秸秆覆盖还田对桑园土壤生物学性状及其细菌群落结构的影响

2020-01-21 14:14林刚云肖健吴银秀黄小丹杨尚东屈达才
南方农业学报 2020年10期
关键词:土壤酶活性桑园

林刚云 肖健 吴银秀 黄小丹 杨尚东 屈达才

摘要:【目的】分析秸稈覆盖还田对桑园土壤肥力及其细菌多样性的影响,阐明秸秆覆盖还田技术对南方桑园土壤肥力及健康的影响机制与应用前景,为构建稳定高产、可持续发展的桑树栽培管理体系提供参考依据。【方法】设秸秆覆盖还田桑园和对照桑园(非秸秆覆盖还田桑园)2个处理,其中,秸秆覆盖还田处理是将水稻秸秆切碎、自然堆沤50~60 d后覆盖于桑树根系两旁,覆盖45 d后分别采集秸秆覆盖还田桑园和对照桑园的土壤样品,利用传统的测定方法和Illumina高通量测序技术分析秸秆覆盖还田对桑园土壤生物学性状及土壤细菌群落结构特征的影响。【结果】秸秆覆盖还田桑园土壤β-葡糖苷酶、氨肽酶和磷酸酶活性及土壤微生物生物量碳(MBC)、微生物生物量氮(MBN)和微生物生物量磷(MBP)含量均显著高于对照桑园土壤(P<0.05,下同);秸秆覆盖还田桑园土壤的菌群Chao1和Shannon指数也显著高于对照桑园土壤。在秸秆覆盖还田桑园和对照桑园土壤中相对丰度大于1.00%的优势细菌门分类数量均为11个,但二者的优势细菌门分类组成比例存在一定差异;相对于对照桑园土壤,秸秆覆盖还田桑园土壤中Bacteroidota的相对丰度急剧增加,而绿弯菌门(Chloroflexi)和Verrucomicrobiota的相对丰度急剧下降。在秸秆覆盖还田桑园和对照桑园土壤中相对丰度大于1.00%的优势细菌属分类数量分别为23和24个;与对照桑园土壤相比,秸秆覆盖还田桑园土壤中虽然慢生根瘤菌属(Bradyrhizobium)、假双头斧形菌属(Pseudolabrys)、Dongia、Candidatus_Udaeobacter和norank_f_JG30-KF-AS9等优势细菌属部分缺失,但富集了类诺卡氏菌属(Nocardioides)、norank_f_Methyloligellaceae、黄杆菌属(Flavobacterium)和微枝形杆菌属(Microvirga)等特有优势菌属。秸秆覆盖还田桑园土壤的特有细菌属为199个、特有细菌种为390个,分别是对照桑园土壤特有细菌属和细菌种的3.75和2.52倍。【结论】秸秆覆盖还田不仅显著提高桑园土壤肥力,还改变桑园土壤优势细菌不同(门、属)分类水平的组成比例,形成更丰富多样的土壤细菌群落结构,而有助于维护桑园土壤健康。

关键词: 秸秆覆盖还田;桑园;土壤酶活性;土壤微生物生物量;细菌群落结构

中图分类号: S154.36                          文献标志码: A 文章编号:2095-1191(2020)10-2339-09

Effects of straw mulching on soil biological properties and bacterial community structure in mulberry plantation

LIN Gang-yun, XIAO Jian, WU Yin-xiu, HUANG Xiao-dan,

YANG Shang-dong, QU Da-cai*

(College of Agriculture, Guangxi University, Nanning  530004, China)

Abstract:【Objective】Analyzing the effects of straw mulching on soil fertility and bacterial diversity in mulberry fields, and clarifying the mechanism and application prospects for straw mulching techniques on soil fertility and health in sou-thern mulberry fields, in order to provide a reference basis for building a stable, high-yield and sustainable mulberry cultivation system. 【Method】Two treatments were set up, including straw mulching and returning to the mulberry field and the control mulberry field(non straw mulching and returning field). The straw mulching treatment was to cut the rice straw to pieces, pile it up naturally for 50-60 d and then cover both sides with the mulberry root system, and collect the straw mulch after 45 d of covering. The soil samples of returning mulberry fields and control mulberry fields were used to analyze the effects of straw mulching on soil biological properties and soil bacterial community structure characteristics using traditional measurement methods and Illumina high-throughput sequence technology. 【Result】The activities of β-glucosidase, amino peptidase and phosphate in mulc-hing soils of mulberry plantation were all significantly higher than those of control(P<0.05, the same below), as well as the microbial biomass carbon(MBC), microbial biomass nitrogen(MBN), microbial biomass phosphorus(MBP). The Chao1 and Shannon indexes of the microbial community of the mulberry soil covered with straws were also significantly higher than those of the control mulberry soil. In both straw mulching mulberry fields and control mulberry fields, there were 11 types of dominant bacteria whose relative abundance was greater than 1.00%, but there were some differences in the composition ratio of dominant bacteria. In addition, compared to the control mulberry soil, the ratios of Bacteroidota increased sharply and Chloroflexi, Verrucomicobiota decreased sharply in mulching soils of mulberry plantation at phylum level. There were 23 and 24 dominant bacterial genera in the soil with relative abundance greater than 1.00% in straw mulching mulberry fields and control mulberry fields, respectively. Moreover, some dominant bacteria, such as Bradyrhizobium,Pseudolabrys,Dongia,Candidatus_Udaeobacter and norank_ f_JG30-KF-AS9 even though lost in mulching soil of mulberry plantation, but Nocardioides,norank_f_Methyloligellaceae,Flavobacterium and Microvirga accumulated in mulching soil as its unique dominant bacteria. There were 199 unique bacteria genera and 390 unique bacteria species in the soil of straw mulching mulberry field, which were 3.75 times and 2.52 times of those in the soil of control mulberry field. 【Conclusion】Straw mulching not only can significantly improve soil fertility, but also change the proportions of soil dominant bacterial community structure(phylum and genus), and form a more diverse soil bacterial community structure. It is helpful for maintaining the health of mulberry field soil.

Key words: straw mulching and returning to field; mulberry field; soil enzyme activity; soil microbial biomass; bacterial community structure

Foundation item: Special Project for Guangxi Innovation Team Construction of National Modern Agricultural Industrial Technology System(nycytxgxcxtd-02-04); University Science and Technology Innovation and Service Capacity Enhancement Project of Guangxi(Guijiaokeyan〔2020〕8)

0 引言

【研究意義】桑树(Morus alba L.)是我国重要的多年生木本经济植物。据统计,我国现有的桑树种质资源居世界第一,约3000份,分属15个种和4个变种(苏超和焦锋,2011;杜伟等,2017)。广西是我国最大的桑蚕生产基地,无论是桑园面积还是蚕茧产量均连续多年位居全国首位,2018、2019年广西桑园面积分别为22.85万和19.72万ha,蚕茧产量分别达40.44万和37.4万t(林强,2020;张志林等,2020)。但目前广西桑园管理偏施氮肥,普遍存在氮、磷、钾肥施用比例失调等问题,造成土壤肥力下降进而导致桑叶产量降低,严重制约了广西桑蚕产业的健康发展(张志林等,2020)。因此,研究并优化桑园施肥技术对提高桑园土壤肥力和桑叶产量及保护生态环境具有重要意义。【前人研究进展】秸秆还田作为改善土壤环境和提高土壤肥力的一种重要方式已被广泛认可(王静静等,2019;刘明等,2020)。秸秆中的碳氮比较高,与燃烧作物秸秆相比,将秸秆还田不仅可减少CO2排放,增加土壤固碳容量(逯非等,2010;Li et al.,2018;彭廷等,2019),还能提高有机质含量与土壤肥力,以及促进土壤团粒结构形成,进而改善土壤结构、透气性、持水能力和阳离子交换量(Lenka and Lal,2013;伍佳等,2019;Su et al.,2020)。此外,在秸秆分解过程中能释放出有利于植物吸收利用的无机磷,而提高土壤养分的有效性(Shan et al.,2010)。赵亚丽等(2015)研究表明,与常规耕作+无秸秆还田相比,深耕+秸秆还田、深松+秸秆还田处理的土壤有机碳含量分别提高14.8%和12.4%,土壤微生物数量分别提高45.9%和33.9%,土壤酶活性分别提高34.1%和25.2%,作物产量分别提高18.0%和19.3%,即土壤深松(耕)结合秸秆还田有利于作物产量及土壤微生物数量和酶活性的提高。王静静等(2019)研究证实,越冬期还田水稻秸秆的腐解呈由慢到快又变慢的变化趋势,且与气温及降水量的变化相关;秸秆还田具有培肥地力的作用,可提高土壤有机质和钾素含量,同时有效调节土壤pH。石含之等(2020)通过研究秸秆还田对土壤有机碳结构的影响,发现秸秆还田能促使红壤和黑土中有机碳疏水性增强,提高土壤团聚体稳定性,对改善土壤结构具有促进作用;此外,外源秸秆的加入会引起土壤中有机碳分解,具体表现为:黑土发生负激发效应,其有机碳结构趋于复杂,稳定性增强;而红壤和褐土发生正激发效应,其有机碳结构简单、稳定性减弱。可见,腐熟分解后的作物秸秆还田不仅能有效提高土壤速效养分含量及作物产量(Han et al.,2013),还有助于提高土壤肥力和维护土壤健康(Gaind and Nain,2007)。自20世纪90年代以来,秸秆还田在我国北方小麦、玉米种植体系中得到大面积推广应用,其累计推广面积已达110亿ha(Qi et al.,2015)。但也有研究表明,新鲜秸秆还田会对土壤健康产生负面影响,可能是作物品质和产量提高的限制因素(Hamada et al.,2011;Yang et al.,2011;杨丽华等,2013)。因此,科学的秸秆还田方法应避免采用新鲜秸秆还田方式。【本研究切入点】目前,在小麦、玉米等大宗作物生产中秸秆还田技术已得到广泛应用,但在我国南方地区鲜见桑园应用秸秆还田技术,尤其缺乏秸秆还田对桑园土壤肥力影响及其改良措施的相关研究。【拟解决的关键问题】基于田间定位试验,系统分析秸秆覆盖还田对桑园土壤肥力及其细菌多样性的影响,旨在阐明秸秆覆盖还田技术对南方桑园土壤肥力及健康的影响机制与应用前景,为构建稳定高产、可持续发展的桑树栽培管理体系提供参考依据。

1 材料与方法

1. 1 试验地概况

试验在广西大学农学院教学实验桑园基地(东经108°17′14″,北纬22°51′17″)进行,试验区域属于亚热带季风气候,阳光充足,雨量充沛;年均气温21.7 ℃,年均降水量1600 mm。桑园土壤类型为赤红壤,土壤pH 5.79,有机质含量6.75 g/kg,全氮含量0.84 g/kg,全磷含量0.53 g/kg,全钾含量14.54 g/kg,碱解氮含量57.45 mg/kg,速效磷含量3.39 mg/kg,速效钾含量81.77 mg/kg。

1. 2 样品采集

试验设秸秆覆盖还田桑园(SR)和对照桑园(CK,非秸秆覆盖还田桑园)2个处理,桑树品种均为桂桑12号,树龄6年。其中,SR处理是将水稻秸秆切碎、自然堆沤50~60 d后覆盖于桑树根系两旁;CK处理除了未进行相应的秸秆覆盖外,其余田间管理措施与SR处理完全相同。覆盖秸秆45 d后(2020年6月18日)分别采集SR处理和CK处理桑园0~30 cm土层的土壤样品。每个处理桑园随机选择6个土壤取样点,用无菌密封袋收集后装入放有冰袋的冰盒中带回实验室,将每份土壤样品分成2份。1份在室内风干后过40目筛,用于土壤理化性质测定;1份过10目筛后用于土壤生物学性状和细菌群落结构分析。

1. 3 土壤生物学性状分析

土壤β-葡糖苷酶、氨肽酶和磷酸酶活性参照杨尚东等(2013)的方法进行测定;土壤微生物生物量碳(MBC)含量采用氯仿熏蒸提取—容量分析法测定(庞师婵等,2020),土壤微生物生物量氮(MBN)含量采用茚三酮比色法测定(Jorrgensen and Brookes,1990),土壤微生物生物量磷(MBP)含量采用磷钼蓝比色法测定(吴金水等,2003)。

1. 4 土壤细菌多样性分析

根际土壤样品总DNA提取、PCR扩增及序列测定均委托上海美吉生物医药科技有限公司完成。具体操作:根据FastDNA? Spin Kit for Soil试剂盒(MP Biomedicals)说明进行总DNA提取,使用NanoDrop 2000分光光度计(Thermo Fisher Scientific)检测其浓度和纯度,同时利用1%琼脂糖凝胶电泳检测DNA提取质量;然后以338F(5'-ACTCCTACGGGAGGCAG CAG-3')和806R(5'-GGACTACHVGGGTWTCTAA T-3')引物对16S rDNA序列V3~V4可变区进行PCR扩增,并进行鉴定、纯化及定量分析。Illumina MiSeq测序:以2%琼脂糖凝胶电泳回收PCR扩增产物,经Extraction Kit纯化、Tris-HCl洗脱及电泳检测后,采用Quantus? Fluorometer(Promega)进行检测定量,以NEXTflex? Rapid DNA-Seq Kit构建文库,然后利用Illumina测序公司的MiSeq平台300PE完成测序。

1. 5 统计分析

试验数据采用Excel 2019和SPSS 21.0进行整理统计,并以上海美吉生物医药科技有限公司的I-Sanger云数据分析平台进行在线分析。

2 结果与分析

2. 1 秸秆覆盖还田桑园土壤生物学性状特征

由表1可知,SR处理桑园土壤β-葡糖苷酶、氨肽酶和磷酸酶活性均显著高于CK处理(P<0.05,下同),表明秸秆覆盖还田能显著提高桑园土壤中涉及碳、氮、磷循环的相关酶活性。在土壤微生物生物量含量方面,SR处理桑园的土壤MBC、MBN和MBP含量也显著高于CK处理,表明秸秆覆盖还田能显著提高桑园土壤中碳、氮和磷的库容,即有助于改良桑园土壤肥力。

2. 2 秸秆覆盖还田桑园土壤细菌OTU聚类分析結果

基于Illumina高通量测序分析,从12个桑园土壤样品中共获得4271个OTUs(表2),依据细菌群落组成不同分类水平可分为39门119纲283目449科820属1620种。其中,从SR处理桑园土壤样品中获得3565个OTUs,其细菌群落组成依据不同分类水平可分为37门114纲271目429科767属1465种;从CK处理桑园土壤样品中获得3017个OTUs,其细菌群落组成依据不同分类水平可分为34门101纲222目350科621属1230种。说明秸秆覆盖还田能改变桑园土壤中不同分类水平的细菌群落组成,且与对照处理相比,秸秆覆盖还田有效提高了桑园土壤中细菌群落不同分类水平的数量。

2. 3 秸秆覆盖还田桑园土壤细菌Alpha多样性分析结果

覆盖率越高说明样本中目的序列被测出的概率越高,而未被测出的概率越低(孙珂岱,2015)。由表3可知,桑园土壤样品分析覆盖率均达98%,说明分析数据真实可信。Alpha多样性指数包括ACE、Chao1、Shannon和Simpson等(洪嘉炜等,2019),其中,Chao1指数表征细菌群落丰富度,Chao1指数越大表明土壤中细菌数目越多,群落丰富度越高(Chao et al.,2000);Shannon指数表征细菌群落多样性,Shannon指数越大表明物种多样性越丰富(张传进等,2020)。由表3可知,SR处理桑园土壤样品的Chao1和Shannon指数均显著高于CK处理,说明秸秆覆盖还田能显著提高桑园土壤细菌的多样性和丰富度。

2. 4 桑园土壤细菌群落组成分析结果

2. 4. 1 桑园土壤细菌门分类水平分析结果 在SR处理和CK处理桑园土壤中相对丰度大于1.00%的优势细菌门分类数量均为11个(图1),但二者的优势细菌门分类组成比例存在一定差异。其中,SR处理桑园土壤优势细菌门的相对丰度排序为变形杆菌门(Proteobacteria,26.26%)>放线菌门(Actinobacteriota,24.76%)>Acidobacteriota(14.26%)>绿弯菌门(Chloroflexi,11.54%)>Bacteroidota(5.57%)>Myxococcota(3.85%)>厚壁菌门(Firmicutes,3.55%)>Gem-matimonadota(1.82%)>Methylomirabilota(1.75%)>Planctomycetota(1.42%)>Verrucomicrobiota(1.12%),其他门类的合计相对丰度为4.10%;CK处理桑园土壤优势细菌门的相对丰度排序为变形杆菌门(Proteobacteria,25.04%)>放线菌门(Actinobacteriota,23.71%)>绿弯菌门(Chloroflexi,15.95%)>Acidobacteriota(14.33%)>厚壁菌门(Firmicutes,3.58%)>Gemmatimonadota(3.48%)>Myxococcota(2.97%)>Bacteroidota(2.40%)>Verrucomicrobiota(1.87%)>Methylomirabilota(1.59%)>Planctomycetota(1.10%),其他门类的合计相对丰度为3.97%。由此可见,秸秆覆盖还田虽然未改变桑园土壤优势细菌门分类水平组成,但部分优势细菌门的相对丰度排序已发生改变。如绿弯菌门类细菌在CK处理桑园土壤中的相对丰度排在变形杆菌门和放线菌门之后,位列第三;但在SR处理桑园土壤中的相对丰度降至11.54%,相对于CK处理桑园土壤(15.95%)下降了4.44%(绝对值),排在Acidobacteriota之后,位列第四。同时,Bacteroidota的相对丰度在覆盖秸秆后急剧增加,由CK处理桑园土壤的2.40%上升至5.57%。秸秆覆盖还田不仅改变桑园土壤中部分优势细菌门分类水平组成比例,还会改变桑园土壤细菌群落结构,最终改善土壤的生物学功能。

2. 4. 2 桑园土壤细菌属分类水平分析结果 在SR处理和CK处理桑园土壤中相对丰度大于1.00%的优势细菌属分类数量分别为23和24个(图2)。其中,SR处理桑园土壤优势细菌属的相对丰度排序为norank_f_norank_o_Vicinamibacterales(5.08%)>no-rank_ f_Vicinamibacteraceae(4.55%)>Gaiella(2.87%)> norank_f_Xanthobacteraceae(2.38%)>norank_f_no-rank_o_norank_c_KD4-96(2.37%)>norank_f_norank_ o_norank_c_MB-A2-108(1.90%)>链霉菌属(Streptomyces,1.85%)>类诺卡氏菌属(Nocardioides,1.84%)>norank_f_JG30-KF-CM45(1.79%)>norank_f_norank_ o_Gaiellales(1.66%)>芽孢杆菌属(Bacillus,1.65%)>norank_f_67-14 (1.49%)>norank_f_SC-I-84(1.44%)>norank_f_Gemmatimonadaceae(1.43%)=norank_f_ no-rank_o_Rokubacteriales(1.43%)>norank_f_Roseiflexaceae(1.32%)>黄杆菌属(Flavobacterium,1.31%)>norank_f_norank_o_norank_c_TK10(1.29%)>鞘脂单胞菌属(Sphingomonas,1.25%)>norank_f_Methyloligellaceae(1.16%)>norank_f_norank_o_IMCC26256 (1.10%)>分枝杆菌属(Mycobacterium,1.09%)>微枝形杆菌属(Microvirga,1.02%),其他属类的合计相对丰度为54.02%(图2-A)。CK处理桑园土壤优势细菌属的相对丰度排序为norank_f_Roseiflexaceae(4.74%)>norank_f_norank_o_Vicinamibacterales(4.73%)>norank_f_Xanthobacteraceae(3.82%)>no-rank_f_norank_o_Gaiellales(3.58%)>Gaiella(3.51%)>芽孢桿菌属(Bacillus,2.97%)>norank_f_Gemmatimonadaceae(2.72%)>norank_f_Vicinamibacteraceae(2.33%)>norank_f_SC-I-84(2.30%)>norank_f_no-rank_o_norank_c_TK10(1.82%)>norank_f_67-14(1.78%)>链霉菌属(Streptomyces,1.64%)>鞘脂单胞菌属(Sphingomonas,1.60%)>norank_f_norank_o_ IMCC 26256(1.56%)>慢生根瘤菌属(Bradyrhizobium,1.44%)>norank_f_norank_o_Rokubacteriales(1.43%)>假双头斧形菌属(Pseudolabrys,1.40%)>norank_f_ norank_o_norank_c_KD4-96(1.36%)>分枝杆菌属(Mycobacterium,1.26%)>norank_f_norank_ o_norank_c_MB-A2-108(1.18%)>norank_f_JG30-KF-AS9(1.15%)>Dongia(1.08%)>Candidatus_Udaeobacter(1.05%)>norank_f_JG30-KF-CM45(1.04%),其他属类的合计相对丰度为46.21%(图2-B)。

与对照相比,秸秆覆盖还田同步改变了桑园土壤优势细菌属分类水平组成及其相对丰度。在SR处理桑园土壤中,norank_f_norank_o_Vicinamibacterales的相对丰度最高(5.08%),但其在CK处理桑园土壤中的相对丰度排名第二,仅为4.73%;norank_ f_Roseiflexaceae在CK处理桑园土壤中相对丰度(4.74%)排名第一,但在SR处理桑园土壤中的相对丰度急剧下降,仅为1.32%,排在第十六位。此外,秸秆覆盖还田还改变了桑园土壤中优势细菌属的群落结构,如慢生根瘤菌属(Bradyrhizobium)、假双头斧形菌属(Pseudolabrys)、Dongia、Candidatus_Udaeobacter和norank_f_JG30-KF-AS9等均为CK处理桑园土壤中的优势细菌属,但经秸秆覆盖还田处理后这些优势细菌属缺失,同时富集了类诺卡氏菌属(Nocardioides)、norank_f_Methyloligellaceae、黄杆菌属(Flavobacterium )和微枝形杆菌属(Microvirga)等细菌属,成为SR处理桑园土壤的特有优势细菌属。

2. 5 桑园土壤细菌群落结构Venn分析结果

基于属和种分类水平的Venn分析结果显示,在属分类水平(图3-A)上,SR处理和CK处理桑园土壤的共有细菌属为568个,SR处理桑园土壤的特有细菌属达199个,CK处理桑园土壤的特有细菌属仅为53个,前者是后者的3.75倍;在种分类水平(图3-B)上,SR处理和CK处理桑园土壤的共有细菌种为1075个,SR处理桑园土壤的特有细菌种达390个,CK处理桑园土壤的特有细菌种为155个,前者是后者的2.52倍。表明秸秆覆盖还田明显改变桑园土壤中细菌属、种分类水平上的群落结构,提高桑园土壤细菌多样性及丰富度,而有助于提高和维护桑园土壤健康。

3 讨论

桑树隶属于桑科(Moraceae)桑属(Morus L.),为多年生的深根性、阔叶型、落叶性小乔木或灌木。桑树除了是养蚕产业不可或缺的原料外,对恢复脆弱生态区植被也具有积极作用,还可作为优质饲料作物,具有极高的经济价值和应用前景。广西是我国桑蚕的主产区,桑园面积约占全国桑园总面积的1/4,连续13年位居全国之首(钟春云,2019)。近年来,由于广西大部分桑园种植年限已超过10年,部分桑农为了获得较高的桑叶产量,而无限制地加大氮肥施用量,导致桑园土壤有机质含量降低、土壤板结及肥力下降,甚至发生严重的土传病害等一系列问题(祁广军等,2015;王帅帅等,2019),严重威胁着广西桑蚕产业的可持续发展。秸秆覆盖还田是将秸秆直接或堆沤后施入土壤中,具有培肥、蓄水、调温、增产及减少污染的作用,能有效改善农田生态环境、培肥地力和提高作物品质与产量。因此,在我国南方桑园开展秸秆覆盖还田技术研究,对构建稳定高产、可持续发展的桑树栽培管理体系具有重要意义。

土壤酶是土壤的重要组成成分,几乎参与土壤中所有有机物和营养元素的循环过程,在稳定土壤结构、分解有机废弃物、有机质形成和养分循环等方面发挥重要作用,其活性高低能客观地反映土壤肥力及其健康状况(Dick et al.,1994;林天等,2005;杨宁等,2014)。土壤微生物生物量是衡量土壤質量、维持土壤肥力和作物生产力的一个重要指标,土壤微生物生物量越大,即土壤通过有机养分矿化提供给植物所需养分的能力越强(任奎瑜等,2020)。本研究结果表明,秸秆覆盖还田桑园土壤中的β-葡糖苷酶、氨肽酶和磷酸酶活性均显著高于对照桑园土壤,MBC、MBN和MBP等土壤微生物生物量也显著高于对照桑园土壤,说明秸秆覆盖还田能显著提高桑园土壤酶活性及土壤中碳、氮和磷的库容,从而有助于提高桑园土壤肥力。

微生物是生态系统中功能最活跃、开发潜力最大的生物资源库(Vanniern et al.,2018)。土壤中丰富的微生物多样性在陆地生态系统中发挥着重要的功能作用(杨尚东等,2019),其微生物群落结构越复杂、物种多样性越丰富时,植物对抗病原菌的综合能力越强(杨尚东等,2020)。本研究结果表明,在秸秆覆盖还田桑园土壤中,指示细菌丰富度的Chao1指数和细菌多样性的Shannon指数均显著高于对照桑园土壤,说明秸秆覆盖还田有助于提高桑园土壤微生物的多样性和丰富度。本研究还发现,秸秆覆盖还田改变了桑园土壤的部分优势细菌门,如Bacteroidota的相对丰度急剧增加,而绿弯菌门(Chloroflexi)和Verrucomicrobiota的相对丰度急剧下降;此外,与对照桑园土壤相比,在秸秆覆盖还田桑园土壤中虽然慢生根瘤菌属(Bradyrhizobium)、假双头斧形菌属(Pseudolabrys)、Dongia、Candidatus_Udaeobacter和norank_f_JG30-KF-AS9等优势细菌属部分缺失,但富集了类诺卡氏菌属(Nocardioides)、norank_f_Methyloligellaceae、黄杆菌属(Flavobacterium)和微枝形杆菌属(Microvirga)等特有优势菌属。其中,类诺卡氏菌属(Nocardioides)细菌不仅能利用C2~C16一系列烷烃及苯酚等作为碳源,具有降解原油、治理污水和生物防护等功能(杜慧竟等,2012),还具有溶磷、产铁载体及固氮等作用(刘冰冰,2014)。黄杆菌属(Flavobacterium)细菌是具有脱氮除磷功能的好氧反硝化微生物(杨浩等,2017)。微枝形杆菌属(Microvirga)是根瘤菌中的一类新种属,具有与黏细菌相似的噬大肠杆菌活性和抗马铃薯晚疫病菌活性,主要分布在干旱或半干旱且偏碱性的土壤中,个别类型菌株可生长在50 ℃左右的地下含水层内,属于中性嗜热微生物,与植物共生时能产生明显的结瘤,即具有显著的固氮活性(赵璞钰等,2017),少数微枝形杆菌属还具有运输铁离子的载体功能。

基于属和种分类水平的Venn分析结果显示,秸秆覆盖还田桑园土壤的特有细菌属为199个,特有细菌种为390个,分别是对照桑园土壤特有细菌属和细菌种的3.75和2.52倍。说明秸秆覆盖还田不仅能改变桑园土壤细菌不同分类水平的组成比例和群落结构,还有助于富集更多的特有细菌种属,提高桑园土壤微生物的多样性及丰富度,而有助于维护桑园土壤健康。

4 结论

秸秆覆盖还田不仅显著提高桑园土壤肥力,还改变桑园土壤优势细菌不同(门、属)分类水平的组成比例,形成更丰富多样的土壤细菌群落结构,而有助于维护桑园土壤健康。

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