Removal of phosphorus in residues of legume or cereal plants determines growth of subsequently planted wheat in a high phosphorus fixing soil

摘要

Legumes have been shown to improve the phosphorus (P) uptake and growth of subsequent cereals even when all legume residues (including roots) are removed, but the mechanism(s) responsible have not been elucidated. Further, this phenomenon has not been investigated in high P-fixing soils, which are prevalent in tropical and subtropical regions where P deficiency in soils is a major global issue. We tested the hypothesis that carbon (C) inputs into the soil from the roots of legumes may increase labile organic C pools and microbial biomass in soil cf. wheat (Triticum aestivum L.), with consequences for subsequent wheat growth. The hypothesis was tested by cultivating wheat after pre-cropping of the soil with either wheat, faba bean (Vicia faba L.), chickpea (Cicer arietinum L.) or an unplanted fallow in a high-P-fixing Ferralsol, with or without P fertiliser addition to the pre-crop, under glasshouse conditions. The addition of P fertiliser resulted in significantly higher P concentrations in the NaHCO3-Pi, NaOH-Pi and HCl-Po pools in the bulk soil after the pre-crop phase, but resulted in significantly lower water-Po, NaHCO3-Po, NaOH-Po and total organic C (TOC) concentrations. Neither of the legumes nor wheat changed the soil P pools, with the exception of the water-Pi pool, which was significantly higher following chickpea where P fertiliser was applied. While the faba bean pre-crops resulted in higher total microbial biomass under nil-P fertiliser conditions, these changes did not result in enhanced growth of the subsequent wheat crop. Notably, a strong negative correlation between subsequent wheat growth and the amount of P removed by the pre-crop was observed both with (r = -0.60) and without (r = -0.80) P fertiliser, suggesting that in high P-fixing soils the depletion of P pools by pre-crops limits the growth of subsequent cereals. Any benefits of legumes to P cycling in rotations on high P-fixing soils likely relies on contributions from the mineralisation of root residues (and shoot residues where these are retained) to soil P availability, rather than any rhizosphere-specific changes in soil P pools induced by legumes.