Effect of sludge and garden waste composting rates on heavy metal content and microbial activity in plantation soil

HU Zihang; ZHAO Xia; DONG Xiaoyun; ZHENG Jingming; JIANG Liwei

doi:10.11833/j.issn.2095-0756.20200262

Volume 38 Issue 1

Jan. 2021

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HU Zihang, ZHAO Xia, DONG Xiaoyun, ZHENG Jingming, JIANG Liwei. Effect of sludge and garden waste composting rates on heavy metal content and microbial activity in plantation soil[J]. Journal of Zhejiang A&F University, 2021, 38(1): 31-37. doi: 10.11833/j.issn.2095-0756.20200262

Citation:

HU Zihang, ZHAO Xia, DONG Xiaoyun, ZHENG Jingming, JIANG Liwei. Effect of sludge and garden waste composting rates on heavy metal content and microbial activity in plantation soil[J]. Journal of Zhejiang A&F University, 2021, 38(1): 31-37. doi: 10.11833/j.issn.2095-0756.20200262

Effect of sludge and garden waste composting rates on heavy metal content and microbial activity in plantation soil

doi: 10.11833/j.issn.2095-0756.20200262

1.
Beijing Key Laboratory for Forest Resources and Ecosystem Processes, Beijing Forestry University, Beijing 100083, China
2.
Survey & Planning Institute of National Forestry and Grassland Administration, Beijing 100013, China

Received Date: 2020-04-08
Rev Recd Date: 2020-10-27

Available Online: 2021-01-21

Publish Date: 2021-01-21

Abstract

Objective This study aims to explore the changes of heavy metal content and soil microbial activity in plantations after 2 years of mixed composting of municipal sludge and garden waste, in order to provide basis for utilization of municipal sludge products in forestry. Method A randomized block design was carried out in an experimental Ulmus pumila plantation. According to the sludge content in the compost, four levels of mixed compost (0, 15, 30 and 60 t·hm⁻² of composted sludge) were set. After two years of application, soil samples were collected for analysis of six heavy metals(Cr, Pb, Cu, Zn, Cd, Ni), as well as seven soil microbial indicators, including microbial carbon and nitrogen, soil respiration and four enzyme activities. The effects of different fertilization rates on soil quality were analyzed. Result After 2 years of sludge composting, the total amount of six kinds of heavy metals increased, but there was no significant difference between different application amounts, indicating that the application of mixed compost had low risk of heavy mental pollution. Soil enzymes, microbial carbon and nitrogen had no obvious change with increment of compost amount, but the soil enzyme index and soil respiration intensity of the fertilized plot were greater than those of the control plot, suggesting that the mixed compost improved soil microbial activity. Conclusion When the amount of sludge compost is lower than 30 t·hm⁻², the elm forest ecosystem could withstand the increased risk of heavy metals, increase soil microbial activity and improve soil quality. [Ch, 1 fig. 4 tab. 22 ref.]
- municipal sludge compost,
- plantation land,
- heavy metals in soil,
- soil microbial activity

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通讯作者: 陈斌, bchen63@163.com

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沈阳化工大学材料科学与工程学院沈阳 110142

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随着城市化的发展，如何处置大量增加的污泥已经成为一个重要问题，国际上污泥的处置方式主要有焚烧、填埋、土地利用等几种，其中污泥土地利用被认为是可持续的主要处置方式，被很多国家采纳^[1]。相对农业土地而言，林地施用污泥更具可行性。第一，林地土壤通常相对贫瘠且较少施肥，而城市污泥含有丰富的植物生长所需养分，可补充林地生态系统中缺乏的营养元素，改善林地土壤质量，促进树木生长^[2]；第二，林业产品不直接进入食物链，健康风险减小^[3]；第三，中国是世界上人工林面积最大的国家，污泥产品在林地施用方面有着较好的前景。但污泥含有重金属等有害物质，可能会对植物、土壤动物和微生物造成影响，污泥产品的林地利用存在不确定的风险^[4]，风险大小往往和污泥类型、施用量等有关^[5]。前人大多采用盆栽实验研究污泥施加对土壤性质的影响^[6-7]，其结果对指导林地施肥作用有限。本研究采用高级厌氧消化污泥与园林废弃物混合堆肥，在北京市平原造林区的榆树Ulmus pumila人工林林地进行了2 a的野外控制施肥试验，探讨污泥堆肥产品对林地土壤质量和安全的影响，以期为污泥堆肥产品的林地应用提供理论依据和技术支持。

3. 讨论

3.1. 施加污泥堆肥对土壤重金属总量的影响

由于污水中的重金属经过处理后浓缩到污泥中，重金属质量分数成为限制污泥土地利用的主要因素。本研究使用的污泥堆肥产品中6种重金属质量分数均未超过CJ/T 362−2011《城镇污水处理厂污泥处置林地用泥质》规定的参考标准值。施肥2 a后，林地土壤重金属总量仅镉超过GB 15618−2018《土壤环境质量农用地土壤污染风险管控标准》中农用土地重金属污染风险筛选值(0.6)，但未超过管制风险值(4.0)，而且未施肥的对照样方土壤也同样镉超标，说明该试验林地的土壤中原来镉就较高^[8]。由于目前没有林地土壤重金属污染控制国家标准，并考虑到林地土壤重金属参考标准值应高于农用土地，本研究表明：在0~60 t·hm⁻²的施加量下，污泥堆肥虽然使土壤中重金属质量分数增加了，但整体上增加量较小，对林地土壤造成的重金属污染风险较低。

3.2. 施加污泥堆肥对土壤重金属形态的影响

施加污泥堆肥2 a后，土壤重金属4种形态质量分数在处理间无显著差异，表明施肥量对土壤中重金属形态分布影响不大。BALDWIN等^[14]采用DPTA浸提法提取施用污泥堆肥的土壤中的重金属元素锌，发现锌在低含量和高含量堆肥处理中的提取态分布无显著差异；MCGRATH等^[15]也得到了相似的结论。其中应当注意的是，锌的弱酸提取态和可还原态质量分数随施加量有增多的趋势，残渣态质量分数却没有显著变化，说明锌主要以弱酸提取态和可还原态形态加入土壤，可能在较大施加量下达到显著水平，造成较大的生态风险。铜的形态分布趋势与锌相反，铜的残渣态质量分数有增多趋势，但其他形态质量分数没有显著变化。因此，在长期影响下，土壤锌有可能向可利用态转化，铜向残渣态转化。其他几种重金属形态质量分数变化与总量变化趋势相同，各处理间均无显著差异。总体来说，通过污泥产品进入土壤中的重金属主要是以稳定的残渣态为主，其他形态较少，对土壤环境影响较低。

3.3. 施加污泥堆肥对土壤酶活性的影响

土壤酶活性不但能反映土壤微生物性能，也可以反映土壤质量和肥力状况^[16]。本研究中，施加污泥堆肥引起4种酶的变化各不相同，其中脱氢酶活性可表征土壤微生物活性的高低，也可用于简单的毒性检测及作为污染监测的指示物。本研究结果显示：脱氢酶活性随施加量的增加而减小，表明污泥堆肥中含有多种重金属，而土壤复合重金属污染可减小脱氢酶活性，与腾应等^[17]研究结果一致。蔗糖酶是可以把土壤中高分子量蔗糖分子分解成能够被植物和土壤微生物吸收利用的葡萄糖和果糖的水解酶，为土壤微生物提供充分能源，其活性反映了土壤有机碳累积与分解转化的规律，对土壤的碳循环起到重要作用。本研究中，蔗糖酶的活性T₁显著大于对照，T₂、T₃处理与对照差异不显著。可能是T₁污泥堆肥的加入增加了土壤有机质质量分数^[8]，从而提高了蔗糖酶活性；而T₂和T₃重金属对微生物活性的抑制作用大于有机质的促进作用，因此蔗糖酶活性降低，体现了污泥混合堆肥对土壤微生物影响的两面性^[18]。脲酶是对土壤有机氮分解转化起重要作用的酶，主要分解有机氮转化过程中形成的尿素，使其转化成矿物态的氮，从而被植物吸收利用。在本研究中，脲酶活性随污泥堆肥施加量提高有增加趋势，但与对照差异不显著，说明施加污泥堆肥2 a后土壤氮质量分数和脲酶活性变化不大，与前人研究结果一致^{[8, 19]}。磷酸酶主要参与将土壤中的有机磷转化成无机磷的过程，能加速有机磷循环速度。从而提高磷素的有效性。施加污泥堆肥后，磷酸酶活性随施加量的增加没有明显变化，这与秦俊梅等^[20]的研究结果较为一致。

评价土壤肥力和重金属造成的风险应整体综合考虑土壤多种酶活性^[4]。土壤酶指数可综合评价土壤活性状况，将土壤酶的实际值均转化为数值0~1，统一了指数量纲，解决了依靠单一酶活性来判断土壤状况的片面性和局限性，可以较为全面地反映不同处理对酶活性的影响^[21]。本研究中施加污泥堆肥可整体提高土壤酶指数，按照其土壤酶指数从大到小排序为：T₁≈T₂、T₃、对照。说明经过2 a，在污泥堆肥中重金属和有机质对微生物活性的综合作用下，不同施肥量下林地的土壤肥力和微生物活性整体上有所提升。应当指出的是，不能排除在此期间各种酶活性有显著增高或降低动态变化。

3.4. 施加污泥堆肥对微生物碳氮和土壤呼吸的影响

施加污泥堆肥2 a后微生物碳和微生物氮质量分数、土壤呼吸速率都随施加量没有显著变化。在以往研究中，少量的施加污泥堆肥后，土壤有机质含量、全氮和微生物碳氮有轻微上升趋势^[20]，与本结果一致。土壤呼吸强度表示土壤微生物对有机质的分解和代谢能力。污泥堆肥的施加改变了土壤有机质质量分数，土壤孔隙度，微生物组成等理化性质强度，会引起土壤呼吸速率的相应变化^[22]。本研究污泥堆肥的施加增大了土壤呼吸强度，在T₁处理下达到最大值，而T₃土壤呼吸强度与对照基本相等。这可能是由于污泥堆肥对土壤微生物影响的两面性造成的，但由于本研究仅为施肥2 a后的测定，不能说明在2 a期间土壤呼吸速率指标的动态变化。

4. 结论

林地施用污泥混合堆肥2 a后，土壤中重金属总量有所增加，但增加量较小，除了镉之外其他5种重金属元素总量都没有超过土壤环境质量标准值(GB 15618−2018)，2 a期间土壤中6种重金属形态分布也没有发生明显变化，表明现有施肥水平对林地土壤的重金属污染风险很低。

污泥堆肥的施用能增加林地土壤的有机质，但同时也引入多种重金属元素，这2种情况都会影响土壤微生物活性和群落组成，从而改变微生物氮质量分数和某些酶活性，体现出混合堆肥对土壤肥力影响的两面性。施肥2 a后的土壤酶指数变化表明，施用污泥堆肥整体上提高了土壤微生物活性。

在污泥施用量＜30 t·hm⁻²的标准推荐值范围内，污泥和园林废弃物混合堆肥有利于提高林地土壤的肥力；但60 t·hm⁻²污泥施用量水平下，相应微生物指标出现下降趋势，因此，当林地污泥产品施用量超过现行相关标准的推荐值时，应加强施用林地的监测和风险评估。

Reference (22)

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项目	pH	有机质质量分数/%	全氮质量分数/ (mg·kg⁻¹)	铬质量分数/ (mg·kg⁻¹)	铅质量分数/ (mg·kg⁻¹)	铜质量分数/ (mg·kg⁻¹)	锌质量分数/ (mg·kg⁻¹)	镉质量分数/ (mg·kg⁻¹)	镍质量分数/ (mg·kg⁻¹)
样地土壤	7.92	1.49	0.39	16.92	17.43	53.02	33.98	2.52	15.35
污泥堆肥	7.68	37.70	32.60	60.90	12.90	319.00	653.00	1.12	2.45
堆肥标准参考值	5.5~8.5	≥18	−	1 000.00	1 000.00	1 500.00	2 000.00	20.00	200.00
说明：污泥混合堆肥林地施用限定值参考CJ/T 362−2011《城镇污水处置厂污泥处置林地用泥质》；−表示没有相应标准值

处理	铬/(mg·kg⁻¹)	铅/(mg·kg⁻¹)	铜/(mg·kg⁻¹)	锌/(mg·kg⁻¹)	镉/(mg·kg⁻¹)	镍/(mg·kg⁻¹)
ck	50.56±1.80 a	25.31±2.27 a	27.94±2.85 c	40.03±1.77 a	2.53±0.53 b	15.35±1.20 b
T₁	52.48±5.58 a	27.22±1.84 a	33.40±0.47 b	46.77±8.92 a	3.11±0.59 ab	16.92±0.41 ab
T₂	51.33±3.77 a	27.26±3.84 a	38.02±2.03 a	47.89±7.34 a	2.83±0.57 ab	17.77±1.15 a
T₃	51.00±4.39 a	27.50±2.65 a	39.12±2.01 a	49.57±7.76 a	3.60±0.16 a	17.11±0.89 ab
GB	250	170	100	300	0.6	190
说明：GB为GB 15618−2018《土壤环境质量农用地土壤污染风险管控标准》中农用土地重金属污染风险筛选值；数值为平均值±标准　　　　　误，同列不同小写字母表示处理间差异显著(P＜0.05)

处理	脱氢酶/(mg·g⁻¹)	蔗糖酶/(mg·g⁻¹)	脲酶/(mg·g⁻¹)	磷酸酶/(mg·g⁻¹)	土壤酶指数
ck	6.29±0.88 a	16.10±0.92 a	0.50±0.04 a	0.47±0.05 a	0.45
T₁	4.22±0.51 b	18.71±2.96 b	0.51±0.10 a	0.44±0.07 a	0.52
T₂	3.11±0.17 bc	14.60±1.92 a	0.48±0.10 a	0.47±0.02 a	0.52
T₃	2.43±0.20 c	13.19±0.25 a	0.54±0.10 a	0.48±0.02 a	0.50
说明：数值为平均值±标准误，同列不同小写字母表示处理间差异显著(P＜0.05)

处理	微生物碳/ (mg·kg⁻¹)	微生物氮/ (mg·kg⁻¹)	土壤呼吸强度/ (mL·g⁻¹·d⁻¹)
ck	44.27±3.40 a	3.93±1.31 a	23.33±10.60 a
T₁	32.22±3.35 a	4.30±0.92 a	26.60±9.90 a
T₂	48.04±2.21 a	3.28±1.44 a	25.20±10.10 a
T₃	34.11±2.19 a	4.01±1.68 a	23.33±7.05 a
说明：数值为平均值±标准误，同列相同小写字母表　　　　　示处理间差异不显著(P＞0.05)