碳氮添加对不同湿度条件下温带森林土壤融化过程中氧化亚氮排放的影响

Effect of carbon and nitrogen addition on nitrous oxide flux during thawing of frozen temperate forest soils with different moisture levels

采用室内土柱培养方法,研究不同土壤湿度(55%和80%土壤充水孔隙度,WFPS)条件下外源碳(葡萄糖,6.4 g C·m^(-2))和2种形态氮(NH_4Cl和KNO_3,4.5 g N·m^(-2))的添加对温带成熟阔叶红松混交林和次生白桦林土壤冻结后融化过程中氧化亚氮(N_2O)排放量的影响。结果表明:冻结过程会激发2种林分土壤融化初期N_2O的排放。随着土壤湿度的增加,2种林分土壤大量消耗硝态氮,反硝化作用强烈,导致融化初期N_2O激发效应的强度大,持续时间长,尤其是白桦林土壤。单施葡萄糖后,2种林分土壤大量消耗铵态氮和硝态氮,进而显著促进2种林分土壤融化初期N_2O的激发排放;随着土壤湿度的增加,葡萄糖对2种林分土壤N_2O累积排放量的促进作用减弱,这可能与高湿度条件下,冻结后融化过程中土壤释放大量溶解性有机碳(DOC)有关。低湿度条件下,2种林分土壤融化过程N_2O排放是铵态氮限制性的,即硝化潜势占主要优势,尤其是白桦林土壤;高湿度条件下,白桦林土壤具有很强的反硝化潜势,并且随着葡萄糖的施加,这种反硝化潜势加强。逐步回归分析显示:2种林分土壤冻结后融化过程N_2O累积排放量受到土壤pH、WFPS及水浸提DOC含量的影响,共同解释其66%的变化,并且与土壤水浸提溶解性有机氮含量呈显著的正相关;阔叶红松混交林土壤冻结后融化过程N_2O累积排放量与微生物生物氮呈显著负相关。综上可推测,温带森林土壤融化过程中N_2O的排放主要依赖于冻结处理后的土壤pH、WFPS以及溶解性有机质释放量的变化。

Packed soilcore incubation experiments were done to study the effects of carbon （glucose, 6.4 g C·m^-2） and nitrogen （NH4Cl and KNO3, 4.5 g N·m^-2） addition on nitrous oxide （N2O） flux during thawing of frozen temperate forest soils with two moisture levels （55% and 80% WFPS, waterfilled pore space） collected from a mature broadleaf and Korean pine mixed forest （BKPF） and adjacent secondary white birch forest （WBF）. The results showed that freezing stimulated N2O fluxes from the two forest soils shortly after thawing. With increasing soil moisture, the magnitude and longevity of the flush of N2O fluxes from the two forest soils were enhanced during the early period of thawing, which was accompanied by great NO3--N consumption by denitrification, especially in the WBF soil. The addition of glucose significantly increased N2O fluxes from the two forest soils during the early period of thawing, with great consumption of NH4＋-N and NO3--N. With increasing soil moisture, the priming effect of glucose on the N2O flux decreased in the two forest soils, which was probably related to a greater release of dissolved organic carbon （DOC） into the soils with high moisture after thawing. Under low soil moisture conditions, N2O fluxes were subjected to the limitation of NH4＋-N pool in the two forest soils during thawing, indicating the relatively great nitrification potential, especially in the WBF soil. However, under high soil moisture conditions, denitrification potential became relatively high in the WBF soil and was enhanced by the addition of glucose. A stepwise regression analysis showed that 66% of the variability in the cumulative N2O emissions from the two forest soils during thawing could be explained by the soil pH, WFPS and waterextractable DOC pool. The cumulative N2O emissions from the two forest soils were significantly and positively correlated to the soil waterextractable dissolved organic nitrogen pool after thawing. The cumulative N2O emission from the BKPF soil was negatively correlated to the soil microbial biomass N. From the experimental results, it can be reasonably concluded that N2O flux during thawing of frozen temperate forest soils depends on soil pH, WFPS, and the amount of dissolved organic matter released into the soil.