氮沉降下纳帕海草甸植被与土壤变化对微生物生物量碳氮的影响

Effects of vegetation and soil changes on microbial biomass carbon and nitrogen in the Napahai meadow under N deposition

为探明高原草甸土壤微生物对短期氮沉降的响应,以纳帕海典型高寒草甸云雾薹草群落为对象,野外原位布设低氮(5 g N·m^(-2)·a^(-1))、中氮(10 g N·m^(-2)·a^(-1))和高氮(15 g N·m^(-2)·a^(-1))3种施氮处理,研究氮沉降引起高寒草甸植物多样性及土壤性质变化对微生物生物量碳氮的影响。结果表明:氮添加显著增加土壤微生物生物量碳氮及其熵值,中氮处理下微生物生物量碳增量最高,达139.3%;微生物生物量碳氮的垂直变化表现为沿土层显著降低,降幅为24.1%~75.1%。氮添加显著提高群落地上生物量,降低Shannon和Simpson多样性,变幅达6.6%~65.4%;氮添加显著降低土壤pH,增加土壤有机质、全氮、铵态氮和硝态氮含量,且在中氮处理下变幅(7.0%~511.1%)最大;土壤pH随土层加深而增大,而其他理化指标则沿土层加深而显著减少,变幅达19.5%~91.2%。结构方程模型表明,土壤铵态氮、硝态氮和有机质对微生物生物量起促进作用,而土壤pH和植物群落Shannon指数对其具有负效应;植物和土壤理化性质共同解释微生物生物量碳氮及其熵55%~77%的变化,其中土壤理化性质对微生物生物量碳氮及其熵的效应值最高(0.56~0.95),其次是植物群落多样性和生物量。因此,氮沉降主要通过提高地上生物量及土壤碳氮养分的可利用性而增加土壤微生物生物量碳氮及其熵,但高氮处理导致土壤酸化及植物多样性降低,而对其产生一定的抑制效应。

To explore the responses of soil microorganisms to short-term nitrogen deposition in alpine meadow,we set three treatments of low nitrogen(5 g N·m^(-2)·a^(-1)),medium nitrogen(10 g N·m^(-2)·a^(-1)),and high nitrogen(15 g N·m^(-2)·a^(-1))addition to investigate the effects of nitrogen-deposition induced alterations in plant diversity and soil physicochemical properties on microbial biomass carbon(MBC)and nitrogen(MBN)in a typical alpine meadow community of Carex nubigena in Napahai.The results showed that nitrogen addition significantly increased soil MBC,MBN,and their quotients,with the increases of MBC being as high as 139.3%under medium nitrogen treatment.Both MBC and MBN showed significant decreases along the soil layer,with a reduction of 24.1%to 75.1%.Nitrogen addition significantly increased aboveground biomass and reduced Shannon and Simpson indices by 6.6%-65.4%.Nitrogen addition significantly decreased soil pH,increased the contents of organic matter,total nitrogen,ammonium nitrogen and nitrate nitrogen,with the highest reduction(7.0%-511.1%)being observed in medium nitrogen treatment.Soil pH increased while other physical and chemical indicators significantly decreased with the increases of soil layer,with a variation range of 19.5%-91.2%.Results of structural equation model showed that microbial biomass was significantly positively correlated with ammonium nitrogen,nitrate nitrogen and organic matter,but negatively correlated with pH and Shannon index.The interaction of plant and soil physicochemical properties explained 55%-77%of the variations in MBC,MBN and their quotient.Soil physicochemical properties had the highest effect value(0.56-0.95)on MBC,MBN and their quotients,followed by plant diversity and aboveground biomass.Therefore,nitrogen deposition increased soil MBC and MBN and their quotient,primarily through improving soil nutrient availability and plant aboveground biomass,whereas MBC and MBN and their quotient were suppressed by high-level nitrogen deposition due to soil acidification and plant diversity losses.