常伊梅林, 唐常源,, 李 杏, 李 銳, 曹英杰

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礦山廢水灌溉區(qū)農(nóng)田土壤N2O的產(chǎn)生及釋放機制研究*

常伊梅林1, 唐常源1,2, 李 杏2**, 李 銳2, 曹英杰2

(1. 中山大學(xué)地理科學(xué)與規(guī)劃學(xué)院 廣州 510275; 2. 中山大學(xué)環(huán)境科學(xué)與工程學(xué)院 廣州 510006)

農(nóng)田系統(tǒng)是溫室氣體N2O的主要排放源, 目前對酸性礦山廢水(acid mine drainage, AMD)灌溉影響下, 農(nóng)田土壤剖面N2O的來源識別、轉(zhuǎn)換機制及其控制因子缺乏深入研究。本文選擇廣東省大寶山礦區(qū)下游沿岸水稻田和甘蔗田兩種典型農(nóng)田, 針對酸性礦山廢水灌溉區(qū)(上壩村)和天然來水灌溉區(qū)(連心村), 對土壤理化性質(zhì)、重金屬含量及包氣帶N2O濃度、同位素特征值進行了測定, 定量計算了硝化和反硝化作用對土壤中N2O的貢獻比和N2O轉(zhuǎn)化為N2的還原比, 評價了其相關(guān)影響因素。結(jié)果表明: 在AMD影響下, 灌區(qū)農(nóng)田土壤剖面N2O濃度均高于同種作物類型天然來水區(qū)土壤, 同種灌溉處理下甘蔗田土壤N2O濃度高于水稻田。甘蔗田表層土壤(0~30 cm)反硝化作用對N2O產(chǎn)生量的貢獻比高于硝化作用, 約71.29% N2O由反硝化作用產(chǎn)生。AMD灌區(qū)甘蔗田土壤剖面中N2O還原成N2的比例隨深度增加逐漸減小, 在N2O濃度峰值處僅有15.54% N2O被還原成為N2, 而天然來水區(qū)N2O還原成N2的平均比率高達49.80%。這表明較弱的土壤N2O還原能力導(dǎo)致較高濃度的N2O殘留在土壤中。相關(guān)性分析表明, AMD灌溉通過改變上壩村土壤的pH、重金屬含量、含水率從而改變了土壤N2O的來源途徑及還原能力。組合同位素特征值溯源法有效地揭示了農(nóng)田土壤N2O的來源和AMD灌區(qū)土壤的潛在生態(tài)風(fēng)險, 為日后的治理修復(fù)工作提供了科學(xué)依據(jù)。

同位素特征值; N2O; 酸性礦山廢水灌溉; 硝化作用; 反硝化作用; 紅壤區(qū)

N2O是一種十分重要的溫室氣體, 在100年的時間尺度里, 其溫室效應(yīng)是CO2的265倍。N2O的排放會造成全球變暖和平流層臭氧的消耗[1]。研究表明, 農(nóng)業(yè)排放占N2O總排放量的79%[2], 其中土壤微生物的硝化和反硝化作用是農(nóng)業(yè)土壤中N2O產(chǎn)生的重要途徑, 其微生物轉(zhuǎn)化過程如圖1所示。農(nóng)業(yè)生產(chǎn)活動中, 灌溉、氮肥的施用、作物類型、耕作方式等都可能改變土壤理化特性, 進而影響微生物介導(dǎo)的N2O的產(chǎn)生和排放[3-5]。因此, 分析土壤剖面各層N2O濃度及N2O同位素特征值的垂直分布規(guī)律, 不僅有助于甄別土壤N2O的來源, 研究其轉(zhuǎn)換過程及其機理, 而且對如何減少農(nóng)田土壤N2O產(chǎn)生和排放具有重要的指導(dǎo)意義。

圖1 土壤微生物產(chǎn)生N2O的過程[6]

研究N2O產(chǎn)生與轉(zhuǎn)換機制的傳統(tǒng)方法包括15N同位素標(biāo)記法、硝化抑制劑等都受到前體(產(chǎn)生N2O的反應(yīng)物)同位素的影響以及標(biāo)記物或者抑制劑擴散不均勻等限制。因此, 目前國內(nèi)外利用N2O在產(chǎn)生及還原過程中,15N會發(fā)生同位素分餾現(xiàn)象, 來解釋N2O的來源[7]。而且針對N2O的氮同位素分餾研究也可以用于野外田間實地條件下N2O殘留比的量化[8-10]。

農(nóng)田土壤特征反映了區(qū)域原有的地球化學(xué)環(huán)境條件的影響。研究表明, 重金屬和pH均會顯著影響氮的礦化、硝化和反硝化作用[11-12]。重金屬還能抑制N2O還原酶在反硝化作用過程中的活性, 使N2O還原成N2的過程受阻而引起土壤中N2O累積, 最終導(dǎo)致N2O排放量的增加[13]。低pH也會降低N2O還原酶活性導(dǎo)致土壤空氣中N2O濃度增高[14]。因此, 酸性礦山廢水(acid mine drainage, AMD)灌溉農(nóng)田不僅會造成土壤團粒結(jié)構(gòu)的破壞, 鹽基陽離子淋濾加劇, 有機質(zhì)分解受抑制, 保水保肥能力嚴(yán)重退化[15], 而且土壤重金屬積累、低pH形成都可能對N2O還原率產(chǎn)生抑制效應(yīng), 從而引發(fā)N2O的大量排放[6]。

紅壤是我國南方廣泛發(fā)育和分布的酸性土壤, 由于其性質(zhì)上存在酸、瘦、黏等弱點, 容易被污染, 導(dǎo)致生態(tài)環(huán)境惡化。Liu等[16]研究表明, AMD灌溉農(nóng)田土壤會對負(fù)責(zé)氮循環(huán)的土壤微生物群落產(chǎn)生影響, 從而影響水稻田產(chǎn)生N2O的潛在量。Zhou等[17]通過對AMD灌區(qū)農(nóng)田土壤進行室內(nèi)培養(yǎng)試驗發(fā)現(xiàn), 受到重金屬污染的水稻田N2O排放量增加, 重金屬對N2O還原為N2的抑制作用大于反硝化過程。李爽等[18]通過對華南紅壤地區(qū)稻田土壤室內(nèi)模擬試驗發(fā)現(xiàn), 加入Fe(Ⅱ)促進了N2O的生成, 提高了N2O還原基因的拷貝數(shù); 該研究增加了對華南紅壤區(qū)稻田體系中的氮、鐵循環(huán)轉(zhuǎn)化的了解。實際農(nóng)田環(huán)境中氮循環(huán)過程十分復(fù)雜, N2O產(chǎn)生和轉(zhuǎn)換過程缺乏單一主控制因子[13]。因此, 在野外田間試驗條件下, 明確各土層N2O及其同位素分布特征, 探究農(nóng)田土壤包氣帶中N2O的產(chǎn)生與轉(zhuǎn)換機制,以及影響控制因子仍是亟待解決的科學(xué)難題。本研究利用組合同位素特征值溯源法, 揭示農(nóng)田土壤N2O的來源和AMD灌區(qū)土壤的潛在生態(tài)風(fēng)險, 為日后的治理修復(fù)工作提供了科學(xué)依據(jù)。

1 材料與方法

1.1 研究區(qū)概況

研究區(qū)位于廣東省北部翁源縣(113.7°~ 113.9°E, 24.4°~24.6°N), 屬于亞熱帶季風(fēng)氣候, 年平均氣溫20.3 ℃, 年均降水量1 782.7 mm, 其上游15 km處的大寶山是一座以鐵銅為主的大型多金屬礦山。選礦產(chǎn)生的尾砂、廢石主要堆放在由兩個尾砂壩攔截形成的大型尾礦庫內(nèi)。長期以來庫內(nèi)匯集的AMD排入橫石河, AMD具有重金屬濃度高、pH低、鹽度高等特征, 嚴(yán)重危害了以橫石河作為灌溉水源的沿岸農(nóng)業(yè)生產(chǎn)和居民用水。研究區(qū)的農(nóng)田分布在河流階地上, 其中上壩村曾經(jīng)長期利用橫石河灌溉, 而連心村則取自于沒有受AMD影響的太平河灌溉。其中橫石河作為農(nóng)業(yè)用水的主要水源, 河水中重金屬Cu、Zn、Pb、Cd最大含量超過國家農(nóng)田灌溉水質(zhì)標(biāo)準(zhǔn)(GB 5084—92)的3.94倍、11.63倍、4.1倍和18倍。而太平河水Cu、Zn、Pb、Cd總量均符合國家農(nóng)田灌溉水質(zhì)標(biāo)準(zhǔn)。

1.2 樣品采集與處理

為了研究AMD灌溉對農(nóng)田土壤N2O產(chǎn)生及轉(zhuǎn)換的影響, 分別在AMD灌溉區(qū)(上壩村)及天然來水灌溉區(qū)(連心村)選取了SG(上壩村甘蔗田)、SS(上壩村水稻田)、LG(連心村甘蔗田)、LS(連心村水稻田)4個采樣地, 分別采集不同深度的N2O土壤氣體及土壤樣品。為了采集水稻田的土壤氣體, 采樣時間選取水稻田排水期。

現(xiàn)場將獲取的非擾動土柱(深度80 cm), 按10 cm間隔進行連續(xù)切分, 分別用聚乙烯自封袋盛裝帶回實驗室, 經(jīng)自然風(fēng)干后過篩備用。同時, 在每個采樣點以10 cm為間隔, 將GVP土壤氣體采樣套裝(AMS, USA)探頭順次擊打入所需深度的土壤中, 通過連接探頭的聚氟管用60 mL注射器采集土壤氣體。在采集氣體樣品時, 為了避免前次取樣存留的氣體影響, 棄掉第一次抽取的60 mL土壤氣體, 將隨后抽取的120 mL土壤空氣樣品注入已抽真空的頂空瓶與真空袋中。每個樣地隨機布設(shè)5個樣點重復(fù)上述土壤和氣體采集過程, 然后將對應(yīng)深度的土樣和氣體樣品分別進行等量混合。

1.3 測定項目與方法

土壤粒徑、重量含水率(以下簡稱含水率)、pH、土壤陽離子交換量(CEC)、全碳(TC)、全氮(TN)、碳氮比(C/N)、硝酸根(NO3-)及銨根(NH4+)的測定方法均參照《土壤農(nóng)化分析》(第3版)[19]。土壤重金屬含量測定采用HF-HNO3-HClO4法消解土壤樣品, 待測液用ICP-OES(5300DV, Perkin-Elmer, USA)測定土壤中Cu、Zn、Pb、Cd、Cr等金屬元素。為了確保分析結(jié)果的可靠與準(zhǔn)確, 每批消解的樣品均含有空白樣, 土壤標(biāo)樣(GSD-12)及3個平行土樣, 所選試劑均為優(yōu)級純。標(biāo)樣的重金屬回收率均高于90%, 平行樣間的標(biāo)準(zhǔn)偏差均在誤差允許范圍內(nèi)。

土壤氣體N2O濃度及其同位素特征值測定方法分別參照Yamamoto等[20]與Toyoda等[21], N2O濃度利用裝備有電子俘獲檢測器(ECD)氣相色譜GC-2014(Shimadzu, JPN)測定。15Nbulk、15Nα、18O同位素特征值利用TraceGas專用取樣瓶, 使用ISOPRIME公司生產(chǎn)的TraceGas-IRMS氣體樣品濃縮系統(tǒng)結(jié)合連接的Isoprime-100質(zhì)譜儀進行測定分析。標(biāo)準(zhǔn)物質(zhì)主要來自美國地質(zhì)調(diào)查局(USGS32、34、35), 采用國際上通用的兩點校正方法對所測定樣品進行校正。

1.4 計算方法

1.4.1 N2O同位素特征值計算

N2O分子式為Nβ=Nα=O, 這種不對稱的直線型結(jié)構(gòu)導(dǎo)致N2O中兩個氮原子所處的能量狀態(tài)不同, 一般用α和β分別表示與氧原子和不與氧原子相連接的氮原子[22]。在同位素分析中,15N所處位置(α位或β位)不同, 其測量值也不同, 這種直線型結(jié)構(gòu)線性分子N2O內(nèi)15N的擇優(yōu)占位稱為位嗜值(site preference, 簡稱SP)[23]。由下式分別計算N2O同位素特征值及位嗜值SP。

式中:15=15N/14N,18=18O/16O; 下標(biāo)sample和standard分別表示樣品和標(biāo)準(zhǔn)樣品。15Nbulk表示N2O氮穩(wěn)定同位素比值。

1.4.2 二元混合模型評估N2O來源貢獻比

為了定量評估N2O各個來源的貢獻比通常利用同位素二元混合模型, 較為常用的為SP-15Nbulk和SP-18O兩種混合模型。其中通過“映射方法”將同位素的分餾及混合過程同步量化, 甄別并確定在被還原成N2前, N2O分別來自硝化與反硝化作用的占比。該方法基于純細菌培養(yǎng)及土培試驗所得到的混合線(mixing line)與還原線(reduction line)的斜率定量計算[24-32]。通過兩種同位素模型方法分析N2O來源途徑, 并計算相關(guān)的貢獻比。

o′nit(1x)′denit(5)

式中:o表示N2O還原前的SP值, 下標(biāo)nit、denit分別表示硝化作用、反硝化作用過程; 而、1x分別表示上述2種過程的相應(yīng)貢獻比。

N2O還原為N2的過程發(fā)生同位素分餾, 根據(jù)同位素分餾方程, 應(yīng)用封閉系統(tǒng)瑞利模型[22], 則:

可近似于:

2 結(jié)果與分析

2.1 供試土壤理化性質(zhì)及其重金屬含量垂向分布特征

供試土壤含水率、基本化學(xué)性質(zhì)及重金屬含量如表1所示。AMD灌溉區(qū)農(nóng)田土壤含水率明顯低于天然來水區(qū)農(nóng)田土壤, 其中甘蔗田表層土壤(0~30 cm)含水率低于水稻田。天然來水區(qū)農(nóng)田土壤pH均值為5.64, 而受到AMD灌溉影響的農(nóng)田土壤pH最低達3.84。AMD灌溉區(qū)農(nóng)田土壤CEC也明顯低于天然來水區(qū)農(nóng)田土壤。土壤(SG、SS、LG、LS)全碳含量都隨深度增加而減小, 其中AMD灌溉區(qū)農(nóng)田中下層土壤(30~80 cm)全碳含量低于天然來水區(qū)農(nóng)田土壤。土壤全氮、硝態(tài)氮和氨態(tài)氮含量垂向分布特征與全碳含量變化趨勢相似。AMD灌溉區(qū)農(nóng)田土壤各層Cu、Zn、Pb、Cd含量明顯高于連心村, 且均超過國家土壤環(huán)境質(zhì)量二級標(biāo)準(zhǔn)。

表1 供試土壤含水率、化學(xué)性質(zhì)及其重金屬含量

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。同列不同小寫字母表示0.05水平差異顯著。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area. Data with different lowercase letters in the same column are significantly different at 0.05 level.

2.2 土壤氣體中N2O濃度垂向分布特征

研究區(qū)農(nóng)田土壤包氣帶N2O濃度垂向分布如圖2所示。不同作物類型的農(nóng)田土壤剖面N2O濃度分布存在顯著性差異(<0.05)。甘蔗田土壤N2O濃度高于水稻田, 在30~50 cm土層出現(xiàn)峰值, AMD灌區(qū)農(nóng)田和天然來水區(qū)分別為4.73 μmol?mol-1與4.25 μmol?mol-1; 而水稻田土壤N2O濃度隨深度的變化不明顯, 平均值分別為0.51 μmol?mol-1、0.43 μmol?mol-1。對于同種作物土壤N2O濃度而言, AMD灌溉區(qū)有別于天然河水灌溉區(qū), 兩者水稻田N2O濃度存在顯著差異(<0.05), 表明AMD灌溉具有增加土壤N2O濃度的趨勢。

圖2 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田土壤包氣帶N2O濃度垂向分布特征

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area.

2.3 土壤包氣帶中N2O的氮氧同位素特征

由圖3可知, AMD灌區(qū)甘蔗田(SG)的SP值隨深度增加逐漸增加, 在地表附近為10.48‰, 在80 cm深處達到16.04‰。天然來水灌溉區(qū)甘蔗田(LG)的SP值也隨深度增加緩慢增加。而水稻田的SP值隨深度增加變化不大, AMD灌區(qū)水稻田(SS)和天然來水灌溉區(qū)水稻田(LS)平均值分別為16.75‰和18.85‰, 兩采樣點在80 cm深處僅為16.30‰。

圖3 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田包氣帶土壤N2O同位素特征值垂向分布特征

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。SP:15N的位嗜值。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area. SP: site preference of15N.

各采樣點位包氣帶剖面N2O同位素特征值(15Nbulk、18O)垂向變化趨勢相似, 其中甘蔗田15Nbulk與18O在30~50 cm土層出現(xiàn)最小極值, 揭示發(fā)生同位素15Nbulk與18O貧化。水稻田N2O同位素特征值隨深度變化較小。同一深度范圍, 不同作物土壤15Nbulk與18O也有所差異, 甘蔗田SG、LG深層土壤(50~70 cm)N2O的15Nbulk與18O比水稻田SS、LS深層土壤貧化。此外, AMD灌區(qū)(SG、SS)表層土壤(0~30 cm)N2O的15Nbulk與18O比天然來水灌溉區(qū)農(nóng)田(LG、LS)表層土壤貧化。而4個采樣點中, AMD灌區(qū)甘蔗田(SG)土壤剖面的N2O的3種同位素豐度值均最貧化。

2.4 硝化/反硝化作用對土壤氣體N2O的貢獻比

本文采用SP-18O模型對微生物硝化作用、反硝化作用和N2O還原過程的貢獻進行定量評估。根據(jù)前人研究數(shù)據(jù)可以綜合確定各個微生物過程的SP范圍(圖4)。其中微生物硝化作用SP值范圍為32.0‰~38.7‰[24-26],18O范圍為35.6‰~55.2‰[24-26]; 真菌反硝化作用SP值范圍為30.2‰~39.3‰[27-29],18O范圍為40.6‰~51.9‰[27-29]; 細菌反硝化作用SP值范圍為-7.5‰~3.7‰[26,30],18O范圍為17.4‰~ 21.4‰[31-32]; 硝化細菌反硝化作用SP值范圍為-13.6‰~1.9‰[24,26],18O范圍為19.8‰~26.5‰[24,26]。

圖4 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田土壤SP-δ18O模型分析的N2O產(chǎn)生和消耗途徑

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。SP:15N的位嗜值。均值混合線: 由端元值(SP=-3.90,18O=21.00)、(SP=34.80,18O=43.60)確定; 還原線: 斜率為0.35。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area. Mixing line is determined by the end-member value (SP=-3.90,18O=21.00), (SP=34.80,18O=43.60). Reduction line is with slope of 0.35. SP: site preference of15N.

在確定混合線的斜率時, 各微生物過程SP范圍值均有重疊部分(細菌反硝化作用與硝化細菌反硝化作用、硝化作用與真菌反硝化作用, 圖4)。根據(jù)大量純細菌培養(yǎng)及土培試驗結(jié)果, 利用端元值(SP=-3.90,18O=21.00)、(SP=34.80,18O=43.60)能合理確定均值混合線(mixing line)[33]。鑒于各微生物過程SP值范圍是確定的, 由此可知, 具有最大及最小斜率的混合線分別為SP=5.75′18O-165.9與SP=0.75′18O-9.33。

圖5 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田土壤硝化/反硝化作用對土壤氣體N2O的貢獻比范圍

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。Nit: 硝化作用; Denit: 反硝化作用。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area. Nit: nitrification; Denit: denitrification.

N2O被還原為N2前, 各點位不同深度土壤硝化和反硝化作用對N2O生成的貢獻比范圍如圖5所示。研究區(qū)水稻田土壤反硝化作用生成N2O的比率隨深度變化不大, 均略大于硝化作用。而甘蔗田硝化與反硝化作用生成N2O的比率隨深度變化明顯(<0.05)。其中AMD灌溉區(qū)甘蔗田(SG)表層土壤(0~30 cm) 71.29%的N2O由反硝化作用產(chǎn)生。隨著深度增加, 硝化作用對N2O的貢獻比有所增加。由此可見, 研究區(qū)水稻田和甘蔗田表層土壤產(chǎn)生的N2O大多來自于反硝化作用, 這與Park等[8]在2011年農(nóng)田表層土壤的研究結(jié)果一致。

假設(shè)硝化作用與反硝化作用產(chǎn)生的N2O是先混合后還原, SP-18O同位素二元混合模型采用平均斜率值0.35作為農(nóng)田土壤SP和18O還原線(reduction line)的斜率[34], 通過公式(6)、(7)、(8)可以得出微生物N2O還原成N2的比例范圍(N2O-reduction ratio)(圖6a)。AMD灌溉區(qū)甘蔗田土壤N2O還原成N2的比例隨深度逐漸減小, 最小值(15.54%)出現(xiàn)在30~40 cm深度土壤層(圖6b), 而該深度土壤的N2O濃度最大且15N和18O比其他深度貧化(圖2-圖3)。由此可見, 該深度土壤較弱的N2O還原能力致使高濃度的N2O還保留在土壤中。而其他3種土壤剖面中N2O還原成N2的比例隨深度變化較小。

圖6 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田土壤N2O還原成N2的比例范圍(a)和均值混合線上各點N2O還原成N2的比例(b)

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area.

3 討論

3.1 N2O同位素分布的影響因素

一般認(rèn)為,15Nbulk與18O能夠提供關(guān)于控制N2O產(chǎn)生和消耗過程的信息[6,35-36]。由于同位素分餾效應(yīng), 無論硝化作用還是反硝化作用生成的N2O中的15N都會貧化, 而部分N2O在反硝化過程中又被進一步還原成N2時,15N會在剩余的N2O中富集[37]。在人為干擾情況下, 肥料充足的農(nóng)田土壤所產(chǎn)生和轉(zhuǎn)換的N2O同位素特征值15Nbulk平均值為-34‰±12.4‰[8]。本研究區(qū)供試土壤N2O同位素特征值15Nbulk相對富集, 其范圍為-19.62‰~6.15‰, 平均值為-1.57‰。通過對比發(fā)現(xiàn), 供試土壤的硝態(tài)氮和氨態(tài)氮含量都較低, 微生物活動受到氮源的限制, 促使N2O進一步還原成N2, 從而引起15N在剩余N2O富集。長期淹水造成水稻田表層土壤含水率較高, 也阻礙了N2O向大氣的擴散以及大氣中氧氣向下層土壤的輸送, 從而在土壤中形成厭氧條件, 有足夠的時間進行N2O還原。

相關(guān)研究亦表明, 農(nóng)田土壤相比森林土壤15Nbulk與18O之間的相關(guān)性更具有統(tǒng)計學(xué)意義[8]。這是因為農(nóng)田土壤頻繁受到諸如土地耕作、氮肥施用、降水等人為因素的干擾, 使產(chǎn)生的N2O很快擴散進入大氣, 減少N2O還原成為N2。而森林土壤屬于非人為干擾的自然狀態(tài), 土壤剖面不同微生物過程產(chǎn)生的N2O處于穩(wěn)定狀態(tài), 短時間內(nèi)不可能發(fā)生顯著的擴散, 進而被還原成為N2導(dǎo)致同位素分餾, 使15Nbulk與18O相關(guān)性較差。研究區(qū)土壤N2O同位素特征值15Nbulk與18O相關(guān)性具有統(tǒng)計學(xué)意義(圖7,2=0.64,<0.000 1,=32)。其中SG土壤剖面擬合結(jié)果更好(2=0.95,<0.000 1,=8)。利用SP值和18O定量計算所得到的N2O還原比結(jié)果與15Nbulk與18O之間相關(guān)性的定性分析結(jié)果一致性良好。

此外, 種植作物與耕作方式不同對同位素的分布也有影響。研究區(qū)甘蔗田(SG、LG)土壤N2O的15N和18O分別比水稻田SS、LS同一深度土壤貧化, 且N2O的反硝化作用貢獻比高于SS、LS。

圖7 酸性礦山廢水灌區(qū)、天然來水區(qū)不同作物田不同深度土壤氣體N2O同位素特征值δ15Nbulk與δ18O線性回歸分析

SG: 酸性礦山廢水灌溉區(qū)甘蔗田; SS: 酸性礦山廢水灌溉區(qū)水稻田; LG: 天然來水灌溉區(qū)甘蔗田; LS: 天然來水灌溉區(qū)水稻田。SG: sugarcane field in acid mine drainage irrigated area; SS: paddy field in acid mine drainage irrigated area; LG: sugarcane field in natural water irrigated area; LS: paddy field in natural water irrigated area.

3.2 AMD灌溉對農(nóng)田土壤N2O產(chǎn)生和釋放的影響

土壤產(chǎn)生N2O是一個復(fù)雜的微生物過程, 其產(chǎn)生和釋放量主要取決于土壤微生物的硝化和反硝化作用的反應(yīng)速率、N2O在反應(yīng)產(chǎn)物中的比例以及N2O擴散到大氣前在土壤中擴散和被還原程度。這一過程受多種因素的影響, 包括土壤水分、通氣條件、可利用氮、溫度、pH等[38]。

土壤剖面不同深度N2O濃度與土壤孔隙含水率、施肥狀況均有關(guān)[7,39-41]。研究區(qū)甘蔗田(SG、LG)地表以下30~50 cm深度范圍, 土壤孔隙含水率存在峰值, 分別高達69.94%、89.15%。已知該深度范圍內(nèi), N2O濃度最大。因此, 包氣帶含水率大的深度范圍N2O濃度也高。丁軍軍等[42]研究也表明, 70%土壤孔隙含水率(WFPS)的水分條件下N2O的排放較高。但是, 70%WFPS并不代表就是最佳土壤孔隙含水率。Bouwman等[43]認(rèn)為, 土壤性質(zhì)決定了N2O排放所需的最佳土壤孔隙含水率。N2O還原比與土壤含水率呈正相關(guān)關(guān)系(表2), 表明農(nóng)田土壤含水率是控制N2O還原比的關(guān)鍵因素之一。在同等施氮條件下有機肥比無機肥(尿素)更能促進反硝化作用, 能明顯減少土壤N2O排放, 而不影響作物產(chǎn)量[38]。因此, 在實際農(nóng)業(yè)生產(chǎn)過程中合理控制土壤含水率及施肥狀況, 有助于減少溫室氣體N2O排放及提高肥料利用效率。

表2 土壤反硝化作用貢獻比(Denit)、還原比(Fr)及N2O濃度()與理化性質(zhì)及重金屬濃度的相關(guān)性分析

**和*分別表示0.01和0.05水平(雙尾)相關(guān)性顯著。** and * indicate significant correlation at 0.01 and 0.05 levels (2-tailed), respectively.

結(jié)合土壤氣體中N2O濃度與同位素特征值分析可知, 同一深度范圍AMD灌區(qū)(SG、SS)土壤剖面N2O的15N和18O比天然來水區(qū)(LG、LS)同一深度土壤貧化, N2O濃度也較高, 且SG、SS土壤N2O的來源反硝化作用貢獻比高于LG、LS。這表明受到AMD灌溉影響的上壩村甘蔗田與水稻田土壤剖面各深度反硝化作用增強, 產(chǎn)生15N顯著貧化的氣體N2O。由表2可知, N2O濃度與pH呈負(fù)相關(guān), 表明土壤pH越低, 反硝化作用對N2O生成的貢獻比越高。這與之前的研究結(jié)果一致, 即較低的pH可抑制硝化作用的發(fā)生[44]。N2O的濃度與反硝化作用貢獻比呈正相關(guān), 與還原比呈負(fù)相關(guān), 表明土壤中的N2O主要來源于反硝化作用, 而且高濃度的N2O將會抑制還原過程(N2O→N2)。反硝化作用貢獻比與Cu、Zn、Pb均存在正相關(guān), 表明AMD灌溉下, 較高的重金屬濃度對反硝化作用產(chǎn)生N2O有促進作用。以往試驗研究結(jié)果也有相同結(jié)果[12], 土壤As、Pb、Cu、Zn含量分別為466 mg×kg-1、187 mg×kg-1、344 mg×kg-1、326 mg×kg-1時, 利用乙炔抑制法在只進行反硝化作用時N2O排放通量為0.237 μg×kg-1×d1, 遠高于空白樣品0.095 μg×kg-1×d-1。Holtan-Hartwig等[45]通過將提取的微生物細胞暴露于重金屬環(huán)境下進行厭氧培養(yǎng)也發(fā)現(xiàn), 重金屬所產(chǎn)生的即刻效應(yīng)導(dǎo)致反硝化速率降低, 而且N2O還原率減小程度遠遠大于N2O產(chǎn)生率的減小程度, N2O產(chǎn)生率在試驗進行8 d后已經(jīng)部分恢復(fù), 而N2O還原率2個月后仍未完全恢復(fù)。本研究中N2O還原比與Zn、Pb、Cd也均呈負(fù)相關(guān), 說明重金屬可能對反硝化作用中的N2O還原酶產(chǎn)生抑制作用。由此可見, AMD灌溉通過改變農(nóng)田土壤的pH、重金屬含量、含水率從而改變了其土壤對N2O的來源途徑及還原能力, 而AMD灌溉對水稻田土壤N2O的還原能力的影響小于對甘蔗田的影響。因此, 酸性礦山廢水灌溉的影響下, 土壤氣體中N2O產(chǎn)生和釋放也與土壤作物類型、種植方式有關(guān)。

4 結(jié)論

本研究利用N2O的3種同位素特征值(15Nbulk、18O、SP)對華南紅壤區(qū)受AMD灌溉影響的農(nóng)田土壤N2O的微生物產(chǎn)生途徑和轉(zhuǎn)換機制進行了探究。AMD灌區(qū)農(nóng)田土壤N2O濃度均稍高于利用天然河流灌溉的同類作物農(nóng)田土壤N2O濃度。AMD灌區(qū)甘蔗田土壤N2O產(chǎn)生主要來源于反硝化作用, 于此同時N2O還原比例隨深度增加而逐漸減小。N2O濃度峰值、反硝化作用貢獻比峰值以及N2O還原比極小值出現(xiàn)的土壤層是一致的, 這說明了該深度范圍較弱的土壤N2O還原能力導(dǎo)致較高濃度的N2O殘留。其他3種土壤N2O還原比例隨深度增加變化較小, 且均大于40%。通過分析各理化性質(zhì)與供試土壤反硝化作用貢獻比、還原比以及N2O濃度相關(guān)性結(jié)果可知, AMD灌溉通過改變農(nóng)田土壤的pH、重金屬含量、含水率, 從而改變了其土壤對N2O的來源途徑及還原能力, 其次土壤中N2O的產(chǎn)生和消耗也與作物類型、種植方式有關(guān)。而利用SP值和18O定量計算的土壤N2O還原比與利用15Nbulk與18O之間相關(guān)性所得的定性結(jié)果具有良好一致性, 為定量分析土壤N2O產(chǎn)生和轉(zhuǎn)化機制打下了科學(xué)基礎(chǔ)。

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Production and release mechanism of N2O in agricultural soils irrigated with acid mine drainage*

CHANG Yimeilin1, TANG Changyuan1,2, LI Xing2**, LI Rui2, CAO Yingjie2

(1. School of Geography and Planning, Sun Yat-sen University, Guangzhou 510275, China; 2. School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou 510006, China)

Acid mine drainage (AMD) is mostly untreated or not up to standard level before directly drained into rivers for irrigation, causing severe pollution of agriculture eco-environments. Metal pollution had been widely reported in extensive fields including the red soil region in South China. As we have known, N2O emitted from agricultural systems was one of the important causes of global greenhouse effects. However, there has been poor knowledge of potential changes in N2O evolution in polluted fields.In this study, four agricultural soil profiles from sugarcane and paddy fields were used to track the changes in N2O emission and sources of heavy metal polluted soils irrigated with AMD (Shangba Village, Wengyuan County, Guangdong Province) and then compared with unpolluted soils irrigated with natural water (Lianxin Village, Wengyuan County, Guangdong Province). The physical / chemical parameters and contents of heavy metals in the soils, N2O concentration and stable nitrogen and oxygen isotope compositions were analyzed to determine the contribution of nitrification and denitrification of N2O and the reduction ratio of N2O. Our results showed that there was slightly higher N2O concentration of the same crop in AMD irrigated area than in unpolluted soil irrigated with natural water, and higher soil N2O concentration in sugarcane fields than in rice fields. The production of N2O from denitrification was 71.29%, which was higher than that from nitrification in surface soil (0–30 cm) in sugarcane fields in areas irrigated with AMD.N2O reduction ratio in the soil profile in AMD irrigation area decreased gradually with increasing depth. There was only 15.54% N2O reduction to N2at the peak of N2O concentration. However, the average ratio of N2O reduction to N2in sugarcane fields irrigated with natural water was as high as 49.80%. Limited N2O reduction led to high levels of N2O residues in the soil. Studies showed that AMD irrigation changed the production and release of N2O by changing pH, heavy metal content and moisture content of agricultural soils. N2O production and reduction studies carried out using combined nitrogen and oxygen isotope compositions clarified potential risks of irrigated agricultural soils with AMD. This provided the scientific basis for future restoration works in polluted soils.

Isotopic signature; Nitrous oxide; Acid mine drainage irrigation; Nitrification; Denitrification; Red soil region

, E-mail: lxbaboon@163.com

X53

A

2096-6237(2019)01-0001-10

10.13930/j.cnki.cjea.180568

常伊梅林, 唐常源, 李杏, 李銳, 曹英杰. 礦山廢水灌溉區(qū)農(nóng)田土壤N2O的產(chǎn)生及釋放機制研究[J]. 中國生態(tài)農(nóng)業(yè)學(xué)報(中英文), 2019, 27(1): 1-10

CHANG Y M L, TANG C Y, LI X, LI R, CAO Y J. Production and release mechanism of N2O in agricultural soils irrigated with acid mine drainage[J]. Chinese Journal of Eco-Agriculture, 2019, 27(1): 1-10

* 廣東省基礎(chǔ)與應(yīng)用基礎(chǔ)研究專項資金項目(2017A030310563)、廣州市科技計劃項目(201510010300)和國家自然科學(xué)基金青年科學(xué)基金項目(41501512)資助

李杏, 主要研究方向為小流域氮循環(huán), 流域地表水、地下水地球化學(xué)研究。E-mail: lxbaboon@163.com

常伊梅林, 主要研究方向為土壤重金屬污染和土壤氮循環(huán)。E-mail: changyimeilin@qq.com

2018-06-19

2018-09-12

* This study was supported by the Natural Science Foundation of Guangdong Province, China (2017A030310563), the Science and Technology Program of Guangzhou, China (201510010300) and the National Natural Science Foundation of China (41501512).

Jun. 19, 2018;

Sep. 12, 2018