| Extracellular enzymes mediate the degradation, transformation and mineralization of soil organic matter, which are important biogeochemical processes and closely related to global climate change. The activities of cellulases, phosphatases and other hydrolases have received extensive study and in many cases stoichiometric relationships and responses to disturbances are well established. But inspite of this, phenol oxidase and peroxidase activities, well known for their association with the key ecosystem functions of lignin degradation, humification, carbon mineralization and dissolved organic carbon (DOC) export, have been measured in only a small subset of soil enzyme studies.The purpose of this work was to investigate the effects of moisture on soil-phenol oxidase (pox), peroxidase (per), phenolics and dissolved organic carbon, and ascertain the relationships between soil oxidase activities and soil organic carbon. The soil samples were collected from the quaternary developed paddy fields located in Changsha (CS), Xianning (XN) and Xiangfan (XF), China, and incubated at 30℃for 42 days using different moisture contents of 40%,70%,100% or 200% water hold capacity (WHC). Impact of various factors (i.e. pH of buffer solution, soil dilution multiple, reaction time and temperature) were determined on soil phenol oxidase and peroxidase activities by L-DOPA method prior to the incubation. The results are as follows:1) Soil phenol oxidase and peroxidase activities decreased significantly with the reduced soil dilution multiple or lengthened reaction time, but the effects of buffer solution pH (pH=4-8) and temperature (15～35℃) were less obvious, even though the higher activities were observed at pH=8 or 30℃.2) Moisture had a remarkable positive effect on soil phenol oxidase and peroxidase activities, and the stimulation to peroxidase was much stronger. At 100%WHC and 200%WHC, a rapid increase in activities of both subjected enzyme was noted in CS and XN soils on the third day of incubation, while it reached to its highest in 5-15 days. Moreover, the phenol oxidase and peroxidase activities were higher at 200%WHC than that of 100%WHC, and the highest increase in phenol oxidase and peroxidase activities was 9 and 20 times in XN soil, while 4 and 7 times in CS soil than the activities at the beginning of the incubation. Similarly, peroxidase activities of XF soil also increased in 8-36 days with the water content of 100%WHC and 200%WHC. However, lower activities with no pronounced change were observed at 40%WHC and 70%WHC during the incubation from all the sites.3) Dissolved organic carbon of CS, XN and XF paddy soils increased rapidly in 15～22 days followed by subsequent falling until the incubation was finished. Higher dissolved organic carbon was observed at 100%WHC and 200%WHC as compared to 40%WHC and 70%WHC in 15～36 days. Phenolics had no significant response to the different moisture regimes throughout the incubation period.4) The relevant analysis between soil oxidase activities and phenolics showed the negative correlations at 40%WHC and 70%WHC, and positive correlations at 100%WHC and 200%WHC, though there were properties differences among CS, XN and XF soils.. But no significant relationship was observed between dissolved organic carbon and oxidase activities or phenolics.These results may have implied the complicated decomposition and transformation processes of soil phenolics and dissolved organic carbon. Furthermore, soil phenol oxidase and peroxidase may also be involved in the partial decomposition of complex recalcitrant phenolics. This process may result in the release of smaller and more soluble phenolics, which supply enough substrates for the humification, especially at the waterlogging paddy soils where the hydrolase activities are inhibited. |
PDF Full Text Request