Humification of three plant residues with Sakurajima volcanic ash as influenced by phosphate addition and low pH condition

The influence of phosphate addition and low pH condition on humification of three plant residues were determined by performing model thermal incubations of the plant residues rice straw (Oryza sativa), broadleaf sawdust (Kashiwa, Quercus dentata), and coniferous sawdust (Japanese cider, Cryptomeria japonica) with Sakurajima volcanic ash: Four experimental set ups were prepared: (Group 1) plant residues only; (Group 2) plant residues with Sakurajima volcanic ash; {Group 3) plant residues with Sakurajima volcanic ash and Na2HPO4; and (Group 4) plant residues with Sakurajima volcanic ash and Na2HPO4 with the pH adjusted to 5. Humic acids were extracted using a mixed solution of 0.02 M Na4P2O7 and 0.1 M NaOH. Extractable Fe and Al oxides were determined using three different extraction methods: dithionite-citrate-bicarbonate (DCB), acid-oxalate (TAMM), and Na-pyrophosphate (NaPP). The pH values obtained for Group 1 samples ranged from 2.75 to 6.21; Group 2 samples were from 3.74 to 4.78; Group 3 samples were near neutral or alkaline, ranging from 6.93 to 7.59; and the values for Group 4 samples ranged from 5.38 to 6.28. The delta logK values decreased with longer incubation times while the RF values increased. Group 3 samples showed a decrease in RF values compared to Group 2. Type A humic acids were produced only from Group 2 samples derived from rice straw and broad leaf sawdust after about 120 incubation days. The amounts of the Fe and Al from all types of extraction methods decreased in Groups 3 and 4 indicating a possible binding up of these components with phosphate ions. It can be concluded that addition of Na2HPO4 increased the pH of the system and at the same time lowers the solubility of Fe and Al present in the Sakurajima volcanic ash. However, adjusting the pH to 5 to maintain lower pH conditions, has slightly increased the solubility of Fe and Al. Therefore, the higher the amount of extractable Fe and Al, the higher the RF values indicating a faster humification process. The relatively small amounts of Fe and Al in higher pH values and their reaction with phosphate rendered these components ineffective in hastening the humification process. Based on the correlation analysis, Al was more effective in hastening the humification process than Fe.