Soil pH is the main factor influencing growth and rhizosphere properties of wheat following different pre-crops

Phosphorus utilization efficiency is very low in soil due to its low solubility and mobility. Legumes have been shown to increase P uptake of the following wheat, but the underlying mechanisms of this effect are unclear. Using three soils with low P availability differing in pH and therefore predominant P form: Mount Bold (pH 4.8), Monarto (pH 7.5) and Langhorne Creek pH 8.8), a rotation of faba bean, white lupin and wheat and unplanted soil in phase 1 and wheat in phase 2 was grown. To distinguish between the pre-crop effect and the residue effect, the residues from the pre-crops in phase 1 were either returned to the pre-crop soil or added to the previously unplanted soil. In the rhizosphere of wheat, P fractions were determined and the community composition of bacteria, fungi, P mobilizers (ALP gene), ammonia-oxidizing bacteria (AOB) and archaea (AOA) were analyzed by polymerase chain reaction (PCR) - denaturing gradient gel electrophoresis (DGGE). There were no differences in wheat root and shoot biomass among treatments in the acidic Mount Bold soil, but in both the alkaline Langhorne Creek and neutral Monarto soils, wheat shoot biomass was highest in wheat and white lupin pre-crop soils. The alkaline Langhorne Creek soil had the highest concentrations of HCl P, whereas NaOH I Pi, NaOH I Po, NaOH II Pi, residual P and total P were highest in the acidic Mount Bold soil. In the soil with residues, the presence of plants in phase 1 of the rotation increased the concentrations of labile P pools and the NaOH extractable P pools in the rhizosphere of wheat compared to wheat grown in the previously unplanted soil, with the increase occurring in both inorganic and organic P fractions. As the amount of residues added in the soil was only 1 g kg(-1), the effect of the residues alone on soil P pools was relatively small. The community composition of all microbial groups investigated differed among soils and within one soil by the pre-crop. Among the pre-crops, white lupin had a negative effect on AMF colonization although resin P concentrations were not higher than in the other pre-crop treatments. Soil P pools and microbial community composition were predominantly affected by soil pH and within a given soil by the pre-crop treatment whereas the residue effect was small. The finding that in two of the soils the presence of plants in phase 1 of the rotation increased P uptake by 6 week-old wheat but also increased the concentrations of labile P pools and the NaOH extractable P pools in the rhizosphere of wheat suggests that pre-crops may enhance P uptake by wheat also in the later stages of growth.