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Effect of elevated CO2 and N level on growth of wheat and field pea

Clayton Butterly1, Roger Armstrong2, Deli Chen3, Nicole Mathers2 and Caixian Tang1

1 Department of Agricultural Sciences, LaTrobe University, Bundoora, VIC 3086. www.latrobe.edu.au/agriculture
Email C.Butterly@latrobe.edu.au; C.Tang@latrobe.edu.au
2
Department of Primary Industries Victoria, Horsham, Vic 3400. www.dpi.vic.gov.au
Email Roger.Armstrong@dpi.vic.gov.au; Nicole.Mathers@dpi.vic.gov.au
3
School of Land & Environment, The University of Melbourne, Parkville, Vic 3010. www.land-environment.unimelb.edu.au
Email delichen@unimelb.edu.au

Abstract

Increases in atmospheric carbon dioxide (CO2) concentration have been shown to enhance photosynthetic gain and net primary productivity and increase water- and N-use efficiency. However, it is uncertain whether this extends to grain crops in semi-arid agricultural systems, such as southern Australia, which are often N and water limited. We investigated the interactive effect of CO2 concentration and nitrogen (N) fertiliser application on carbon (C) and N partitioning at the soil free-air CO2 enrichment (SoilFACE) facility. Wheat and field pea were grown under elevated CO2 (eCO2; 550 ppm) or ambient CO2 (aCO2; 370 ppm) with two N levels (40 and 100 mg N/kg soil over 4 applications) in soil columns containing a Vertosol. We report greater effects of eCO2 on shoot biomass than root biomass of wheat and field pea. Wheat produced more roots at 25-50 cm under eCO2. Conversely, N level only affected wheat shoot growth. Elevated CO2 increased the C:N ratio of wheat root and shoot regardless of N level and this could affect C cycling.

Key Words

Elevated CO2, SoilFACE, biomass partitioning, carbon, nitrogen, C:N.

Introduction

Elevated atmospheric CO2 concentrations have been shown to increase plant productivity and are attributed to direct stimulation of photosynthesis and increased water-use efficiency due to reduced evapotranspiration (Kimball et al., 2002). Studies indicate that crop biomass and yield can increase by up to 30% under eCO2, with a greater response in legumes than non-legumes due to the stimulation of N fixation, which fulfils the additional N requirement under eCO2 (Ainsworth and Long, 2005). However, most studies have only examined the effects of eCO2 on above-ground components, such as shoot development, biomass and yield. Few studies have considered below-ground C and N partitioning within soil-plant systems, particularly under field conditions (Ko et al., 2010; Manderscheid et al., 2009; Mitchell et al., 1993). The response of plants to eCO2 will depend on abiotic factors such as temperature and availability of water and nutrients. In Australian dryland agricultural systems non-legumes rely almost entirely on N fertiliser to meet their growth requirements. In these systems responses to eCO2 will be determined by soil N status and are likely to be quantitatively smaller in situations where N is not optimal. This study aimed to quantify the changes in biomass partitioning of wheat and field pea under eCO2 and different N levels, and to assess the implications for C and N cycling.

Methods

Experimental details and design

We conducted a column experiment at the SoilFACE facility, Department of Primary Industries, Horsham. SoilFACE consists of eight bunkers (3.7 m wide) with either aCO2 (370 ppm) or eCO2 (550 ppm) concentrations in a randomised 4 x 2 design. The eCO2 is achieved using FACE technology as described by Mollah et al. (2009) and the level of elevation is based on the predicted atmospheric CO2 concentration in the year 2050. A Vertosol soil (Isbell, 1996) was collected from a roadside, sieved (<4 mm), and mixed 1:1 with triple washed white sand. Briefly, the Vertosol had the following properties; pHCaCl, 7.7; total C 14.2 g/kg, total N 0.8 g/kg. Twelve kg (dry weight equivalent) of the soil: sand mix was added to plastic columns (15 cm ID x 60 cm long), which equated to a bulk density of 1.2 g/cm3. The soil: sand mix contained the following basal nutrients in mg/kg; CaCl2, 186; CoCl2, 0.4; CuSO4, 6; FeCl3, 0.6; K2SO4, 103; KH2PO4, 70.4; MgSO4, 122; MnSO4, 6; Na2B4O7, 1.6; Na2MoO4, 0.4 and ZnSO4, 8. Soil columns were adjusted to 80% field capacity and sown with either wheat (Triticum aestivum L. cv. Yitpi) or field pea (Pisum sativum L. cv. PBA Twilight) on the 14th June 2011. Group E inoculum was used for field pea. After 3 weeks, wheat and field pea plants were thinned to 3 and 5 plants/column, respectively. Two N levels, 40 or 100 mg N/kg soil, were added as Ca(15NO3)2 split over four equal applications at sowing and at 3 times during the growing season. Columns only received water when N was added (10% Field Capacity) and in the final 3 weeks before sampling (20% FC).

Sampling and analyses

Columns were removed from SoilFACE and destructively sampled at peak biomass (2nd November 2011). Plant shoots were removed, rinsed with reverse osmosis (RO) water and dried at 70C for 3 days. The soil was separated into depths of 0-10 cm, 10-25 cm and 25-50 cm. Roots were carefully removed, washed with RO water, and root length and surface area were determined on fresh samples using WinRHIZO Pro 2003b (Regent Instruments, Quebec, Canada). Roots were then dried at 70C for 3 days, ground (<0.5 mm) using a Retsch ZM200 centrifugal mill, thoroughly mixed and subsamples were ground further using a Retsch MM400 mixer mill (Retsch GmbH, Haan, Germany) for analyses. Total C and N, and 15N content were determined using an isotope-ratio mass spectrometer (IRMS) (Hydra 20-20, SerCon, Crewe, UK).

Statistical analyses

For each species, a two-way analysis of variance (ANOVA) in a randomised complete block split-plot design was used to test the effects of CO2 concentration (main-plots) and N level (sub-plots) on shoot and root variables. Root variables were analysed separately for each depth to improve the accuracy and interpretation. Significant (P = 0.05) differences between means were established using least significance difference (LSD) test.

Results

Shoot growth

Nitrogen supply and eCO2 concentration significantly increased shoot growth (P = 0.05), and altered N variables of wheat and field pea. For wheat, plant height was greater under eCO2 but lower at higher N levels (Table 1). In contrast, the height of field pea was not affected by either CO2 concentration or N supply. A significant (P = 0.05) positive interaction between CO2 concentration and N supply was observed for wheat shoot growth. Similarly, greater dry matter production occurred under eCO2 for field pea, but shoot growth was not affected by N supply, presumably because it is a legume. The shoot N concentration and total N uptake of wheat were greater at 100 mg N/kg than 40 mg N/kg, and this resulted in a significant decrease in the C:N ratio. Furthermore, eCO2 decreased the N concentration in wheat shoot and this decrease was greater at the higher rate of N addition. The C:N of wheat shoot grown under eCO2 increased by 6.3 and 5.7 for the low N level and high N level, respectively. In contrast to wheat, N concentration in field pea shoot, total N uptake and C:N were not significantly different (P = 0.05) between 40 mg N/ kg and 100 mg N/kg (Table 1). For field pea, N concentration within the shoot was not altered by CO2 concentration. Hence, the significant increase in N uptake by field pea at eCO2 occurred due to greater biomass accumulation. A decrease in the C:N of field pea shoot occurred when the plants were grown under eCO2, but only for plants grown at the low N level.

Table 1. Effect of eCO2 and N supply on shoot variables.

CO2 conc.

(ppm)

N rate (mg/kg soil)

Height

(cm)

Dry Matter
(g/column)

N conc.

(g/kg plant)

Total N uptake

(mg/column)

C:N

Wheat

           

370

40

51.9

27.7

10.3

286

39.9

550

40

58.3

37.0

8.9

329

46.2

370

100

45.4

28.9

20.1

561

21.3

550

100

53.2

43.2

15.5

669

27.0

LSD (P=0.05)

4.8

2.9

2.0

55

3.4

CO2

n.s.

**

*

n.s.

*

N

*

*

***

***

***

CO2 x N

n.s.

*

n.s.

n.s.

n.s.

Field Pea

           

370

40

47.0

26.0

23.2

603

17.8

550

40

56.0

32.3

26.5

859

15.7

370

100

50.0

25.9

23.7

614

17.4

550

100

53.7

35.2

23.0

812

17.5

LSD (P=0.05)

-

3.7

-

163

0.6

CO2

n.s.

*

n.s.

*

*

N

n.s.

n.s.

n.s.

n.s.

n.s.

CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

***, **, * and n.s. represent P<0.001, P<0.01, P<0.5, P>0.5, respectively.

Root growth

CO2 concentration and N supply affected roots less than shoots (Table 2). No significant effect of CO2 or N levels on any root variable of field pea was observed. Wheat root growth was increased under eCO2 but was only significant (P = 0.05) in the 25-50 cm layer. Furthermore, there was no effect of N level on root mass in any layer. CO2 concentration and N supply altered the morphology of roots in the 0-10 cm and 25-50 cm layers but had no effect on the middle (10-25 cm) layer. In the low N treatment a decrease in wheat root length occurred in the 0-10 cm layer under eCO2. In most cases, wheat root length was greater in the 0-10 cm and 25-50 cm layers at 100 mg N/kg than 40 mg N/kg, however this was not significant (P = 0.05) for aCO2 in the 0-10 cm layer. A similar response to N level was observed for specific root length. Furthermore, specific root length in the 25-50 cm layer was significantly greater under aCO2 and 100 mg N/kg than all of the other treatments. The surface area of wheat roots grown under eCO2 was lower in the soil surface layer. In contrast, root surface area was greater at 100 mg N/kg than 40 mg N/kg in both 0-10 cm and 25-50 cm layers. Root N concentration and total N uptake were greater at 100 mg N/kg than 40 mg N/kg and this resulted in lower C:N ratios. Conversely, eCO2 did not significantly alter root N concentration and total N uptake, although in most cases lower values were observed under eCO2. Subsequently, the C:N ratio was significantly higher under eCO2, although this was not significant (P = 0.05) in the 0-10 cm layer.

Table 2. Effect of eCO2 and N supply on root variables.

CO2 conc.

(ppm)

N rate

(mg/kg soil)

Root

mass
(g)

Root

length

(cm)

SRa

length

(cm/g)

Surface area (cm2 /column)

N conc.

(g/kg plant)

Total N

uptake (mg /column)

C:N

Wheat 0-10 cm

             

370

40

6.36

3521

552

316

4.9

31

76.2

550

40

7.02

2851

404

269

3.7

26

104.7

370

100

6.55

3737

572

343

9.1

59

38.2

550

100

7.09

3508

494

314

8.0

57

45.0

LSD (P=0.05)

-

281

53

26

1.5

8

15.4

CO2

n.s.

*

n.s.

*

n.s.

n.s.

n.s.

N

n.s.

*

*

*

**

***

***

CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

Wheat 10-25 cm

             

370

40

5.08

4601

913

375

8.0

41

49.6

550

40

5.22

4775

917

385

6.3

33

63.0

370

100

5.22

4436

849

378

13.5

70

29.5

550

100

5.53

4857

880

418

10.3

57

37.7

LSD (P=0.05)

-

-

-

-

1.0

7

3.0

CO2

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

*

N

n.s.

n.s.

n.s.

n.s.

***

***

***

CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

Wheat 25-50 cm

             

370

40

7.57

4755

629

418

8.5

64

46.0

550

40

8.75

4831

552

428

7.0

62

54.6

370

100

6.82

5581

820

487

14.0

95

28.7

550

100

8.46

5392

637

473

12.0

102

32.0

LSD (P=0.05)

0.65

527

72

24

1.2

12

4.8

CO2

*

n.s.

*

n.s.

n.s.

n.s.

*

N

n.s.

*

**

*

***

**

***

CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

Field Pea 0-10 cm

             

370

40

4.67

2250

482

224

24.0

112

16.7

550

40

4.73

2267

478

251

25.6

121

16.3

370

100

4.69

2518

533

259

24.7

116

17.1

550

100

4.83

3004

619

336

25.4

123

16.6

CO2, N, CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

Field Pea 10-25 cm

             

370

40

4.57

3706

807

421

23.9

109

17.4

550

40

4.72

4060

857

498

24.5

115

17.2

370

100

4.59

3665

798

389

23.3

107

18.1

550

100

4.57

3047

667

351

23.6

108

17.3

CO2, N, CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

Field Pea 25-50cm

             

370

40

5.18

2622

505

312

24.1

125

17.4

550

40

5.46

3672

680

422

24.1

131

18.0

370

100

5.21

3086

595

353

24.1

126

17.7

550

100

5.49

2782

507

331

25.5

140

16.5

CO2, N, CO2 x N

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

n.s.

aSpecific root (SR) length is the root length per unit dry root mass (cm/g).

***, **, * and n.s. represent P<0.001, P<0.01, P<0.5, P>0.5, respectively.

Discussion and conclusions

Our study showed that eCO2 increased the above-ground biomass of wheat and field pea. A 24-35% increase in the shoot biomass of field pea was observed but was unaffected by N supply. Therefore, N fixation was able to meet the greater N demand under eCO2 and field pea was able to maintain similar tissue N concentrations under aCO2 and eCO2. In contrast, the increase in wheat shoot biomass under eCO2 was greater at high levels of applied N (49%) than at low N levels (34%). Other studies have recorded increases in wheat shoot biomass under eCO2 even at low N levels, although smaller in magnitude (Mitchell et al., 1993). The effect of soil N status on above-ground biomass of wheat was greater than the CO2 effect and is consistent with other studies (Ko et al., 2010). Total N uptake under eCO2 was greater for both field pea and wheat. For wheat, this indicated greater access to soil N, however this was not sufficient to meet the N demand under eCO2 and wheat tissue N concentration decreased considerably. The contribution of N fertiliser and N fixation are currently being assessed (Armstrong, pers comm). This study showed that eCO2 resulted in an increase in shoot: root ratio of both species. In fact, eCO2 only increased root biomass for wheat and only the deeper (25-50 cm) soil layer. The effect of elevated CO2 concentration on root growth is inconsistent but is generally positive if water is not limiting (Wechsung et al., 1999; Li et al., 2003) and/or if soil N status is low (Mitchell et al., 1993), although there are reports of decreases (Li et al., 2003). The lack of an effect of eCO2 on below-ground biomass allocation may indicate that the plants within the current study were drought stressed during the growing season. An increase in root: shoot ratio to overcome the decrease in tissue N concentration was expected; however this was not observed. Reduced mass flow and N uptake under drought may also explain the lower C:N ratio of wheat shoot at the high soil N level. eCO2 tended (P = 0.062) to decrease wheat root N concentration and significantly (P = 0.05) increased the C:N ratio. These roots could decompose slower than roots grown under aCO2, particularly those additional roots accumulated at lower soil depths. Further, specific root length of wheat decreased under eCO2 indicating coarser roots. Martens et al., (2009) indicated that despite no effect of eCO2 on root biomass, significant increases in soil organic 14C under eCO2 were revealed using repeated 14CO2 pulse-labelling. This requires further investigation. This study confirmed the dependence of wheat on soil N status to respond to eCO2. Field pea appears to take advantage of the positive effect of eCO2 better than wheat, provided conditions for efficient symbiosis and N fixation are met.

Acknowledgements

This research was supported by an Australian Research Council Linkage Project (LP 100200757). SoilFACE was funded by DPI Victoria and the Grains Research & Development Corporation.

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