不同根系分泌物对土壤N2O排放及同位素特征值的影响

【目的】探究植物根系分泌的主要组分（有机酸、氨基酸、糖类）对土壤N₂O排放及其微生物过程的影响,为选择适宜的植物进而控制土壤N₂O排放提供支撑。【方法】通过室内试验分别添加草酸、丝氨酸、葡萄糖于土壤中模拟根系的3种主要分泌物,每种分泌物设置两个浓度水平：低浓度（150 μg C·d ^-1）和高浓度（300 μg C·d ^-1）,另设置添加蒸馏水的对照组,共7个处理。将土壤置于120 mL玻璃瓶中进行培养,24 h内采集气体样品7次,每次培养2 h,获取N₂O排放速率、日累积排放量和同位素特征值（δ ¹⁵N ^bulk、δ ¹⁸O和SP（site preference,SP=δ ¹⁵N ^α-δ ¹⁵N ^β））。【结果】添加3种根系分泌物组分后,土壤N₂O排放速率均逐渐升高,且均高于对照。高浓度处理组N₂O累积排放量为：葡萄糖（（3.2±1.3）mg·kg ^-1·d ^-1）处理>丝氨酸（（2.6±0.5）mg·kg ^-1·d ^-1）处理>草酸（（1.4±0.2）mg·kg ^-1·d ^-1）处理,低浓度处理组为：草酸（（2.7±1.3）mg·kg ^-1·d ^-1）处理>丝氨酸（（1.8±0.4）mg·kg ^-1·d ^-1）处理>葡萄糖（（1.6±0.8）mg·kg ^-1·d ^-1）处理;添加根系分泌物的不同处理间土壤N₂O的δ ¹⁸O值无明显差异,并稳定在24.1‰—25.6‰,且均显著高于对照（（20.1±1.5）‰）;土壤N₂O的δ ¹⁵N ^bulk值与添加根系分泌物的种类有关,其中草酸处理组为（-20.06±2.22）‰、丝氨酸处理组为（-22.33±1.10）‰、葡萄糖处理组为（-13.86±1.11）‰、对照组为（-23.14±3.72）‰。各处理土壤N₂O的SP值的变化范围为13.13‰—15.03‰,根系分泌物浓度越高,SP值越低。综合分析不同处理4个指标（N₂O排放速率、N₂O的δ ¹⁵N ^bulk、δ ¹⁸O和SP值）的不同时刻的检测值与日均值的校正系数,添加根系分泌物后第16小时各处理4个指标的校正系数最接近于1。【结论】在NH+ 4-300 mg N·kg ^-1的土壤环境下根系分泌物促进N₂O的排放,且在培养期间（24 h）土壤N₂O排放速率逐渐升高。高浓度处理组葡萄糖对土壤N₂O排放速率促进效果最强,低浓度处理组草酸对土壤N₂O排放速率促进效果最强。与对照组相比,根系分泌物的添加使N₂O的δ ¹⁸O值显著升高;与对照组相比,葡萄糖的添加使δ ¹⁵N ^bulk值显著升高。根系分泌物浓度越高,反硝化作用对N₂O的贡献越大。

Effects of Different Root Exudates on Soil N2O Emissions and Isotopic Signature

【Objective】The aim of this study was to investigate the effects of the main plant root exudates (organic acids, amino acids, sugars) on N₂O emission and its microbial processes, so as to provide a support for selecting suitable plants to control soil N₂O emissions. 【Method】Three main components of root exudates, including oxalic acid, serine and glucose, were added in the soil, and two levels of concentration were set for each component: low concentration (150 μg C·d ^-1) and high concentration (300 μg C·d ^-1). There were totally 7 treatments with a control treatment treated with distilled water, and all treatments were incubated in 120 mL glass bottles in incubator. The gas samples were sampled 7 times within 24 hours after 2-hour incubation each time. After sampling, the N₂O emission rate, daily cumulative emission and isotope signatures (δ ¹⁵N ^bulk, δ ¹⁸O and SP (intramolecular ¹⁵N site preference, δ ¹⁵N ^α-δ ¹⁵N ^β)) were measured, then the optimal sampling time was determined according to their daily variation rules. 【Result】The N₂O emission rate of soils increased gradually after adding three components of root exudates, which were higher than the control treatment. The cumulative emission of N₂O in the high concentration treatment was: Glucose treatments ((3.2±1.3) mg·kg ^-1·d ^-1)>Serine treatments ((2.6±0.5) mg·kg ^-1·d ^-1)>Oxalic acid treatments((1.4±0.2) mg·kg ^-1·d ^-1), low concentration treatment: Oxalic acid ((2.7±1.3) mg·kg ^-1·d ^-1)>Serine ((1.8±0.4) mg·kg ^-1·d ^-1)>Glucose ((1.6±0.8) mg·kg ^-1·d ^-1); the values of δ ¹⁸O of N₂O were not different among different root exudate treatments and were stable at 24.1‰-25.6‰, but significantly higher than the control treatment ((20.1±1.5) ‰); the δ ¹⁵N ^bulk value of N₂O was related to the component of root exudates added, which was (-20.06±2.22) ‰ of oxalic acid treatment, (-22.33±1.10) ‰ of serine treatment, (-13.86±1.11) ‰ treated of glucose treatment, and (-23.14±3.72) ‰ of the control treatment. The SP value of N₂O of each treatment ranged from 13.13‰ to 15.03‰, and the higher the root exudate concentration, the lower the SP value; after a comprehensive analysis of the correction coefficients of four indexes (N₂O emission rate, the value of δ ¹⁵N ^bulk, δ ¹⁸O and SP) at each sampling time and their daily mean values of 7 treatments, the correction coefficients of all treatments were closest to 1 at the 16th hour after the addition of root exudates. 【Conclusion】In the soil environment with NH+ 4- 300 mg N·kg ^-1, the root exudates promoted N₂O emission and the N₂O emission rate increased gradually during the culture time (24 hours). The promotion effect of glucose in high concentration group was the strongest, while that of oxalic acid in low concentration group was the strongest. Compared with the control treatment, the addition of root exudates significantly increased the δ ¹⁸O value of N₂O; the addition of glucose significantly increased the δ ¹⁵N ^bulk value of N₂O. The higher the concentration of root exudates, the stronger the contribution of denitrification to N₂O was detected.