土壤微生物量氮对小麦各生育期氮素形态的调控

  【目的】  土壤中氮素的有效性很大程度上影响着作物对氮的吸收。明确各形态氮素对作物吸氮量的贡献，研究调控土壤氮素形态的因素，为培育氮素高效和作物高产的土壤提供理论依据。  【方法】  试验基于河南新乡的“国家潮土土壤肥力与肥料效益监测基地”长期定位试验，以不施肥 (CK)、施NPK化肥 (NPK) 和1.5倍NPK化肥并配施有机肥 (1.5MNPK) 3个处理的土壤作为低肥力 (F1)、中肥力 (F2) 和高肥力 (F3) 土壤进行小麦盆栽试验。3个肥力土壤处理施肥方法相同，盆钵埋于土壤内，盆钵顶部露出地面5 cm。分别在小麦拔节期、孕穗期和成熟期采集土壤和植株样品，测定小麦产量、各生育期吸氮量，分析土壤有机氮、矿质氮 (铵态氮和硝态氮)、固持氮库 (微生物量氮和固定态铵) 含量差异，并通过结构方程模型 (SEM) 建立各形态氮素与小麦吸氮量的相关关系。  【结果】  3个肥力水平土壤矿质氮含量在小麦生长期内总体呈下降趋势，收获期土壤矿质氮含量在F1、F2、F3中分别比播种前显著下降了2.9、1.8和6.8 mg/kg。从拔节期到收获期，土壤微生物量氮在F1先增加后降低，在F3中持续增加，在F2中先降低后增加。土壤固定态铵含量在拔节期前和孕穗期后均无显著变化，但从拔节期到孕穗期，3个肥力土壤中固定态铵含量均显著提高。而固持氮库在不同肥力土壤间差异明显，其从播种前到拔节期在F1中增加了10.6 mg/kg，而在F2和F3中分别降低了14.3和32.2 mg/kg；从拔节期到孕穗期都显著增加；从孕穗期到收获期在F1中降低了2.4 mg/kg，而在F2和F3中分别增加8.2和8.7 mg/kg。小麦的产量和吸氮量均在F3中最高，F1中最低；氮素表观平衡在F1中最高，F3中最低。SEM分析结果表明，固持氮库可直接正向调控小麦吸氮量，有机氮库通过固持氮库和矿质氮库之间的变化而间接调控小麦吸氮量。  【结论】  包含微生物量氮和固定态铵的固持氮库可直接正向调控小麦吸氮量，有机氮库通过影响固持氮库和矿质氮库间接调控小麦吸氮量。由于固定态铵在拔节前和孕穗期后含量较为稳定，在高肥力土壤上微生物量氮随着小麦生育期的推进显著增加，可促进小麦的生长和氮素吸收，减少肥料氮的残留量，较高的微生物量氮又可作为氮库来固存易损失的矿质氮和肥料氮。

Regulation of soil microbial biomass nitrogen on nitrogen forms in different growth stages of wheat

  【Objectives】  The availability of soil nitrogen (N) significantly affects plant uptake of the nutrient. Here, we studied the changes of different forms of N in soils under wheat growth to regulate N uptake of crop. The aim was to understand the effect of soil fertility on N supply capacity.  【Methods】  A wheat pot experiment was conducted with soils having low, medium, and high fertility (denoted as F1, F2, and F3, respectively). The tested soils were collected from the long-term experimental station at “National Long-term Monitoring Station of Fluvo-Aquic Soil Fertility and Fertilizer Efficiency” in Xinxiang, Henan Province. The three soil fertility levels were respectively collected from the treatments of no fertilizer (CK), NPK fertilizer (NPK), and 1.5 times of NPK fertilizer combined with organic fertilizer (1.5MNPK). All treatment pots received the same quantity of fertilizer and transferred to the field with the top 5 cm above the soil surface. Both soil and plant samples were collected at the jointing stage, booting stage, and after wheat harvest. Soil organic N, mineral N (ammonium and nitrate), and immobilized N (microbial biomass N and fixed ammonium) were analyzed. Further, the grain yield, N uptake, and N balance were studied. A structural equation model (SEM) was employed to clarify the contribution of various forms of soil N to crop N uptake.  【Results】  Mineral N contents decreased during wheat growth. After wheat harvest, the values decreased by 1.8–6.8 mg/kg compared with those before sowing. From jointing stage to harvesting stage, soil microbial biomass N increased first and then decreased in F1, while decreased first and then increased in F2, and continuously increased in F3. Soil fixed ammonium content was significantly increased in F1, F2 and F3 from jointing stage to booting stage, and did not change significantly before jointing stage and after booting stage. The immobilized N pool increased by 10.6 mg/kg in F1 but decreased by 14.3 mg/kg in F2 and 32.2 mg/kg in F3 before the jointing stage, and significantly increased in all the three fertility fields from jointing to booting stage; and decreased by 2.4 mg/kg in F1 and increased by 8.2 mg/kg in F2 and 8.7 mg/kg in F3 from booting to harvesting stage. The highest wheat yield, N uptake, and the lowest N balance were observed in F3. Conversely, the lowest N uptake and highest N balance were observed in F1. Structural equation model analysis showed that immobilized N could directly and positively regulate N uptake by wheat, and organic N could regulate N uptake by influencing immobilized N indirectly.  【Conclusions】  Immobilized N pool directly and positively regulate wheat N uptake. The organic N pool affects wheat N uptake through the transformation of immobilized and mineral N. As the fixed ammonium content is relatively stable, microbial biomass N plays a crucial role in wheat growth by increasing soil available N. On the other hand, it can hold extra mineral N which is easily lost.