Effects Of C/N Ratio On Conversion Of Organic Carbon Compounds In Aerobic Composting And Its Mechanism

Aerobic composting is an effective way to realize the harmless and resource utilization of agricultural waste,which can not only realize the effective recycling of resources,but also reduce the air,water and soil pollution caused by improper waste disposal.However,in the process of aerobic composting,due to the existence of lignocellulose and other refractory rate-limiting organic substances,the aerobic composting raw materials decompose slowly,the composting cycle is too long,and the maturity effect is poor.In this experiment,five different initial C/N treatments were set up:15:1,20:1,25:1,30:1,35:1,marked as T1,T2,T3,T4,T5.The effects of C/N ratio on organic carbon transformation during aerobic composting were studied by measuring the degradation of lignocellulose,the activities of related enzymes,the community succession of functional microorganisms producing β-glucosidase,and the changes of transcriptional activity of functional genes in dominant microbial genera.The relationship between organic carbon conversion,lignocellulose degradation,β-glucosidase production and lignocellulose degradation under initial C/N ratio was investigated.The main test results are as follows:1.C/N significantly affected the content of organic carbon and its components during aerobic composting.1.1 The degradation of total organic carbon(TOC)mainly occurred in the first 12 days of aerobic composting.With the increase of initial C/N,the degradation amount of TOC,the degradation rate and the ratio of TOC to TOC increased.In the whole process of aerobic composting,the organic carbon content of T5 treatment with high C/N was significantly higher than that of other treatments,and T1 treatment with low C/N was significantly lower than that of other treatments(P<0.05),while the organic carbon content of T5 treatment decreased the most,the degradation rate was the highest,and the loss of organic carbon during composting was the largest.1.2 Significant increase of dissolved organic carbon(DOC)mainly occurred in the first 12 days of composting.Treatment T5(C/N=35)had the largest increase of DOC,and the content of DOC began to decrease from the 12th day to the 45th day.Treatment T3(C/N=25)had the least decrease.After 45 days of composting,the content of hot water soluble organic carbon(HWC)in T1～T5 increased with the increase of C/N,and there were significant differences among the treatments(P<0.05),while the degradation rate of HWC in T3 treatment was the highest during the whole aerobic composting process.After 45 days of composting,the content of alkali-soluble organic carbon(AOC)of T1 treatment decreased by 16.34 g/kg,and the degradation rate was 12.33%,which was significantly higher than that of other treatments(P<0.05).However,during the whole process of aerobic composting,the content of AOC of T1 treatment was significantly higher than that of other treatments(P<0.05)until the end of composting.The content of AOC decreased with the increase of initial C/N ratio,and low C/N ratio significantly increased the content of AOC.1.3 The effect of C/N on the content of high,middle and low labile organic carbon was different.At the end of aerobic composting,the content of high and low labile organic carbon decreased,but the content of middle labile carbon increased,and the degradation rate of low and high labile carbon in T3 with C/N=25 was significantly lower than that in other treatments(P<0.05).And the largest increase in labile organic carbon.1.4 At the end of composting,the degradation rate of humic acid reached 14.20%～45.32%,and the difference among treatments was significant(P<0.05).The degradation rate of T3 treatment with C/N 30 was the highest,which was 3.2 times higher than that of T2 treatment with the lowest;After 45 days of composting,the total humic acid content of T2 and T3 with C/N of 20 and 25 was significantly higher than that of other treatments(P<0.05).2.C/N had a significant effect on the degradation of lignocellulose during aerobic composting2.1 The severe degradation of cellulose mainly occurred in the first 33 days of composting,accounting for 92.0%～97.3%of the total degradation rate.After 45 days of composting,the cellulose degradation rates of T1～T5 treatments were 41.3%,43.3%,55.9%,52.3%and 44.0%respectively,and the degradation rate of T3 treatment was the highest,which was 6.9%～35.2%higher than that of other treatments.2.2 The degradation of hemicellulose was mainly concentrated in the mature stage of compost(18～42 days),and the degradation rate reached 49.35%～66.36%of the total degradation rate;At the end of composting,the degradation rates of cellulose were 35.35%,40.91%,50.93%,44.05%and 37.06%,respectively.The degradation rates of T3 and T4 with C/N of 25 and 30 were significantly higher than those of T1 with the lowest C/N and T5 with the highest C/N(P<0.05).And increase by 24.5%～44.08%and 7.67%～24.61%compared with other treatment.2.3 The degradation of lignin mainly occurred in the high temperature stage and the mature stage(9～42 days),and the degradation rate of lignin reached 6.13%～16.58%,accounting for 93.17%～97.01%of the total degradation rate of the lignin.At the end of composting,the lignin degradation rates of the three treatments were 6.44%,8.70%,17.09%,12.98%and 11.86%,respectively.The lignin degradation rates of T4 and T5 treatments had no significant difference,but they were significantly higher than those of T1 and T2 treatments(P<0.05).The lignin degradation rate of T3 treatment was significantly higher than that of other treatments(P<0.05),and increased by 31.69%～165.60%compared with other treatments.2.4 During the whole aerobic composting process,the degradation rate of cellulose was the highest,followed by hemicellulose and lignin.The degradation rates of cellulose and hemicellulose of T3 and T4 were significantly higher than those of other treatments(P<0.05),which indicated that C/N=25 and C/N=30 were more beneficial to the degradation of cellulose and hemicelulose.The degradation rate of lignin in T3 treatment was the highest and reached significant level(P<0.05),indicating that the degradation rate of lignin was higher under this C/N condition.3.C/N significantly changed the enzyme activity during aerobic composting.3.1 The activities of manganese peroxidase(MNP),lignin peroxidase(LIP),polyphenol oxidase(PPO)and peroxidase(POD)involved in lignocellulose degradation had the same change trend,and there were significant differences among treatments(P<0.05).With the increase of initial C/N,it took longer time for the enzyme activities to reach the peak value;In the whole aerobic composting process,the activities of four enzymes in T3 treatment with C/N of 25 were always significantly higher than those in other treatments(P<0.01),which was the reason for the highest lignocellulose degradation rate in T3 treatment.3.2 The activities of LIP and PPO were significantly positively correlated with the degradation of lignocellulose(lignin,hemicellulose,cellulose)(P<0.01).At the early stage of composting,the activities and degradation rates of enzymes under low C/N treatment were higher than those under high C/N treatment.At the late stage of composting,the activities and degradation rates of enzymes under high C/N were higher than those under low C/N;The activity of MNP was positively correlated with the degradation of lignin(r=0.219*)and only participated in the degradation of lignin.4.Different initial C/N affected the microbial community function ofβ-glucosidase production during aerobic composting4.1 The Transcription level and transcription efficiency of bacterial and fungalβ-glucosidase gene in T1 and T2 with low initial C/N were higher than those in high C/N treatment,and the transcription level and transcription efficiency of bacterial and fungal β-glucosidase gene in T1 and T2 with high initial C/N were higher than those in GH1B-b1 and GH1B-c1.The transcriptional level and efficiency of β-glucosidase genes such as GHlB-dl and GH3E-d3 were lower than those of low C/N treatment.4.2 The transcriptional efficiency and gene abundance of GH3 β-glucosidase genes were low during the high temperature period,and the transcriptional efficiency of GH1 β-glucosidase genes increased during the high temperature period of T3,T4 and T5.The β-glucosidase gene of GH1 bacteria family played a more important role in lignocellulose degradation when T3,T4 and T5 were treated at high temperature.4.3 In the aerobic composting process,the biological effect of the functional microorganism with high β-glucosidase gene abundance and low transcription efficiency in cellulose degradation is greater than that of the microorganism with high transcription efficiency and low gene abundance.