把余玲, 田霄鴻, 萬 丹, 李 錦, 王淑娟

(西北農(nóng)林科技大學資源環(huán)境學院，農(nóng)業(yè)部西北植物營養(yǎng)與農(nóng)業(yè)環(huán)境重點實驗室，陜西楊凌 712100)

玉米植株不同部位還田土壤活性碳、氮的動態(tài)變化

把余玲, 田霄鴻*, 萬 丹, 李 錦, 王淑娟

(西北農(nóng)林科技大學資源環(huán)境學院，農(nóng)業(yè)部西北植物營養(yǎng)與農(nóng)業(yè)環(huán)境重點實驗室，陜西楊凌 712100)

探討玉米植株不同部位腐解對還田土壤活性碳、 氮動態(tài)變化的影響。采用室內(nèi)培養(yǎng)方法，通過動態(tài)監(jiān)測土壤微生物量碳(SMBC)、微生物量氮(SMBN)、可溶性碳(DOC)和礦質(zhì)氮含量，研究等量玉米根茬、秸稈、莖及葉4個部位在連續(xù)7季還田(秸稈+根茬還田)和不還田土壤(僅根茬還田)中的腐解轉(zhuǎn)化特征。結(jié)果表明，秸稈腐解的最初7 d是土壤活性碳、 氮動態(tài)變化的高峰期；腐解期間(62 d)SMBC、SMBN含量表現(xiàn)為添加秸稈始終高于根茬，葉分別在前28 d、14 d內(nèi)高于莖，后期則低于莖，秸稈介于莖、葉之間；土壤DOC、礦質(zhì)氮含量為葉>秸稈>莖>根茬；培養(yǎng)結(jié)束時，各處理SMBC和礦質(zhì)氮含量均較起始(0 d)顯著提高，DOC含量基本保持不變，SMBN含量顯著下降。與不還田土壤相比，還田土壤對新鮮殘體的腐解影響不顯著，且兩者間土壤活性氮組分的差異較碳組分明顯。腐解期間土壤活性碳、 氮的動態(tài)變化主要取決于各器官碳、 氮等化學組分的差異性，等量秸稈較根茬更有利于補充土壤活性碳、氮數(shù)量，土壤活性氮組分對還田土壤的響應較碳組分靈敏。

玉米殘體； 微生物量碳； 微生物量氮； 可溶性有機碳； 礦質(zhì)氮

作物秸稈作為農(nóng)田生態(tài)系統(tǒng)中土壤有機物歸還的主要來源，已廣泛應用于農(nóng)業(yè)生產(chǎn)實踐[1-3]，其在土壤中的腐解主要取決于殘體來源和成分[4]。殘體來源因受人為利用和管理措施影響，其各部分歸還到土壤中的數(shù)量和比例有所不同[5-6]，尤其在機械化程度較高地區(qū)，作物收獲后根茬幾乎全部留在土壤中被腐解[7-8]。此外，同一作物各器官因生長條件不同，其成分間存在高度差異性[7]。不少研究表明，秸稈腐解不僅顯著提高土壤有機質(zhì)含量[3, 9-10]，而且提高包括土壤微生物量、可溶性碳等活性有機質(zhì)的含量[11-13]，進而影響土壤微生物對氮素的固持與釋放[1,14]。有研究指出，與作物秸稈相比，根茬對土壤結(jié)構(gòu)的改善及有機碳的貢獻作用更顯著，且對根際影響最大[15-18]。Puget等研究發(fā)現(xiàn)，雖然秸稈施入土壤后能迅速分解并為下茬作物提供氮源，但根茬可能更有利于短期土壤結(jié)構(gòu)的改善及長期土壤有機質(zhì)的累積[17]。因此，在農(nóng)業(yè)實踐中，如何協(xié)調(diào)秸稈和根茬腐解在土壤養(yǎng)分供應中所起的作用是值得關(guān)注的重要問題。

隨著農(nóng)業(yè)機械化的普及，關(guān)中平原小麥、玉米秸稈還田面積越來越大，即使秸稈不還田，仍有大量根茬留在土壤中被腐解。在這種情況下，多年秸稈還田與不還田土壤相比，對作物殘體腐解影響究竟能產(chǎn)生多大差異的報道尚不多見，且對同一作物不同部位(秸稈、根茬、莖、葉)各自的腐解特性，以及腐解過程中土壤活性碳、 氮等養(yǎng)分動態(tài)變化的研究很少。因此，本試驗采用室內(nèi)培養(yǎng)方法，初步研究玉米各部位殘體(根茬、秸稈、莖、葉)還田和不還田后，土壤微生物量碳、 氮、可溶性碳及礦質(zhì)氮的動態(tài)變化，旨在進一步探討作物各部位殘體腐解過程中養(yǎng)分供應與土壤肥力的關(guān)系，為合理還田與農(nóng)田養(yǎng)分科學管理提供依據(jù)。

1 材料與方法

1.1 供試材料

1.2 培養(yǎng)試驗

1.2.1 試驗設計 以上述2種土壤(還田土、不還田土)和4種玉米植株不同部位(根茬、秸稈、莖、葉)為研究因素，另設不加玉米殘體土壤作為對照，共組成10個處理，每個處理重復3次。

1.2.2 培養(yǎng)過程 稱土250 g(烘干土)裝入1 L塑料培養(yǎng)盆中，加蒸餾水至田間持水量(WHC)的60%，在20℃下預培養(yǎng)4 d，以恢復土壤微生物活性。然后，將2.5 g玉米根茬、秸稈、莖、葉殘體分別施入相應土壤中，并依各殘體C、N含量，加入適量尿素溶液調(diào)節(jié)C/N至25 ∶1(使土壤含水量調(diào)至70% WHC)，同時設不加殘體的2種土樣作為對照，充分混合均勻，置于培養(yǎng)箱中，25±1℃恒溫培養(yǎng)62 d，每隔5 d采用稱重法補充水分。在培養(yǎng)的第0(6 h后)、 3、 7、 14、 28、 42、 62 d分別從各培養(yǎng)盆中取樣，測定土壤微生物量碳、微生物量氮、可溶性碳和礦質(zhì)氮含量。

1.3 測定項目與方法

表1 土壤及玉米植株樣品的基本性質(zhì)Table 1 Basic properties of soil samples and maize residues

采用Microsoft Excel 2007、SigmaPlot 12.0軟件對數(shù)據(jù)進行預處理及作圖，用SAS 8.0軟件進行方差分析及LSD0.05差異顯著性檢驗。

2 結(jié)果與分析

2.1玉米秸稈和根茬腐解過程中土壤活性碳、氮組分含量的動態(tài)變化

圖1 玉米秸稈和根茬腐解過程中土壤微生物量碳含量的動態(tài)變化Fig.1 Dynamics of soil microbial biomass carbon contents during maize straw and root decomposition [注(Note): 豎線長度代表最小顯著差異值 (P<0.05) Vertical bars means the least-significant differences at the 0.05 probability level.]

圖3 玉米秸稈和根茬腐解過程中土壤微生物量氮含量的動態(tài)變化Fig.3 Dynamics of soil microbial biomass nitrogen contents during maize straw and root decomposition

2.2玉米莖、葉腐解過程中土壤活性碳、氮的動態(tài)變化

表2可見，培養(yǎng)最初7 d內(nèi)，各處理土壤活性碳、氮含量波動幅度較大，微生物量碳顯著增加，微生物量氮先增加后降低，土壤可溶性碳及礦質(zhì)氮先降低后增加；添加葉土壤微生物量碳、氮在培養(yǎng)28 d、14 d內(nèi)高于添加莖土壤，之后低于添加莖土壤，且這一變化在秸稈不還田土中較還田土出現(xiàn)時間提前，添加葉土壤可溶性碳含量始終高于添加莖土壤，土壤礦質(zhì)氮含量除第0 d外均為葉>莖，這可能與殘體剛施入土壤后，葉較莖更易被腐解，土壤礦質(zhì)氮被固持更快有關(guān)，之后隨著腐解的進行，礦質(zhì)氮又逐漸被釋放出來。

還田土壤微生物量碳含量在添加莖條件下，除培養(yǎng)第7 d、 62 d外均低于不還田土處理，而添加葉處理下為前42 d高于不還田土壤，之后趨勢相反；添加莖、葉條件下，還田土壤微生物量氮含量在培養(yǎng)第3 d、 62 d高于不還田土，其余時期均顯著低于不還田土；土壤可溶性碳和礦質(zhì)氮含量基本上為還田土高于不還田土。培養(yǎng)結(jié)束時，兩種培養(yǎng)土中莖、葉處理土壤微生物量碳、礦質(zhì)氮含量均較起始(0 d)顯著增加，可溶性碳含量基本保持不變，微生物量氮含量顯著下降。

3 討論

3.1玉米各器官(根茬、秸稈、莖、葉)腐解過程中土壤活性碳、氮含量變化的實質(zhì)

C/N低的有機物料更能夠促進土壤微生物量的提高而加快碳素和氮素的循環(huán)[21]。本研究中，玉米各部位殘體在添加量一致條件下，腐解期間秸稈處理(C/N=74.9)土壤微生物量碳、氮及礦質(zhì)氮含量始終顯著高于根茬(C/N=103.9)，這可能是由于秸稈較根茬碳、氮含量較高，C/N較低(表1)，且含有較多可溶性碳，從而更易被微生物群落吸收、礦化及循環(huán)；葉處理(C/N=64.5)土壤微生物量碳、氮在培養(yǎng)前期高于莖(C/N=93.6)，后期則低于莖，不還田土(僅根茬還田)較還田土(秸稈+根茬還田)出現(xiàn)時間提前，另外土壤礦質(zhì)氮含量均為葉>莖，且土壤礦質(zhì)氮的增加量與殘體C/N呈顯著負相關(guān)(r=-0.76*)，這與王春陽等的研究結(jié)果相似[22]。原因可能是莖含碳量高于葉，含氮量又低于葉，這使得后期微生物對葉碳分解利用減弱時，對莖碳的利用強度仍能持續(xù)，且在不還田土中強度較大，此外也不排除氯仿釋放的碳，除微生物體中的碳外還包括殘體碳。δ13C標記試驗發(fā)現(xiàn)，土壤-玉米殘體混合物處理中土壤微生物量碳約75%來源于玉米殘體[23]。在殘體腐解過程中，有合理數(shù)量的底物碳能穿過土壤微生物量并最終以土壤微生物殘留物的形式存在，這部分碳是土壤養(yǎng)分、能源的一個重要供應庫[11]。由此說明，殘體碳作為土壤微生物量碳的主要來源，在腐解過程中為土壤微生物補充了豐富的易利用有機碳源，同時也反映出微生物利用玉米各部分殘體養(yǎng)分的特異性。

表2 玉米莖和葉腐解過程中土壤活性碳、 氮組分含量的動態(tài)變化 (mg/kg)Table 2 Dynamics of soil carbon, nitrogen components during maize stem and leaf decomposition

注(Note): 同列數(shù)據(jù)后不同字母表示處理間差異顯著(P<0.05) Values followed by different small letters mean significantly different in the same column at the 0.05 level (P<0.05).

3.2玉米各器官(根茬、秸稈、莖、葉)腐解過程中土壤活性碳、氮含量變化特性

3.3 秸稈還田土壤對新鮮殘體腐解特性的影響

培養(yǎng)期間，還田土中各處理微生物量碳、可溶性碳及礦質(zhì)氮含量均高于不還田土，微生物量氮含量則相反，大部分時期差異未達顯著水平，兩種土壤間活性氮組分的差異較碳組分明顯，表明7季秸稈還田土對新鮮殘體的腐解影響不顯著，且氮組分對培養(yǎng)土壤是否還田的響應較碳組分靈敏。原因可能是土壤原有有機碳組成已相對穩(wěn)定，殘體腐解過程中土壤活性碳組分主要受殘體影響，而氮組分主要受土壤控制；此外，不還田土較還田土有機質(zhì)含量低，微生物活性較弱，殘體施入土壤后激發(fā)效應反而更強烈，氮素更易被固持[32]。

在土壤有機質(zhì)不斷形成和分解過程中，激發(fā)效應的存在是不可忽略的。研究發(fā)現(xiàn)，以復雜、不溶性化合物形式提供的殘體碳可能更易引起激發(fā)效應[33]；此外，與養(yǎng)分充足土壤相比，較為瘠薄土壤中的激發(fā)效應反而更為激烈[32]。本研究采用的室內(nèi)培養(yǎng)條件與田間實際情況存在較大差異，在自然條件下作物凋落物會或多或少不斷投入到土壤中[33]，活根的存在也刺激產(chǎn)生根際激發(fā)效應[34-35]，新鮮有機質(zhì)腐解的最初階段作物與微生物爭奪養(yǎng)分的效應等[36]。由此從本試驗結(jié)果中得出是否存在正負激發(fā)效應還比較困難。因此，有必要采用同位素標記法定量研究殘體腐解期間土壤微生物對施入殘體碳、氮的固持與釋放，進一步驗證激發(fā)效應是否存在，以評價土壤活性有機質(zhì)組分在土壤碳、氮循環(huán)中的作用。

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Labilecarbonandnitrogendynamicchangesinsoilsincorporatedwithdifferentpartsofmaizeplants

BA Yu-ling, TIAN Xiao-hong*, WAN Dan, LI Jin, WANG Shu-juan

(CollegeofNaturalResourceandEnvironment,KeyLabofPlantNutritionandtheAgri-EnvironmentinNorthwestChina,MinistryofAgriculture,NorthwestA&FUniversity,Yangling712100,China)

An incubation experiment was carried out to investigate the labile carbon and nitrogen dynamic changes in soils added with different parts of maize plants (straw, root, stem and leaf). The straw-amended soils had been incorporated with both straw and root residues, and the control soils with only root residues in consecutive seven-seasons of summer maize and winter wheat rotation system in Guanzhong Plain, Shaanxi province, China. The soil microbial biomass carbon (SMBC), soil microbial biomass nitrogen (SMBN), dissolved organic carbon (DOC), mineral nitrogen are determined regularly over 62 days, incubation. The results show that soil labile carbon and nitrogen change rapidly in the first 7 days. The contents of SMBC and SMBN amended with straw are significantly (P<0.05) higher than those with root. The SMBC and SMBN contents are greater in soils added with leaves than with stems at the first 28 d and 14 d incubation, and opposite afterwards. The SMBC and SMBN contents in soils added with straws are in between of the leaves and stems additions. The soil DOC and mineral nitrogen contents are in the order: leaf > straw > stem > root. At the end of the incubation, both the SMBC and mineral nitrogen contents increased significantly, soil DOC contents kept unchanged and the SMBN contents declined in all straw parts treatments. Compared to the non-added soils, the straw-added soils had no significant effect on the decomposition of fresh residues, and the differences in soil labile N between the two soils are greater than those in soil labile C. Therefore, soil labile C and N dynamics are influenced primarily by the ratio of C to N in the different straw parts. A same amount of straw is more efficient in replenishing soil C and N than roots after incorporated into soil, and the soil labile N is more sensitive than C to straw addition.

maize residue; microbial biomass C; microbial biomass N; dissolved organic C; mineral N

2012-12-20接受日期2013-06-04

國家科技支撐計劃項目(2012BAD14B11)；國家自然科學基金項目(40971179，31071863)；西北農(nóng)林科技大學“創(chuàng)新團隊建設計劃”項目(2010)資助。

把余玲(1988—)，女，甘肅蘭州人，碩士研究生，主要從事植物營養(yǎng)研究。E-mail: bayuling@163.com * 通信作者 E-mail: txhong@hotmail.com

S153.6+21

A

1008-505X(2013)05-1166-08