牛粪好氧发酵添加聚天冬氨酸固持氮素的机理

Mechanism of nitrogen retention by polyaspatric acid during cow manure composting

【目的】为有效控制堆肥过程中氮素流失,采用聚天冬氨酸(PASP)作为氮素固持材料,分步研究牛粪好氧发酵添加聚天冬氨酸固持氮素的效果及机理。【方法】首先,采用干湿分离牛粪为原料,设置不同PASP添加量处理[0 g/t(CK)、120 g/t(A)、240 g/t(B)]进行堆肥试验,并针对堆体理化性质、氮素含量及腐熟度指标进行定期监测;其次,依据后期制备生物炭中PASP的担载量,设置堆肥过程空白生物炭(BC-80 g/kg)、生物炭担载PASP(BP-80 g/kg)添加处理,堆肥初始时(第1天)及高温期结束时(第22天)时,采用扫描电镜(SEM)观察PASP形貌、傅里叶红外变换光谱(FTIR)表征官能团变化,X射线光电子能谱(XPS)表征氮素结合形态。【结果】堆肥初始时,相较于BC,BP表面出现颗粒化特征形貌;高温堆肥22天处理后,BP表面该形貌特征进一步被强化,且结合PASP添加组的堆体pH持续高于CK,表明堆肥过程中PASP未被完全分解;综合堆体理化指标、氮素含量变化及微观表征结果分析发现,PASP通过侧链—COOH与铵态氮形成络合结构,增加高温期堆体特别是堆肥第5~10天铵态氮含量(P<0.05)的同时,增加了微生物对铵态氮的利用难度,导致其高温期积温较CK降低40.0℃~91.9℃,并促使其提前4.0~4.6天进入后腐阶段(P<0.05),有效控制了高温期氮素的流失风险;堆体DOM(水溶性有机物)红外光谱分析结果表明,PASP促使堆体相对芳香化程度增强(P<0.05),且基于其在高温期对氮素的固持作用,提升降温期微生物水溶性总氮供应能力,间接降低矿化作用强度,促进含氮有机物与芳香物质合成腐殖质,控制降温期的氮素损失风险;并最终表现为堆肥第39天时,PASP处理氨气累积挥发量显著降低26.62%~37.02%(P<0.05),其全氮含量较堆前增加7.09%~13.64%(CK下降1.23%,P>0.05)。【结论】PASP通过侧链—COOH与铵态氮络合作用及对堆体腐殖化程度的提升作用,综合控制氮素损失风险。同时,添加PASP可降低高温期积温,促使堆体提前4.0~4.6天进入后腐阶段,这也是其控制高温堆肥氮素流失的主要机制。对干湿分离牛粪好氧堆肥而言,PASP在120 g/t低用量时可获得较为理想的氮素截留效果。

【Objectives】Polyaspatric acid(PASP)is added during aerobic composting to decrease the nitrogen loss.The effect and mechanism of the nitrogen retention by PASP were studied in this paper.【Methods】The dehydrated cow dung was used as composting material.First,the PASP was added in rate of 0 g/t(CK),120 g/t(A)and 240 g/t(B)during composting experiment.The physicochemical indexes,N content and maturity of the compost were measured periodically since the beginning of the composting.Then,according to the loading amount of PASP in biochar by special process,the PASP loaded biochar(BP)and the blank biochar(BC)were adding into dehydrated cow dung with the same dosage(80 g/kg)separately,and sealed in mesh bags for fermenting in the composting stack.The morphology were scanned by electron microscopy(SEM),the change of functional groups was measured using fourier transform infrared spectrometer(FTIR),and the nitrogen binding forms of PASP and humification degree were observed using X-ray photoelectron spectroscopy(XPS).【Results】At the beginning of composting,comparing with BC,the morphology of granulation existed in the surface of BP.The specific morphology was intensified by composting during high temperature period.The pH value of PASP added group was higher than that of CK,all indicated that PASP was only partially decomposed during composting.According to the variations of physicochemical indexes,N content and microstructure characteristic,the content of ammonium N was increased during the high temperature period,especially from the fifth to the tenth day.It was tend to attribute the phenomenon to the chelating of ammonium N by the carboxyl from the side chain of PASP.Meanwhile,the phenomenon caused difficult use of ammonium by microbial.Then,the humification period arrived 4.0–4.6 days earlier while the accumulated temperature during high temperature period was reduced by 40.0℃–91.9℃,compared with the CK treatment.Eventually,the risk of N loss during high temperature was reduced.The analysis results about the FTIR of DOM in compost pile showed that the relative degree of aromatization could be strengthen by the addition of PASP,and the supply ability of available nitrogen during humification period could be promoted by the function of nitrogen retention of PASP in the high temperature period,which led to the decline of mineralization intensity.Thus,the risk of nitrogen loss during humification period was reduced by the combination of nitrogenous compounds and aromatic compound.Finally,the total amount of ammonia volatilization had been lowered by 26.62%–37.02%compared with CK treatment(P<0.05),and the total N amount of the PASP treatment had been increased by7.09%–13.64%at the 39 th day compared with the beginning of compost,while CK treatment had been lowered by 1.23%(P>0.05).【Conclusions】The PASP could retain ammonium ions through the direct complexation between carboxyl and ammonium.At the same time,addition of PASP could decrease the cumulative temperature during the high temperature period,and shorten 4.0–4.6 days of the post humification period,which is the most important mechanism for reducing nitrogen loss by PASP.The most economic dosage for the dehydrated cow manure compost is adding PASP 120 g/t.