Dissolved Organic Matter Release From Sludge Biochar And Its Effect On Soil Acid Phosphatase Activity

A growing number of studies have found that biochar can effectively increase soil nutrient elements(such as N,P,K,Mg,etc.)and organic matter content,improve soil water retention,fertilizer retention and aeration,regulate soil pH value and cation exchange capacity,etc.,thus improving soil fertility and solving the increasingly serious problem of soil degradation.However,most of the studies have ignored the effects of biochar on soil enzymes,which limits the accurate assessment of soil fertility and ecosystem health under the effects of biochar.Under the condition of phosphorus starvation,plants are induced to secrete acid phosphatase to hydrolyze phosphate groups on organophosphorus substrates for absorption and utilization by plants.When biochar is used as a soil modifier,the dissolved substances released into the soil mainly include(heavy)metals,polycyclic aromatic hydrocarbons(PAHs),dissolved organic matter(DOM),etc.These substances may directly act on soil acid phosphatase,reduce its activity,and thus pose a potential threat to soil health.The effect of soluble components in sludge biochar(SBC)on acid phosphatase activity was systematically studied in this paper.In the previous characterization experiments,it was found that SBC could dissolve a large amount of DOM.Whether DOM dissolved by SBC will have a negative effect on soil acid phosphatase activity is the core issue to be explored in this paper.In this study,DOM derived from sludge biomass(SBMD),DOM dissolved by SBC at 200?(SBCD200),DOM dissolved by SBC at 300?(SBCD300),and DOM dissolved by SBC at 450?(SBCD450?),and its properties were analyzed.The components of SBCD were identified by three-dimensional fluorescence coupled with parallel factor analysis(EEM-PARAFAC).Compared with the existing literature,the research hypothesis was put forward:the properties of SBCD and the types and contents of SBCD components are controlled by pyrolysis temperature.Since the pyrolysis process will decompose or volatilize the large molecular weight SBCD,the differences of SBCD components and contents under different pyrolysis temperatures may lead to different inhibitory degrees on the activity of acid phosphatase.On the basis of this hypothesis,this experiment simulated the changes of enzyme activity after the interaction of acidic phosphatase with SBCD,When the pH of the solution was 3.The relationship between the composition of SBCD and the changes of acid phosphatase activity was emphatically analyzed,and the following conclusions were obtained:(1)With the increase of pyrolysis temperature,the physicochemical properties and the dissolved substances of SBC changed.SBC is weakly alkaline,and its pH mainly ranges from 8.58to 9.19.The specific surface area of SBC increased from 5.95 m²·g^-1 at 200?to 35.0 m²·g^-1 at450?.The yield of SBC decreased from 88.7%to 70.5%,while the volatiles decreased from47.9%to 36.5%.The degree of carbonization of SBC increased and the surface polar functional groups decreased.In addition,the dissolution of SBCD increased first and then decreased with the increase of pyrolysis temperature.The dissolved organic carbon(DOC)of SBMD,SBCD200 and SBCD300 increased gradually with the increase of pyrolysis temperature from 6.43 mg·g^-1 to 15.71mg·g^-1.The DOC of SBCD450 decreased to 7.45 mg·g^-1.The(heavy)metals Mn,Cr,Fe,Zn,As and Ni are enriched in SBC with the increase of pyrolysis temperature,and the contents of Cr,Zn and Ni is higher,but the(heavy)metal content in SBCD is lower.Therefore,the application of SBC in soil environment has certain potential risks.However,the environmental risk of(heavy)metals dissolved by SBC is low or negligible under current research conditions.(2)The properties and components of SBCD are affected by pyrolysis temperature.With the increase of pyrolysis temperature,the aromaticity of SBCD increased significantly(p?0.001),the degree of humification increased first and then decreased,the degradation accelerated,and the bioavailability increased.To sum up,during the pyrolysis process,the large-molecular weight DOM in SBCD degrades into small-molecular weight DOM that is easy to be bioutilized.In addition,three fluorescence components(C1,C2,C3)in SBCD were identified by EEM-PARAFAC.Among them,group C1 and C2 were divided into humic acid substances;The C3group is classified as tryptophan.With the increase of pyrolysis temperature,the relative abundance of C1 increased from 38.5%in SBMD to 51.3%in SBCD450.The relative abundance of C2 increased from 23.8%in SBMD to 38.6%in SBCD450.The relative abundance of C3 in SBMD decreased from 37.7%to 10.0%in SBCD450.It can be seen that with the increase of pyrolysis temperature,the humus substances in SBCD increase,while the tryptophan substances decrease.(3)SBCD can inhibit the activity of acid phosphatase,and the degree of inhibition depends on its source and concentration.The specific activity of acid phosphatase in SBMD and SBCD200was most inhibited.The specific activity of acid phosphatase in SBCD300 and SBCD450 was less affected.In addition,the increase of SBCD concentration further increased the inhibition of acid phosphatase activity.When the concentration of SBCD was 12.5 mg C·L^-1,the activity of acid phosphatase in SBMD and SBCD200 decreased about 92.3%to 30.55?35.1%.The specific activity of acid phosphatase in SBCD300 and SBCD450 was still between 73.2 and 75.0%.In SBMD and SBCD200,the fluorescence intensity of tryptophan residues decreased greatly with the increase of SBCD concentration,while in SBCD300 and SBCD450,the effect of the increase of SBCD concentration on the fluorescence intensity of tryptophan residues decreased relatively little.The possible reason is that one of the components in SBMD and SBCD200 perturbates the active site of the enzyme and causes local conformational change.In combination with the research in Chapter 5,it was found that C3 component may be an important factor affecting the inhibitory activity of SBCD against acid phosphatase.There was a negative linear correlation between the F_max of C3 component and the activity of acid phosphatase,that is,the specific activity of acid phosphatase decreased when the F_max of C3 component increased.Therefore,the C3 component may have a greater effect on the activity of acid phosphatase.