旱澇交替下控制灌溉對(duì)稻田節(jié)水及氮磷減排的影響

高世凱，俞雙恩，王 梅，曹睿哲，郭 蓉

旱澇交替下控制灌溉對(duì)稻田節(jié)水及氮磷減排的影響

高世凱，俞雙恩※，王 梅，曹睿哲，郭 蓉

（1. 河海大學(xué)南方地區(qū)高效灌排與農(nóng)業(yè)水土環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室，南京 210098；2. 河海大學(xué)水利水電學(xué)院，南京 210098）

該文研究控制灌排技術(shù)對(duì)稻田水氮磷動(dòng)態(tài)變化及節(jié)水減排效應(yīng)的影響。于2015年5—10月在河海大學(xué)江寧校區(qū)節(jié)水園，在有底側(cè)坑內(nèi)進(jìn)行水稻栽培試驗(yàn)，于水稻分蘗期、拔節(jié)孕穗期、抽穗開(kāi)花期和乳熟期4個(gè)生育階段進(jìn)行控水試驗(yàn)，以常規(guī)控制灌溉為對(duì)照，測(cè)定稻田淹排水銨態(tài)氮（NH4+-N）、硝態(tài)氮（NO3--N）和總磷濃度變化。結(jié)果表明：旱轉(zhuǎn)澇處理淹水初期稻田水中銨態(tài)氮（NH4+-N）、硝態(tài)氮（NO3--N）和總磷濃度顯著高于澇轉(zhuǎn)旱處理，這個(gè)時(shí)期地表和地下排水應(yīng)該引起注意?？刂乒嗯艞l件下灌水量減少7.4%～18.5%，排水量減少23.0%～43.5%，NH4+-N負(fù)荷減少18.5%～54.5%，NO3--N負(fù)荷減少16.8%～57.7%，總磷負(fù)荷減少34.2%～58.3%；其中拔節(jié)孕穗期和抽穗開(kāi)花期在保證節(jié)水減排的同時(shí)，也能實(shí)現(xiàn)較高的產(chǎn)量；因此，控制灌排技術(shù)具有較好的節(jié)水減排效果，對(duì)南方稻作區(qū)灌排實(shí)踐具有指導(dǎo)意義。

土壤水分；氮；磷；旱澇交替；控制灌排；水稻；農(nóng)田水深

0 引 言

水稻是南方地區(qū)最主要的糧食作物，生育期內(nèi)的降雨較多，排水量較大[1-2]。農(nóng)田排水造成氮磷等營(yíng)養(yǎng)物質(zhì)流失，不僅降低了水分和肥料的利用效率，還導(dǎo)致了附近河流、湖泊和水庫(kù)的水體富營(yíng)養(yǎng)化[3-4]。然而稻田具有人工濕地的功能，延長(zhǎng)降雨和灌溉水層在田間的滯留時(shí)間，有利于提高水分利用率和凈化水質(zhì)[5-6]。因此實(shí)施水稻節(jié)水灌溉技術(shù)和控制排水技術(shù)對(duì)于節(jié)約水資源、減輕環(huán)境污染具有重要意義。

已有研究表明，控制排水有利于提高肥料和水分利用率，從而降低農(nóng)田氮磷等對(duì)河流和地下水的污染[7-11]?？刂婆潘疁p少農(nóng)田氮磷損失的途徑主要有控制排水流量和排水強(qiáng)度、控制降雨期間和雨后的排水時(shí)間以及雨水滯留在田間和排水溝中的時(shí)間。瞿思堯等[12-13]的研究發(fā)現(xiàn)，連續(xù)對(duì)農(nóng)田實(shí)施控制排水措施可以使排水量大約降低30%；Williams等[14]研究表明，與常規(guī)排水方式相比，控制排水可以使硝態(tài)氮（NO3--N）降低約20%左右，而溶解磷大約可降低35%。近年來(lái)，干濕交替、間歇灌溉、控制灌溉等水稻節(jié)水灌溉技術(shù)逐漸替代傳統(tǒng)的淹水灌溉。但現(xiàn)有的大部分節(jié)水灌溉技術(shù)，灌水下限較高，同時(shí)限制了雨后蓄水深度，以避免產(chǎn)生淹水脅迫[8,15-17]。這些灌溉技術(shù)雖然保持了較高的產(chǎn)量，但也造成灌水量和排水量較大，雨水利用率相應(yīng)降低。水稻控制灌排綜合考慮了節(jié)水灌溉與控制排水的協(xié)同效應(yīng)，在保證水稻產(chǎn)量的前提下，在保持或者甚至低于現(xiàn)有節(jié)水模式的灌水下限，適當(dāng)增加雨后蓄水深度，達(dá)到水稻節(jié)水、減排、控污、高產(chǎn)的目的。已有的關(guān)于水分脅迫對(duì)稻田氮磷流失影響的研究[1,5,12]，多是針對(duì)單一受旱或受澇條件下開(kāi)展的，對(duì)旱澇交替情況下稻田氮磷動(dòng)態(tài)變化影響的研究涉及較少。此外，大多研究是針對(duì)水稻全生育期的氮磷變化規(guī)律。本文探討旱轉(zhuǎn)澇（drought then flooding，HZL）和澇轉(zhuǎn)旱（flooding then drought，LZH）條件下控制灌排技術(shù)對(duì)不同生育階段氮磷濃度變化規(guī)律的影響及節(jié)水減排效應(yīng)，對(duì)合理制定農(nóng)田水位調(diào)控下稻田控制灌排模式、控制農(nóng)業(yè)面源污染具有重要意義。

1 材料與方法

1.1 試驗(yàn)區(qū)基本情況及農(nóng)藝措施

試驗(yàn)于2015年5—10月在河海大學(xué)南方地區(qū)高效灌排與農(nóng)業(yè)水土環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室江寧校區(qū)節(jié)水園區(qū)進(jìn)行。試驗(yàn)區(qū)（31°86′N(xiāo)，118°60′E）屬于亞熱帶濕潤(rùn)氣候，年均降雨量1 021.3 mm，其中5—9月降雨量占年平均降雨量的60%以上，年均蒸發(fā)量900 mm，平均無(wú)霜期237 d，年平均氣溫15.7 ℃，日照時(shí)數(shù)2 212.8 h。節(jié)水園區(qū)共有32個(gè)規(guī)格為2.5 m×2.0 m×2.0 m（長(zhǎng)×寬×深）的蒸滲測(cè)坑（28個(gè)有底，4個(gè)無(wú)底），按南北方向布置，共2排，每排16個(gè)，地面設(shè)移動(dòng)式雨棚，中間地下設(shè)有廊道及設(shè)備室，有底測(cè)坑可以實(shí)時(shí)控制地下水位。試驗(yàn)所用土壤為黏壤土，測(cè)坑內(nèi)0～30 cm土層土壤容重為1.46 g/cm3，總孔隙度為44.9%，田間持水率為25.28%，pH值為6.97，有機(jī)質(zhì)為2.19%，全氮為0.91 g/kg，全磷為0.32 g/kg，速效氮為27.65 mg/kg，速效磷為12.5 mg/kg。

試驗(yàn)供試水稻品種為南粳9108，5月13日泡種，15日催芽，5月17日下種，6月16日移栽，栽插密度為

20 cm×14 cm，每穴3根籽苗。移植前1周，對(duì)試驗(yàn)測(cè)坑進(jìn)行泡田，以便于耕耙和插秧。水稻生育期內(nèi)共施肥3次，施肥情況見(jiàn)表1。

表1 研究區(qū)農(nóng)田施肥狀況Table1 Farmland fertilization condition in experimental area

1.2 試驗(yàn)設(shè)計(jì)

考慮南方地區(qū)水稻生長(zhǎng)特點(diǎn)及氣候條件，不同處理控水試驗(yàn)設(shè)計(jì)詳見(jiàn)表2。選取水稻分蘗期、拔節(jié)孕穗期、抽穗開(kāi)花期和乳熟期4個(gè)生育階段進(jìn)行控水試驗(yàn)，共8個(gè)處理，每個(gè)處理設(shè)3個(gè)重復(fù)，在24個(gè)有底的蒸滲測(cè)坑內(nèi)進(jìn)行試驗(yàn)。控制灌溉（CK）水深達(dá)到灌水下限值（-200 mm），立即灌水至灌水上限（分蘗期為20 mm，其他生育期為50 mm）；CK在分蘗期的雨后蓄水上限為100 mm，其他生育期為200 mm，當(dāng)雨后水深值超過(guò)蓄水上限，立即排水至蓄水上限。HZL處理，控水期開(kāi)始排干水深為0，控水5 d后達(dá)到控水下限（-500 mm），第6天灌水至淹水上限（分蘗期為200 mm，其他生育期為 250 mm），然后讓其自然消退，淹水5 d后將田面水排到灌水上限；LZH各處理，控水開(kāi)始當(dāng)天立即灌水至淹水上限（分蘗期為200 mm，其他生育期為250 mm），然后讓其自然消退，淹水5 d后將田面水排盡，直至水深達(dá)到控水下限（-500 mm），再灌水至該生育期灌水上限；除控水期外，HZL和LZH處理的其他時(shí)間水深與控制灌溉（CK）相同?？厮陂g，降雨時(shí)關(guān)閉雨棚。各處理田面有水層時(shí)，應(yīng)保持2 mm/d的田間滲漏量，田面無(wú)水層時(shí)，禁止地下排水。

1.3 測(cè)定指標(biāo)與方法

1）灌水時(shí)間。每天大約08:30用直尺測(cè)量農(nóng)田水深，當(dāng)水深值達(dá)到或接近控制的下限值（測(cè)量值與控制設(shè)定值的差值≤50 mm（地下水）或≤10 mm（水層））時(shí)，按處理要求灌水。

2）水樣采集及分析方法。各生育期地表水按照取水間隔（淹水1、3、5 d）采用50 mL醫(yī)用注射器，不擾動(dòng)土壤層，隨機(jī)抽取測(cè)坑內(nèi)中部地表水；地下水在田面無(wú)水層時(shí)，在水深為0、–250、–500 mm左右時(shí)采集地下排水的尾水（距田間地面1.7 m）；當(dāng)田面有水層時(shí)，與地表水同一時(shí)間取水，采集的水樣注入塑料瓶并做好標(biāo)記，低溫保存于冰箱中，進(jìn)行冷藏（3 ℃）處理，并在24 h內(nèi)進(jìn)行水質(zhì)分析。用納氏試劑光度法測(cè)定銨態(tài)氮（NH4+-N），紫外分光光度法測(cè)定NO3–-N，鉬銻抗分光光度法測(cè)定總磷（total P，TP）濃度[18]。測(cè)定儀器為日本島津公司的UV2800紫外分光光度儀。試驗(yàn)為完全隨機(jī)區(qū)組設(shè)計(jì)，用SPSS 19.0軟件進(jìn)行統(tǒng)計(jì)分析，依據(jù)F–檢驗(yàn)和Leastsignificant difference（LSD）方法進(jìn)行顯著性分析（α=0.05）。

3）地表氮磷流失量。它指淹水5 d后將田面水排到灌水適宜上限的氮磷流失量。

表2 各處理控水方案Table2 Water control program of each treatment

2 結(jié)果與分析

在水稻分蘗期、拔節(jié)孕穗期、抽穗開(kāi)花期和乳熟期4個(gè)生育階段進(jìn)行控水試驗(yàn)，得到淹水期間各處理地表水濃度見(jiàn)圖1。濃度顯然受到淹水持續(xù)時(shí)間的影響。由于淹水時(shí)間的延長(zhǎng)，通過(guò)微生物相互作用、土壤顆粒沉降和植物吸收，各處理地表水中的濃度顯著減少（P＜0.05）。與淹水1 d相比，淹水5 d處理HZL-1、HZL-2、HZL-3和HZL-4的濃度減少了55.2%、53.1%、51.0%、32.5%（P＜0.05），濃度減少了38.8%、52.2%、40.4%、26.9%（P＜0.05）；處理LZH-1、LZH-2、LZH-3和LZH-4的濃度減少了50.9%、42.2%、54.74%、42.1%（P＜0.05），濃度減少了35.1%、62.2%、62.1%、16.4%（P＜0.05）。長(zhǎng)時(shí)間淹水導(dǎo)致生物反硝化活動(dòng)增強(qiáng)，各生育期地表水濃度顯著大于濃度（P＜0.05）。淹水結(jié)束時(shí)，HZL-1的濃度顯著大于LZH-1（P＜0.05），產(chǎn)生這種現(xiàn)象的原因可能主要是較高的濃度經(jīng)硝化生成，導(dǎo)致濃度降低較慢。在乳熟期，HZL處理的N濃度在5 d略微升高，濃度呈現(xiàn)先升高后降低的趨勢(shì)，產(chǎn)生這種現(xiàn)象的原因可能一方面是乳熟期植物吸收氮素減少，另一方面是受旱復(fù)水后前期硝化作用較強(qiáng)，隨淹水時(shí)間的延長(zhǎng)，硝化作用減弱。在淹水初期，HZL處理的濃度顯著大于LZH處理（P＜0.05），這可能是因?yàn)橥寥篮式档鸵种屏宋⑸锖兔傅幕钚?，?dǎo)致了無(wú)機(jī)氮在干旱條件下積累，而旱后復(fù)水促進(jìn)了氮的礦化和硝化。因此，水稻旱后受澇時(shí)，延長(zhǎng)淹水時(shí)間，更有利于減少稻田排水中的濃度，分蘗期旱后淹水5 d后，濃度還較高，應(yīng)避免這個(gè)時(shí)期的排水。此外，由于乳熟期旱后淹水初期濃度較高，應(yīng)避免這個(gè)時(shí)期的排水。

圖1 地表水的濃度變化Fig.1 Change in-N concentration in surface water

圖2 控水期間地下水濃度變化Fig.2 Change ofconcentration in underground water during water control process

2.2 地表水及地下水TP濃度變化

2.2.1 地表水TP濃度變化

地表水TP濃度變化見(jiàn)圖3。分蘗期、拔節(jié)孕穗期、抽穗開(kāi)花期和乳熟期各處理的TP濃度變化呈現(xiàn)逐漸降低趨勢(shì)。與淹水1 d相比，淹水5 d處理HZL-1、HZL-2、HZL-3和HZL-4的TP濃度減少了38.8%、52.2%、40.4%、26.9%（P＜0.05）；LZH-1、LZH-2、LZH-3和LZH-4的TP濃度減少了31.8%、55.0%、62.9%、49.9%（P＜0.05）。HZL處理的TP濃度顯著大于LZH處理，這可能是因?yàn)楹缔D(zhuǎn)澇的土壤環(huán)境能夠提高土壤磷素的有效性，增強(qiáng)了磷素的溶解活性。

圖3 地表水TP變化Fig.3 Change of TP concentration in surface water

2.2.2 地下水TP濃度變化

稻田地下水TP濃度變化見(jiàn)圖4。LZH-1、LZH-2和LZH-3的TP濃度總體呈下降趨勢(shì)，HZL-1、HZL-2、HZL-3和HZL-4處理在控水1 d的TP濃度顯著大于控水前（P＜0.05），這是因?yàn)楦珊凳雇寥李w?？p隙變大導(dǎo)致復(fù)水后不易被土壤吸附的磷入滲到地下水中。處理HZL-2和HZL-3第1天與第5天的TP濃度差異不顯著（P＞0.05），產(chǎn)生這種現(xiàn)象是由于拔節(jié)孕穗期和抽穗開(kāi)花溫度較高，導(dǎo)致較大的土壤縫隙，使地表磷滲入較多。

圖4 控水期間地下水TP濃度變化Fig.4 Change of TP concentration in underground water during water control process

2.3 控制灌排條件下各生育期氮磷流失負(fù)荷

地表氮磷流失負(fù)荷見(jiàn)表3。延長(zhǎng)稻田淹水時(shí)間，能夠有效減少農(nóng)田氮磷流失負(fù)荷。與CK處理相比，HZL-1、HZL-2和HZL-3處理負(fù)荷減少了51.3%、30.33%、26.2%（P＜0.05），TP負(fù)荷分別減少了34.2%、40.9%、41.7%、（P＜0.05）； HZL-1、HZL-2、HZL-3、HZL-4處理N負(fù)荷分別減少了34.6%、45.6%、49.0%、16.8%（P＜0.05）；LZH-1、LZH-2、LZH-3和LZH-4處理負(fù)荷減少了54.5%，18.5%、30.0%、30.9%（P＜0.05），負(fù)荷減少了57.7%、48.5%、39.4%、31.9%（P＜0.05），TP負(fù)荷減少了55.3%、50.0%、58.3%、42.9%（P＜0.05）。HZL-4處理與CK處理的、TP負(fù)荷差異均不顯著（P＞0.05），這可能是HZL-4處理的淹水起始濃度較大和乳熟期作物對(duì)和TP的吸收降低。HZL和LZH處理的負(fù)荷在分蘗期差異顯著（P＜0.05），在拔節(jié)孕穗期、抽穗開(kāi)花期和乳熟期不顯著（P＞0.05）。HZL和LZH處理的分蘗期和拔節(jié)孕穗期的TP負(fù)荷差異不顯著（P＞0.05），而抽穗開(kāi)花期和乳熟期TP負(fù)荷顯著（P＜0.05），這主要是隨著水稻生育生長(zhǎng)階段的進(jìn)行，磷素逐漸被土壤吸附，而旱后復(fù)水導(dǎo)致磷流失增加。因此，控制灌排模式能夠從源頭上減少農(nóng)田氮磷排放量。

表3 不同生育期稻田地表氮磷流失負(fù)荷Table3 Nitrogen and phosphorus loss from paddy field under different growth stages

2.4 控制灌排對(duì)灌水量、排水量和產(chǎn)量的影響

控制灌排稻田能夠顯著減少稻田灌水量（除乳熟期LZH外）和排水量（P＜0.05）（表4）。與CK處理相比，控制灌排處理的灌水量減少了7.4%～18.5%（P＜0.05），排水量減少了23.0%～43.5%（P＜0.05）。由于控制灌排提高了雨后淹水深度，降低了灌水下限，導(dǎo)致了分蘗期處理產(chǎn)量顯著減少（P＜0.05），而拔節(jié)孕穗期，抽穗開(kāi)花期保持較高的產(chǎn)量。因此，在拔節(jié)孕穗期和抽穗開(kāi)花期實(shí)施控制灌排，能夠?qū)崿F(xiàn)節(jié)水減排，也能保持較高的產(chǎn)量。在分蘗期和乳熟期實(shí)施控制灌排時(shí)，應(yīng)降低此生育階段的雨后蓄水深度和提高灌水下限。

表4 不同處理灌水量、排水量和水稻產(chǎn)量Table4 Irrigation amount, drainage amount and rice grain yield under different treatments

3 討 論

4 結(jié) 論

1）稻田灌排條件下延長(zhǎng)淹水時(shí)間顯著減少了氮磷濃度。地表和地下排水中的濃度顯著大于在各生育階段的淹水初期，HZL處理的地表水、和TP濃度顯著大于LZH處理，此階段應(yīng)避免地表排水。當(dāng)水分脅迫從旱轉(zhuǎn)為澇時(shí)，增加了地下水氮磷淋失的風(fēng)險(xiǎn)。

3）控制灌排通過(guò)對(duì)稻田水位的調(diào)控，灌水量減少7.4%～18.5%，排水量減少23.0%～43.5%。在拔節(jié)孕穗期和抽穗開(kāi)花期實(shí)施控制灌排，不僅能夠?qū)崿F(xiàn)節(jié)水減排，也能保持較高的產(chǎn)量。

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Effect of controlled irrigation and drainage on saving water and reducing nitrogen and phosphorus loss in paddy field under alternate drought and flooding condition

Gao Shikai, Yu Shuang′en※, Wang Mei, Cao Ruizhe, Guo Rong
(1. Key Laboratory of Efficient Irrigation-Drainage and Agricultural Soil-Water Environment in Southern China, Ministry of Education, Hohai University, Nanjing 210098, China; 2. College of Water Conservancy and Hydropower, Hohai University, Nanjing 210098, China)

Attempts to reduce the nutrient losses in drainage water have led to the promotion of controlled irrigation and drainage. Under the controlled irrigation and drainage condition, alternate drought and flooding condition (drought then flooding (HZL) and flooding then drought (LZH)) often occur. It is unclear about the change of nitrogen and phosphorus concentration in paddy water affected by controlled irrigation and drainage under the alternate stress. Therefore, this study based on farmland water depth as the control index aimed to investigate dynamic changes of nitrogen and phosphorus concentration in underground water and surface water of paddy field under the controlled irrigation and drainage in the HZL and LZH condition. Moreover, the effect of controlled irrigation and drainage on the discharge-reducing and water saving was studied. The experiments were conducted in specially designed experimental pits in Jiangning Water-saving Experiment Station at the Key Laboratory of Efficient Irrigation-Drainage and Agricultural Soil-Water Environment in Southern China, Ministry of Education, Nanjing (31°86′N(xiāo), 118°60′E) during the rice growing season of 2015 (May to October). Eight controlled irrigation and drainage treatments were designed for the experiment. For the HZL treatments, rice experienced the drought stress at first, and when the field water depth dropped to the lower limit (-500 mm) the irrigation water was added with an auto-irrigation system until the upper limit of water depth (200 mm at the tillering stage and 250 mm at the other stages) was reached. For the LZH treatments, rice experienced the flooding stress at first, and then irrigation water was immediately added to the upper limit of water depth (200 mm at tillering stage and 250 mm at the other stages); the field surface water was then drained and the field water depth dropped to the lower limit (-500 mm). The controlled irrigation was considered as the control with irrigation lower limit of -200 mm and the upper limit of 20 mm for the tillering stage and 50 mm for the other stages. Ammonia N (NH4+-N), nitrate N (NO3?-N), and total phosphorus (TP) in the water samples were analyzed. The results showed that when the HZL treatments had higher NH4+-N, NO3?-N and TP concentration during the earlier period of flooding, therefore, the surface and underground drainage should be noticed. Extending the flooding days could decrease nitrogen and phosphorus concentration in the controlled irrigation and drainage. The NH4+-N was the major form of N in the surface drainage and percolation water. The TP concentration in the surface water followed a decreasing trend during the flooding. Compared to the controlled irrigation, under the controlled irrigation and drainage, the irrigation amount was reduced by 7.4%-18.5%, the drainage amount was reduced by 23.0%-43.5%, the NH4+-N load was reduced by 18.5%-54.5%, the NO3?-N load was reduced by 16.8%-57.7% and the TP load was reduced by 34.2%-58.3%. Meanwhile, most of the controlled irrigation and drainage treatments could keep high rice grain yield. In sum, the controlled irrigation and drainage could achieve a positive effect on grain yield, water saving and N-P loss reduction at the jointing-booting stage and heading and flowering stage. Therefore, the controlled irrigation and drainage technology has a good effect on water saving and N and P loss reduction and provides a guide for the irrigation and drainage.

soil moisture; nitrogen; phosphorus; alternate drought and flooding; controlled irrigation and drainage; rice; farmland water depth

10.11975/j.issn.1002-6819.2017.05.018

S143.1; S274.3

A

1002-6819(2017)-05-0122-07

高世凱，俞雙恩，王 梅，曹睿哲，郭 蓉. 旱澇交替下控制灌溉對(duì)稻田節(jié)水及氮磷減排的影響[J]. 農(nóng)業(yè)工程學(xué)報(bào)，2017，33(5)：122－128.

10.11975/j.issn.1002-6819.2017.05.018 http://www.tcsae.org

Gao Shikai, Yu Shuang′en, Wang Mei, Cao Ruize, Guo Rong. Effect of controlled irrigation and drainage on saving water and reducing nitrogen and phosphorus loss in paddy field under alternate drought and flooding condition[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2017, 33(5): 122－128. (in Chinese with English abstract) doi：10.11975/j.issn.1002-6819.2017.05.018 http://www.tcsae.org

2016-05-05

2016-12-10

國(guó)家自然科學(xué)基金“農(nóng)田水位調(diào)控下水稻旱澇交替脅迫機(jī)理、稻田氮磷流失規(guī)律及節(jié)水控污灌排模式”（51479063）；“南方地區(qū)土地開(kāi)發(fā)整理的適宜水面率研究及其優(yōu)化調(diào)控”（41401628）。

高世凱，男，河南駐馬店人，博士生，主要從事灌溉理論及節(jié)水灌溉技術(shù)研究。南京 河海大學(xué)水利水電學(xué)院，210098。

Email：igaoshikai@163.com

※通信作者：俞雙恩，男，安徽安慶人，教授，博士，主要從事灌溉排水及技術(shù)研究。南京 河海大學(xué)水利水電學(xué)院，210098。Email：seyu@hhu.edu.cn