Wheat barley and chickpea varietal root responses to N P K and S and implications for management
Author: Chris Guppy, Sheikh M. F. Rabbi, and Richard Flavel, University of New England | Date: 20 Feb 2017
Take home message
After accounting for other varietal choice factors (climate, disease, yield, rotation etc.) the P recovery strategies of wheat and barley varieties differ. In contrast to northern wheat lines, the southern lines tested so far have far less plasticity in response to bands of P, and recover less P from those bands. There remains potential to target varieties to make better use of the prevailing nutritional environment.
No response to bands of N, K or S were observed in northern wheat lines. Limited evidence that wheat roots respond to N, S or K meaning co-placement strategies of these nutrients must include P to be successful.
Minimal early season root plasticity was observed in the 6 chickpea varieties tested. Yorker was the most responsive to a P band.
GRDC recently invested in quantifying the variation in root response of 10 northern wheat and 5 barley lines to P, K, S and N nutrition. A modification to the project included responses of 6 chickpea lines to P and K nutrition, and finally, an investigation of 10 southern wheat lines for P response.
Roots should respond with greater plasticity (extra biomass/root length) when encountering patches of immobile nutrients such as P, and perhaps K. Mobile nutrients such as N and S may not result in proliferation responses as, by the time roots are grown the nutrients may have moved. Research has suggested cereal roots respond to P and N, but there is little evidence of response to S and K.
The data acquired was used to generate a table identifying which varieties may respond well in low soil P, high soil P, stratified P (high surface and low subsoil P) and where growers have placed subsoil banded P and this can be found on the GRDC website. These are conditions growers may face under a range of fertiliser application strategies. For example, low soil P may arise with a history of cropping with minimal P replacement. High soil P may occur where land is recently cleared, or fertiliser application has been maintained with both surface and subsoil placement. Stratified P scenarios arise with regular application of starter P rates or controlled traffic scenarios, and deep placement is a recent development following recognition of subsoil P resource depletion.
Materials and methods
Ten common northern Australian wheat varieties, five southern lines (including Federation) thus far and five barley varieties were investigated. These varieties represent the most grown, and most likely to be grown, in the major central-eastern Australian cereal areas over the next 3-5 years.
Root plasticity experiments
Five hundred grams of sieved (<5 mm) P and N responsive Red Ferrosol subsoil from Kingaroy was packed in a closed cell polyethylene tubes (aka ‘pool noodles’). The soil moisture was maintained at 80% of field capacity throughout the trial. A 4 cm P band (150 mg P kg-1) was placed 3 cm below the surface of the soil. The P was supplied as MAP and the soil above and below the P band was adjusted to the N concentration in the band (68 mg N kg-1) using urea. Basal K, S and Ca were supplied. A negative control containing only N and basal nutrients (No-P), and a positive control containing P at the banded rate throughout the entire soil volume were also established (Uniform P). One germinated seed of wheat was planted approximately 3 mm below the soil surface of each tube. The treatments were replicated 4 times and grown in a glasshouse with day and night time temperatures of 25°C and 14°C, respectively. All plants were grown to the 4th leaf stage. The plants were then harvested, dried at 40°C in oven and weighed. Shoots were digested and analysed for P. Roots were extracted, washed and scanned on a flatbed at 600 dpi (i.e. 42 µm) and analysed with WinRhizo® v. 2009c software to determine root length and average root diameter of each section. Six chickpea varieties were also examined under the above conditions, with a growth period of 25 days.
Sulfur, potassium and nitrogen
Similar soil systems were established and wheat lines contrasting in response to P were selected to identify responses to a band. The Red Ferrosol subsoil was used for N and K, whilst two, low S soils from the New England, contrasting in texture were used for S responses. S and K were placed in bands, whilst N was tested in a variety of ways including; continuous nitrate injection at slightly elevated background rates each day and ammonium addition with a nitrification inhibitor. Recovery of S, K and N was measured, and root length in the zones where nutrients were concentrated was measured.
All varieties responded to P application in the band, and grew faster as a consequence. All varieties showed plasticity of roots (i.e. proportional RLD increase in band compared to the surface) and produced between 3 and 18 cm cm-3 of root in the P enriched patch (3-7 cm) in P band treatments (Fig. 1).
Figure 1. Example of the increase in root length density (RLD) of wheat varieties over 13 cm following either no P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (Uniform P).
(Kennedy and Suntop are protected under the Plant Breeders Rights Act 1994.)
The greatest increase in RLD between the surface and the band was observed in Kennedy (Fig. 2). However, statistically speaking there was no difference between the northern varieties in their ability to proliferate roots in response to encountering banded P. This lack of significant difference is a function of the large variation in root system expression in cereals. The extra root length produced in the P band was up to 4 times higher, and in some varieties 8 times higher. Barley and wheat were not different in their plasticity responses although barley varieties were less plastic at the high end, then wheat varieties.
However, when southern varieties are included in the analysis we found significant differences in root proliferation and recovery of P from the band. Whilst shoot dry weights generated by the 4th leaf stage were similar across all varieties, the allocation of carbon to root systems when encountering a band of P was lower in the southern lines tested thus far (Fig 2 and 3). Phosphorus uptake within the band was also considerably lower in southern wheat lines (Fig 3).
Figure 2. Plasticity of 15 wheat and 5 barley varieties over 13 cm following either no-P application (No-P), a band containing 150 mg P kg-1 (P band) or a uniform profile with 150 mg P kg-1 throughout (High P), and comparison of the P use efficiency of varieties barley and northern wheat when encountering the band of P.
(Kennedy, Spitfire, Gregory, Longreach Dart, Crusader, Baxter and Suntop are protected under the Plant Breeders Rights Act 1994.)
Figure 3. Analysis of variation in plasticity and phosphorus recovery from P enriched bands of 15 wheat lines. Lines circled in red are southern lines. Federation (green) is the original Australian wheat line.
Sulfur, potassium and nitrogen
In a lighter texture soil (sandy loam), plants grown with banded S took up 40% more S than control plants, but 40% less S than plants adequately supplied with S. There was no evidence of root proliferation in the S fertiliser band. This suggests the roots do not proliferate in response to banded S, but that by simply having some volume of the soil fertilised, in which the roots were growing, does allow for more S uptake than unfertilised soil. The larger the volume of soil fertilised with S, the better. This effect might be more pronounced in a lighter textured soil where sulfate from a band is more likely to leach, hence increasing the volume of soil effectively fertilised, and the amount of roots growing in soil with higher S concentrations.
In a heavier textured clay soil, we observed no increase in S uptake in response to a banded application of S. Only plants grown in soil fully fertilised with S showed an increase in S uptake. Interestingly we did observe an increase in root proliferation in the band of S; this was accompanied by increased P uptake and the result suggests that the form in which we applied S (i.e. gypsum) may have increased the availability of P. In essence we may have induced a P proliferation response.
There was no root response to application of K or N (in either form) by any of the range of wheat varieties tested.
Root plasticity of wheat and barley varieties
Root architectural variation, or plasticity in wheat, barley and chickpea lines in response to banded P application had not been measured prior to this experiment. All cereal varieties showed root plasticity in the P band by producing more branch roots. These seminal root branches in the P band produced thin roots with higher specific surface area. The diameter of roots in the P band was significantly lower than that of surface layers, suggesting that lateral roots increased specific surface area only where P supply was highest. This study revealed that variation in this trait is small for varieties selected for the northern cereal zone. Comparison of these with modern southern varieties, and, more importantly, with Federation, landrace or wild wheat lines may indicated that there is in fact considerable difference in plasticity and enriched band root recovery in wheat. The results are preliminary however, and need verification with the remaining lines to be tested (including Mace). However, it is suggestive that indirect selection for plasticity has occurred over time. This may have arisen due to differences in the farming systems they are selected for, particularly the nature of rainfall and hence the distribution of surface or subsoil rooting patterns. If confirmed it may provide an incredibly useful screening tool as southern farming systems also run down P and increasingly band applications.
There was limited capacity for early stage lateral root expression in response to banded P in the 6 chickpea varieties tested. Chickpeas were grown for longer time periods than cereals, yet still failed to respond with increased root allocation to P enriched soil. This may account for some of the variable field responses to banded fertiliser applications by chickpea.
The lack of response to enriched S and K bands was not unexpected. It may be that co-location of S and K with P is required to increase root recovery of these nutrients and further research is required to test this as field K (and to a much lesser extent S) responses are increasingly observed. The lack of response to N however was surprising given the previous published reports of proliferation in response to N application. We can offer little explanation for this lack of response at this stage. Logically it is unsurprising, but many scientists before us have observed these responses. Placing P with the N most likely increases N recovery also, but it seems proliferation may be simply a P response.
In summary, we hypothesised that there would be variation amongst 15 commonly grown wheat and barley varieties in their ability to proliferate and respond to banded P applications. All varieties possess the ability to decrease root diameters and increase root length density when encountering enriched P patches, however, northern lines do this to a much greater extent. The question remains what level of root plasticity is required for plants to gain most from heterogeneously distributed nutrients in soil and how we can manage the soil or fertiliser placement to optimise these gains against changing environments.
The research undertaken as part of this project is made possible by the significant contributions of growers through the support of the GRDC, the authors would like to thank them for their continued support.
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