施肥對(duì)麥田土壤可溶性有機(jī)氮的影響

梁　斌，李俊良，楊學(xué)云，周建斌,3, *

1 青島農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院，青島　266109　　2 西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院，楊凌　712100　　3 農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室，楊凌　712100

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施肥對(duì)麥田土壤可溶性有機(jī)氮的影響

梁斌1,2，李俊良1，楊學(xué)云2，周建斌2,3, *

1 青島農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院，青島2661092 西北農(nóng)林科技大學(xué)資源環(huán)境學(xué)院，楊凌7121003 農(nóng)業(yè)部西北植物營(yíng)養(yǎng)與農(nóng)業(yè)環(huán)境重點(diǎn)實(shí)驗(yàn)室，楊凌712100

利用長(zhǎng)期定位試驗(yàn)，研究施肥和小麥生長(zhǎng)對(duì)土壤可溶性有機(jī)氮(EON)的影響。長(zhǎng)期不同施肥土壤包括不施肥(No-F)、施用化肥(NPK)和有機(jī)肥與化肥配施(MNPK)3種。EON含量范圍為7.5—29.3 kg/hm2，No-F、NPK和MNPK土壤中EON分別占可溶性總氮的40%、56%和56%。長(zhǎng)期有機(jī)肥與化肥配施顯著提高0—15 cm土層EON含量，但對(duì)30 cm以下土層EON含量無影響。在小麥開花期，可溶性有機(jī)氮的含量及其相對(duì)含量顯著高于拔節(jié)期和收獲期。雖然施用氮肥對(duì)當(dāng)季EON含量無顯著影響，但同位素示蹤微區(qū)試驗(yàn)表明，土壤耕層(0—15 cm)中仍有0.4%—2.8%的可溶性有機(jī)氮來源于當(dāng)季施入的肥料氮?？梢姡瘜W(xué)氮肥向可溶性有機(jī)氮的轉(zhuǎn)化緩慢，但農(nóng)田土壤中可溶性有機(jī)氮含量與礦質(zhì)態(tài)氮含量相當(dāng)，發(fā)生淋溶損失的風(fēng)險(xiǎn)大。

長(zhǎng)期定位施肥試驗(yàn)；小麥生長(zhǎng)期；淋溶；15N標(biāo)記

土壤可溶性有機(jī)氮(Extractable organic N, EON)雖僅占土壤全氮的很小部分，但近年來的研究表明，它是土壤氮庫中最活躍的組分之一，對(duì)土壤氮素循環(huán)影響很大[1]。在林地土壤中可溶性有機(jī)氮占可溶性總氮的比例可高達(dá)90%以上[2]，土壤EON與土壤氮素遷移和供應(yīng)的關(guān)系不可忽視[3]?？扇苄杂袡C(jī)氮含量與土壤氮素礦化和土壤微生物量氮顯著相關(guān)[4]，研究表明土壤不溶性有機(jī)氮向EON的轉(zhuǎn)化是土壤中有機(jī)氮礦化的限制因子[3]?？扇苄杂袡C(jī)氮除了是土壤微生物氮素的重要來源之外[5]，一些低分子量的EON可以直接或者通過菌根被植物吸收利用[3]。在一些降雨量大或灌溉地區(qū)，可溶性有機(jī)氮的淋溶損失是氮素?fù)p失的重要途徑之一[6-7]，在林地生態(tài)系統(tǒng)中EON是氮素?fù)p失的主要形態(tài)[8]。綜上說明EON在土壤氮素組成、轉(zhuǎn)化、供應(yīng)和損失方面都具有重要的意義。

土壤可溶性有機(jī)氮含量及其行為易受土地利用方式、施肥狀況和種植作物等因素影響。目前對(duì)農(nóng)田土壤可溶性有機(jī)氮含量的影響因素研究相對(duì)較少，且得出的一些結(jié)果不盡一致。比如，Currie等[9]研究表明，施用化學(xué)氮肥提高土壤中EON含量，McDowell等[10]也得出類似的結(jié)論。但Vestgarden等[11]卻發(fā)現(xiàn)，連續(xù)九年施用化學(xué)氮肥(每年30 kg/hm2)使土壤溶解性有機(jī)氮含量顯著降低；Gundersen等[12]報(bào)告指出，施用氮肥并不影響溶解性有機(jī)氮的含量。因此，有必要進(jìn)一步研究施肥對(duì)農(nóng)田土壤中可溶性有機(jī)氮的影響。在林地中，可溶性有機(jī)氮是氮素?fù)p失的主要形態(tài)[8]，那么在農(nóng)田中可溶性有機(jī)氮的淋溶情況也是值得關(guān)注的問題。因?yàn)橛袡C(jī)氮的淋溶不但關(guān)系到氮肥的利用狀況，還可能帶來一系列生態(tài)環(huán)境問題。

本研究利用已經(jīng)進(jìn)行了19a的田間試驗(yàn)，研究了長(zhǎng)期不同施肥處理對(duì)麥田土壤耕層及0—100 cm剖面可溶性有機(jī)氮的影響以及短期內(nèi)氮肥向可溶性有機(jī)氮轉(zhuǎn)化情況，揭示施肥對(duì)土壤有機(jī)氮含量及其淋溶特性的影響，同時(shí)闡明了小麥不同生長(zhǎng)階段對(duì)土壤表層EON含量的影響，以期為完善土壤氮素循環(huán)理論、有效調(diào)控土壤氮素供應(yīng)提供依據(jù)。

1　材料與方法

1.1試驗(yàn)設(shè)計(jì)

長(zhǎng)期定位試驗(yàn)開始于1990年，種植制度為小麥單作，小麥?zhǔn)斋@后休閑至下季小麥種植。設(shè)對(duì)照(No-F，不施肥)、施用NPK化肥(NPK)、有機(jī)肥配施NPK化肥(MNPK)3個(gè)處理。其中氮肥為尿素，磷肥為過磷酸鈣，鉀肥為氯化鉀。NPK處理施用量分別為N 135.0 kg/hm2、P 47.1 kg/hm2、K 56 kg/hm2。MNPK處理中過磷酸鈣和氯化鉀的施用量與NPK處理相同，氮肥用量與NPK處理相同，其中70%的氮來源于牛廄肥，30%的氮由尿素提供，各施肥處理的肥料均于小麥播種前一次性施用。小區(qū)面積為 399 m2(21 m×19 m)。土壤經(jīng)19年不同施肥處理后其0—20 cm土層基本理化性狀見表1。土壤質(zhì)地為重壤土，土壤顆粒<0.002 mm、0.002—0.02 mm和 >0.02 mm的粘粒、粉粒和砂粒含量分別為168、516、316 g/kg。

2009年10月小麥種植施肥前，在每處理土壤內(nèi)用PVC管設(shè)置氮同位素示蹤微區(qū)試驗(yàn)，PVC管長(zhǎng)63 cm，直徑為24.5 cm，其中60 cm打入土中，3 cm留在地表之上。微區(qū)設(shè)施氮肥(+N)和不施氮肥(CK)兩處理，重復(fù)3次，其中施入的氮肥為15N標(biāo)記的尿素(豐度為19.58%)。在小麥種植前將所有處理土柱內(nèi)0—15 cm土層土壤取出施入微區(qū)以外相同的磷、鉀肥，在+N處理中按165 kg N/hm2的量加入標(biāo)記尿素，CK處理不加氮肥，施肥后回填到原來PVC管中。于2009年10月18日播種，播種量為每PVC管30粒，小麥出苗后間苗至20株。

表1　長(zhǎng)期不同施肥處理0—20 cm土層理化性狀

以上數(shù)據(jù)為平均值(標(biāo)準(zhǔn)差)(n=3);同一行不同小寫字母表示差異達(dá)顯著水平

1.2土壤樣品的采集

于小麥拔節(jié)期(2010年3月26日)、開花期(2010年5月4日)和收獲期(2010年6月15日)在微區(qū)試驗(yàn)處理內(nèi)采集土壤樣品(0—15 cm)，利用土鉆在每土柱內(nèi)采集混合樣，過2 mm篩，測(cè)定其中可溶性總氮、礦質(zhì)氮含量及其15N豐度。于小麥?zhǔn)斋@期在各長(zhǎng)期定位試驗(yàn)處理中按10 cm一層采集0—100 cm土壤剖面樣品，測(cè)定土壤剖面可溶性有機(jī)氮和礦質(zhì)氮含量。

圖1　小麥不同生長(zhǎng)階段土壤中(0—15 cm)來源于當(dāng)季氮肥的可溶性有機(jī)氮百分比Fig.1　Percent of soil extractable organic N derived from 15N-labeled fertilizer in soils (0—15 cm) under long-term different fertilization managements during stem elongation (ET), flowering (FT), and harvest (HT) stage of wheat

1.3樣品測(cè)定與數(shù)據(jù)分析

土壤樣品采集過篩后，用0.5 mol/L硫酸鉀浸提(土水比1∶4)，浸提液中可溶性總氮用過硫酸鉀氧化—紫外分光光度計(jì)比色法測(cè)定，礦質(zhì)氮利用流動(dòng)分析儀測(cè)定，可溶性有機(jī)氮含量為可溶性總氮和礦質(zhì)氮含量之差，可溶性有機(jī)氮相對(duì)含量是指可溶性有機(jī)氮占可溶性總氮含量的百分比。微區(qū)試驗(yàn)處理中一部分土壤浸提液經(jīng)過硫酸鉀氧化后擴(kuò)散[13]，測(cè)定其中可溶性總氮的15N豐度，另一部分浸提液直接擴(kuò)散，測(cè)定其中礦質(zhì)態(tài)氮的15N豐度。擴(kuò)散后的15N豐度用質(zhì)譜儀測(cè)定，樣品15N豐度測(cè)定由美國(guó)加利福尼亞大學(xué)戴維斯分校穩(wěn)定同位素研究所完成。可溶性有機(jī)氮中的15N含量為可溶性總氮和礦質(zhì)態(tài)氮中15N含量之差，來源于肥料的可溶性有機(jī)氮百分比用Ndff(%)表示，施入肥料向可溶性有機(jī)氮轉(zhuǎn)化率用Con(%)表示，計(jì)算公式如下：

Ndff(%)=可溶性有機(jī)氮15N原子百分超/肥料15N豐度

(1)

Con(%)=可溶性有機(jī)氮含量×Ndff/施氮量

(2)

圖表中的數(shù)據(jù)，用SAS Version 8.1 for Windows 作方差分析，若差異顯著，采用LSD 法進(jìn)行多重比較。

2　結(jié)果與分析

2.1施肥對(duì)土壤可溶性有機(jī)氮含量的影響

土壤可溶性有機(jī)氮的含量為7.5—29.3 kg/hm2(表2)，占全氮的比例為0.6%—0.8%，其中來源于當(dāng)季施入肥料氮的比例為0.5%—2.8%(圖1)，僅占施入氮肥的0.03%—0.24%。長(zhǎng)期不同施肥顯著影響EON含量(P< 0.01)(表3)。與No-F相比，長(zhǎng)期施用化肥使EON顯著增加34%—89%，平均增幅為55%(表2)。長(zhǎng)期有機(jī)無機(jī)配施土壤可溶性有機(jī)氮含量范圍為12.2—25.6 kg/hm2，平均為19.0 kg/hm2，顯著高于NPK處理(表2)。MNPK和NPK土壤EON相對(duì)含量分別為40%—81%和39%—81%，平均皆為56%(圖2)。No-F土壤可溶性有機(jī)氮的相對(duì)含量為40%，顯著低于NPK和MNPK土壤(圖2)。

表2土壤耕層(0—15 cm)可溶性有機(jī)氮含量(kg/hm2)

Table 2Soil extractable organic N content in 0—15 cm layer of soils under long-term different fertilization managements at different growth stage of wheat

施氮處理Ntreatments采樣時(shí)Samplingtimes不施肥Nofertilization施用化肥ApplicationofinorganicNPKfertilizer有機(jī)肥配施化肥CombinedapplicationofinorganicNPKandmanure不施氮拔節(jié)期7.5(1.7)Ac12.4(1.7)Bb14.0(1.9)CaNoapplicationofN開花期9.2(1.1)Ac17.4(1.7)Ab25.6(1.1)Aa收獲期9.2(0.6)Ac12.4(1.3)Bb16.1(0.1)Ba平均8.6c14.0b18.5a施氮拔節(jié)期8.4(0.7)Ab12.8(0.9)Ba12.2(2.4)BaApplicationofN開花期10.6(3.5)Ac15.3(1.3)Ab29.3(4.5)Aa收獲期10.5(0.7)Ac15.1(0.2)Ab17.0(1.7)Ba平均9.9c14.4b19.5a

表中數(shù)據(jù)為平均值(標(biāo)準(zhǔn)差)(n=3); 同一行不同小寫字母和同一列不同大寫字母表示差異在0.05水平顯著

表3　不同施肥和生長(zhǎng)時(shí)期對(duì)土壤可溶性有機(jī)氮影響的F檢驗(yàn)

2.2小麥不同生長(zhǎng)階段可溶性有機(jī)氮含量

3個(gè)采樣時(shí)期中，開花期EON含量最高，此時(shí)期NPK和MNPK土壤EON含量較拔節(jié)期分別提高48%和82%。小麥生長(zhǎng)對(duì)No-F土壤EON含量無顯著影響(表2)。生長(zhǎng)時(shí)期顯著影響EON相對(duì)含量。小麥開花期可溶性有機(jī)氮相對(duì)含量最高，各處理范圍為38%—81%，平均為64%；小麥?zhǔn)斋@期可溶性有機(jī)氮相對(duì)含量平均為50%；拔節(jié)期可溶性有機(jī)氮的相對(duì)含量最低，各處理平均為39%(圖2)。

圖2　長(zhǎng)期不同施肥和小麥不同生長(zhǎng)時(shí)期土壤耕層(0—15 cm)可溶性有機(jī)氮相對(duì)含量Fig.2　Effect of long-term fertilization and growth stage of wheat on percent of soil extractable organic N in soil extractable N

2.3可溶性有機(jī)氮在土壤0—100 cm剖面的分布

長(zhǎng)期不同施肥主要影響0—30 cm土層EON含量，對(duì)30 cm以下土層EON含量影響不大(圖3)。No-F、NPK和MNPK土壤CK處理0—100 cm土壤剖面EON累積量分別為43.1、51.6、55.2 kg/hm2(圖4)；MNPK和NPK土壤中EON累積量無顯著差異，兩者均顯著高于No-F土壤。在種植前未施氮肥處理不同土壤40—100 cm累積的可溶性有機(jī)氮分別占可溶性總氮的43%—50%。種植前施用氮肥使土壤EON增加14%—34%，No-F、NPK和MNPK土壤分別達(dá)到58.1、67.2、63.2 kg/hm2(圖4)。

圖3　長(zhǎng)期不同施肥土壤不施氮(a)和施氮(b)處理0—100 cm剖面可溶性有機(jī)氮含量Fig.3　Soil extractable organic N content in 0—100 cm layers of soils under long-term different fertilization managements

圖4　長(zhǎng)期不同施肥土壤可溶性有機(jī)氮在0—100 cm剖面的累積Fig.4　Accumulative soil extractable organic N in 0—100 cm layers of soils under long-term different fertilization managements

3　討論

3.1土壤可溶性有機(jī)氮的含量

在本試驗(yàn)中，長(zhǎng)期不同施肥土壤耕層EON含量為8—27 kg/hm2，No-F，NPK和MNPK 3種施肥處理的土壤EON占可溶性總氮的比例分別為13%—67%，38%—82%和44%—57%。Jensen等[14]研究表明，在沙土和沙壤土可溶性有機(jī)氮含量范圍分別為：8—20、15—30 kg/hm2。Mcneill等[15]的研究中可溶性有機(jī)氮占可溶性總氮的比例為55%—66%。可見，農(nóng)田土壤中可溶性有機(jī)氮含量與礦質(zhì)態(tài)氮含量相當(dāng)，是農(nóng)田土壤中一個(gè)重要的氮庫。

3.2長(zhǎng)期不同施肥對(duì)土壤可溶性有機(jī)氮的影響

與長(zhǎng)期施用化肥相比，長(zhǎng)期有機(jī)肥配施化肥顯著提高土壤EON含量以及可溶性有機(jī)氮占可溶性總氮的百分比，其他學(xué)者也得出相同的結(jié)論[26-27]。增加的可溶性有機(jī)氮一方面來源于施入的有機(jī)肥[28]，另一方面來源于增加的作物根系脫落物等殘?bào)w[18-19]。另外，長(zhǎng)期有機(jī)無機(jī)配施土壤中微生物量氮是施用化肥土壤的1.3倍，微生物量的增加也可提高EON含量[25]。

3.3短期施用氮肥對(duì)土壤可溶性有機(jī)氮的影響

施入土壤的氮素除被作物吸收、微生物固持和損失外，還有一部分可在生物和非生物因素下轉(zhuǎn)化為可溶性有機(jī)氮。在林地酸性土壤中，Dail等[29]研究指出，在對(duì)照、輻射滅菌和高溫滅菌土壤中分別大約有30%、40%和55%所加入的硝態(tài)氮轉(zhuǎn)化為土壤可溶性有機(jī)氮。Compton等[30]和Perakis等[31]也得出，硝態(tài)氮加入土壤之后，有很大一部分迅速地轉(zhuǎn)化為土壤可溶性有機(jī)氮。Davidson等[32]研究發(fā)現(xiàn)，硝態(tài)氮加入土壤之后，在鐵錳等化合物的作用下轉(zhuǎn)化為亞硝態(tài)氮，而亞硝態(tài)氮與土壤有機(jī)物結(jié)合轉(zhuǎn)化為可溶性有機(jī)氮。但其他學(xué)者[33-34]通過試驗(yàn)，并沒有發(fā)現(xiàn)大量硝態(tài)氮向可溶性有機(jī)氮的轉(zhuǎn)化。在本研究中，各小麥生長(zhǎng)時(shí)期各土壤中有0.5%—2.8%的可溶性有機(jī)氮來源于當(dāng)季施用的肥料氮，占當(dāng)季施入氮肥的0.5%以下。說明化學(xué)氮肥向可溶性有機(jī)氮的轉(zhuǎn)化比較緩慢，沒有發(fā)生快速大量轉(zhuǎn)化的情況。肥料氮在施肥當(dāng)季轉(zhuǎn)化為土壤可溶性有機(jī)氮的機(jī)理包括：(1)在肥料氮施入土壤之后，通過土壤微生物的固持與轉(zhuǎn)化，部分肥料氮以可溶性有機(jī)氮的形態(tài)釋放到土壤中[1]；(2)施入的肥料氮可通過作物吸收及其分泌分泌物轉(zhuǎn)化為土壤可溶性有機(jī)氮[35]。

3.4小麥生長(zhǎng)階段對(duì)土壤可溶性有機(jī)氮的影響

作物的生長(zhǎng)對(duì)土壤可溶性有機(jī)氮含量有顯著的影響[36]。本研究中，在小麥開花期土壤可溶性有機(jī)氮含量和占可溶性有機(jī)氮的比例皆為最高。這說明旺盛生長(zhǎng)的作物增加土壤可溶性有機(jī)氮的含量，其他學(xué)者也得出相同的結(jié)論[37-38]。這是因?yàn)樵谕⑸L(zhǎng)階段，作物根系、根系脫落物和土壤微生物量都較高所致。研究表明，在作物開花期根系脫落物的碳可達(dá)根系碳含量的兩倍[39]。一方面，根系的分泌物及脫落物本身含有大量的可溶性有機(jī)氮；另一方面，較多的有機(jī)碳為土壤微生物提供了大量的能源物質(zhì)，從而增加了土壤微生物的數(shù)量，而微生物數(shù)量與土壤可溶性有機(jī)氮含量呈顯著正相關(guān)關(guān)系[20,37]。

3.5可溶性有機(jī)氮在土壤剖面的分布

長(zhǎng)期有機(jī)肥配施化肥顯著提高0—15 cm土層可溶性有機(jī)氮含量，但對(duì)20 cm以下土壤可溶性有機(jī)氮含量無影響，MNPK土壤40—100 cm剖面中累積的EON與No-F和NPK土壤相當(dāng)。說明長(zhǎng)期有機(jī)肥配施化肥僅增加土壤耕層可溶性有機(jī)氮含量。但Dyke等[40]指出，與單施化肥相比，施用有機(jī)肥使更多的可溶性有機(jī)氮淋溶到土壤下層。結(jié)果不同的原因可能與Dyke等[40]的研究中有機(jī)肥的施用量更高(每年有機(jī)肥提供的氮量為240 kg/hm2)和試驗(yàn)進(jìn)行的時(shí)間更長(zhǎng)(156a)有關(guān)。在種植前未施氮肥處理不同土壤40—100 cm累積的可溶性有機(jī)氮分別占可溶性總氮的43%—50%。Madou等[41]研究表明，土壤中通過淋溶損失的氮素中土壤可溶性有機(jī)氮占17%—32%，說明可溶性有機(jī)氮的淋溶損失是氮素?fù)p失的重要途徑之一。Van Kessel等[7]也報(bào)道指出，可溶性有機(jī)氮是農(nóng)田土壤中氮素淋溶損失的重要形態(tài)，尤其是在降雨量氮或灌溉地區(qū)。因此，在評(píng)價(jià)農(nóng)田氮素淋溶損失時(shí)，應(yīng)該考慮可溶性有機(jī)氮的損失。

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Effect of fertilization on extractable organic nitrogen in wheat monoculture cropping systems

LIANG Bin1,2, LI Junliang1, YANG Xueyun2, ZHOU Jianbin2,3,*

1CollegeofResourcesandEnvironmentalSciences,QingdaoAgricultureUniversity,Qingdao266109,China2CollegeofResourcesandEnvironmentalSciences,NorthwestAgricultureandForestryUniversity,Yangling712100,China3KeyLaboratoryofPlantNutritionandtheAgri-EnvironmentinNorthwestChina,MinistryofAgriculture,Yangling712100,China

Soil extractable organic nitrogen (SON) is an important nutrient pool involved in N transformations, and the content and conversion of SON are affected by fertilization practices. However, many gaps remain in our understanding of SON, especially in agricultural soil. The effects of long-term (1990—2009) fertilization on SON at elongation, flowering, and harvest stages in wheat were evaluated in a loess soil (Eum-Orthic Anthrosol) in northwestern China. The treatments included no fertilization (No-F), application of inorganic NPK fertilizer (NPK), and combined application of inorganic NPK and manure (MNPK). Using15N tracer techniques,15N-labeled urea (165 kg N/hm2) was applied to microplots within each treatment to investigate the effect of short-term addition of N on content of SON during the wheat-growing season in wheat monoculture cropping systems. The SON content was 7.5—29.3 kg/hm2and accounted for 40%, 56%, and 56% of total extractable N in No-F, NPK, and MNPK, respectively. Compared with No-F, application of inorganic NPK fertilizer increased SON content significantly (55% on average) in the 0—15 cm soil layer. Soil extractable organic N content in the MNPK treatment was significantly higher (by 32%—35%) than that in the NPK treatment in the 0—15 cm layer. Long-term fertilization had no effect on SON content below 30 cm. SON was highest at flowering and was significantly higher during flowering than at the elongation stage in NPK and MNPK (by 48% and 82%, respectively). In relation to No-F, fertilization treatments increased the SON significantly in the 0—100 cm soil profile, SON was 43.1, 51.6, 55.2 kg/hm2in No-F, NPK, and MNPK, respectively. Addition of N had no significant effect on SON content in the 0—15 cm soil layer during the same growing season; however, 0.4%—2.8% of SON was derived from the15N-labeled fertilizer applied before seeding, representing 0.03%—0.24% of the fertilizer, and short-term addition of N increased SON in the 0—100 cm soil profile by 35%, 30%, and 14% in No-F, NPK, and MNPK, respectively. We conclude that the conversion of inorganic N to extractable organic N was slow. However, long-term fertilization increased SON content in the topsoil, and SON is a significant nitrogen pool in agriculture soils.

long-term fertilization; wheat growth stage; leaching;15N labeling

國(guó)家自然科學(xué)基金資助項(xiàng)目(31372137, 31401947)

2014-12-12; 網(wǎng)絡(luò)出版日期:2015-10-30

Corresponding author.E-mail: jbzhou@nwsuaf.edu.cn

10.5846/stxb201412122482

梁斌，李俊良，楊學(xué)云，周建斌.施肥對(duì)麥田土壤可溶性有機(jī)氮的影響.生態(tài)學(xué)報(bào),2016,36(14):4430-4437.

Liang B, Li J L, Yang X Y, Zhou J B.Effect of fertilization on extractable organic nitrogen in wheat monoculture cropping systems.Acta Ecologica Sinica,2016,36(14):4430-4437.