Changes in soil P pools during legume residue decomposition

In rotations, cereals after legumes often have higher P uptake than after cereals, and it has been suggested that legume residues may play an important role in this effect by mobilising soil P in the legume phase and by the P added with the residues. However, little is known about the changes in P pools during legume residue decomposition. Residues from faba bean, white lupin or chickpea (shoots or roots) with varying P concentrations were added to a loamy sand soil with a low available P concentration, and the concentration of various soil P pools were assessed by soil P fractionation on days 0, 14, 28 and 56. Residue addition significantly increased cumulative respiration which was positively correlated with amount of C added with residues (r = 0.54, p < 0.05), and negatively correlated with the C/P ratio (r = −0.58, p < 0.05). The size of the P pools changed over time and was affected by both residue P concentration and plant part (root or shoot). In the first two weeks, microbial P increased and resin P and NaHCO3–Pi decreased with low-P residues (0.6–1.8 mg kg−1) while the reverse was true for high-P residues (6.5–8.3 mg kg−1). In medium-P residues (2.9–3.3 mg kg−1), there was a balance between mineralisation and immobilisation. Decreases of NaOH–Po occurred earlier with low-P and medium-P residues (d0 to d14) than with high-P residues (d14–d28). The increase in residual P with all residues indicated that part of mineralised P was converted into stable P within 14d; but later (d28–d56), the concentration of residual P strongly decreased. In the period from d28 to d56, there was an increase in NaOH–Po and HCl–P with all residues, indicating net conversion of P into stable organic and inorganic P. Changes in P pools between roots and shoots occurred mainly in the initial phase. The concentration of NaOH–Po increased from d0 to d14, but then decreased from d14 to d28 with addition of shoot residues whereas the reverse was found with roots. These changes were generally more pronounced in low-P than in medium-P residues. This study demonstrates that changes and transformations in soil P pools over time were dependant on residue P concentration and plant part.