长期施肥下黑土不同团聚体氮组分的植物有效性差异

【目的】 土壤团聚体组分形成机制不同，其所含有的氮的转化和有效性也不同。阐明不同团聚体中氮素有效性差异，可为科学施肥、培育高效的土壤结构、提高氮肥利用率提供重要的理论依据。 【方法】 依托吉林黑土长期定位试验，于2014年进行了室内土壤培养试验和黑麦草盆栽试验。供试土壤选择的定位试验处理包括不施肥 (CK)、氮磷钾 (NPK)、氮磷钾 + 秸秆 (NPKS)、氮磷钾 + 农家肥 (NPKM)。采集1000 g土样于2 L塑料瓶内，加入15N丰度为20.12%的尿素0.247 g，置于25℃培养箱中恒温控湿培养40天。培养完成后将土样风干，将有机物分为粗游离颗粒有机物 (cfPOM，> 250 μm)、微团聚体有机物 (iPOM，53～250 μm) 和矿物结合有机物 (MOM，< 53 μm)，矿物结合有机物又进一步分为团聚体内矿物结合有机物 (MOMi) 和团聚体外矿物结合有机物 (MOMo)，分析了不同团聚体组分中15N的固持量。称取各粒级土壤样品40 g，分别与20 g细石英砂混匀，于80 mL小塑料盆中，每盆黑麦草定苗7株于温室内培养，20天时加入适量磷、钾营养液。培养30天后，分别采集黑麦草地上部和根系，烘干、称重、研磨，测定养分含量及15N丰度。 【结果】 在NPK处理和NPKM处理的土壤中，植株生物量分别在MOMo和cfPOM下最高，分别为每盆100.2 mg和99.8 mg。黑麦草尿素氮含量在MOMo的NPK处理最大，在其他三个组分均表现为CK > NPK > NPKS > NPKM，其中cfPOM组分氮固持量与黑麦草氮含量表现一致；黑麦草吸收的氮素主要来自cfPOM和MOMo组分中 (0.1～0.21 mg/pot)，在其他组分下不足0.05 mg/pot；cfPOM、iPOM、MOMi和MOMo中的氮素利用率分别为14.1%～19.3%、5.5%～15.4%、3.1%～4.9%和12.7%～23.6%，在NPKM处理下，以cfPOM组分中最高，为19.3%，在NPK处理下以MOMo组分最高，为23.6%。 【结论】 施用有机肥可促进外源氮肥保存在粗游离颗粒中，其固持的氮有效性最高，有利于后季养分的供应。单施化肥处理团聚体外部矿物结合有机物中固持氮的有效性最高；团聚体内部矿物结合有机物组分虽固持氮含量较高，但氮有效性很低。

Difference in plant availability of nitrogen in aggregates of black soil under long-term fertilization

【Objectives】 The formation mechanism of soil aggregates is different, and the conversion and availability of the contained nitrogen (N) in these aggregates are different as well. The differences in N availability among soil aggregates were investigated in this study to provide theoretical basis for rational fertilization, build-up of good soil structure and the improvement of N fertilizer use efficiency. 【Methods】 An incubation experiment and a pot experiment were conducted in 2014. The soils used were collected from the long-term fertilization experiment of black soil in Jilin province, and involved treatments included: no fertilization (CK), chemical NPK fertilizers (NPK), chemical NPK fertilizers and straw returning (NPKS), and chemical NPK fertilizers and manure (NPKM). Urea with a 15N abundance of 20.12% was mixed into 1000 g of soil and incubated within 2 L volume of plastic bottles for 40 days at 25℃. The incubated soil was separated into 4 aggregate fractions: coarse free particulate organic matter (> 250 μm, cfPOM), micro-aggregate organic matter (53?250 μm, iPOM), outer mineral-associated organic matter (MOMo, <53 μm), and inner mineral-associated organic matter (MOMi, < 53 μm). Residing differences of fertilizer N in aggregates were analyzed. Ryegrass (Loliumperenne L.) was planted in each soil fraction for 30 days, then the biomass, fertilizer N uptake and fertilizer N use efficiency of ryegrass were analyzed. 【Results】 The plant biomass was the highest in MOMo (100.2 mg/pot) and cfPOM (99.8 mg/pot) in treatments of NPK and NPKM, respectively. The urea N content of ryegrass in three soil fractions was in the order of CK > NPK > NPKS > NPKM. The resided nitrogen in cfPOM was consistent with the nitrogen content of ryegrass, except that it was highest in NPK treatment for the fraction of MOMo. N absorbed by ryegrass was mainly from cfPOM and MOMo (0.1?0.21 mg/pot), and less than 0.05 mg/pot from other fractions. The nitrogen use efficiencies were 14.1%?19.3%, 5.5%?15.4%, 3.1%?4.9%, and 12.7%?23.6% in cfPOM, iPOM, MOMi, and MOMo, respectively. In NPKM treatment, N use efficiency was highest in cfPOM (19.3%), and that in NPK was highest in MOMo (23.6%) . 【Conclusions】 The application of organic fertilizer promotes exogenous N to be stored in large aggregates and held the highest nitrogen efficiency of large aggregates, which is beneficial to the nutrient supply in the later season. The application of chemical fertilizer facilitates the use efficiency of N immobilized by outer mineral-associated organic matter. The residual nitrogen availability is low in inner mineral-associated organic matter though the residual amount is high.