Distribution, Activity And Ecological Indicator Function Of Enzymes In Wetland And Paddy Soils

Global climate change due to the increase of atmospheric CO2, CH4 and other greenhouse gas concentration is a common concern of mankind; global carbon cycle has also been one of core in current climate change and regional sustainable development. Characteristics of overall material cycle in ecosystem may be reflected by the characteristics of the carbon cycle. Soil enzymes are released by soil microorganisms, plant roots and soil animals. Soil enzymes play an important role in carbon cycle and were participated in all the complex biochemical processes in the soil necessary for the life processes of microorganisms in the soil system and the stabilization of soil structure, the decomposition of polymeric material (e.g. cellulose, chitin, and proteins), organic matter formation and nutrient cycling. In addition to as substrates of enzymatic reactions, soil organic matter is also the supports for soil micro-organisms, enzymes and minerals. Being the most complex system in the soil solid phase, soil organic matter is the material basis for soil fertility.Long-term field irrigation experiment and short-term laboratory experiment (25 days) were carried out to evaluate the effect of pulp wastewater on soil enzyme activities and respiration of a managed wetland. Long-term irrigation increased the pH, electrical conductivity (EC), total organic carbon (TOC), total Kjeldahl nitrogen (TKN), available phosphorus and potassium in the soil. A significant increase ofβ-glucosidase (115.3%) was found in wastewater treated soil after 4 years irrigation. Acid and alkaline phosphatase activities were increased by 26.2% and 45.9%, respectively. The protease was 12.6% higher than the control whereas no significant change was observed in urease (an increase of only 4.2%), and this inhibition effect was believed to be due to the increase in concentration of heavy metals resulting from the wastewater. However, there was also a significant increase in the concentration of Na, an important factor for aggravating salinity problems, which cannot be overlooked. But for short-term irrigation, all the enzyme activities and respiration were increased compared to the control, and were well correlated to CODcr values of the irrigated wastewater. The activity of all the enzymes were significantly inter-correlated which indicated that wastewater irrigation has similar effects on the activity of these enzymes involved in C, N and P cycling in soils. It is suggested that pulp wastewater can be used to irrigate wetland with a continuous monitoring of the effluent quality to avoid contamination.The effects of Hg2+and Cd2+ on the activities of urease, invertase, and amylase in diffetent time and different concemtration by indoor soil cultivating method were also studied. Results showed that the activities of soil urease, invertase, and amylase heavy metals were inhibited markedly, but this inhibitory effect differed among enzymes. During the treatment of 45 days, the activities of urease, invertase, and amylase were decreased with the increase of the concentrations of Hg2+. There were significant lofarithmic correlations between the concentration of heavy metals and the activities of these three enzymes (r2>0.902). The inhibitory effect was also characterized by the term of ecological dose (ED50). It indicated that urease activity was more sensitive than the other two emzymes to heavy metals contamination. So, urease activity may be a suitable early warning index to be used in the characterization the soil pollution condition of heavy metals.Specific location of extracellular enzymes within the soil matrix was related to soil organic matter (SOM) quality and carbon turnover. A physical fractionation procedure was used to investigate the distribution of degradative enzymes in various sized fractions of a paddy soil. The distribution of enzymes in the physically isolated fractions depended on the enzymes investigated. Activities of polysaccharides degrading enzymes were greatest in POM fractions, consistent with the rapid turnover of POM fractions. Absolute activities of catalase, dehydrogenase and invertase were bimodally distributed with the highest activity in the POM and clay-sized fractions and lowest in the silt-sized fraction. Polyphenol oxidase and urease were highest in the clay-sized fraction, which contain C pools with very slow turnover rates. It was concluded that the slow enzymes production and carbon turnover rates in mineral-dominated fractions were corresponded to the immobilization and protection effects of enzymes and their carbon substrates.However, soil enzymatic reactions occur in a heterogeneous rather than homogenous environment. A certain proportion of free enzymes may undergo stabilization through adsorption on soil minerals or through incorporation into humic material, despite affecting their catalytic potential, may enable enzyme activity to remain active for long periods of time in soil. To better understanding of enzyme stabilization and the subsequent catalytic process in soil environment, adsorption, immobilization and activity ofβ-glucosidase on various soil colloids were studied. Higher adsorption amounts and lower adsorption affinity were found on fine soil colloids, which were attributed to their higher surface area and the content of iron oxides. The percentages ofβ-glucosidase desorbed from external surfaces of the coarse soil colloids (27.6-28.5%) were higher than those from the fine soil colloids (17.5-20.2%).β-Glucosidase immobilized on the coarse inorganic and organic soil colloids remained 72.4 and 69.8% of activity, respectively, which indicated that the facilitated effect of soil organic matters in the inhibition of enzyme activity. The residual activity for the fine soil clay is 79-81%. After 30 d of storage at 25℃the freeβ-glucosidase retained 66.2%of its initial activity, whereas the soil colloidal particles immobilized enzyme retained 77.1-82.4% of its activity. Due to the protective effect of supports, soil colloidal particles immobilized enzymes were less sensitive to pH and temperature changes than free enzyme. The kinetic and thermodynamic parameters ofβ-glucosidase immobilized on different-sized colloidal particles from a paddy soil were also studied. Immobilization ofβ-glucosidase decreased the Vmax values and increased the Km values, which indicated that the immobilized enzyme has an apparently lower affinity for its substrate due to structural changes ofβ-glucosidase or less accessibility of substrate to the active site of immobilized enzymes. The values of Ea,△Ha and Q10 for the immobilized enzymes were smaller than that with free enzyme, implying that the immobilized enzymes are less temperature sensitive. Furthermore, mean values of Q10 were ranged from 1.32 to 1.50. These results indicated the higher stablity ofβ-glucosidase after immobilization on various soil colloidal particles. Data obtained in this study are helpful for the further researching on enzymatic mechanism in carbon cycling and soil carbon storage.