短期氮添加降低亚热带黄山松林和罗浮栲林土壤微生物固氮速率

Short-term nitrogen addition reduces soil microbial nitrogen fixation rate in subtropical Pinus taiwanensis and Castanopsis faberi forests

生物固氮是陆地生态系统氮素的重要来源,但不同森林生态系统土壤微生物固氮速率对氮沉降的响应仍不明确。本研究在亚热带黄山松林和罗浮栲林开展野外氮添加模拟大气氮沉降试验。以尿素为氮源,设置了3个氮添加水平:0(对照)、40(低氮)和80 kg N·hm^(-2)·a^(-1)(高氮)。测定了表层(0~10 cm)土壤化学性质、微生物生物量碳、酶活性和nifH基因拷贝数,并利用^(15)N标记法测定土壤微生物固氮速率。结果表明:与对照相比,氮添加显著降低了黄山松林和罗浮栲林土壤微生物固氮速率,降幅分别为29%~33%和10%~18%。氮添加显著降低了两种林分土壤微生物氮获取酶(β-1,4-N-乙酰氨基转移酶)活性和nifH基因拷贝数。黄山松林土壤微生物固氮速率与可溶性有机碳含量存在显著正相关关系,而罗浮栲林土壤微生物固氮速率与铵态氮含量存在显著负相关关系。总之,黄山松林土壤微生物固氮功能对氮添加的响应比罗浮栲林更敏感,且影响土壤微生物固氮速率的因素在两种林分中存在差异。本研究可为氮沉降对森林生态系统生物固氮的影响提供见解,并为森林经营管理提供理论依据。

tBiological nitrogen(N)fixation is an important source of N in terrestrial ecosystems,but the response of soil microbial N fixation rate to N deposition in different forest ecosystems still remains uncertain.We conducted a field N addition experiment to simulate atmosphere N deposition in subtropical Pinus taiwanensis and Castanopsis faberi forests.We set up three levels of nitrogen addition using urea as the N source:O(control),40(low N),and 80 g N·hm^(-2)·a^(-1)(high N)to examine the chemical properties,microbial biomass C,enzyme activities,and nifH gene copies of top soils(0-10 cm).We also measured the microbial N fixation rate using the ^(15)N labeling method.Results showed that N addition significantly reduced the soil microbial N fixation rate in the P.taiwanensis and C.faberi forests by 29%-33% and 10%-18%,respectively.Nitrogen addition significantly reduced N-acquiring enzyme(i.e.,β-1,4-N-acetylglucosaminidase)activity and nifH gene copies in both forest soils.There was a significant positive correlation between the microbial N fixation rate and soil dissolved organic C content in the P.taiwanensis forest,but a significant negative relationship between the rate of soil microbial nitrogen fixation and NH_(4)^(+)-N content in the C.faberi forest.Overall,soil microbial N fixation function in the P.taiwanensis forest was more sensitive to N addition than that in the C.faberi forest,and the factors affecting microbial N fixation varied between the two forest soils.The study could provide insights into the effects of N addition on biological N fixation in forest ecosystems,and a theoretical basis for forest management.