玉米植株不同部位还田土壤活性碳、氮的动态变化

把余玲, 田霄鸿, 万 丹, 李 锦, 王淑娟

(西北农林科技大学资源环境学院，农业部西北植物营养与农业环境重点实验室，陕西杨凌 712100)

玉米植株不同部位还田土壤活性碳、氮的动态变化

把余玲, 田霄鸿*, 万 丹, 李 锦, 王淑娟

(西北农林科技大学资源环境学院，农业部西北植物营养与农业环境重点实验室，陕西杨凌 712100)

探讨玉米植株不同部位腐解对还田土壤活性碳、 氮动态变化的影响。采用室内培养方法，通过动态监测土壤微生物量碳(SMBC)、微生物量氮(SMBN)、可溶性碳(DOC)和矿质氮含量，研究等量玉米根茬、秸秆、茎及叶4个部位在连续7季还田(秸秆+根茬还田)和不还田土壤(仅根茬还田)中的腐解转化特征。结果表明，秸秆腐解的最初7 d是土壤活性碳、 氮动态变化的高峰期；腐解期间(62 d)SMBC、SMBN含量表现为添加秸秆始终高于根茬，叶分别在前28 d、14 d内高于茎，后期则低于茎，秸秆介于茎、叶之间；土壤DOC、矿质氮含量为叶>秸秆>茎>根茬；培养结束时，各处理SMBC和矿质氮含量均较起始(0 d)显著提高，DOC含量基本保持不变，SMBN含量显著下降。与不还田土壤相比，还田土壤对新鲜残体的腐解影响不显著，且两者间土壤活性氮组分的差异较碳组分明显。腐解期间土壤活性碳、 氮的动态变化主要取决于各器官碳、 氮等化学组分的差异性，等量秸秆较根茬更有利于补充土壤活性碳、氮数量，土壤活性氮组分对还田土壤的响应较碳组分灵敏。

玉米残体； 微生物量碳； 微生物量氮； 可溶性有机碳； 矿质氮

作物秸秆作为农田生态系统中土壤有机物归还的主要来源，已广泛应用于农业生产实践[1-3]，其在土壤中的腐解主要取决于残体来源和成分[4]。残体来源因受人为利用和管理措施影响，其各部分归还到土壤中的数量和比例有所不同[5-6]，尤其在机械化程度较高地区，作物收获后根茬几乎全部留在土壤中被腐解[7-8]。此外，同一作物各器官因生长条件不同，其成分间存在高度差异性[7]。不少研究表明，秸秆腐解不仅显著提高土壤有机质含量[3, 9-10]，而且提高包括土壤微生物量、可溶性碳等活性有机质的含量[11-13]，进而影响土壤微生物对氮素的固持与释放[1,14]。有研究指出，与作物秸秆相比，根茬对土壤结构的改善及有机碳的贡献作用更显著，且对根际影响最大[15-18]。Puget等研究发现，虽然秸秆施入土壤后能迅速分解并为下茬作物提供氮源，但根茬可能更有利于短期土壤结构的改善及长期土壤有机质的累积[17]。因此，在农业实践中，如何协调秸秆和根茬腐解在土壤养分供应中所起的作用是值得关注的重要问题。

随着农业机械化的普及，关中平原小麦、玉米秸秆还田面积越来越大，即使秸秆不还田，仍有大量根茬留在土壤中被腐解。在这种情况下，多年秸秆还田与不还田土壤相比，对作物残体腐解影响究竟能产生多大差异的报道尚不多见，且对同一作物不同部位(秸秆、根茬、茎、叶)各自的腐解特性，以及腐解过程中土壤活性碳、 氮等养分动态变化的研究很少。因此，本试验采用室内培养方法，初步研究玉米各部位残体(根茬、秸秆、茎、叶)还田和不还田后，土壤微生物量碳、 氮、可溶性碳及矿质氮的动态变化，旨在进一步探讨作物各部位残体腐解过程中养分供应与土壤肥力的关系，为合理还田与农田养分科学管理提供依据。

1 材料与方法

1.1 供试材料

1.2 培养试验

1.2.1 试验设计 以上述2种土壤(还田土、不还田土)和4种玉米植株不同部位(根茬、秸秆、茎、叶)为研究因素，另设不加玉米残体土壤作为对照，共组成10个处理，每个处理重复3次。

1.2.2 培养过程 称土250 g(烘干土)装入1 L塑料培养盆中，加蒸馏水至田间持水量(WHC)的60%，在20℃下预培养4 d，以恢复土壤微生物活性。然后，将2.5 g玉米根茬、秸秆、茎、叶残体分别施入相应土壤中，并依各残体C、N含量，加入适量尿素溶液调节C/N至25 ∶1(使土壤含水量调至70% WHC)，同时设不加残体的2种土样作为对照，充分混合均匀，置于培养箱中，25±1℃恒温培养62 d，每隔5 d采用称重法补充水分。在培养的第0(6 h后)、 3、 7、 14、 28、 42、 62 d分别从各培养盆中取样，测定土壤微生物量碳、微生物量氮、可溶性碳和矿质氮含量。

1.3 测定项目与方法

表1 土壤及玉米植株样品的基本性质Table 1 Basic properties of soil samples and maize residues

采用Microsoft Excel 2007、SigmaPlot 12.0软件对数据进行预处理及作图，用SAS 8.0软件进行方差分析及LSD0.05差异显著性检验。

2 结果与分析

2.1玉米秸秆和根茬腐解过程中土壤活性碳、氮组分含量的动态变化

图1 玉米秸秆和根茬腐解过程中土壤微生物量碳含量的动态变化Fig.1 Dynamics of soil microbial biomass carbon contents during maize straw and root decomposition [注(Note): 竖线长度代表最小显著差异值 (P<0.05) Vertical bars means the least-significant differences at the 0.05 probability level.]

图3 玉米秸秆和根茬腐解过程中土壤微生物量氮含量的动态变化Fig.3 Dynamics of soil microbial biomass nitrogen contents during maize straw and root decomposition

2.2玉米茎、叶腐解过程中土壤活性碳、氮的动态变化

表2可见，培养最初7 d内，各处理土壤活性碳、氮含量波动幅度较大，微生物量碳显著增加，微生物量氮先增加后降低，土壤可溶性碳及矿质氮先降低后增加；添加叶土壤微生物量碳、氮在培养28 d、14 d内高于添加茎土壤，之后低于添加茎土壤，且这一变化在秸秆不还田土中较还田土出现时间提前，添加叶土壤可溶性碳含量始终高于添加茎土壤，土壤矿质氮含量除第0 d外均为叶>茎，这可能与残体刚施入土壤后，叶较茎更易被腐解，土壤矿质氮被固持更快有关，之后随着腐解的进行，矿质氮又逐渐被释放出来。

还田土壤微生物量碳含量在添加茎条件下，除培养第7 d、 62 d外均低于不还田土处理，而添加叶处理下为前42 d高于不还田土壤，之后趋势相反；添加茎、叶条件下，还田土壤微生物量氮含量在培养第3 d、 62 d高于不还田土，其余时期均显著低于不还田土；土壤可溶性碳和矿质氮含量基本上为还田土高于不还田土。培养结束时，两种培养土中茎、叶处理土壤微生物量碳、矿质氮含量均较起始(0 d)显著增加，可溶性碳含量基本保持不变，微生物量氮含量显著下降。

3 讨论

3.1玉米各器官(根茬、秸秆、茎、叶)腐解过程中土壤活性碳、氮含量变化的实质

C/N低的有机物料更能够促进土壤微生物量的提高而加快碳素和氮素的循环[21]。本研究中，玉米各部位残体在添加量一致条件下，腐解期间秸秆处理(C/N=74.9)土壤微生物量碳、氮及矿质氮含量始终显著高于根茬(C/N=103.9)，这可能是由于秸秆较根茬碳、氮含量较高，C/N较低(表1)，且含有较多可溶性碳，从而更易被微生物群落吸收、矿化及循环；叶处理(C/N=64.5)土壤微生物量碳、氮在培养前期高于茎(C/N=93.6)，后期则低于茎，不还田土(仅根茬还田)较还田土(秸秆+根茬还田)出现时间提前，另外土壤矿质氮含量均为叶>茎，且土壤矿质氮的增加量与残体C/N呈显著负相关(r=-0.76*)，这与王春阳等的研究结果相似[22]。原因可能是茎含碳量高于叶，含氮量又低于叶，这使得后期微生物对叶碳分解利用减弱时，对茎碳的利用强度仍能持续，且在不还田土中强度较大，此外也不排除氯仿释放的碳，除微生物体中的碳外还包括残体碳。δ13C标记试验发现，土壤-玉米残体混合物处理中土壤微生物量碳约75%来源于玉米残体[23]。在残体腐解过程中，有合理数量的底物碳能穿过土壤微生物量并最终以土壤微生物残留物的形式存在，这部分碳是土壤养分、能源的一个重要供应库[11]。由此说明，残体碳作为土壤微生物量碳的主要来源，在腐解过程中为土壤微生物补充了丰富的易利用有机碳源，同时也反映出微生物利用玉米各部分残体养分的特异性。

表2 玉米茎和叶腐解过程中土壤活性碳、 氮组分含量的动态变化 (mg/kg)Table 2 Dynamics of soil carbon, nitrogen components during maize stem and leaf decomposition

注(Note): 同列数据后不同字母表示处理间差异显著(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、秸秆、茎、叶)腐解过程中土壤活性碳、氮含量变化特性

3.3 秸秆还田土壤对新鲜残体腐解特性的影响

培养期间，还田土中各处理微生物量碳、可溶性碳及矿质氮含量均高于不还田土，微生物量氮含量则相反，大部分时期差异未达显著水平，两种土壤间活性氮组分的差异较碳组分明显，表明7季秸秆还田土对新鲜残体的腐解影响不显著，且氮组分对培养土壤是否还田的响应较碳组分灵敏。原因可能是土壤原有有机碳组成已相对稳定，残体腐解过程中土壤活性碳组分主要受残体影响，而氮组分主要受土壤控制；此外，不还田土较还田土有机质含量低，微生物活性较弱，残体施入土壤后激发效应反而更强烈，氮素更易被固持[32]。

在土壤有机质不断形成和分解过程中，激发效应的存在是不可忽略的。研究发现，以复杂、不溶性化合物形式提供的残体碳可能更易引起激发效应[33]；此外，与养分充足土壤相比，较为瘠薄土壤中的激发效应反而更为激烈[32]。本研究采用的室内培养条件与田间实际情况存在较大差异，在自然条件下作物凋落物会或多或少不断投入到土壤中[33]，活根的存在也刺激产生根际激发效应[34-35]，新鲜有机质腐解的最初阶段作物与微生物争夺养分的效应等[36]。由此从本试验结果中得出是否存在正负激发效应还比较困难。因此，有必要采用同位素标记法定量研究残体腐解期间土壤微生物对施入残体碳、氮的固持与释放，进一步验证激发效应是否存在，以评价土壤活性有机质组分在土壤碳、氮循环中的作用。

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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

国家科技支撑计划项目(2012BAD14B11)；国家自然科学基金项目(40971179，31071863)；西北农林科技大学“创新团队建设计划”项目(2010)资助。

把余玲(1988—)，女，甘肃兰州人，硕士研究生，主要从事植物营养研究。E-mail: bayuling@163.com * 通信作者 E-mail: txhong@hotmail.com

S153.6+21

A

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