Effects Of Sludge Composting On Soil Enzyme Dynamics And Thermodynamic Characteristics

With the rapid development of urbanization,the production of urban domestic sludge is increasing.The composted sludge is rich in nitrogen and phosphorus nutrients and organic matter,which can be used to improve the quality of soil,thus realizing the resourcefulness,harmlessness,stabilization and reduction of sludge.Soil enzymes are important players in soil nutrient-element cycling and can reflect the processes of soil material transformation and energy metabolism.The kinetic and thermodynamic parameters of soil enzymes can characterize the biochemical reactions and energy changes of sludge-applied soil,which are important for clarifying the mechanism of soil nutrient transformation after composted sludge application.In this study,we investigated the effects of different incubation temperatures(5℃,15℃,25℃ and 35℃)and composted sludge additions(0%(CK),2%(W1),5%(W2),10%(W3),15%(W4)and 20%(W5))on soil β-1,4-glucosidase(BG),cellobiose hydrolase(CBH),N acetyl-D-glucosaminidase(NAG)and alkaline phosphate monoesterase(ALP)enzymatic activities and their kinetic and thermodynamic parameters.The following main findings were obtained:(1)Composted sludge addition significantly increased soil enzyme activities per unit organic carbon,with BG and ALP,CBH,and NAG soil enzyme activities per unit organic carbon reaching maximum values at the sludge addition treatments of W2,W3,and W4,respectively.The composted sludge addition treatment significantly increased the average soil enzyme activity per unit,which increased 0.11～0.14 times under the W2 treatment compared to the CK treatment.The carbon to phosphorus ratio and nitrogen to phosphorus ratio of soil enzymes showed an increasing trend with the increase of sludge addition,while the carbon to nitrogen ratio showed a decreasing trend.There was a significant positive correlation between vector length and vector angle(P<0.001).Soil enzyme activities per unit organic carbon for BG,CBH,NAG and ALP increased with increasing incubation temperature,and the temperature sensitivity Q10 was greater than 1 for all,indicating that increasing temperature would increase enzyme activities.Soil total phosphorus(TP),nitrate nitrogen(NO3--N),organic carbon(SOC)and fast-acting phosphorus(AP)contents were important factors affecting soil enzyme activities and their enzyme stoichiometric ratios.(2)Soil enzyme Mie constant Km decreased significantly with increasing incubation temperature,and potential maximum rate Vmax and catalytic efficiency Kcat increased significantly with increasing incubation temperature.Soil enzyme Vmax showed a trend of increasing and then decreasing with the increase of sludge volume.Soil enzyme Vmax reached the maximum at the treatment of sludge addition of W2 and W4 for BG,CBH,ALP and NAG,respectively,and the maximum increase of soil enzyme Vmax was 2.66 times compared with that of CK.With the increase of sludge volume,the soil enzyme Km of BG,CBH and NAG showed an overall trend of decreasing and then increasing,and the Km of ALP showed an overall trend of increasing and then decreasing,and the maximum decrease of Km of ALP at different incubation temperatures ranged from 4.59%to 39.10%compared with CK.The soil enzyme Kcat of BG,ALP and NAG showed a trend of increasing and then decreasing,and Kcat of CBH showed a trend of increasing and then decreasing.Soil enzyme Kcat and soil enzyme activity showed a significant or highly significant positive correlation(P<0.05),which can well characterize the enzyme activity.Soil enzyme kinetic parameters were driven by the combination of NO3--N,TP and SOC.The temperature sensitivity Q10(Vmax)of soil enzyme Vmax for BG,CBH,NAG and ALP was basically greater than 1.The Q10(Vmax)of BG was maximum at the temperature interval of 5-15℃ and decreased with increasing incubation temperature,and the soil enzyme Vmax of CBH was most sensitive to temperature.The Q10(Vmax)maximum values were 1.69,2.07,1.71,and 1.82,respectively.The range of Q10(Km)was from 0.16 to 1.79,and most of the values were between 0.50 and 1.10,which showed no consistent pattern and fluctuated in different temperature intervals.The Q10(Kcat)values were all greater than 1 and ranged from 1.00 to 6.34.(3)With the increase of sludge amount,the soil enzyme activation energy Ea showed an overall decreasing trend,indicating that the addition of sludge reduced the reaction energy barrier and faster reaction rate.Compared with CK,the maximum decrease of Ea for BG,CBH,NAG,and ALP was 41.50%,22.81%,35.92%,and 63.09%,respectively.The linear fits between soil enzyme activation energy Ea and potential maximum rate of enzymatic reaction Vmax at different levels of sludge addition all reached significant or highly significant positive correlation(P<0.05).The soil enzyme activation energy Ea was driven by the combination of AP,TP and TN.The activation enthalpy ΔH showed a trend of decreasing and then increasing with increasing sludge volume,the Gibbs free energy ΔG showed an overall trend of decreasing and then increasing,the activation entropy ΔS of BG and ALP showed an overall trend of decreasing,while the activation entropy ΔS of CBH and NAG showed an overall trend of decreasing and then increasing.The formation of transition state complexes between enzymes and substrates is an energy-absorbing process and the molecular conformational complexity of BG,CBH,ALP and NAG was the least and the reaction rate was the highest at the sludge addition levels of W2 and W3,respectively.With the increase of temperature,the activation enthalpy ΔH of the four enzymes showed a decreasing trend,and the Gibbs free energy ΔG all showed an increasing trend.The activation entropy ΔS showed an overall increasing trend.The activation enthalpy ΔH varied more than activation entropy ΔS at different sludge volume addition levels,and the enzymatic reaction was enthalpy-controlled process.