1. Proportion of intermediate diameter roots
Nitrogen and water uptake from different soil layers in relation to wheat root traits under different water regimes using 15N and H218O tracers
Shiva Bakhshandeh1*, Michael Kertesz1, Paola Elisa Corneo1, Feike A. Dijkstra1
1Department of Environment Sciences, University of Sydney, Sydney, NSW 2570, Australia. *
Introduction
Water and nitrogen are essential for plant growth and their limitation can strongly impact on final yield. Plants rely on surface roots for nutrient uptake, but rely on extensive and deep root systems for water uptake, especially in semi-arid, rainfed crop production systems. We assessed uptake of nitrogen and water at tillering stage affected by different water regimes in different wheat genotypes and different soil layers.
Figure 1
249 took up more water than CIMMYT from bottom soil (average increase of 53%), but NH4+ uptake was an average 51% higher from the topsoil.
During soil surface drying, plants water uptake shifts to bottom soil, however, plants ammonium uptake also decreased.
Hypothesis
During surface soil drying, plant water uptake shifts from the top to the bottom soil, but N uptake is maintained in the top soil because of an increase in root length.
Figure 2
Water uptake rate was positively related to both root length (RL) and proportion of intermediate diameter roots and negatively related to proportion of fine diameter roots.
Methods
Wheat genotypes: 249, CIMMYT.
Water treatments: Top watering (simulating rainfall during growing season) and bottom watering (simulating surface soil drying).
Dual isotope labelling ((15NH4)2SO4 & H218O) in top and bottom soil to estimate ammonium and water uptake.
NH4+ uptake rate was positively related to both root and shoot biomass and negatively related to specific root length (SRL).
Result
Table 1
Effects of soil layer, genotype and water treatment on extractable NH4+, NO3-, DIN
Figure 3
Plant water uptake was not related to NH4+ uptake, and predominantly occurred in the bottom and top soil, respectively.
Figure 4
There was a positive relationship between transpiration rate and water uptake calculated with the H218O tracer
Soil layer
Genotype
Water treatment
NH4+
(mgpot-1)
NO3-
DIN
Top soil
CIMMYT
Top watering
3.8 ± 0.5
4.2 ± 0.6
8.0 ± 1.1
Bottom watering
6.5 ± 0.6
22.5 ± 1.5
28.4 ± 1.3
249
3.0 ± 0.2
10.2 ± 6.3
14.0 ± 7.0
5.5 ± 0.5
30.3 ± 6.3
36.6 ± 7.5
Bottom soil
3.4 ± 0.1
7.5 ± 2.8
11.0 ± 2.7
5.4 ± 0.6
8.7± 1.3
14.1 ± 0.9
3.6 ± 0.3
4.5 ± 0.4
8.1 ± 0.7
3.5 ± 0.5
7.2 ± 2.6
11.7 ± 3.5
ANOVA P-values
0.04
0.0005
0.0008
0.01
0.3
0.4
Watering
<0.0001
0.0002
Soil layer × Genotype
0.9
0.1
Soil layer × Watering
0.02
0.001
0.003
Genotype × Watering
0.7
0.8
Soil layer × Genotype × Watering
0.2
1.0
Table 2
Effects of soil layer, genotype and water treatment on root traits
Soil layer
Genotype
Water treatment
Root biomass (g pot-1) RL
(m)
SRL
(m g-1)
Proportion of fine
diameter roots
(< 0.1 mm)
Proportion of intermediate diameter roots
( mm)
Top soil
CIMMYT
Top watering
1.5 ± 0.1
120 ± 22
79 ± 13
0.09 ± 0.008
0.81 ± 0.01
Bottom watering
1.4 ± 0.1
65 ± 6
73 ± 8
0.13 ± 0.002
0.74 ± 0.01
249
2.2 ± 0.1
116 ± 5
70 ± 2
0.08 ± 0.006
0.80 ± 0.01
1.3 ± 0.1
71 ± 8
68 ± 7
0.11 ± 0.008
0.78 ± 0.01
Bottom soil
1.7 ± 0.2
202 ± 26
125 ± 6
0.07 ± 0.002
0.85 ± 0.01
1.5 ± 0.2
177 ± 6
121 ± 9
0.07 ± 0.003
0.84 ± 0.01
171 ± 16
110 ± 5
0.05 ± 0.002
0.86 ± 0.01
2.0 ± 0.2
198 ± 17
102 ± 9
0.04 ± 0.004
0.83 ± 0.01
ANOVA P-values
0.4
<
0.03
0.9
0.1
0.0008
Watering
0.5
0.003
Soil layer × Genotype
0.2
0.8
0.3
Soil layer × Watering
0.002
0.0005
Genotype × Watering
0.7
1.0
Soil layer × Genotype × Watering
0.001
Conclusion
Surface soil drying decreased both water and NH4+ uptake at the top soil. However, increased water availability in deep soil layer increase water uptake but not NH4+ uptake.
Water and NH4+ uptake could be improved by breeding for genotypes with higher root length and greater proportion of intermediate diameter roots in the deeper soil layers and more root biomass in the surface soil layers.