曹彬彬^,, 朱熠辉, 姜禹含, 师江澜, 田霄鸿^,西北农林科技大学资源环境学院/农业农村部西北植物营养与农业环境重点实验室,陕西杨凌 712100

Effects of Lime and Straw Addition on SOC Sequestration in Tier Soil

CAO BinBin^,, ZHU YiHui, JIANG YuHan, SHI JiangLan, TIAN XiaoHong^,College of Natural Resource and Environment, Northwest A＆F University /Key Laboratory of Plant Nutrition and the Agro-environment in Northwest China, Ministry of Agriculture and Rural Affairs, Yangling 712100, Shaanxi

通讯作者: 田霄鸿,E-mail: txhong@hotmail.com

责任编辑: 李云霞
收稿日期:2020-02-10接受日期:2020-06-24网络出版日期:2020-10-16

基金资助:

国家重点研发计划.2016YFD0200308 陕西省重点研发计划.2019ZDLNY01-05-01 “十二五”国家科技支撑计划.2012BAD14B11

Received:2020-02-10Accepted:2020-06-24Online:2020-10-16
作者简介 About authors
曹彬彬,E-mail: caobb0606@163.com。







摘要
【目的】研究作物秸秆与石灰配施对土壤CO₂排放、土壤有机碳（SOC）固持、土壤无机碳（SIC）转化的影响机制,以及SOC固持对初始SOC含量的响应。【方法】--采用室内恒温培养试验及稳定同位素技术（¹³C）,选用经16年不同碳氮水平管理,且长期进行冬小麦-夏休闲种植的2个供试土壤样品:S₀N₀土壤（不进行秸秆还田+不施用氮肥）和S₁N₁土壤（高量秸秆还田+高量施用氮肥:240 kg·hm^-2）,将S₀N₀土壤和S₁N₁土壤分别在添加秸秆（12 g·kg^-1）或不添加秸秆以及添加石灰（3 g·kg^-1）或不添加石灰的情况下于25℃黑暗条件中培养120 d。【结果】未添加秸秆和石灰时,S₁N₁土壤的CO₂累积释放量比S₀N₀土壤高出42.9%;添加等量秸秆不仅提高了S₀N₀土壤和S₁N₁土壤的CO₂累积释放量（81.6%,70.4%）,而且S₀N₀土壤CO₂累积释放量的增加幅度高于S₁N₁土壤,这说明秸秆的添加对初始SOC含量低的土壤即S₀N₀土壤的原SOC矿化影响更大。但是无论添加秸秆与否,石灰的加入使S₀N₀土壤和S₁N₁土壤的CO₂累积释放量分别降低了428.11和528.52 mg·kg^-1。与空白土壤相比,添加秸秆使S₀N₀土壤和S₁N₁土壤的SOC含量分别提高了2.95和3.19 g·kg^-1;但是与单独添加秸秆相比,同时添加秸秆和石灰使S₁N₁土壤的SOC显著降低了1.36 g·kg^-1,而对S₀N₀土壤的SOC含量没有影响。利用¹³C稳定同位素技术发现,添加秸秆能促使新形成SOC;其中,S₀N₀土壤中新形成的SOC含量比S₁N₁土壤高出0.77 g·kg^-1;然而与单独添加秸秆相比,同时添加石灰和秸秆后新形成的SOC与其相差无几,说明石灰的加入对秸秆的腐解不会造成影响。在S₀N₀土壤和S₁N₁土壤中,添加秸秆使SOC净固持量分别提高了3 066.3和2 480.53 mg·kg^-1;同时添加石灰和秸秆对S₀N₀土壤的SOC净固持量无显著影响,但是S₁N₁土壤的SOC净固持量则呈现下降的趋势。石灰的加入使S₀N₀土壤和S₁N₁土壤的CO₂释放量分别降低了469和529 mg·kg^-1,同时使SIC含量分别提高了443和566 mg·kg^-1。【结论】初始SOC含量低的土壤具有更高的固碳潜力;添加钙源能够与土壤CO₂通过化学反应生成无机碳—碳酸钙的方式从另一个角度达到土壤固碳减排的目标。
关键词： 土壤碳;土壤呼吸;添加秸秆;施用石灰;同位素技术

Abstract
【Objective】Soil organic carbon (SOC) sequestration is crucial for improving soil fertility and agricultural production sustainability. The soil inorganic carbon (SIC) is closely related to SOC with regarding with inter-transformation, which has also great effect on carbon sequestration. Crop straw return has been recognized as one of the most important organic amendment improving soil organic carbon sequestration in farmland. Meanwhile, the addition of lime also contributes greatly to increasing SIC, thereby affecting the SOC sequestration. However, the mechanism of simultaneous incorporation of crop straw and lime affecting on the CO₂ emission, SOC and SIC dynamics are not well understood, and how the SOC sequestration responds to the initial level of SOC is not clear, particularly after straw return. 【Method】 The incubation experiment and stable isotope technique (¹³C) were used in the study. The two tested soils were collected from a field with continuously cropping of winter wheat for 16 years, which was subjected to differential crop residue and nitrogen managements over long-term, including (1) S₀N₀ soil (no straw return+ nitrogen fertilizer application: 0); (2) S₁N₁ soil (high amount of straw return+ nitrogen fertilizer application: 240 kg·hm^-2). And then the two soils were both incubated with or without addition of straw and lime for 120 days under 25℃. 【Result】 The study showed that the soil cumulative CO₂ emission was observed 42.9% higher in S₁N₁ soils than that in S₀N₀ soils, when without straw and lime addition. In both soils, the straw addition alone increased the soil cumulative CO₂ emission by averages of 81.6% and 70.4%, respectively, compared with straw absence. Meanwhile, the increase of the cumulative CO₂ emission in S₀N₀ soils was higher than in S₁N₁ soils. This showed that straw addition had a greater impact on the native OC mineralization in soil with low initial SOC. Lime addition decreased soil cumulative CO₂ emission in both soils whether straw addition or not. Straw addition increased the SOC by 2.95 g·kg^-1 and 3.19 g·kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively, while reduced the SOC by 1.36 g·kg^-1 in S₁N₁ soils and did not affect the SOC in S₀N₀ soils when combining with lime addition. Using¹³C stable isotope technology, the newly formed organic carbon (OC) in the two soils both significantly increased after straw addition, therein which increased 25.8% in S₀N₀ soils when compared with S₁N₁ soils. However, the conjoint addition of lime and straw did not modify the newly formed SOC when compared with the addition of straw alone, which showed that the lime addition had no effect on the decomposition process of straw in the soil. Over all, straw addition alone increased the SOC net sequestration by 3 066.3 mg?kg^-1and 2 480.53 mg?kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively. The conjoint addition of lime and straw had no significant effect on the SOC net sequestration in S₀N₀ soils, but there was a decreasing trend on the SOC net sequestration in S₁N₁ soils. Lime addition reduced the cumulative CO₂ emission by 469 mg?kg^-1 and 529 mg?kg^-1 in S₀N₀ soils and S₁N₁ soils, respectively, which approximately equaled to the increases in SIC (by 443 mg?kg^-1 and 566 mg?kg^-1, respectively).【Conclusion】 In conclusion, the soil with low initial SOC content had higher potential in SOC sequestration. Lime addition might be an effective method to affect soil carbon sequestration and reduce soil CO₂ emission through chemical reactions.
Keywords：soil carbon;soil respiration;straw return;lime;isotope technology


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本文引用格式
曹彬彬, 朱熠辉, 姜禹含, 师江澜, 田霄鸿. 添加石灰和秸秆对塿土有机碳固持的影响[J]. 中国农业科学, 2020, 53(20): 4215-4225 doi:10.3864/j.issn.0578-1752.2020.20.010
CAO BinBin, ZHU YiHui, JIANG YuHan, SHI JiangLan, TIAN XiaoHong. Effects of Lime and Straw Addition on SOC Sequestration in Tier Soil[J]. Scientia Acricultura Sinica, 2020, 53(20): 4215-4225 doi:10.3864/j.issn.0578-1752.2020.20.010

0 引言

【研究意义】土壤碳库是地球上最大的碳库,约含3 500—4 800 Pg C,分别是大气和植被碳库的4倍和8倍^[1]。土壤碳库包括约62%的土壤有机碳（SOC）和38%的土壤无机碳（SIC）,两种碳库对全球碳循环和气候变化均具有重要影响^[2]。由于SOC在维持土壤质量、作物高产稳产及农业可持续发展中起着至关重要的作用,它一直是众多研究关注的重点,而对SIC关注相对较少^[3,4]。在干旱半干旱地区,SIC储量往往是SOC储量的2—10倍,而且SIC在固碳和减缓气候变化方面也具有巨大潜力^[5,6]。因此,研究农田管理措施对土壤碳固持的影响时,很有必要同时考虑这些措施对SOC和SIC两者的影响。我国农业生产中每年会产生10.4亿吨以上的作物秸秆^[7],秸秆就地还田具有培肥地力、保护生态环境等多方面的意义^[8]。【前人研究进展】秸秆等有机物料输入农田土壤的多寡及其随后的降解过程所产生的物质循环,会决定SOC固持的方向及程度^[9,10]。秸秆还田后既通过激发效应对原SOC的矿化产生显著影响^[11],同时秸秆自身腐解时也会新形成SOC,不过LI等^[12]研究发现大部分秸秆碳最终会被矿化成CO₂释放到大气中。因此,原SOC的矿化与新形成SOC之间的盈亏关系决定了SOC净固持量^[13]。另外,SOC固持受到很多因素影响,其中土壤性质尤其是原SOC含量的高低可能会起重要作用。KEITH等^[14]认为SOC固持能力可能随着原SOC含量增加而增加;而KIRKBY等^[15]则发现初始SOC含量越高,其产生的激发效应（priming effect,PE）会越强烈,SOC固持效率越低。同时农田土壤SOC与SIC在一定条件下是可以相互作用和相互转化的:SOC→CO₂→HCO₃^-(aq)→CaCO₃(s)^[16],具体表现为SOC通过降解释放出的CO₂能转化成SIC被固存下来;反之,含钙物质（石灰（CaO）、石灰石（CaCO₃）、白云石（CaMg(CO₃)₂））的投入在提高SIC含量的同时,可能会通过对土壤pH、微生物活性的影响,最终对SOC含量起到增加或降低的作用^[17,18]。【本研究切入点】由于在有关SOC固持与初始SOC含量之间关系的研究中添加的外源物料及土壤本身性质的不同,造成结果之间产生很大差异,使得原SOC含量与SOC固持之间的关系尚未明确。因此有必要采用土壤原SOC含量不同的同一类型土壤来探讨秸秆添加对其土壤固碳潜力的影响。同时,目前大多数研究为了提高土壤的固碳潜力,往往通过长期秸秆还田配施其他外源物料来实现^[19,20,21],但是关注点往往仅在SOC固持方面。【拟解决的关键问题】假设在秸秆还田条件下,添加外源含钙物质会降低土壤CO₂的释放,促进SIC的形成,同时提高SOC含量。因此,本研究采用长期秸秆还田量和氮肥施用量差异很大的两个供试土壤样品,通过室内恒温培养试验研究添加钙源和秸秆时土壤CO₂、SIC及SOC的特征,旨在揭示秸秆与外源含钙物质配合添加对土壤原SOC矿化及秸秆腐解产生的影响,并量化对新形成SOC及SIC的贡献,以期为农田土壤固碳减排提供理论依据。

1 材料与方法

1.1 供试材料

本研究中使用的供试土壤样品采自西北农林科技大学农作一站进行不同有机物料和矿质氮管理的长期定位试验地,土壤类型属于塿土（旱耕土垫人为土）,采用冬小麦-夏休闲的种植制度。长期定位试验起始于2002年,近20年中采用不同的有机物料投入量和氮肥施用量,共包含9个田间处理,本研究选择其中碳氮投入差异很大的2个田间处理,在本文中分别称为S₀N₀土壤:不进行秸秆还田（S₀）+不施用氮肥（N₀）,具体为在2002—2017年间均不投入小麦秸秆和化学氮肥;S₁N₁土壤:高量秸秆还田（S₁）+施氮肥（N₁）,具体为2002—2016年进行小麦秸秆覆盖还田,每年覆盖量为4 500 kg?hm^-2;2016—2017年进行小麦秸秆高量还田,每年秸秆还田量约为15 000 kg?hm^-2;2002—2017年每年施氮量均为240 kg?hm^-2。

在2018年夏休闲时期,从上述两个田间处理的土壤耕层（0—20 cm）采集试验所用的土壤样品,自然风干后分为两部分,一部分用于测定土壤基本理化性质;剩余样品除去可见的石块和植物残体,研磨过2 mm筛后用于室内培养试验。

室内培养试验所用的玉米秸秆采自西北农林科技大学斗口试验站,玉米植株成熟后将整株采回,将其地上部分（不包括穗部）在75℃下烘干粉碎至约2 mm长,备用。供试玉米秸秆含碳量为430.6 g·kg^-1,含氮量为5.6 g·kg^-1。本研究采用¹³C稳定同位素技术,选取的S₀N₀土壤（δ¹³C=-24.5‰）和S₁N₁土壤（δ¹³C=-25.1‰）均为种植小麦（C₃作物）超过10年的耕层土壤,将该土壤定义为C₃土壤。在该土壤中添加玉米秸秆（C₄作物;δ¹³C=-14.088‰）,由于同位素分馏作用使C₃土壤和C₄植物体的δ¹³C差异很大,能够通过¹³C稳定同位素技术区分碳的来源^[21,22,23]。

本试验采用的石灰为实验室分析用剂氧化钙（分析纯;CaO含量≥97%）。

供试土壤样品的理化性质见表1。

Table 1
表1
表1培养前两个供试土壤样品的基本理化性质
Table 1Basic physical and chemical properties of the two soils before incubation

土样 Soil	pH	SOC (g?kg-1)	TN (g?kg-1)	DOC (mg?kg-1)	MBC (mg?kg-1)	CaCO3 (g?kg-1)	NO3--N (mg?kg-1)	NH4+-N (mg?kg-1)	δ13C (‰)
S0N0	8.21a	7.98b	0.77b	7.49b	177.8b	68.6a	6.86b	0.45b	-24.514
S1N1	7.99b	12.61a	1.05a	195.7a	447.0a	65.5b	28.59a	1.38a	-25.132

同列数据后不同小写字母表示不同土样在P<0.05水平上差异显著
Different lowercase letters in the same column indicate significant difference between different soils at P<0.05

新窗口打开|下载CSV

1.2 试验设计及方法

本试验采用室内恒温培养试验,共有3个研究因素,分别为土壤碳氮水平（S₀N₀土壤;S₁N₁土壤）、玉米秸秆（M₀:不添加玉米秸秆;M₁:添加玉米秸秆）和石灰（L₀:不添加石灰;L₁:添加石灰）,共组成8个处理,表示为S₀N₀M₀L₀、S₀N₀M₀L₁、S₀N₀M₁L₀、S₀N₀M₁L₁、S₁N₁M₀L₀、S₁N₁M₀L₁、S₁N₁M₁L₀、S₁N₁M₁L₁,重复3次。其中玉米秸秆添加量为12 g?kg^-1土,石灰用量为3 g?kg^-1土。所有处理添加等量的氮磷肥,以补充秸秆腐解过程中所需N、P量,使秸秆的腐解情况达到理想的状态。

将250 g土壤（干重）置于1 L的培养罐中进行培养,将玉米秸秆和石灰分别按照不同处理与250 g土壤充分混匀,氮磷肥溶于蒸馏水后以溶液的形式加入,同时用蒸馏水调节土壤含水量为田间持水量的70%;随后将装有20 mL的1 mol·L^-1 NaOH溶液的小塑料瓶悬挂于培养罐中加盖密封后放置于恒温培养箱中进行随机排列。在25℃恒温的黑暗条件中培养120 d。

1.3 测定指标及方法

为了测定土壤CO₂的释放量,在培养的第2、3、4、8、7、9、12、16、21、26、37、48、64、84、104和120 d,取下装有的已吸收CO₂的20 mL 1 mol·L^-1 NaOH溶液的小塑料瓶,用20 mL的0.5 mol·L^-1的BaCl₂对吸收CO₂的NaOH溶液进行沉淀,将酚酞作为指示剂,用0.5 mol·L^-1的HCl进行反滴定,测定土壤CO₂的释放量^[24]。试验期间打开培养罐通气以保证气体交换。每次CO₂测定结束后,更换NaOH溶液并进入下一个培养时期。

培养试验结束后,将培养罐中的土壤样品风干后测定SOC、SIC及SOC的δ¹³C值等指标。

SOC采用重铬酸钾-浓硫酸外加热法测定^[25];土壤碳酸钙采用气量法进行测定^[25];玉米秸秆的δ¹³C值直接采用元素分析-同位素质谱仪（Delta plus, Finnigan MAT, Bremen, Germany）进行测定;土壤样品则先用HCl溶液进行预处理,除去土壤无机碳,烘干,磨细至<0.105 mm再采用元素分析-同位素比质谱仪（Delta plus, Finnigan MAT, Bremen, Germany）测定其中SOC的δ¹³C,计算公式如下:

δ¹³C（‰）=[(R_sample/R_standard)-1]×1000

式中,R_sample=¹³C_sample/¹²C_sample,R_standard=¹³C_standard/¹²C_standard^[26]。

1.4 计算方法

（1）根据培养前后土壤及秸秆的δ¹³C值,采用下列公式计算培养结束后土壤中来源于秸秆的有机碳（New OC）和土壤原有机碳（Native SOC）的含量:

f_new(%)=(δ¹³C_after-δ¹³C_before)/(δ¹³C_straw-δ¹³C_before)

New OC=f_new(%)×C _total

Native SOC=(1-f_new)×C _total

式中,δ¹³C_straw表示秸秆的δ¹³C值;δ¹³C_before表示培养前土壤的δ¹³C值;δ¹³C_after表示培养结束后土壤SOC的δ¹³C值;f_new表示培养结束后SOC中源自秸秆碳的比例;C_total表示培养结束后SOC含量;New OC表示培养结束后土壤中源自秸秆的碳量;Native SOC表示培养结束后土壤中原土壤有机碳量。

（2） SOC净固持量(mg?kg^-1)=最终SOC-初始SOC。

1.5 数据处理

本研究的原始数据采用Microsoft Excel 2007进行整理计算,试验数据表示均为具有标准误差的3次重复的平均值。使用DPS V7.05专业版软件对SIC、SOC采用三因素方差分析（Three-way ANOVA）进行F检验后,进行多重比较（最小显著差法LSD）,比较不同处理间在P<0.05水平的显著性差异;使用Excel 2010软件和Sigmaplot 12.5对数据进行绘图。

2 结果

2.1 土壤CO₂释放速率及累积释放量的变化

在培养期间,添加秸秆后的S₀N₀土壤和S₁N₁土壤的CO₂释放速率均高于不添加秸秆。在培养的第2天,未添加石灰的处理均达到土壤CO₂释放速率的高峰,之后随着培养时间的延长释放速率开始降低。同时,无论添加秸秆与否,S₁N₁土壤CO₂释放速率始终高于S₀N₀土壤。未添加秸秆时,加入石灰使CO₂释放速率降低,S₀N₀土壤培养到第3天达到CO₂释放速率的顶峰,而S₁N₁土壤则在第16天达到CO₂释放速率的最大值。然而,同时添加秸秆和石灰时S₀N₀土壤CO₂释放速率的顶峰晚于S₁N₁土壤2 d。与单独添加秸秆相比,秸秆与石灰配施使S₀N₀土壤和S₁N₁土壤的CO₂释放速率分别降低了18%和20%。因此,无论添加秸秆与否,石灰的加入均降低了土壤CO₂释放速率（图1-a）。

图1

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图1不同处理土壤CO₂释放速率（a）和CO₂累积释放量（b）

曲线外的误差棒代表各处理间5%水平的LSD值
Fig. 1Soil CO₂ emission rate (a), cumulative CO₂ emission (b) under different treatments

The error bars outside the curve represent LSD0.05


培养期间土壤CO₂累积释放量受到培养时间、土壤碳氮水平、秸秆和石灰添加的主效应的显著影响（表2）。在不添加秸秆时,S₁N₁土壤CO₂累积释放量始终高于S₀N₀土壤;添加秸秆增加了S₀N₀土壤和S₁N₁土壤的CO₂累积释放量,同时S₁N₁土壤的CO₂累积释放量仍高于S₀N₀土壤（图1-b,表3）,并且均随着培养时间的延长而增加。不管添加秸秆与否,石灰的加入均显著降低了土壤CO₂累积释放量（表3）,使S₀N₀土壤和S₁N₁土壤CO₂累积释放量分别降低了19.9%和18.2%（图1-b）。

Table 2
表2
表2培养时间作为因素对CO₂累积释放量进行四因素重复测量方差分析
Table 2Four-way repeated measure ANOVA with incubation time as factor for cumulative CO₂ release

	时间 Time	时间×土壤 Time×soil fertility	时间×秸秆 Time× straw	时间×石灰 Time× lime	时间×土壤×秸秆 Time×soil fertility×straw	时间×土壤×石灰 Time×soil fertility×lime	时间×秸秆×石灰 Time×straw ×lime	时间×土壤×秸秆×石灰 Time×soil fertility×straw×lime
	P	P	P	P	P	P	P	P
CO2累积释放量 Cumulative CO2 emission	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001

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Table 3
表3
表3秸秆和石灰的添加对CO₂排放量、SOC、SIC的影响
Table 3Effects of straw and lime amendment on cumulative CO₂ emission, SOC and SIC

处理 Treatment		CO2释放量 CO2 emission (mg?kg-1)	增加量 Increase amount (mg?kg-1)	SOC (mg?kg-1)	增加量 Increase amount (mg?kg-1)	SIC (mg?kg-1)	增加量 Increase amount (mg?kg-1)
S0N0	M0	600b	-	7280b	-	8380a	-
	M1	3564a	2964	10480a	3200	8390a	10
S1N1	M0	1193b	-	12440b	-	7823a	-
	M1	4027a	2833	14435a	1995	7783a	-40
S0N0	L0	2316a	-	8825a	-	8160b	-
	L1	1847b	-469	8940a	115	8610a	443
S1N1	L0	2912a	-	13575a	-	7520b	-
	L1	2383b	-529	13300b	-275	8086a	566

L₀:不添加石灰;L₁:添加石灰;M₀:不添加秸秆;M₁:添加秸秆。同列不同小写字母表示不同处理在P<0.05水平上的差异显著。表4同
L₀: No lime; L₁: Add lime; M₀: No maize straw; M₁: Add maize straw. Different letters in the same column indicate significant difference between different treatments at P<0.05. The same as Table 4

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2.2 土壤SOC含量及净固持量、新形成SOC和SIC含量的变化

本研究中,SOC含量受到土壤碳氮水平、秸秆及石灰添加的交互效应的显著影响。首先,在不添加任何外源物料的情况下,S₁N₁土壤的SOC含量高于S₀N₀土壤的69.7%（图2）。与不添加秸秆相比,添加秸秆使S₀N₀土壤和S₁N₁土壤的SOC含量分别提高了2.95和3.19 g·kg^-1（图2）;同时,S₁N₁土壤在添加秸秆后SOC含量仍高于S₀N₀土壤。但是,石灰的加入对土壤SOC含量的影响甚微（表3）。在秸秆还田条件下,添加石灰较不添加石灰相比使S₁N₁土壤SOC含量显著降低了8.71%;而对S₀N₀土壤SOC含量影响不显著（图2）。

图2

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图2不同处理下土壤有机碳（SOC）含量的差异

误差棒代表平均值的标准误差。不同小写字母表示不同处理在P<0.05水平上的差异显著
Fig. 2Difference of SOC content under different treatments

Error bars represent standard errors of the mean values. Different lowercase letters indicate significant difference between different treatments at P<0.05


δ¹³C值的变化可以计算出土壤原SOC和新形成的SOC在总SOC中所占比例。由表4可以看出,在不添加任何外源物料情况下,S₀N₀土壤SOC的δ¹³C值高于S₁N₁土壤的6.9%。当添加C₄玉米秸秆后,SOC的δ¹³C值均有增加的趋势。通过¹³C质量守恒定律计算可得土壤原SOC和新形成的SOC在分解期间对总SOC的贡献比例及贡献量的变化情况,添加秸秆在S₀N₀土壤和S₁N₁土壤中均会新形成SOC;同时,S₀N₀土壤中新形成的SOC含量高出S₁N₁土壤0.77 g·kg^-1。在添加秸秆时,加入石灰对土壤新形成SOC含量没有显著影响。

Table 4
表4
表4不同处理下土壤新碳的形成及原有土壤有机碳的分解的差异
Table 4Differences between soil straw-derived organic carbon formation and decomposition of native organic carbon under different treatments

处理 Treatment			丰度值 δ13C 13C abundance (‰)	土壤有机碳 (Ctotal) Final SOC (g?kg-1)	源自秸秆碳比例(fnew) New OC proportion (%)	源自秸秆碳 New OC (g?kg-1)	源自原土壤碳比例(fnative) Native SOC proportion (%)	源自土壤有机碳 Native SOC (g?kg-1)
S0N0	L0	M0	-23.54	7.35e	0	0	100	7.35b
		M1	-21.61	10.30d	28.9	2.98a	71.1	7.32b
	L1	M0	-23.52	7.22e	0	0	100	7.22b
		M1	-21.49	10.66d	32.4	3.45a	67.6	7.21b
S1N1	L0	M0	-25.28	12.48c	0	0	100	12.48a
		M1	-23.48	14.67a	15.1	2.21b	84.9	12.46a
	L1	M0	-25.42	12.40c	0	0	100	12.40a
		M1	-23.46	14.20b	15.2	2.15b	84.8	12.04a

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本研究中,添加秸秆使S₀N₀土壤和S₁N₁土壤的SOC净固持量分别增加了3 066.3和2 480.53 mg?kg^-1,有利于土壤有机碳的固持。无论添加秸秆与否,在S₀N₀土壤中,石灰的加入对SOC净固持量无显著影响;但是在S₁N₁土壤中添加秸秆时,石灰的加入则使土壤SOC净固持量显著降低了55%（图3）。

图3

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图3不同处理对SOC净固持量的影响

初始SOC含量:S₀N₀ 7.98 g?kg^-1;S₁N₁ 12.61g?kg^-1。柱上小写字母表示不同处理的最终SOC在P<0.05水平上差异显著;大写字母表示不同处理的SOC净固持量在P<0.05水平上差异显著
Fig. 3Differences between soil organic carbon net sequestrations under different treatments

Initial soil organic carbon content: S₀N₀: 7.98 g?kg^-1, S₁N₁: 12.61g?kg^-1. Lowercase letters in the figure indicate significant differences of final SOC between different treatments at P<0.05; Uppercase letters indicate significant differences of soil organic carbon net sequestration between different treatments at P<0.05


对于SIC而言,培养试验结束后,土壤碳氮水平和石灰的主效应对SIC含量的影响显著。S₁N₁土壤的SIC含量低于S₀N₀土壤的7.3%。添加秸秆对土壤S₀N₀土壤和S₁N₁土壤的SIC含量在均无显著影响。但是无论添加秸秆与否,石灰的加入均显著提高了S₀N₀土壤和S₁N₁土壤的SIC含量（表3）。与单独添加秸秆相比,秸秆和石灰添加下S₀N₀土壤和S₁N₁土壤的SIC分别增加了7.4%和7.6%（图4,表3）。

图4

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图4不同处理下土壤无机碳（SIC）含量的差异

误差棒代表平均值的标准误差;大写字母表示S₀N₀土壤和S₁N₁土壤处理间在P<0.05水平上差异显著;*表示添加石灰与不添加石灰处理间在P<0.05水平上差异显著
Fig. 4Difference of SIC content under different treatments

Error bars represent standard errors of the mean values. Uppercase letters indicate significant differences between the two fertility soil treatments at P<0.05. Asterisk above the bars indicates statistically significant differences between lime-unamended and corresponding lime-amended soils

3 讨论

3.1 秸秆和石灰添加对不同土壤CO₂释放量的影响

本研究表明,未添加任何外源物料时,S₁N₁土壤与S₀N₀土壤的CO₂累积释放量之间存在显著差异,前者比后者高出42.9%（图1）,说明土壤基础呼吸的强弱高度依赖于初始SOC含量,因为S₁N₁土壤能够为微生物提供满足生长代谢的碳氮等养分,土壤微生物活性更高,此前众多研究也有类似结论^{[14, 27-28]}。

当土壤中添加等量秸秆之后,与对照土壤相比,S₀N₀土壤和S₁N₁土壤的CO₂释放速率和累积释放量均显著增加。但是,S₀N₀土壤CO₂累积释放量的增加幅度高于S₁N₁土壤（图1）。这说明相对于S₁N₁土壤,添加的秸秆对初始SOC含量低的土壤的原SOC的矿化影响更大。主要原因可能是,无论对于哪种土壤,添加秸秆均会促进土壤微生物生长及酶活性大幅增加,从而“激发”原SOC的分解即发生正激发效应,一般来说,惰性有机碳成分含量低的秸秆等外源物料均表现为正激发效应^[29]。本研究中S₀N₀土壤与S₁N₁土壤的表观激发效应分别为3 192、3 088 mg?kg^-1;二者相差很小,但是S₀N₀土壤产生了相对更高的PE值,这可能与“化学计量学（stoichiometric decomposition）”策略和“微生物氮挖掘（microbial N-mining）”策略有关:由于土壤微生物的生长本身存在固定的碳氮养分需求,如果土壤环境中的碳氮能够满足微生物生长代谢所需养分时,微生物的活性最高,对外源有机物料的分解速率最大。相反,在氮的有效性较低的条件下,微生物则会通过分解惰性的有机质来获取需要的氮源^[30]。因此在本研究中由于S₀N₀土壤本身能被微生物利用的养分数量更少,同时本研究中未额外添加外源氮素等养分,土壤本身养分会以较快速率耗竭,此时土壤微生物中K-策略菌起主导作用,添加秸秆则会刺激K-策略菌更倾向于去分解利用更为惰性的原SOC中的养分,来满足自身生长,从而加速原SOC的矿化,产生更大的正激发效应^[31,32]。而S₁N₁土壤中的碳氮养分充足,能够满足微生物生长所需,此时土壤中r-策略菌开始起主导作用,添加秸秆后该类微生物优先利用外源投入的秸秆,对其进行分解利用^[28,33-34],而减少对原SOC的分解。因此添加秸秆后,初始SOC含量低的土壤会产生更高的PE。

在土壤中仅添加石灰而不添加秸秆时,会导致土壤CO₂释放量的降低,在S₀N₀土壤和S₁N₁土壤中的降幅分别是35%和15.4%;同时与单独添加秸秆相比,秸秆与石灰配施时,S₀N₀土壤和S₁N₁土壤的CO₂累积释放量也分别降低了15.7%和18.9%（图1）。以上表明无论添加秸秆与否,钙源添加均能降低土壤CO₂累积释放量。

3.2 秸秆和石灰添加对新形成有机碳的影响

就添加秸秆对新形成有机碳而言,在两个供试土壤中,与不添加秸秆的空白土壤相比,添加秸秆后均能促使新形成SOC,且S₀N₀土壤和S₁N₁土壤中新形成的SOC含量分别提高了28.9%和15.1%（表4）,这可能是因为S₁N₁土壤含有丰富的碳源,微生物活性相对较高;添加秸秆后,微生物会快速分解新鲜秸秆,加速秸秆碳的周转,故残留的秸秆碳相对低^[33]。KIRKBY等^[15]在对4种初始SOC含量不同的耕地土壤添加秸秆后,也发现土壤初始SOC含量越低,新形成的有机碳含量越高,这与我们的研究结果是一致的。

与对照相比,石灰与秸秆同时添加显著增加了S₁N₁和S₀N₀土壤SOC含量31%和72%;但是与单独添加秸秆相比,秸秆和石灰添加显著降低了S₁N₁土壤SOC含量的3.2%,对S₀N₀土壤无显著影响（图2）;表明石灰对土壤SOC的影响与土壤性质和农田管理措施有关^[35,36]。石灰与秸秆配施仍然会增加土壤中新形成SOC（表4）,其新形成的SOC含量与单独添加秸秆时大致相当,说明了添加石灰对秸秆在土壤中的腐解过程不会造成明显影响。

3.3 秸秆和石灰添加对SOC净固持量的影响

本研究中,添加秸秆后S₀N₀土壤和S₁N₁土壤的SOC净固持量分别提高了3 066.3和2 480.53 mg?kg^-1（图3）;这是因为土壤中添加等量秸秆后,有机碳的矿化量主要来源于SOC自身被微生物矿化、秸秆腐解及秸秆添加对土壤原SOC引起PE等三者产生的CO₂^[37]。同时,秸秆分解后有一些秸秆碳转化成“新”有机碳固持于土壤中。本研究中SOC净固持量的增加说明了新形成的SOC含量抵消甚至超过了原SOC的矿化量。虽然S₀N₀土壤和S₁N₁土壤的SOC净固持量的增加幅度相差不大,但是S₀N₀土壤的SOC净固持量数值更高,这可能是因为:与S₁N₁土壤相比,S₀N₀土壤的初始SOC含量距离碳饱和水平相对较远,因此更利于SOC固持^[38]。以上结果表明,在同一质地的土壤中,初始SOC含量越低的土壤越利于SOC固持。

在不添加秸秆的情况下,SOC的净固持量取决于SOC矿化量。与空白土壤相比,单独添加石灰使S₀N₀土壤和S₁N₁土壤的SOC净固持量没有发生显著变化;但是这与土壤CO₂累积释放量的现象相矛盾:添加石灰显著降低了土壤CO₂累积释放量;这可能是因为石灰的存在使SOC矿化的CO₂中一部分通过其他途径吸收或反应,最终引起土壤CO₂累积释放量的降低。但是在添加秸秆后,SOC的净固持量取决于SOC的矿化量和新SOC的形成量之间的平衡;与单独添加秸秆相比,同时添加秸秆和石灰使S₁N₁土壤的SOC净固持量呈现下降趋势,而对S₀N₀土壤的SOC净固持量仍无显著影响,这可能归因于石灰和秸秆影响了S₀N₀土壤和S₁N₁土壤的微生物活性,进而对SOC的固持能力产生了影响,但具体影响的微生物群落和活性还有待于进一步研究。

3.4 石灰降低土壤CO₂释放量的机制

无论添加秸秆与否,加入石灰均降低了土壤CO₂释放量,难道是石灰的存在影响了有机碳的矿化作用吗?本研究认为没有,研究结果显示添加石灰后引起S₀N₀土壤和S₁N₁土壤CO₂的减少量为469和529 mg?kg^-1,同时土壤SIC含量分别提高了443和566 mg?kg^-1（表3）;这一现象说明添加石灰对土壤CO₂释放量的影响机制是:CaO首先与土壤中的水反应生成Ca(OH)₂,再与土壤CO₂反应,最终形成CaCO₃固持于土壤中^[39,40]。这种现象在两个供试土壤及添加或不添加秸秆时均出现,表明含钙物质与土壤CO₂的反应与土壤性质和有机物料的添加无关;也进一步说明了石灰对土壤CO₂释放量降低的影响可能是钙源吸收土壤CO₂后通过化学反应生成CaCO₃所造成的,对SOC的矿化过程没有产生影响。

4 结论

无论添加秸秆与否,S₁N₁土壤CO₂累积释放量始终高于S₀N₀土壤;添加等量秸秆后S₀N₀土壤产生的PE略高于S₁N₁土壤,说明秸秆的添加对初始SOC含量低的土壤原SOC矿化影响更大。无论是否添加秸秆,添加石灰显著降低了土壤CO₂累积释放量。添加秸秆后,S₀N₀土壤中新形成的SOC含量高于S₁N₁土壤,而石灰的加入对新形成SOC的数量没有明显影响。添加秸秆均促进了S₀N₀土壤和S₁N₁土壤的SOC净固持量,同时初始SOC含量低的土壤净固持率更大;但是加入石灰和秸秆配施则降低了SOC净固持量。加入石灰使土壤CO₂释放量的减少量与SIC的增加量大致相等,因此推测石灰对土壤CO₂释放量的影响机制可能是钙源吸收部分土壤CO₂生成了SIC。由此可见,初始SOC含量低的土壤具有更高的固碳潜力;添加钙源能够与土壤CO₂进行化学反应,从而实现土壤固碳减排的目标。

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