大氣氮沉降對三峽庫區消落帶土壤呼吸的影響

何立平,田茂平,吳 紅,林俊杰,蘭 波

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大氣氮沉降對三峽庫區消落帶土壤呼吸的影響

何立平1,2*,田茂平1,吳 紅1,林俊杰1,蘭 波1

(1.重慶三峽學院,三峽庫區水環境演變與污染防治重慶高校市級重點實驗室,重慶 404100；2.三峽庫區生態環境保護和災害防治重慶市協同創新中心,重慶 404100)

以三峽庫區消落帶落干期土壤為研究對象,采用室內模擬培養的方法,探討了大氣氮沉降通量及其組成對土壤呼吸的影響.結果表明,土壤呼吸速率對氮添加的響應為短期效應. 1倍當前大氣氮沉降添加下,無機氮和有機氮對土壤累積CO2釋放分別表現為無影響和抑制作用.除NH4+-N在2倍氮沉降添加下表現為抑制作用外, 2、3倍氮沉降添加均促進了土壤累積CO2釋放.與硝態氮相比, 2、3倍氮沉降添加的銨態氮對土壤累積CO2釋放具有抑制作用.

大氣氮沉降；三峽庫區；消落帶；土壤呼吸

土壤碳庫是陸地生態系統碳庫的主體,全球約有684~724Pg 碳是以有機質的形態儲存于地表0~30cm的土壤中.土壤碳庫儲量約是陸地植被總碳儲量的1.5倍,與大氣CO2碳儲量相當[1].因此,即使表層土壤有機碳庫輕微的礦化,也可能引起大氣CO2濃度的顯著升高,影響全球氣候變化[2-3].

土壤碳氮循環存在耦合關系[4].作為全球氣候變化驅動因素之一,大氣氮沉降是影響陸地生態系統有機碳生物地球化學循環的重要因素[5].過去60年,人類活動已導致全球大氣氮沉降顯著增加[6-7].并且,隨著全球人口的持續膨脹和能源消耗的進一步加大,未來數十年,全球大氣氮沉降將持續加劇[8].因此,大氣氮沉降對土壤生態系統CO2釋放的影響已成為關注的焦點.

目前,針對大氣氮沉降對土壤生態系統CO2釋放的影響,已開展了大量的室內室外模擬研究.從大氣氮沉降組成的角度,這些研究可分為無機氮和有機氮沉降模擬研究.無機氮沉降模擬研究采用了NH4+-N[9-10]、NO3--N[11-13]和NH4NO3[14-16]3種氮組成,而有機氮沉降模擬研究采用了CO(NH2)2作為氮組成[17-19].目前這些模擬研究還存在以下2方面的問題:一方面,這些研究針對的是不同土壤生態系統(草地、森林、農田、濕地)且氮添加量各異,因此得到了大氣氮沉降對土壤CO2釋放具有促進作用[16,18]、抑制作用[9-10,14]、無影響[20-21]以及土壤CO2釋放與大氣氮沉降通量有關的不同結果[18,22-23].因此,要準確評估大氣氮沉降對土壤生態系統CO2釋放的影響,仍需進一步探討大氣氮沉降對其他土壤生態系統CO2釋放影響,為后續評估模型的建立提供數據支撐.另一方面,實際大氣氮沉降組成既包括有機氮也包括無機氮[24],而相關研究僅模擬討論了單一無機或者有機氮沉降對土壤CO2釋放的影響.因此,這一科學問題仍需針對更多的土壤生態系統和實際的大氣氮沉降組成進行更加細致深入的研究.

消落帶是水陸生態系統的交界地帶,土壤生態環境特殊.針對大氣氮沉降對消落帶土壤CO2釋放的影響研究目前尚未見報道.三峽水庫蓄水后,形成了世界上面積最大的消落帶,本研究以三峽庫區消落帶落干期土壤為研究對象,采用室內模擬培養的方法,探討落干期不同大氣氮沉降通量及其組成對土壤CO2釋放的影響,為評估大氣氮沉降對土壤生態系統CO2釋放的影響和預測未來全球氣候變化的趨勢提供新的科學依據.

1 材料和方法

1.1 土壤樣品采集及預處理

三峽水庫最高水位為175m,最低水位為145m.水庫冬蓄夏泄的調蓄水制度,在兩岸形成了落差30m,面積約400km2的消落帶.消落帶1~4月和5~7月分別為淹水期和落干期[25].自2009年三峽水庫首次蓄水至175m水位以來,消落帶已經歷了6個周期的干濕交替過程.本研究于2016年7月在三峽庫區消落帶萬州段(30o49’26’’N~30o49’38’’N, 108o24’45’’E~108o26’16’’E)采集表層0~10cm土壤樣品.樣品采集后立即放入4℃保溫箱中保存,迅速送往實驗室.土壤經冷凍干燥,去除石礫、動植物殘體,過8目篩后混勻裝入棕色磨口瓶中置于干燥器儲存備用.供試土壤基本理化性質見表1.

表1 消落帶土壤基本理化性質 Table 1 Basic physicochemical properties of soil in water level fluctuating zone

1.2 實驗設計

三峽庫區萬州段消落帶落干期平均氣溫為25℃[25],因此本研究以25℃,好氧培養進行室內模擬.無機氮沉降以NH4+-N、NO3--N模擬,而有機氮沉降以CO(NH2)2模擬.此外,考慮到我國實際大氣氮沉降無機氮和有機氮的比例為72%和28%[26],本研究以NH4NO3(72%)和CO(NH2)2(28%)模擬真實大氣氮沉降.目前三峽庫區大氣氮沉降通量約為50kgN/ (hm2·a),且仍存在逐漸加劇的趨勢[26],因此,本研究模擬大氣氮沉降通量分別為1, 2, 3倍實際大氣氮沉降通量.最終,培養實驗包括4種大氣氮沉降組成(包含NH4+-N、NO3--N、CO(NH2)2和NH4NO3(72%)+ CO(NH2)2(28%)), 3個大氣氮沉降通量(50,100, 150kgN/(hm2×a)),共計12個處理,每個處理設置3個平行,外加3個對照3個空白,共計42個土壤樣品.

1.3 土壤培養

每個處理分別稱取預處理后的消落帶土壤樣品20g,平鋪于250mL棕色培養瓶底部,調整土壤含水率為最大田間持水量(WHC)的50%,加蓋密封預培養24h,然后將土壤含水率調整至WHC的60%進行正式培養.加蓋密封正式培養前向各培養瓶中放置盛裝3mL(1mol/L)NaOH溶液的CO2捕獲瓶.土壤培養期間每日打開瓶蓋通氣20min,并定期通過重量法調整土壤含水率,使其保持穩定.各培養瓶分別于土壤培養的1, 2, 4, 6, 8, 15, 22, 29, 36d打開培養瓶更換CO2捕收液后繼續密封培養[27].土壤培養結束后破壞性取樣分析土壤溶解性有機碳(DOC)含量.

1.4 分析方法

土壤呼吸速率采用堿液吸收法測定;土壤DOC使用0.5mol/L K2SO4提取后用TOC分析儀(TOC-LCPH/CPN, Shimadzu)測定[28];土壤有機質(SOM)采用重鉻酸鉀外加熱法測定[29];總氮采用凱氏法測定;土壤WHC和pH值分別采用環刀法和酸度計法測定;土壤粒徑組成采用比重計法測定.

1.5 數據統計

采用單因素方差(One-way ANOVA)和LSD多重檢驗法分析不同處理下土壤呼吸速率、累積釋放量、DOC差異的顯著性;采用單樣本檢驗法分析不同氮添加量及其組成變化下土壤呼吸速率、累積釋放量及DOC含量與對照之間的顯著性差異;進行方差分析之前,首先對數據是否正態分布和方差是否齊性進行檢驗,如不滿足,則進行非參數檢驗.所有統計分析均在SPSS 18.0 (SPSS Inc, Chicago, IL, USA)中進行,方差分析及檢驗,顯著性標準均為<0.05.使用SigmaPlot12.5軟件作圖.

2 結果與討論

2.1 氮添加量及其組成對土壤呼吸速率的影響

對照及不同氮添加量時,土壤呼吸速率均隨著培養時間的增加逐漸下降,最終保持穩定(圖 1).培養第1d,不同氮添加量下,土壤呼吸速率特征分別為:(1)NH4Cl: 1倍>0, 3倍>2倍[圖1 (A),<0.05]; (2)NaNO3: 3倍>0, 1, 2倍[圖1 (B),<0.05];(3)CO (NH2)2:3倍>0倍>2倍>1倍[圖1 (C),<0.05];(4)CO (NH2)2+NH4NO3: 2, 3倍>0倍>1倍[圖1 (D),< 0.05].培養第1d,不同氮組成添加下土壤呼吸速率特征分別為:(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 (圖1,<0.05).

4種氮組成添加下,土壤培養第1d,呼吸速率對氮添加量存在響應,而隨著培養時間的延長,這種響應逐漸消失(圖1).這表明土壤呼吸速率對氮添加的響應為短期效應. 研究表明對于微生物生長同時受到碳氮限制的森林土壤,氮添加對土壤呼吸并沒有短期的促進作用[30],這與本研究結果是不一致的.消落帶土壤微生物生長沒有受到碳限制是導致不同結果的原因,即三峽庫區消落帶土壤在干濕交替過程中存在新碳輸入,消落帶土壤有機質含量14.78g/ kg(表1),顯著高于當地土壤有機質含量(7.59g/kg)[31].在微生物不受碳限制的條件下,氮添加短期改變了微生物活性,影響土壤呼吸速率[32-33],隨著微生物可利用碳逐漸分解, 呼吸速率趨于穩定.

2.2 氮添加量及其組成對消落帶土壤累積CO2釋放的影響

土壤培養36d后不同氮添加量處理的累積CO2釋放特征分別為:(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).不同組成處理的CO2累積釋放特征分別為:(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添加對土壤累積CO2釋放分別表現為無影響、抑制和促進作用(表2).研究表明NH4+-N增加對土壤CO2釋放存在抑制作用[9-10].這與本研究結果是不一致的.消落帶土壤存在新碳輸入,土壤C/N為56.8(表1),遠高于微生物生長最佳比例(25),因此氮添加量增加能夠逐漸降低土壤C/N引起微生物群落結構和土壤呼吸的變化[32,34-35].這是造成上述結果差異的原因,有待進一步研究.

1倍氮沉降的NO3--N添加下,土壤培養36d后CO2累積釋放量與對照無顯著性差異(0.05),而2, 3倍氮沉降的NO3--N添加下均表現為顯著性大于對照(表2,<0.05).這表明目前大氣氮沉降量下,土壤CO2釋放對NO3--N沉降無響應,而隨著未來氮沉降量的進一步增加氮沉降將促進土壤CO2釋放.研究表明:NO3--N增加能夠抑制微生物對土壤有機質的分解,增加土壤碳儲量[11-13],這顯然與本研究結果是不一致的.消落帶土壤在干濕交替過程中存在新碳輸入是造成結果差異的原因.NO3--N對老碳分解有抑制作用,相反對新碳分解有促進作用[36].因此,NO3--N含量的增加促進了新碳的分解,最終導致土壤呼吸作用增加.

1倍氮沉降添加下,土壤培養36d后CO2累積釋放量在NH4+-N和NO3--N之間無顯著性差異(0.05),而2, 3倍氮沉降添加下表現為NH4+-N

表2 氮添加量及其組成對土壤CO2累積釋放的影響(mg/kg) Table 2 The effects of nitrogen addition flux and its composition on soil CO2 cumulative release (mg/kg)

注:土壤培養時間36d,不同小寫字母表示CO2累積釋放量在不同的氮添加量之間存在顯著性差異(<0.05); 不同大寫字母表示CO2累積釋放量在不同的氮添加組分之間存在顯著性差異(<0.05).

1倍氮沉降的CO(NH2)2及CO(NH2)2+NH4NO3添加下,土壤培養36d后CO2累積釋放量均顯著小于對照,而在2, 3倍氮沉降的氮添加下則相反(表3,0.05).這表明目前大氣氮沉降量下,CO(NH2)2和CO(NH2)2+NH4NO3添加對土壤呼吸均有抑制作用,而隨著未來氮沉降量的持續增加,將逐漸從抑制作用轉換為促進作用.CO(NH2)2對土壤呼吸的影響,目前存在2種觀點:第一種觀點認為低氮通量(約1倍氮沉降)添加能夠促進土壤CO2釋放,而高氮通量(約4倍氮沉降)添加則相反[17];第二種觀點認為, CO(NH2)2添加能夠促進土壤CO2釋放[18-19].本研究結果與這兩種觀點均不一致.消落帶土壤生態環境特殊,土壤性質差異可能是造成結果差異的原因[37].目前關于CO(NH2)2+NH4NO3添加對土壤CO2釋放的影響研究還未見報道,但科研人員研究了NH4NO3添加對土壤CO2釋放的影響.主流觀點認為,NH4NO3添加對土壤CO2釋放具有抑制作用[14-15,38],也有研究得到了相反的結果[16].本研究發現CO(NH2)2+NH4NO3添加下,土壤呼吸作用表現為3倍氮沉降<2倍氮沉降,而僅存在CO(NH2)2添加下,則相反(表3,0.05).由此可見,NH4NO3添加量的增加抑制了土壤CO2釋放,這和主流研究結果是一致的.這是由于高濃度的NH4NO3對土壤真菌具有抑制作用造成的[32].要進一步闡釋CO(NH2)2和CO(NH2)2+ NH4NO3添加對土壤呼吸的影響,仍需探討氮添加過程中土壤微生物群落結構的變化,有待進一步的研究.

除NH4Cl外,土壤累積CO2釋放均表現為2, 3倍氮沉降>0, 1倍氮沉降(表2,0.05).可見,雖然當前大氣氮沉降對土壤CO2釋放沒有促進作用,但是隨著大氣氮沉降的持續增加將逐漸轉變為促進作用,這無疑增加了全球氣候進一步變暖的風險.大量的研究表明大氣氮沉降持續增加將抑制微生物不受氮素限制的森林土壤呼吸[39-40],這與本研究結果恰恰相反.本研究中土壤C/N為56.8(表1),遠高于微生物生長最佳比例(25),高濃度的氮輸入(2, 3倍氮沉降)短期內能夠顯著降低土壤C/N,從而促進了土壤呼吸.

2.3 氮添加量及其組成對土壤DOC含量的影響

土壤培養36d后,不同氮添加量處理的DOC含量特征分別為:(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).不同組成處理的DOC含量特征分別為:(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.

對照土壤DOC含量表現為培養前>培養后,而氮添加下則相反(表1,表3,0.05).可見,氮添加促進了土壤有機質的分解.這與Cusack等[41]的研究結果是一致的.氮添加能夠增加與有機質分解相關的酶活性[42],從而促進土壤有機質的分解. CO(NH2)2進入土壤后水解產物為NH4+-N,與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含量表現為NH4Cl、CO(NH2)2、CO(NH2)2+ NH4NO3> NaNO3(表3,0.05).除NO3--N為相反的趨勢外, DOC含量隨著其余氮組成添加量的增加而減小.這表明NH4+-N含量增加對土壤有機質分解有抑制作用,而NO3--N增加則具有促進作用.NH4+-N含量增加導致的土壤微生物群落結構變化是抑制有機質分解的原因,有待進一步研究.NO3--N對新碳分解有促進作用[36],因此土壤NO3--N添加量與DOC含量正相關.

表3 氮添加量及其組成對土壤培養后DOC含量的影響(mg/kg) Table 3 The effects of nitrogen addition flux and its composition on soil DOC content after soil incubation (mg/kg)

注: 土壤培養時間36d,不同小寫字母表示 DOC含量在不同的氮添加量之間存在顯著性差異(0.05); 不同大寫字母表示, DOC含量在不同的氮添加組分之間存在顯著性差異(0.05).

3 結論

3.1 土壤呼吸速率對氮添加的響應為短期效應.

3.2 1倍氮沉降添加下無機氮對土壤呼吸無影響,而有機氮表現為抑制作用.除NH4+-N在2倍氮沉降添加下抑制了土壤呼吸外,與0, 1倍氮沉降添加相比2, 3倍氮沉降添加均促進了土壤呼吸作用.

3.3 氮添加促進了土壤有機質分解,使得DOC含量增加,從而影響土壤CO2釋放.高通量氮添加下, NO3--N比NH4+-N更有利于土壤有機質分解,促進土壤CO2釋放.

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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,講師,博士,主要從事環境土壤學研究.發表論文10余篇.

2018-08-15

國家自然科學基金資助項目(31770529);重慶市教委科學技術研究項目(KJ1710260);三峽庫區水環境演變與污染防治重慶高校市級重點實驗室開放基金(WEPKL2016LL-03,WEPKL2016ZD-01, WEPKL2016ZZ-01);教育部春暉計劃項目(Z2015133);萬州科技人才專項(2016-1)

*責任作者, 講師, hlp_weird@163.com