Research pursues the secrets of P mobilisation
While wheat grown after a pulse break crop takes up more phosphorus than wheat following wheat, the actual mechanism is not well understood.
La Trobe University PhD student Daniel Espinosa is seeking answers that will help growers use more of the phosphorus bank that is otherwise locked up in soils.
Mr Espinosa, whose research is part of the GRDC Soil Biology Initiative, says interactions between soil properties, plant biology and microbial activity are all likely to determine the amount of soil phosphorus that is mobilised by legumes.
While legumes differ in their ability to mobilise soil phosphorus reserves, legume break crops nonetheless contribute significantly to increase the phosphorus uptake of following wheat crops.
“Research suggests that mobilisation of phosphorus by legumes occurs from sparingly available soil phosphorus compounds that wheat cannot use,” he says.
“It is likely the extra phosphorus becomes available to wheat from legume residue decomposition. It is also possible that changes in soil microbial communities with legume crops occur, which may enhance soil phosphorus availability for the wheat crop. However, so far it has not been possible to detect any surplus available phosphorus in the soil after a legume break crop.”
Mr Espinosa says different legume crops have different strategies to mobilise phosphorus from the soil, therefore the benefit for the following wheat also varies among legume break crops.
“Acidifying the soil around the roots, and organic acid release from roots, are one of the ways that make legumes efficient in mobilising phosphorus from inorganic soil phosphorus pools.
“It is likely that any additional phosphorus that is mobilised by the break crop is refixed by the soil or immobilised by microbial activity.
“As a result, I used an intercropping approach to see if wheat plants growing with legume crops could take up extra phosphorus when the root systems of both crops are intermingled and compared this to a wheat and break crop grown together but their root systems kept separate.
“It was proposed that surplus mobilised phosphorus from the break crop’s root activity would be scavenged and taken up by the intermingled wheat roots.”
The research found that wheat plants did scavenge and use extra mobilised phosphorus when they were grown as companion crops with legumes.
The trial found phosphorus uptake of wheat was increased by 114 per cent with chickpeas and by 70 per cent with white lupins due to an increase in shoot biomass and higher phosphorus concentrations in wheat shoots (Figure 1).
Wheat growing with field peas had the lowest shoot biomass overall, but phosphorus uptake still increased by 69 per cent, despite the reduced growth with the barrier in place.
While growers may not be in a hurry to mix their wheat with another crop, Mr Espinosa says the finding points the way to how future research can examine how legumes mobilise soil phosphorus so the process can actually be utilised in cropping systems.
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A fact sheet on Crop Nutrition Phosphorus Management is available at:
GRDC Project Code UA00119