不同放牧强度下土壤氨氧化和反硝化微生物的变化特征

摘要

土壤硝化及反硝化功能微生物在氮素可利用性、硝酸盐淋溶和氧化亚氮温室气体排放等方面起着关键作用,在指示不同放牧强度对生态系统的影响及预测草地生态系统退化状况等方面具有重要意义.以内蒙古干旱半干旱草原不同放牧强度(轻度、中度和重度)的长期试验样地为对象,应用定量PCR和限制性末端片段长度多态性(Terminal restriction fragment lengthpolymorphism,T-RFLP)的方法,研究土壤氨氧化古菌(ammonia-oxidizing archaea,AOA)、氨氧化细菌(ammonia-oxidizingbacteria,AOB)和反硝化细菌的丰度、群落结构和多样性对不同放牧强度的响应.结果表明,土壤pH和铵态氮含量分别在7.90-8.18和6.37-35.92 mg/kg之间,中度放牧处理显著增高了土壤pH(P=0.03),而铵态氮含量在重度放牧处理中最高(P=0.02).不同放牧强度下土壤异养呼吸相比未放牧处理均显著降低(P=0.02).土壤AOA-amoA和AOB-amoA基因丰度范围分别为每克干土(4.94-7.60)×109个拷贝数和(0.68-3.75)×106个拷贝数,放牧处理对AOA-amoA基因丰度无显著影响,中度放牧处理显著降低了AOB-amoA基因丰度(P=0.04);反硝化微生物nosZ基因丰度随在轻度放牧处理中最低(P=0.03).土壤铵态氮含量是影响AOA-amoA和AOB-amoA基因丰度的主要因子,而nosZ基因丰度主要受反硝化底物含量及土壤通气状况的影响.冗余分析表明由放牧所引起的可利用性氮含量的变化是导致氨氧化和反硝化微生物群落结构显著变化的主要因素.%Soil nitrifiers and denitrifiers play key roles in determining soil nitrogen (N) availability,nitrate leaching,and N2O emission,and thus they could be indicative to the effects of grazing intensity on grassland ecosystems as well as grassland degradation.In this study,soil samples were collected from a long-term field experiment with different grazing intensities (low-level,middle-level,and high-level grazing) in arid and semi-arid grasslands in Inner Mongolia.We analyzed the responses of ammonia-oxidizing archaea (AOA),ammonia-oxidizing bacteria (AOB),and denitrifiers,in terms of abundance,community composition,and diversity,at different grazing intensities using real-time PCR and terminal restriction fragment length polymorphism (T-RFLP) approaches.The results showed that soil pH and ammonium content ranged from 7.90-8.18 and 6.37-35.92 mg/kg,respectively.Middle-level grazing significantly increased soil pH (P=0.03),whereas soil ammonium content was recorded as the highest in the high-level grazing treatment (P =0.02).Soil heterotrophic respiration was markedly lower owing to the effects of grazing intensity than that in the non-grazing treatment (P=0.02).AOA-amoA and AOB-amoA gene abundances ranged from (4.94-7.60)× 109 and (0.68-3.75)× 106 copies/g dry soil,respectively.AOA-amoA gene abundance showed no significant change in any of the treatments,whereas middlelevel grazing strongly decreased AOB-amoA gene abundance (P =0.04).The abundance of nosZ gene (coding for nitrous oxide reductase) was significantly decreased in the low-level grazing treatment (P =0.03).The abundances of AOA and AOB were significantly influenced by ammonium content,whereas nosZ gene abundance was influenced by substrate content and soil aeration.Redundancy analysis showed that the variation in N availability induced by grazing was the major factor influencing the community composition of ammonia oxidizers and denitrifiers.