| The acceleration of urbanization has led to a sharp increase in municipal sludge production.Land use of sludge aerobic composting is an effective way to reduce,stabilize and recycle municipal sludge.It is of great significance to clarify the characteristics and mechanism of soil organic carbon and nitrogen mineralization under the condition of compost sludge addition for improving soil carbon sink potential and soil quality.In this study,the degraded grassland soil in the loess hilly region was used as the research object.The effects of different sludge compost addition ratios(0,2.0%,5.0%,10.0%,15.0%and 20.0%)and culture temperatures(5℃,15℃,25℃ and 35℃)on soil organic carbon and nitrogen mineralization characteristics,temperature sensitivity(Q10),soil chemical properties,fluorescence spectral characteristic parameters and soil enzyme activities were studied by indoor culture experiments.The evolution driving mechanism of soil unit organic carbon and nitrogen mineralization after different temperatures and sludge compost application and the main controlling factors for municipal sludge resource utilization and soil carbon pool stability were revealed,which provided theoretical support for municipal sludge treatment and soil organic carbon sequestration.The main conclusions are as follows:(1)The unit soil organic carbon mineralization rate(Rc),soil net nitrogen mineralization rate(R-Net),and soil unit organic carbon cumulative mineralization(Cm-c)increased significantly(p<0.05)when the amount of compost sludge added was 0-10.0%.There was no significant difference in the mineralization-related indicators when the compost sludge application reached 10.0%-20.0%.The increase in temperature had a very significant positive effect on Rc and R-Net(p<0.001).Compared with 5℃ culture,the mean values of Rc and R-Net increased by 5.4-7.8 and 5.7-11.2 times at 35℃.Q10 increased significantly with the increase of sludge addition in the range of 0%～10.0%(p<0.05),and decreased significantly with the increase of sludge addition in the range of 10.0%～20.0%.Multivariate analysis of variance showed that composting sludge addition and temperature increase had a significant interaction on the mineralization characteristics of soil organic carbon and nitrogen,and temperature was the most important factor affecting carbon and nitrogen mineralization.(2)The addition of sewage sludge and the increase of temperature had positive effects on soil fluorescence characteristic parameters.The content of soil water-soluble organic matter(DOM)was significantly positively correlated with temperature(p<0.001).The content of soil DOM increased significantly with the increase of sludge addition ratio in the gradient of 0～10%,and the conversion of soil DOM was limited when the sludge addition ratio reached 10～20%.The fluorescence index(FI)was significantly correlated with temperature and compost sludge addition(p<0.001).The increase of temperature and compost sludge addition ratio would promote the transformation of soil humus from organic matter decomposition to microbial activity decomposition.The biological source index(BIX)was significantly correlated with sludge addition(p<0.001),and the source was mainly endogenously generated by microbial activities.The humification index(HIX)was significantly correlated with temperature and sludge addition(p<0.001).The increase of temperature would increase the activity of soil organic matter and increase the content of soil humus and microorganisms to further promote the humification of soil organic matter.(3)The addition of compost sludge had a promoting effect on soil enzyme activity and its temperature sensitivity,which increased exponentially in the temperature range of this study.Cellobiohydrolase(CBH),β-glucosidase(BG)and N-acetylglucosaminidase(NAG)all increased with the increase of sludge addition ratio,and alkaline phosphatase(ALP)had no significant difference with the enzyme rate constant of sludge addition.The Q10 of each compost sludge ranged from 0.85 to 3.39,which varied with different temperature ranges.The Q10 of BG,NAG and ALP was significantly negatively correlated with temperature(p<0.01).The Q10 of BG,CBH,NAG and ALP was significantly positively correlated with culture time(p<0.01).The Q10 of CBH,BG and NAG was significantly positively correlated with the amount of compost sludge added.The Q10 of CBH,BG and NAG was not significantly correlated with SOC,TN and TP.Soil enzyme stoichiometric ratios Cenz:Nenz,Cenz:Penz and Nenz:Penz ratios increased with temperature,and Cenz:Penz and Nenz:Penz ratios were less than 1.(4)There was a certain response relationship between composting sludge addition and temperature increase on soil enzyme,carbon and nitrogen mineralization and fluorescence characteristic parameters.Under different sludge addition,soil C:N decreased significantly with the increase of composting sludge addition ratio(p<0.01),and soil C:P and N:P increased significantly with the increase of composting sludge addition ratio(p<0.01).There was a significant positive correlation between the cumulative release of soil CO2 and the cumulative mineralization of soil nitrogen(p<0.01).Soil enzymes were positively correlated with incubation temperature(T),Rc,Cm-c and Cm-n(p<0.001),and negatively correlated with NH4+-N and ammonification rate(Rnh4)(p<0.001).Cenz:Nenz,Cenz:Penz and Nenz:Penz were positively correlated with T,W,Rc,Cm-c and Cm-n(p<0.01),and negatively correlated with NH4+-N and Rnh4(p<0.01).The mineralization of FI,BIX and HIX in soil at low temperature was mainly dominated by exogenous input of self-hydrolysis of soil nutrients.With the increase of temperature,the contribution of humus and biogenic sources in soil gradually changed from terrestrial to microbial autochthonous source.FI,BIX and HIX were significantly correlated with temperature,cumulative carbon mineralization and cumulative nitrogen mineralization,and positively correlated with compost sludge addition,Cenz:Nenz,Cenz:Penz,Nenz:Penz,NAG,BG,CBH,and ALP(p<0.01). |
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