Impact of axial root growth angles on nitrogen acquisition in maize depends on environmental conditions

Abstract

Backgrounds and aims: Crops with reduced requirement for nitrogen (N) fertilizer would have substantial benefits in developed nations, while improving food security in developing nations. This study employs the functional structural plant model SimRoot to test the hypothesis that variation in the growth angles of axial roots of maize (Zea mays L.) is an important determinant of N capture.

Methods: Six phenotypes contrasting in axial root growth angles were modelled for 42 d at seven soil nitrate levels from 10 to 250 kg ha(-1) in a sand and a silt loam, and five precipitation regimes ranging from 0·5× to 1·5× of an ambient rainfall pattern. Model results were compared with soil N measurements of field sites with silt loam and loamy sand textures.

Key results: For optimal nitrate uptake, root foraging must coincide with nitrate availability in the soil profile, which depends on soil type and precipitation regime. The benefit of specific root architectures for efficient N uptake increases with decreasing soil N content, while the effect of soil type increases with increasing soil N level. Extreme root architectures are beneficial under extreme environmental conditions. Extremely shallow root systems perform well under reduced precipitation, but perform poorly with ambient and greater precipitation. Dimorphic phenotypes with normal or shallow seminal and very steep nodal roots performed well in all scenarios, and consistently outperformed the steep phenotypes. Nitrate uptake increased under reduced leaching conditions in the silt loam and with low precipitation.

Conclusions: Results support the hypothesis that root growth angles are primary determinants of N acquisition in maize. With decreasing soil N status, optimal angles resulted in 15-50 % greater N acquisition over 42 d. Optimal root phenotypes for N capture varied with soil and precipitation regimes, suggesting that genetic selection for root phenotypes could be tailored to specific environments.

Keywords: Root architecture; Zea mays; leaching; nitrogen acquisition; precipitation; root growth angles; soil texture.

Fig. 1.

Simulated maize root system with different branching angles after 42 d. Top row: very steep, steep, normal; bottom row: normal–very steep, shallow–very steep, shallow.

Fig. 2.

Simulated nitrate uptake after 42 d shown in single plots for respective root systems growing in a silt loam (A) and, on next page, in a sandy soil (B). Results are given in mmol per plant with colours from light grey to dark green for increasing nitrate uptake. The soil nitrate level is shown on the x-axis and the precipitation factor on the y-axis.

Fig. 2.

Simulated nitrate uptake after 42 d shown in single plots for respective root systems growing in a silt loam (A) and, on next page, in a sandy soil (B). Results are given in mmol per plant with colours from light grey to dark green for increasing nitrate uptake. The soil nitrate level is shown on the x-axis and the precipitation factor on the y-axis.

Fig. 3.

Initial nitrate concentration and after 21 and 42 d for the sandy soil (solid line) and the silt loam (dashed line). Simulations were run with 200 kg N ha⁻¹ and a precipitation factor of 1·0 for both soils.

Fig. 4.

Root profiles after 42 d growing under low (factor 0·5, left two panels) and high precipitation (factor 1·5, right two panels), and soil water nitrate concentration (blue line) and nitrate uptake by the roots (red line). Results are shown for the shallow root system in sand and an initial N concentration of 50 kg ha⁻¹. It is clearly visible how nitrate uptake coincides with nitrate concentration in the soil profile, and how precipitation influences the position and width of the nitrate peak.

Fig. 5.

D₉₀ is the depth where 90 % of a specific property is reached or accumulated on top of that depth. Shown are the D₉₀ values of soil nitrate and roots and the cumulated length of roots below D₉₀ nitrate for the sandy soil and (A) the shallow root system growing in low fertile soil with 25 kg N ha⁻¹ and a precipitation factor of 0·5 and (B) a precipitation factor of 1·5. Bottom row: simulations run with 200 kg N ha⁻¹ and a precipitation factor of 1·0, and (C) for the shallow–very steep root architecture and (D) for the very steep root architecture. The scale for root length below D₉₀ nitrate (secondary vertical axis) was adjusted to the respective maximum depth of D₉₀ roots (primary vertical axis).

Fig. 6.

(A) D₅₀ for soil N-${\text{NO}}_{\text{3}}^{\text{−}}$ and (B) total N-${\text{NO}}_{\text{3}}^{\text{−}}$ (kg ha⁻¹) in the soil profile over time (days after germination). The D₅₀ for soil N-${\text{NO}}_{\text{3}}^{\text{−}}$ indicates the depth at which 50 % of the total amount of nitrate in the soil profile (0–60 cm) is reached. Soil types are fertilized silt loam (mineral fertilizer N 146 kg ha⁻¹), unfertilized silt loam and fertilized loamy sand (mineral fertilizer N 125 kg ha⁻¹, organic manure 785 kg ha⁻¹). All fields were sown to maize. Red lines indicate results from simulations with fertilized silt loam and sandy soil under maize with precipitation factor 1, which means that simulated precipitation was equal to measured precipitation.