The role of particulate organic matter in phosphorus cycling

In tropical cropping systems, a better understanding of P transformations and short-term P cycling during decomposition of incorporated residues is pertinent. The primary objectives of this study were (i) to assess P immobilization in decomposing residues of sorghum (Sorghum bicolor (L.) Moench.) and crotalaria (Crotalaria juncea L.), (ii) to establish the role of soil fungi in the process, and (iii) to determine the contribution of P immobilization in explaining differences in soil P availability from plant residues in two weathered soils (Ultisol and Alfisol). Phosphorus associated with particulate organic matter (POM-P) was measured at different residue decomposition stages. Irradiated and nonirradiated residues were utilized to evaluate the origin of fungal colonization of particulate organic matter (POM). Results indicate a significant release of P from the residues at early stages of decomposition. An average of 93 and 76% of the initial POM-P was released after 5 d of decomposition in the Ultisol and Alfisol, respectively; and no significant differences were found between the residue types. After this initial 5-d period, significantly higher values of POM-P were observed for sorghum (5.8 and 7.9 mg P kg(-1)) as compared with crotalaria (1.9 and 2.8 mg P kg(-1)) in the two soils, suggesting a higher P immobilization in decomposing sorghum residue. Phosphorus immobilization represented up to 30% of the P in the residues and it could account for observed differences in P availability in the amended soils. A correspondence between patterns of P accumulation and fungal colonization of POM suggested that soil fungi might be responsible for this P immobilization. Further research is required however, to identify the mechanisms regulating this fungal colonization and P immobilization of plant residues in tropical acid soils.