[Objectives] Stabilities of organic nitrogen in different aggregates are different.The response mechanism of soil nitrogen fixation to a long-term different fertilization was investigated by analyzing organic nitrogen distribution in different stable aggregates.[Methods] The investigation was based on the long-term experiment in black soil.Soil samples were collected from four treatments:no fertilization (CK),chemical fertilization (NPK),chemical fertilization and straw (NPKS),and chemical fertilization and manure (NPKM).The soil samples were incubated for 40 days and the 15N-labeled urea was applied in the same amount as in the long-term field experiment.Wet sieve method was used to separate aggregates into coarse free particulate organic matter (＞ 250 μm,cfPOM),micro-aggregate organic matter (53-250 μm),and mineral-associated organic matter (＜ 53 μm,MOM).The micro-aggregate organic matter was then subdivided into fine free particulate organic matter (ffPOM) and physically protected intra-aggregate particulate organic matter (iPOM),while the mineral-associated organic matter was subdivided into inter-and external-aggregate MOM.The composition of the tested aggregates and their mean weight diameter (MWD) were investigated,and the proportion of the fixed nitrogen in these aggregates was analyzed.[Results] The amounts of15N-labeled nitrogen fixed by soil in the NPKM and NPKS treatments were significantly increased by about 10％ than those in the CK and NPK.The mean weight diameter (MWD) in the NPKS and NPKM (0.34 mm) was significantly higher than those in the NPK and CK (0.23-0.25 mm).Compared with the CK and NPK treatments,the weight percentage of cfPOM was significantly increased by about 10％,while the weight percentage of MOM was 6.0％-10％ lower in the NPKM and NPKS treatments.The nitrogen fertilizer resided in cfPOM was in the range of 18-36 mg/kg,and the nitrogen fertilizer resided in MOM was in the range of 18-30 mg/kg.[Conclusions] Combined application of chemical fertilizers with manure or straw promoted the accumulation of fertilizer nitrogen in large aggregates,and reduced the accumulation in the clay particles.This might be one of mechanisms for the enhanced nitrogen availability and absorption by manure application.%[目的]不同粒级团聚体中有机氮库的稳定性不同,通过分析长期施肥后有机氮库在不同团聚体中的分布特征,可阐明土壤氮素同持对不同施肥的响应机制.[方法]采集黑土长期定位试验中的4个处理土壤,包括不施肥(CK)和施用氮磷钾(NPK)、氮磷钾+秸秆(NPKS)、氮磷钾+有机肥(NPKM),模拟田间施肥量加入15N标记的尿素培养40天,采用Six湿筛法将团聚体分为粗游离颗粒有机物(cfPOM,＞250μm)、微团聚体有机物(53～250 μm)、矿物结合有机物(MOM,＜53 μm)三大类,其中微团聚体有机物进一步分为细游离颗粒有机物(ffPOM)和物理保护有机物(iPOM),矿物结合有机物分为团聚体内矿物结合有机物(MOM团内)和团聚体外矿物结合有机物(MOM团外),研究了团聚体分布特征、平均重量直径(MWD)和各团聚体中氮的固持量及固持效率.[结果]NPKM和NPKS处理的粗游离颗粒含量较CK和NPK处理显著提高约10％,而矿物结合有机物含量显著降低约6％～10％,NPKM和NPKS处理下平均重量直径为0.34 mm,显著大于NPK和CK的值(0.23～0.25 mm).NPKM处理的土壤肥料氮固持量较CK和NPK处理显著提高了6.0％和10.5％;粗游离颗粒氮固持量为18～36 mg/kg,矿物结合有机氮固持量为18～30 mg/kg.[结论]长期有机无机肥配施或无机肥结合秸秆还田可促进肥料氮在粗游离颗粒中的积累,降低在矿物结合有机物中的积累,这可能是有机无机肥配施增强土壤氮的有效性、促进作物氮吸收的原因之一.
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