川中丘陵區(qū)不同下墊面集水區(qū)氮磷流失特征

徐光志,邵志江,汪 濤,黃美玉,李明明

川中丘陵區(qū)不同下墊面集水區(qū)氮磷流失特征

徐光志1,2,邵志江1,2,汪 濤1*,黃美玉1,2,李明明1,2

(1.中國科學(xué)院成都山地災(zāi)害與環(huán)境研究所,四川 成都 610041；2.中國科學(xué)院大學(xué),北京 100049)

通過對(duì)2019～2020年不同下墊面集水區(qū)(農(nóng)田集水區(qū)與復(fù)合集水區(qū))徑流及氮磷濃度的連續(xù)逐日定位監(jiān)測(cè),研究川中丘陵區(qū)不同下墊面集水區(qū)氮磷徑流流失過程與強(qiáng)度,探討下墊面對(duì)集水區(qū)氮磷徑流流失特征的影響.結(jié)果表明:不同集水區(qū)的徑流過程因下墊面不同而存在明顯差異,農(nóng)田集水區(qū)內(nèi)的水田和坑塘的攔蓄作用滯緩了匯流過程,而復(fù)合集水區(qū)中居民點(diǎn)?公路等不透水下墊面縮短了匯流時(shí)間,使得復(fù)合集水區(qū)的降雨徑流量峰值更高,響應(yīng)速度較農(nóng)田集水區(qū)快12～25min,年徑流深較農(nóng)田集水區(qū)多28.1%;次降雨徑流過程中磷濃度變化較氮更劇烈,濃度峰值出現(xiàn)時(shí)間較氮早約1.2h,在降雨后期磷濃度下降速度更快,降幅更大;復(fù)合集水區(qū)的氮磷平均事件濃度(EMC)?峰值濃度均高于農(nóng)田集水區(qū),且兩集水區(qū)氮流失形態(tài)均以硝酸鹽氮為主,占總氮的65.9%;磷流失以顆粒態(tài)為主,占總磷的67.5%;復(fù)合集水區(qū)的氮磷流失負(fù)荷分別是農(nóng)田集水區(qū)的3.01和4.03倍,氮磷流失強(qiáng)度分別是農(nóng)田集水區(qū)的1.88和2.51倍.因此,復(fù)合集水區(qū)內(nèi)氮磷隨徑流流失的防控可能是未來川中丘陵區(qū)面源污染治理的重點(diǎn).

氮；磷；流失強(qiáng)度；下墊面；集水區(qū)

隨著點(diǎn)源污染的有效控制,農(nóng)業(yè)面源污染問題日益突出[1],種植業(yè)總氮(TN)、總磷(TP)排放占到全國水污染物質(zhì)總排放量的23.66%和24.16%[2].氮、磷作為農(nóng)業(yè)面源的特征污染物[3],其遷移轉(zhuǎn)化過程受地域和季節(jié)的影響,往往隨著降雨、融雪過程發(fā)生,具有隨機(jī)性,分散性等特點(diǎn)[4].目前,對(duì)地塊尺度氮磷隨徑流流失的過程和強(qiáng)度已有充分研究[5],但由于地塊到受納水體的距離較長,氮磷在隨徑流的遷移過程中可沿程消減[6],僅以地塊尺度的流失強(qiáng)度核算農(nóng)業(yè)面源污染,勢(shì)必會(huì)高估農(nóng)業(yè)面源污染排放負(fù)荷[7].因此,基于集水區(qū)尺度氮磷徑流流失的長期定位監(jiān)測(cè)對(duì)面源污染的排放特征調(diào)查、污染負(fù)荷評(píng)估、防控措施實(shí)施有著重要意義.

國內(nèi)外已有不少關(guān)于小流域或集水區(qū)尺度的氮磷徑流流失特征研究的報(bào)道.研究發(fā)現(xiàn),小流域氮磷輸出過程因種植模式的不同而存在明顯差異[8],氮磷輸出強(qiáng)度隨降雨強(qiáng)度的增加而增強(qiáng)[9],且暴雨徑流的發(fā)生會(huì)增加下游富營養(yǎng)化風(fēng)險(xiǎn)[10].同時(shí),國內(nèi)諸多學(xué)者在紫色土丘陵區(qū)[10-11]、喀斯特巖溶區(qū)[12]、紅壤丘陵區(qū)[13]、東北黑土區(qū)[14]等地均開展了大量研究.但是,不同區(qū)域的研究結(jié)果差異較大,這可能與小流域尺度氮磷徑流流失影響因素復(fù)雜有關(guān).已有研究表明,降雨量[15]、極端降雨事件[16]、土壤類型[17]、下墊面特征[14]及人類活動(dòng)[18]等因素均會(huì)影響小流域地表徑流氮磷流失過程.

近10a來長江流域的富營養(yǎng)化狀態(tài)僅得到輕微緩解,農(nóng)業(yè)面源污染擴(kuò)大的趨勢(shì)仍未徹底解決[1,19-20].川中丘陵區(qū)位于長江上游腹地,土壤類型以紫色土為主,土壤質(zhì)地較粗、土層淺薄,受水力侵蝕嚴(yán)重[21],氮磷養(yǎng)分在雨季流失嚴(yán)重,而氮素可隨徑流沿程遷移[22],易對(duì)水環(huán)境造成污染[23].研究發(fā)現(xiàn),土地利用情況及其景觀格局與氮磷流失密切相關(guān),果園和住宅地是氮磷的主要輸出源,農(nóng)用地的高氮磷負(fù)荷輸出與肥料大量使用密切相關(guān),而林地有助于改善水質(zhì)[24-26].下墊面特征作為控制小流域水文過程的關(guān)鍵因子,深刻影響著氮磷徑流流失過程[27-28].然而,已有的研究因各小流域的氣候氣象,地形地貌等環(huán)境條件不同,導(dǎo)致各研究結(jié)果差異較大,難以進(jìn)行直接的對(duì)比分析.同時(shí),野外控制實(shí)驗(yàn)開展難度大的特點(diǎn),使得在相同降水條件下的氮磷徑流流失對(duì)比研究相較缺乏.

因此,本文擬通過對(duì)同一流域內(nèi)不同下墊面集水區(qū)的氮磷徑流流失過程進(jìn)行長期連續(xù)逐日定位監(jiān)測(cè),研究相同降水條件下不同集水區(qū)的氮磷流失過程與強(qiáng)度,揭示下墊面分布對(duì)集水區(qū)氮磷徑流流失過程與強(qiáng)度的影響,以期為川中丘陵區(qū)面源污染防控提供科學(xué)依據(jù).

1 材料與方法

1.1 研究區(qū)域概況

研究區(qū)域位于四川省綿陽市鹽亭縣大興回族鄉(xiāng),地處川中東北部(105°27￠E,31°16￠N),嘉陵江水系彌江支流上游,海拔高度為405～535m.該區(qū)域?qū)儆趤啛釒駶櫦撅L(fēng)區(qū),年均氣溫17.3℃,最高氣溫40.0℃,最低氣溫-5.1℃;多年平均降水量826mm,年內(nèi)降雨分布極不均勻,夏季(6～8月)降水量占全年降水比在60%以上,且多暴雨.區(qū)域內(nèi)的土壤為蓬萊鎮(zhèn)組石灰性紫色土,土層厚度在20～80cm之間,土壤質(zhì)地以中壤居多,土壤粒徑較粗,粉砂粒(粒徑大于0.002mm)占比76.68%,土壤透水性好,壤中流豐富[21].該地區(qū)以林地和耕地為主,主要農(nóng)作物有玉米、小麥、水稻等,林地以榿、柏人工混交林為主.

1.2 觀測(cè)試驗(yàn)設(shè)計(jì)

在研究區(qū)域內(nèi)選擇兩個(gè)下墊面情況存在顯著差異、分水嶺明顯、匯水方向明確的集水區(qū)作為研究對(duì)象,分別稱為復(fù)合集水區(qū)(CC)和農(nóng)田集水區(qū)(AC).復(fù)合集水區(qū)與農(nóng)田集水區(qū)分布在同一個(gè)小流域內(nèi),依據(jù)分水嶺及微地形劃分(DEM見圖1),邊界相鄰.兩個(gè)集水區(qū)為川中丘陵區(qū)的一個(gè)縮影,其土地利用模式與農(nóng)業(yè)結(jié)構(gòu)具有代表性,土地利用狀況如圖1所示.

圖1 不同集水區(qū)DEM及土地利用分布

復(fù)合集水區(qū)總面積16.69hm2,林地?坡耕地和居民點(diǎn)所占的比例分別為60.4%、26.1%和10.9%.農(nóng)田集水區(qū)總面積10.42hm2,坡耕地、林地、水田所占的比例分別為43.2%、26.6%和27.2%.水塘位于農(nóng)田集水區(qū)低洼處,蓄存部分林地徑流用于水田灌溉.

表1 不同集水區(qū)土地利用情況

1.2.1 集水區(qū)流量監(jiān)測(cè) 在兩個(gè)集水區(qū)出口(圖1)建設(shè)永久性流量觀測(cè)直角三角堰,利用薄壁直角三角堰法計(jì)算2019年1月1日～2020年12月31日各出口逐日流量(GB/T 21303-2017)[29].堰口水位采用電容式水位計(jì)(新西蘭Odyssey)自動(dòng)監(jiān)測(cè)記錄(頻率為10min/次,精度為1mm).

1.2.2 逐日氮磷濃度監(jiān)測(cè) 2019年1月1日～2020年12月31日,利用水樣采集器在兩個(gè)集水區(qū)堰口分別采集混合水樣(至少3點(diǎn)混合),用經(jīng)過稀硫酸處理并以蒸餾水洗凈的聚乙烯瓶收集250mL混合水樣,分析各種形態(tài)氮磷濃度.采樣頻率為1次/d,采樣時(shí)間為上午08:00.

1.2.3 次降雨過程中氮磷流失監(jiān)測(cè) 當(dāng)降雨產(chǎn)流發(fā)生后,利用人工采樣的方法,每5min用經(jīng)過稀硫酸處理并以蒸餾水洗凈的聚乙烯瓶采集1次水樣;當(dāng)降雨強(qiáng)度明顯減小后,每10min采集1次水樣;降雨停止后,每30min采集1次水樣,持續(xù)2h.兩堰口處水樣采樣時(shí)間始終保持同步.

所采水樣立即送入實(shí)驗(yàn)室于4℃冰箱冷藏,并于48h完成分析;若未能立即分析,則加入1～2滴濃硫酸后,置于-20℃冰柜冷凍保存,并于1周內(nèi)分析完畢.

1.3 樣品分析

水樣送入實(shí)驗(yàn)室后,分別取10mL原液于25mL比色管中,采用堿性過硫酸鉀消解分光光度法(HJ 636-2012)[30]測(cè)定TN濃度;采用鉬酸銨分光光度法(GB 11893-89)[31]測(cè)定TP濃度.另取經(jīng)0.45μm濾膜過濾后的水樣于10mL自動(dòng)進(jìn)樣管中,采用Seal AA3+流動(dòng)分析儀分別測(cè)定銨態(tài)氮(NH4+-N)、硝態(tài)氮(NO3--N)、可溶性總氮(TDN)、磷酸鹽(PO43--P)、可溶態(tài)總磷(TDP)濃度.

1.4 數(shù)據(jù)分析與統(tǒng)計(jì)

采用事件平均濃度(EMC)衡量次降雨氮磷流失過程,計(jì)算公式如下:

式中:q與q1為兩相鄰時(shí)刻與+1時(shí)的徑流量,m3/s;c與c+1,對(duì)應(yīng)與+1時(shí)刻污染物的濃度,mg/L; Δ表示兩相鄰時(shí)刻的時(shí)間差.

次降雨和年內(nèi)氮磷流失量(, mg/L),計(jì)算式見式(2),式中參數(shù)意義與式(1)相同.

數(shù)據(jù)統(tǒng)計(jì)分析及繪圖利用Origin 8.0和SPSS 22軟件完成.利用單因素方差分析檢驗(yàn)顯著性(=0.05),相關(guān)性的判斷選用皮爾遜相關(guān)系數(shù).

2 結(jié)果與分析

2.1 降雨徑流特征

2.1.1 年降雨徑流特征 2019～2020年降雨事件特征見表2. 2019和2020年降水年內(nèi)極不均勻,每年降雨主要集中在6～9月,分別占當(dāng)年降水量的67.7%,和81.7%.

表2 2019～2020年降雨特征統(tǒng)計(jì)表

2019～2020年兩集水區(qū)降雨徑流過程如圖2所示,日降雨-徑流過程存在明顯響應(yīng)關(guān)系,并表現(xiàn)出夏秋季徑流量大而春冬季少的季節(jié)性特征.

復(fù)合集水區(qū)與農(nóng)田集水區(qū)年均徑流總量分別約為18.9×104和8.5×104m3,年均徑流深分別為81.5和113.3mm,徑流系數(shù)分別為0.16、0.12.復(fù)合集水區(qū)逐日徑流量過程高于農(nóng)田集水區(qū),且復(fù)合集水區(qū)徑流量峰值更高.這可能與復(fù)合集水區(qū)內(nèi)居民點(diǎn)、公路等不透水下墊面分布較多有關(guān),導(dǎo)致匯水時(shí)間縮短,入滲下降,徑流量增加[25].

圖2 2019～2020年兩集水區(qū)降雨-徑流過程

2.1.2 次降雨徑流特征 選取2018年7月30日、2018年8月21日、2019年8月6日共3次產(chǎn)流事件研究兩集水區(qū)次降雨徑流過程,各事件命名分別為0730次、0821次、0806次,3次降雨事件的基本情況見表3.

兩集水區(qū)出口處次降雨徑流過程如圖3所示,復(fù)合集水區(qū)徑流過程呈現(xiàn)陡升陡降態(tài)勢(shì),徑流響應(yīng)更迅速,峰值出現(xiàn)時(shí)刻和最大雨強(qiáng)時(shí)段同步,洪水持續(xù)時(shí)間較短,一般在1h左右.復(fù)合集水區(qū)出口處流量峰值與降雨量、降雨強(qiáng)度和土壤含水量密切相關(guān), 3場(chǎng)降雨中0730次降雨量和最大雨強(qiáng)最大,其徑流峰值也最高,為1.13m3/s; 0821次降雨的最大雨強(qiáng)約為0806次降雨的1/2,徑流峰值為0.48m3/s; 0730次降雨總量和最大雨強(qiáng)最小,且降雨前有4d無雨期,土壤含水率較低,因而徑流峰值最小,為0.10m3/s.

表3 降雨事件基本情況

與復(fù)合集水區(qū)相比,農(nóng)田集水區(qū)的3次降雨徑流過程呈緩升緩降的特點(diǎn),對(duì)強(qiáng)降雨的反應(yīng)較為遲鈍,其徑流峰形較寬,洪水持續(xù)時(shí)間較長,均大于3h,徑流峰值出現(xiàn)時(shí)間也比復(fù)合集水區(qū)晚12～25min.相同降雨下,農(nóng)田集水區(qū)的3次徑流峰值分別為0.0096, 0.010, 0.015m3/s,僅為復(fù)合集水區(qū)的1%～10%,且徑流峰值與降雨強(qiáng)度和降雨量之間均無顯著正相關(guān)性(>0.05).此外,兩集水區(qū)的退水過程均呈現(xiàn)先快后慢的趨勢(shì),但農(nóng)田集水區(qū)退水時(shí)間長于復(fù)合集水區(qū).這可能與復(fù)合集水區(qū)不透水下墊面比例顯著高于農(nóng)田集水區(qū)有關(guān).

圖3 3次典型降雨-徑流過程

2.2 典型次降雨過程中氮磷流失過程及特征

以0806次降雨事件為例,繪制氮磷流失過程,如圖4,兩集水區(qū)內(nèi)各形態(tài)氮的流失過程均呈現(xiàn)出上升趨勢(shì),其中NO3--N為主要的流失形態(tài),占TN濃度的83.08%.在產(chǎn)流前30min (08:15～08:45),農(nóng)田集水區(qū)和復(fù)合集水區(qū)的TN濃度在較低水平穩(wěn)定波動(dòng),分別為(5.90±0.82) mg/L和(5.86±0.57) mg/L;在降雨產(chǎn)流后期,農(nóng)田集水區(qū)氮素流失升至較高水平后回落,TN濃度明顯高于產(chǎn)流前期,為(8.26±1.68) mg/L,而復(fù)合集水區(qū)同時(shí)段的TN流失量依然保持在較高濃度,為(8.39±2.71) mg/L,其流失峰值也高于農(nóng)田集水區(qū).

兩集水區(qū)徑流中磷流失過程和特征也較為相似.其中,TP對(duì)降雨過程流失響應(yīng),較TN而言更加迅速,且有明顯的上升和下降過程,但TDP始終保持在較低水平,表明兩集水區(qū)的磷流失以顆粒態(tài)為主,其占比達(dá)90%以上.農(nóng)田集水區(qū)的TP在降雨產(chǎn)流后的15min達(dá)到峰值0.83mg/L;復(fù)合集水區(qū)的磷流失濃度在10min即達(dá)到峰值,為1.22mg/L,呈現(xiàn)出響應(yīng)更快,峰值更高的特點(diǎn).

圖4 0806次降雨氮磷流失過程

3次降雨事件中的氮磷流失EMC濃度和流失量見表4.兩集水區(qū)在3次降雨下的氮流失均以NO3--N為主,占TN的(65.9±16.3)%;磷流失均以顆粒態(tài)為主,占流失總量的(67.5±19.9)%,且復(fù)合集水區(qū)各形態(tài)氮磷流失平均濃度和流失量顯著大于農(nóng)田集水區(qū)(<0.05,成對(duì)樣本檢驗(yàn)).3次不同降雨條件下兩集水區(qū)內(nèi)各形態(tài)氮磷的EMC和流失量均存在顯著差異(<0.05),表明氮磷流失過程與流失量受降雨影響顯著.

表4 3次典型降雨事件氮磷流失EMC濃度及流失量

最大雨強(qiáng),平均雨強(qiáng)和NO3--N、TDN、TN的EMC之間均呈顯著正相關(guān)性(<0.05),說明降雨強(qiáng)度越大,氮流失強(qiáng)度越大;降雨歷時(shí)與各形態(tài)氮磷EMC均呈負(fù)相關(guān)性,與各形態(tài)氮磷流失量呈正相關(guān)性,且降雨量也與TN、TP的流失量呈正相關(guān)性,表明EMC隨降雨歷時(shí)增大而減小;氮磷流失量隨降雨量和降雨歷時(shí)增大而增大.

2.3 氮磷流失負(fù)荷與強(qiáng)度

2019～2020年不同集水區(qū)氮磷流失強(qiáng)度變化曲線見圖5,氮磷流失強(qiáng)度變化呈現(xiàn)季節(jié)性波動(dòng),豐水期(5～9月)氮磷流失量較高.其中,農(nóng)田集水區(qū)在豐水期氮磷流失量分別占全年流失量的91.87%和92.10%,而復(fù)合集水區(qū)的占比略低,分別為89.84%和81.23%.復(fù)合集水區(qū)日氮磷流失強(qiáng)度均普遍高于農(nóng)田集水區(qū)(<0.05). Pearson相關(guān)分析可知,兩集水區(qū)氮磷流失強(qiáng)度過程均與徑流過程存在極顯著性相關(guān)(<0.01),表明降雨徑流過程是集水區(qū)氮磷流失的重要驅(qū)動(dòng)力.

圖5 集水區(qū)氮磷流失強(qiáng)度動(dòng)態(tài)變化

表5 兩集水區(qū)氮磷流失負(fù)荷與強(qiáng)度對(duì)比

不同集水區(qū)氮磷流失狀況見表5,復(fù)合集水區(qū)的年均氮磷流失負(fù)荷遠(yuǎn)高于農(nóng)田集水區(qū),2019和2020年復(fù)合集水區(qū)氮流失負(fù)荷是農(nóng)田集水區(qū)的3.29倍和2.73倍,氮流失強(qiáng)度是農(nóng)田集水區(qū)的2.06倍和1.70倍;復(fù)合集水區(qū)磷負(fù)荷是農(nóng)田集水區(qū)的3.86倍和4.19倍,磷流失強(qiáng)度是農(nóng)田集水區(qū)的2.41倍和2.61倍.逐月比較相同集水區(qū)不同年份之間的氮磷流失負(fù)荷與流失強(qiáng)度,發(fā)現(xiàn)不同年份之間無顯著性差異(>0.05),說明年際間的降雨氣候差異并不會(huì)顯著改變集水區(qū)年內(nèi)的流失負(fù)荷與強(qiáng)度.

3 討論

3.1 不同下墊面集水區(qū)氮磷流失過程差異分析

地形地貌和土地利用情況的不同是影響集水區(qū)水文過程差異的重要因素[27].相同條件下農(nóng)田坡度越大,降雨沖刷越劇烈,養(yǎng)分流失強(qiáng)度也越劇烈[32-33].復(fù)合集水區(qū)內(nèi)的林地占比大,其最高點(diǎn)至集水區(qū)出口斷面的比降為0.20,高于農(nóng)田集水區(qū)的0.17,地形更陡,匯流過程中受重力勢(shì)能影響更大,匯流時(shí)間更短[34];而農(nóng)田集水區(qū)的整體地形較復(fù)合集水區(qū)平緩,耕地坡度約6°,土壤厚度較林地更大,蓄水能力更強(qiáng),其壤中流補(bǔ)給或是徑流消退較慢的重要原因[35].此外,復(fù)合集水區(qū)的不透水下墊面,如道路和居民點(diǎn)等,會(huì)加快產(chǎn)匯流過程,而農(nóng)田集水區(qū)內(nèi)的坑塘?水田反而會(huì)攔蓄徑流,削減徑流量峰值,延長退水時(shí)間[36].

已有研究表明,養(yǎng)分輸出濃度會(huì)隨著農(nóng)田和城鎮(zhèn)面積占比的增加而增加[37],而不同土地利用類型對(duì)養(yǎng)分輸出貢獻(xiàn)也存在著差異,其中農(nóng)業(yè)地區(qū)對(duì)氮的徑流負(fù)荷貢獻(xiàn)更多,城市流域?qū)α纵敵鲐?fù)荷的貢獻(xiàn)更多[38-39].朱波等[24]在石盤丘小流域的研究發(fā)現(xiàn)居民點(diǎn)和坡耕地對(duì)氮流失負(fù)荷的貢獻(xiàn)分別為38%和15%,對(duì)磷流失負(fù)荷貢獻(xiàn)分別為25%和18%.在本研究中,復(fù)合集水區(qū)氮磷流失負(fù)荷及強(qiáng)度均高于農(nóng)田集水區(qū),且磷負(fù)荷較氮負(fù)荷相對(duì)增量更大.這可能與復(fù)合集水區(qū)內(nèi)居民點(diǎn)生活污水未被集中處理而直接排入溝渠有關(guān).據(jù)調(diào)查,川中丘陵區(qū)居民點(diǎn)分散生活污水中TN、TP濃度分別高達(dá)32.52和3.03mg/L[40],同時(shí),由于居民點(diǎn)不透水下墊面較多,會(huì)加快產(chǎn)匯流過程,進(jìn)一步增大了氮磷流失強(qiáng)度.此外,本研究農(nóng)田集水區(qū)中分布有坑塘和連片水田,不僅能攔蓄徑流,削減洪峰,而且能消納上部來水中的部分營養(yǎng)元素,從而減少農(nóng)田集水區(qū)的氮磷流失.

3.2 與其他小流域氮磷流失強(qiáng)度的對(duì)比

通過文獻(xiàn)調(diào)研,獲得長江流域不同小流域或集水區(qū)的氮磷徑流流失強(qiáng)度,結(jié)果見表6,復(fù)合集水區(qū)氮磷流失強(qiáng)度差異較大.本研究復(fù)合集水區(qū)氮素流失強(qiáng)度最高,其次為全城塢村復(fù)合集水區(qū).這可能與兩個(gè)集水區(qū)都有較高的農(nóng)村生活輸出有關(guān)[40-41].但是,瀲水河[42]和石盤丘[24,43]復(fù)合型集水區(qū)盡管也有農(nóng)村生活輸出,其氮流失強(qiáng)度卻明顯低于本研究.這可能與流域內(nèi)土地利用類型的空間分布有關(guān).以石盤丘復(fù)合集水區(qū)為例,該集水區(qū)沿高程自上而下依次分布果園、坡耕地、水田,雖然集水區(qū)內(nèi)旱坡地、果園氮素流失強(qiáng)度較高,但其下階梯狀連片水田能有效攔截來自坡面上端的養(yǎng)分[24,38],導(dǎo)致氮素流失強(qiáng)度較本研究低.

不同農(nóng)田集水區(qū)之間氮磷流失強(qiáng)度也差異明顯.以農(nóng)業(yè)為主的新政[11]小流域氮流失強(qiáng)度較本文研究的農(nóng)田集水區(qū)略低,但磷流失強(qiáng)度是本研究的4.55倍.這可能與下墊面狀況及農(nóng)業(yè)活動(dòng)差異相關(guān),本文的農(nóng)田集水區(qū)以坡耕地為主,占比43.2%,而新政小流域以經(jīng)濟(jì)果林為主,占比55.6%[11],而與傳統(tǒng)糧食作物相比果林施肥量更大,表施的施肥方式使得以顆粒態(tài)流失為主的磷,受降雨沖刷作用更劇烈[10],流失強(qiáng)度更大.史書等[44]按農(nóng)田空間格局的整體性強(qiáng)弱將王家溝小流域劃分為A和B兩處農(nóng)田集水區(qū).整體性更好的B集水區(qū)的氮磷流失強(qiáng)度與本研究相比更低,其水稻田景觀破碎指數(shù)更低,對(duì)徑流中養(yǎng)分的攔截效果更好[45],說明良好的空間格局分布能有效地發(fā)揮水田作為面源污染“匯”的優(yōu)勢(shì).

表6 不同小流域氮磷流失強(qiáng)度對(duì)比

注: a為本文研究結(jié)果.

4 結(jié)論

4.1 不同集水區(qū)內(nèi)的降雨-徑流過程存在明顯差異.復(fù)合集水區(qū)次降雨過程中的徑流響應(yīng)時(shí)間較農(nóng)田集水區(qū)快12～25min,徑流量峰值更高,年徑流深多出28.1%,這與兩集水區(qū)內(nèi)地形地貌及土地利用情況的差異密切相關(guān).

4.2 不同集水區(qū)內(nèi)氮磷流失濃度特征存在顯著差異,復(fù)合集水區(qū)的氮磷流失EMC濃度?濃度峰值?流失量均高于農(nóng)田集水區(qū).徑流過程是氮磷流失的內(nèi)在驅(qū)動(dòng)力,氮流失以NO3--N為主,占TN的65.9%;磷流失以顆粒態(tài)為主占TP流失量的67.5%.

4.3 復(fù)合集水區(qū)氮磷流失負(fù)荷及強(qiáng)度遠(yuǎn)高于農(nóng)田集水區(qū),其年均氮磷流失負(fù)荷是農(nóng)田集水區(qū)的3.01和4.03倍,氮磷流失強(qiáng)度是農(nóng)田集水區(qū)的1.88和2.51倍.

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Patterns of nitrogen and phosphorus losses in two catchments with contrasting underlying surfaces.

XU Guang-zhi1,2, SHAO Zhi-jiang1,2, WANG Tao1*, HUANG Mei-yu1,2, LI Ming-ming1,2

(1.Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China；2.University of Chinese Academy of Sciences, Beijing 100049, China)., 2022,42(7)：3334～3342

To investigate the effects of different underlying surfaces on nitrogen (N) and phosphorus (P) losses via surface flow at a catchment scale, continuous daily monitoring of the surface flow and the concentrations of N and P was conducted in an agricultural catchment (AC) and a compound catchment (CC) in a hilly area of central Sichuan from 2019 to 2020. Results demonstrated a significant difference in the runoff process between the two catchments. The peak value of the surface flow was larger, the response speed was12 to 25min faster, and the annual runoff depth was 28.1% greater in CC than in AC. These results highlighted the effects of the contrasting underlying surfaces on the speed of runoff process, with the process being slowed down by the paddy fields and ponds in AC but speeded up by the impervious surfaces of the residential areas and roads in CC. During each rainfall event, the P concentration in runoff dropped more drastically, particularly in the late stage of the event, and the timing of peak P concentration was about 1.2hours earlier than those of N, respectively. Both the event mean concentration (EMC) and peak concentration of P or N were higher in CC than in AC. Nitrate-N was the main form of N losses in both catchments, accounting for 65.9% of the total N loss; while particulate P was the primary P form in the runoff, contributing to 67.5% of the total P loss. The N and P loads in CC were 3.01 and 4.03 times larger than those in AC, respectively, and the loss intensity of N and P in CC were 1.88 and 2.51times greater than those in AC, respectively. Therefore, the control of N and P losses from the compound catchments could be critical for the non-point source pollution control in hilly areas of central Sichuan in the future.

nitrogen；phosphorus；loss intensity；underlying surface；catchment

X52

A

1000-6923(2022)07-3334-09

徐光志(1997-),男,湖北武漢人,中國科學(xué)院大學(xué)碩士研究生,主要從事農(nóng)業(yè)面源污染與生態(tài)控制研究.發(fā)表論文1篇.

2021-12-02

四川省科技計(jì)劃資助項(xiàng)目(2021YFN0131,2022YFS0500)

* 責(zé)任作者, 副研究員, wangt@imde.ac.cn