient requirement for plant growth. To examine plant N uptake efficiency, 21.7 mg of 15N tracer (15NH415NO3, 99 % atom 15N) was also applied into the soil. Sampling, measurements, and analyses Plant and soil samples were collected 10 weeks after sowing (at the stem elongation stage, Zadoks code 34 (Zadoks et al. 1974)). Plant samples were divided into leaf, stem, and root. Two grams (fresh weight) of the root samples from each plant was carefully washed and cut into 0.5- to 1-cm-long segments, dipped in 10 % KOH at 90 °C for 20 min, acidified in 2 % HCl for 5 min, and stained with 0.01 % acid fuchsin in lactophenol

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(a)

The data were statistically analyzed using Microsoft Excel and SPSS 16.0 software. The effects of AM fungus and CO2 treatments on variables were analyzed using two-way ANOVA. Differences between treatments were considered significant when the P value was less than 0.05 by Duncan’s test.

20 18

Relative humidity (%)

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80 70

Results

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AM fungal colonization and plant biomass

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PAR (µmol m-2 s -1)

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The AM fungal colonization was not observed in the noninoculated wheat roots. CO2 elevation significantly increased root colonization by R. irregularis (Table 1). Both R. irregularis inoculation and elevated CO2 increased plant total dry weight, and the positive effect of the AM fungus was more pronounced under elevated CO2 resulting in significant interactive effect between AM and CO2 treatments (Table 1). For the leaf dry weight, both R. irregularis inoculation and CO2 elevation had positive effects. Stem dry weight was increased by the AM fungus and CO2 elevation, and the positive effect of R. irregularis was more pronounced under elevated CO2. Root dry weight was not affected by CO2 treatment but was increased by R. irregularis inoculation, and the positive effect of the AM fungus was more pronounced under CO2 elevation (Table 1).

08-May

28-Apr

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200 14-Mar

CO2 concentration (ppm)

Statistical analysis

Cell 700 ppm

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Mean temperature (•C)

Cell 400 ppm

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Fig. 1 Actual mean temperature (a), relative humidity (b), photosynthetic active radiation (PAR) (c) and CO2 concentration (d) during the experimental period in the two greenhouse cells with CO2 concentrations of 400 and 700 ppm, respectively

(Kormanik et al. 1980). AM fungal colonization was microscopically calculated by the percentage of root segments using the gridline intercept method (Giovannetti and Mosse 1980). The oven-dried (70 °C for 48 h) leaf, stem, and root and the natural air-dried soil samples were ground to a fine powder. Plant and soil C, N, and 15N concentrations were analyzed using the Dumas dry combustion method in a system consisting of an ANCA-SL Elemental Analyzer coupled to a 20-20 Mass Spectrometer (Europa Scientific Ltd, Creve, UK). 15 N recovery rate was calculated as the ratio of the amount of 15 N in the plant tissue to the amount of 15N applied to the soil. Plant C to N ratio was calculated as an estimation of plant nitrogen use efficiency (NUE).

C and N concentrations Leaf C concentration was unaffected by any of the treatments, although R. irregularis inoculation tended to increase it under both ambient and elevated CO2 concentrations (Fig. 2a). Stem C concentration was significantly affected by interactions between the AM fungus and CO2 treatment, being lower for the AM plants under ambient CO2 but higher for the AM plants under elevated CO2, compared to the respective non-AM plants (Fig. 2b). Root C concentration was unaffected by R. irregularis inoculation while it was significantly increased by CO2 elevation, and the increase was more pronounced in the AM plants (Fig. 2c). At ambient CO2, AM fungal inoculation increased N concentration in leaves but decreased it in stems (Fig. 2d, e). On the contrary, under elevated CO2, the N concentrations in the leaves and stems of the AM plants were lower than those of the non-AM plants. Figure 2d also shows that leaf N concentration was interactively affected by the two treatments; R. irregularis inoculation increased leaf N concentration under ambient CO2 but decreased it under elevated CO2. Root N concentration was not affected any of the treatments (Fig. 2f).

Mycorrhiza Table 1 Root colonization and plant growth of wheat plants inoculated (M+) or not (M−) with Rhizophagus irregularis (AM) at ambient and elevated CO2 treatments CO2

Inoculation Root colonization (%)

Ambient M− M+ Elevated M− M+ Source of variation AM CO2 AM×CO2

41.5±1.13 49.9±2.45

*

Leaf dry weight (g plant−1)

Stem dry weight (g plant−1)

Root dry weight (g plant−1)

Total dry weight (g plant−1)

4.88±0.41 5.33±0.35 5.21±0.40 7.82±0.91

1.73±0.15 2.65±0.31 1.97±0.18 4.56±0.49

1.26±0.15 1.89±0.09 1.17±0.09 2.99±0.44

7.86±0.44 9.87±0.69 8.36±0.57 15.4±1.53

* * NS

** ** *

** NS *

** ** *

Values are means±SE NS not significant by Duncan’s test *P

Arbuscular mycorrhiza improve growth, nitrogen uptake, and nitrogen use efficiency in wheat grown under elevated CO2.

Effects of the arbuscular mycorrhizal (AM) fungus Rhizophagus irregularis on plant growth, carbon (C) and nitrogen (N) accumulation, and partitioning ...
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