Changes in phosphorus pools in three soils upon addition of legume residues differing in carbon/phosphorus ratio

Previously we showed that addition of legume residues affected the size of different soil phosphorus (P) pools in an alkaline loamy sand soil. Here, we tested whether the changes in soil P pools induced by residue addition are generally applicable or whether they are dependent on certain soil properties. Three legume residues differing in P concentration, faba bean (Vicia faba L.) (high P), chickpea (Cicer arietinum L.) (medium P), and white lupin (Lupinus albus L.) (low P), were added at a rate of 20 g residue kg–1 soil to three different soils with low Colwell-P concentration: Mt Bold (sandy clay loam, high organic carbon (C) content, pH 5.1), Monarto (loamy sand, low organic C content, pH 7.5), and Langhorne Creek (sandy loam, low organic C content, pH 8.1). Soil P pools were assessed by sequential P fractionation on days 0 and 42. In residue-amended soils from day 0 to day 42, the concentrations of water-soluble and microbial P decreased, whereas the concentrations of NaHCO3-Pi (inorganic P) and NaOH-Po (organic P) increased. The magnitude of these changes differed among soils, being greatest in the Mt Bold soil. Residue addition had little or no effect on the concentrations of NaOH-Pi and residual P, which also did not change significantly over time. Principal component analysis of the data showed that the size of the P pools was related to soil properties; high concentrations of HCl-P were associated with high pH and calcium concentrations, high concentrations of NaOH-P and residual P were correlated with high aluminium, silt, organic C, and total nitrogen and P. In the unamended soil on day 0, the concentration of NaHCO3-Pi was correlated with the clay content, whereas on day 42, the concentrations of the labile P pools were related to amount of P added with the residues. It can be concluded that most effects of residue addition to soils on microbial activity and growth and soil P pools can be generalised across the three soil used in this study, but that the size of the P pools is affected by soil properties such as organic C content, pH, and texture.

Md Alamgir and Petra Marschner