大气氮沉降对三峡库区消落带土壤呼吸的影响

何立平,田茂平,吴 红,林俊杰,兰 波



大气氮沉降对三峡库区消落带土壤呼吸的影响

何立平1,2*,田茂平1,吴 红1,林俊杰1,兰 波1

(1.重庆三峡学院,三峡库区水环境演变与污染防治重庆高校市级重点实验室,重庆 404100；2.三峡库区生态环境保护和灾害防治重庆市协同创新中心,重庆 404100)

以三峡库区消落带落干期土壤为研究对象,采用室内模拟培养的方法,探讨了大气氮沉降通量及其组成对土壤呼吸的影响.结果表明,土壤呼吸速率对氮添加的响应为短期效应. 1倍当前大气氮沉降添加下,无机氮和有机氮对土壤累积CO2释放分别表现为无影响和抑制作用.除NH4+-N在2倍氮沉降添加下表现为抑制作用外, 2、3倍氮沉降添加均促进了土壤累积CO2释放.与硝态氮相比, 2、3倍氮沉降添加的铵态氮对土壤累积CO2释放具有抑制作用.

大气氮沉降；三峡库区；消落带；土壤呼吸

土壤碳库是陆地生态系统碳库的主体,全球约有684~724Pg 碳是以有机质的形态储存于地表0~30cm的土壤中.土壤碳库储量约是陆地植被总碳储量的1.5倍,与大气CO2碳储量相当[1].因此,即使表层土壤有机碳库轻微的矿化,也可能引起大气CO2浓度的显著升高,影响全球气候变化[2-3].

土壤碳氮循环存在耦合关系[4].作为全球气候变化驱动因素之一,大气氮沉降是影响陆地生态系统有机碳生物地球化学循环的重要因素[5].过去60年,人类活动已导致全球大气氮沉降显著增加[6-7].并且,随着全球人口的持续膨胀和能源消耗的进一步加大,未来数十年,全球大气氮沉降将持续加剧[8].因此,大气氮沉降对土壤生态系统CO2释放的影响已成为关注的焦点.

目前,针对大气氮沉降对土壤生态系统CO2释放的影响,已开展了大量的室内室外模拟研究.从大气氮沉降组成的角度,这些研究可分为无机氮和有机氮沉降模拟研究.无机氮沉降模拟研究采用了NH4+-N[9-10]、NO3--N[11-13]和NH4NO3[14-16]3种氮组成,而有机氮沉降模拟研究采用了CO(NH2)2作为氮组成[17-19].目前这些模拟研究还存在以下2方面的问题:一方面,这些研究针对的是不同土壤生态系统(草地、森林、农田、湿地)且氮添加量各异,因此得到了大气氮沉降对土壤CO2释放具有促进作用[16,18]、抑制作用[9-10,14]、无影响[20-21]以及土壤CO2释放与大气氮沉降通量有关的不同结果[18,22-23].因此,要准确评估大气氮沉降对土壤生态系统CO2释放的影响,仍需进一步探讨大气氮沉降对其他土壤生态系统CO2释放影响,为后续评估模型的建立提供数据支撑.另一方面,实际大气氮沉降组成既包括有机氮也包括无机氮[24],而相关研究仅模拟讨论了单一无机或者有机氮沉降对土壤CO2释放的影响.因此,这一科学问题仍需针对更多的土壤生态系统和实际的大气氮沉降组成进行更加细致深入的研究.

消落带是水陆生态系统的交界地带,土壤生态环境特殊.针对大气氮沉降对消落带土壤CO2释放的影响研究目前尚未见报道.三峡水库蓄水后,形成了世界上面积最大的消落带,本研究以三峡库区消落带落干期土壤为研究对象,采用室内模拟培养的方法,探讨落干期不同大气氮沉降通量及其组成对土壤CO2释放的影响,为评估大气氮沉降对土壤生态系统CO2释放的影响和预测未来全球气候变化的趋势提供新的科学依据.

1 材料和方法

1.1 土壤样品采集及预处理

三峡水库最高水位为175m,最低水位为145m.水库冬蓄夏泄的调蓄水制度,在两岸形成了落差30m,面积约400km2的消落带.消落带1~4月和5~7月分别为淹水期和落干期[25].自2009年三峡水库首次蓄水至175m水位以来,消落带已经历了6个周期的干湿交替过程.本研究于2016年7月在三峡库区消落带万州段(30º49’26’’N~30º49’38’’N, 108º24’45’’E~108º26’16’’E)采集表层0~10cm土壤样品.样品采集后立即放入4℃保温箱中保存,迅速送往实验室.土壤经冷冻干燥,去除石砾、动植物残体,过8目筛后混匀装入棕色磨口瓶中置于干燥器储存备用.供试土壤基本理化性质见表1.

表1 消落带土壤基本理化性质 Table 1 Basic physicochemical properties of soil in water level fluctuating zone

1.2 实验设计

三峡库区万州段消落带落干期平均气温为25℃[25],因此本研究以25℃,好氧培养进行室内模拟.无机氮沉降以NH4+-N、NO3--N模拟,而有机氮沉降以CO(NH2)2模拟.此外,考虑到我国实际大气氮沉降无机氮和有机氮的比例为72%和28%[26],本研究以NH4NO3(72%)和CO(NH2)2(28%)模拟真实大气氮沉降.目前三峡库区大气氮沉降通量约为50kgN/ (hm2·a),且仍存在逐渐加剧的趋势[26],因此,本研究模拟大气氮沉降通量分别为1, 2, 3倍实际大气氮沉降通量.最终,培养实验包括4种大气氮沉降组成(包含NH4+-N、NO3--N、CO(NH2)2和NH4NO3(72%)+ CO(NH2)2(28%)), 3个大气氮沉降通量(50,100, 150kgN/(hm2×a)),共计12个处理,每个处理设置3个平行,外加3个对照3个空白,共计42个土壤样品.

1.3 土壤培养

每个处理分别称取预处理后的消落带土壤样品20g,平铺于250mL棕色培养瓶底部,调整土壤含水率为最大田间持水量(WHC)的50%,加盖密封预培养24h,然后将土壤含水率调整至WHC的60%进行正式培养.加盖密封正式培养前向各培养瓶中放置盛装3mL(1mol/L)NaOH溶液的CO2捕获瓶.土壤培养期间每日打开瓶盖通气20min,并定期通过重量法调整土壤含水率,使其保持稳定.各培养瓶分别于土壤培养的1, 2, 4, 6, 8, 15, 22, 29, 36d打开培养瓶更换CO2捕收液后继续密封培养[27].土壤培养结束后破坏性取样分析土壤溶解性有机碳(DOC)含量.

1.4 分析方法

土壤呼吸速率采用碱液吸收法测定;土壤DOC使用0.5mol/L K2SO4提取后用TOC分析仪(TOC-LCPH/CPN, Shimadzu)测定[28];土壤有机质(SOM)采用重铬酸钾外加热法测定[29];总氮采用凯氏法测定;土壤WHC和pH值分别采用环刀法和酸度计法测定;土壤粒径组成采用比重计法测定.

1.5 数据统计

采用单因素方差(One-way ANOVA)和LSD多重检验法分析不同处理下土壤呼吸速率、累积释放量、DOC差异的显著性;采用单样本检验法分析不同氮添加量及其组成变化下土壤呼吸速率、累积释放量及DOC含量与对照之间的显著性差异;进行方差分析之前,首先对数据是否正态分布和方差是否齐性进行检验,如不满足,则进行非参数检验.所有统计分析均在SPSS 18.0 (SPSS Inc, Chicago, IL, USA)中进行,方差分析及检验,显著性标准均为<0.05.使用SigmaPlot12.5软件作图.

2 结果与讨论

2.1 氮添加量及其组成对土壤呼吸速率的影响

对照及不同氮添加量时,土壤呼吸速率均随着培养时间的增加逐渐下降,最终保持稳定(图 1).培养第1d,不同氮添加量下,土壤呼吸速率特征分别为:(1)NH4Cl: 1倍>0, 3倍>2倍[图1 (A),<0.05]; (2)NaNO3: 3倍>0, 1, 2倍[图1 (B),<0.05];(3)CO (NH2)2:3倍>0倍>2倍>1倍[图1 (C),<0.05];(4)CO (NH2)2+NH4NO3: 2, 3倍>0倍>1倍[图1 (D),< 0.05].培养第1d,不同氮组成添加下土壤呼吸速率特征分别为:(1)1倍氮添加: NH4Cl >NaNO3, CO(NH2)2+NH4NO3> CO(NH2)2;(2)2倍氮添加: CO(NH2)2+ NH4NO3>NaNO3> CO(NH2)2> NH4Cl;(3)3倍氮添加: NaNO3, CO(NH2)2,CO(NH2)2+ NH4NO3> NH4Cl (图1,<0.05).

4种氮组成添加下,土壤培养第1d,呼吸速率对氮添加量存在响应,而随着培养时间的延长,这种响应逐渐消失(图1).这表明土壤呼吸速率对氮添加的响应为短期效应. 研究表明对于微生物生长同时受到碳氮限制的森林土壤,氮添加对土壤呼吸并没有短期的促进作用[30],这与本研究结果是不一致的.消落带土壤微生物生长没有受到碳限制是导致不同结果的原因,即三峡库区消落带土壤在干湿交替过程中存在新碳输入,消落带土壤有机质含量14.78g/ kg(表1),显著高于当地土壤有机质含量(7.59g/kg)[31].在微生物不受碳限制的条件下,氮添加短期改变了微生物活性,影响土壤呼吸速率[32-33],随着微生物可利用碳逐渐分解, 呼吸速率趋于稳定.

2.2 氮添加量及其组成对消落带土壤累积CO2释放的影响

土壤培养36d后不同氮添加量处理的累积CO2释放特征分别为:(1)NH4Cl: 3倍>0, 1倍>2倍;(2) NaNO3: 2, 3倍>0, 1倍;(3)CO(NH2)2: 3倍>2倍>0倍>1倍;(4)CO(NH2)2+ NH4NO3: 2倍>3倍>0倍>1倍(表3,<0.05).不同组成处理的CO2累积释放特征分别为:(1)1倍氮沉降添加:NH4Cl, NaNO3> CO (NH2)2+ NH4NO3> CO(NH2)2;(2) 2倍氮沉降添加: CO(NH2)2+ NH4NO3> NaNO3> CO(NH2)2> NH4Cl;(3)3倍氮沉降添加: CO(NH2)2,NaNO3> NH4Cl, CO(NH2)2+NH4NO3.

1, 2, 3倍氮沉降量的NH4+-N添加对土壤累积CO2释放分别表现为无影响、抑制和促进作用(表2).研究表明NH4+-N增加对土壤CO2释放存在抑制作用[9-10].这与本研究结果是不一致的.消落带土壤存在新碳输入,土壤C/N为56.8(表1),远高于微生物生长最佳比例(25),因此氮添加量增加能够逐渐降低土壤C/N引起微生物群落结构和土壤呼吸的变化[32,34-35].这是造成上述结果差异的原因,有待进一步研究.

1倍氮沉降的NO3--N添加下,土壤培养36d后CO2累积释放量与对照无显著性差异(0.05),而2, 3倍氮沉降的NO3--N添加下均表现为显著性大于对照(表2,<0.05).这表明目前大气氮沉降量下,土壤CO2释放对NO3--N沉降无响应,而随着未来氮沉降量的进一步增加氮沉降将促进土壤CO2释放.研究表明:NO3--N增加能够抑制微生物对土壤有机质的分解,增加土壤碳储量[11-13],这显然与本研究结果是不一致的.消落带土壤在干湿交替过程中存在新碳输入是造成结果差异的原因.NO3--N对老碳分解有抑制作用,相反对新碳分解有促进作用[36].因此,NO3--N含量的增加促进了新碳的分解,最终导致土壤呼吸作用增加.

1倍氮沉降添加下,土壤培养36d后CO2累积释放量在NH4+-N和NO3--N之间无显著性差异(0.05),而2, 3倍氮沉降添加下表现为NH4+-N

表2 氮添加量及其组成对土壤CO2累积释放的影响(mg/kg) Table 2 The effects of nitrogen addition flux and its composition on soil CO2 cumulative release (mg/kg)

注:土壤培养时间36d,不同小写字母表示CO2累积释放量在不同的氮添加量之间存在显著性差异(<0.05); 不同大写字母表示CO2累积释放量在不同的氮添加组分之间存在显著性差异(<0.05).

1倍氮沉降的CO(NH2)2及CO(NH2)2+NH4NO3添加下,土壤培养36d后CO2累积释放量均显著小于对照,而在2, 3倍氮沉降的氮添加下则相反(表3,0.05).这表明目前大气氮沉降量下,CO(NH2)2和CO(NH2)2+NH4NO3添加对土壤呼吸均有抑制作用,而随着未来氮沉降量的持续增加,将逐渐从抑制作用转换为促进作用.CO(NH2)2对土壤呼吸的影响,目前存在2种观点:第一种观点认为低氮通量(约1倍氮沉降)添加能够促进土壤CO2释放,而高氮通量(约4倍氮沉降)添加则相反[17];第二种观点认为, CO(NH2)2添加能够促进土壤CO2释放[18-19].本研究结果与这两种观点均不一致.消落带土壤生态环境特殊,土壤性质差异可能是造成结果差异的原因[37].目前关于CO(NH2)2+NH4NO3添加对土壤CO2释放的影响研究还未见报道,但科研人员研究了NH4NO3添加对土壤CO2释放的影响.主流观点认为,NH4NO3添加对土壤CO2释放具有抑制作用[14-15,38],也有研究得到了相反的结果[16].本研究发现CO(NH2)2+NH4NO3添加下,土壤呼吸作用表现为3倍氮沉降<2倍氮沉降,而仅存在CO(NH2)2添加下,则相反(表3,0.05).由此可见,NH4NO3添加量的增加抑制了土壤CO2释放,这和主流研究结果是一致的.这是由于高浓度的NH4NO3对土壤真菌具有抑制作用造成的[32].要进一步阐释CO(NH2)2和CO(NH2)2+ NH4NO3添加对土壤呼吸的影响,仍需探讨氮添加过程中土壤微生物群落结构的变化,有待进一步的研究.

除NH4Cl外,土壤累积CO2释放均表现为2, 3倍氮沉降>0, 1倍氮沉降(表2,0.05).可见,虽然当前大气氮沉降对土壤CO2释放没有促进作用,但是随着大气氮沉降的持续增加将逐渐转变为促进作用,这无疑增加了全球气候进一步变暖的风险.大量的研究表明大气氮沉降持续增加将抑制微生物不受氮素限制的森林土壤呼吸[39-40],这与本研究结果恰恰相反.本研究中土壤C/N为56.8(表1),远高于微生物生长最佳比例(25),高浓度的氮输入(2, 3倍氮沉降)短期内能够显著降低土壤C/N,从而促进了土壤呼吸.

2.3 氮添加量及其组成对土壤DOC含量的影响

土壤培养36d后,不同氮添加量处理的DOC含量特征分别为:(1)NH4Cl、CO(NH2)2及CO(NH2)2+ NH4NO3: 1倍>2倍>3倍>0倍;(2)NaNO3: 3倍>2倍>1倍>0倍(表4,0.05).不同组成处理的DOC含量特征分别为:(1)1倍氮添加: NH4Cl > CO(NH2)2, CO(NH2)2+NH4NO3>NaNO3;(2)2倍氮添加:NH4Cl > CO(NH2)2+ NH4NO3> CO(NH2)2>NaNO3;(3)3倍氮添加: NH4Cl, NaNO3> CO(NH2)2+ NH4NO3> CO(NH2)2.

对照土壤DOC含量表现为培养前>培养后,而氮添加下则相反(表1,表3,0.05).可见,氮添加促进了土壤有机质的分解.这与Cusack等[41]的研究结果是一致的.氮添加能够增加与有机质分解相关的酶活性[42],从而促进土壤有机质的分解. CO(NH2)2进入土壤后水解产物为NH4+-N,与NO3--N相比, NH4+-N更易被土壤微生物吸收,因此在1倍氮沉降添加下,土壤DOC含量表现为NH4Cl、CO(NH2)2、CO(NH2)2+ NH4NO3> NaNO3(表3,0.05).除NO3--N为相反的趋势外, DOC含量随着其余氮组成添加量的增加而减小.这表明NH4+-N含量增加对土壤有机质分解有抑制作用,而NO3--N增加则具有促进作用.NH4+-N含量增加导致的土壤微生物群落结构变化是抑制有机质分解的原因,有待进一步研究.NO3--N对新碳分解有促进作用[36],因此土壤NO3--N添加量与DOC含量正相关.

表3 氮添加量及其组成对土壤培养后DOC含量的影响(mg/kg) Table 3 The effects of nitrogen addition flux and its composition on soil DOC content after soil incubation (mg/kg)

注: 土壤培养时间36d,不同小写字母表示 DOC含量在不同的氮添加量之间存在显著性差异(0.05); 不同大写字母表示, DOC含量在不同的氮添加组分之间存在显著性差异(0.05).

3 结论

3.1 土壤呼吸速率对氮添加的响应为短期效应.

3.2 1倍氮沉降添加下无机氮对土壤呼吸无影响,而有机氮表现为抑制作用.除NH4+-N在2倍氮沉降添加下抑制了土壤呼吸外,与0, 1倍氮沉降添加相比2, 3倍氮沉降添加均促进了土壤呼吸作用.

3.3 氮添加促进了土壤有机质分解,使得DOC含量增加,从而影响土壤CO2释放.高通量氮添加下, NO3--N比NH4+-N更有利于土壤有机质分解,促进土壤CO2释放.

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Effects of atmospheric nitrogen deposition on soil respiration in the water level fluctuating zone of the Three Gorges Reservoir area.

HE Li-ping1,2*, TIAN Mao-ping1, WU Hong1, LIN Jun-jie1, LAN Bo1

(1.Key Laboratory of Water Environment Evolution and Pollution Control in Three Gorges Reservoir, Chongqing Three Georges University, Chongqing 404100, China；2.Collaborative Innovation Center of Ecological Environment Protection and Disaster Prevention in the three Gorges Reservoir area, Chongqing Three Georges University, Chongqing 404100, China)., 2019,39(3)：1132~1138

An indoor incubation experiment was conducted to reveal the effect of atmospheric nitrogen deposition on soil respiration of water level fluctuating zone in the Three Gorges Reservoir area. The results showed that the response of soil respiration rate to nitrogen addition was a short term effect. Under the present atmospheric nitrogen deposition flux, soil cumulative CO2release was not changed by inorganic nitrogen addition, while, it was inhibited by organic nitrogen addition. Except soil cumulative CO2release was depressed by ammonium addition of double present atmospheric nitrogen deposition flux, it was promoted by all the simulated atmospheric nitrogen deposition compositionunder both double and triple present atmospheric nitrogen deposition flux. Compared with ammonium, nitrate was more conducive for promoting cumulative CO2release under double and triple present atmospheric nitrogen deposition flux.

atmospheric nitrogen deposition；Three Gorges Reservoir area；water level fluctuating zone；soil respiration

X142

A

1000-6923(2019)03-1132-07

何立平(1982-),男,四川南充人,讲师,博士,主要从事环境土壤学研究.发表论文10余篇.

2018-08-15

国家自然科学基金资助项目(31770529);重庆市教委科学技术研究项目(KJ1710260);三峡库区水环境演变与污染防治重庆高校市级重点实验室开放基金(WEPKL2016LL-03,WEPKL2016ZD-01, WEPKL2016ZZ-01);教育部春晖计划项目(Z2015133);万州科技人才专项(2016-1)

*责任作者, 讲师, hlp_weird@163.com