氮添加和降水改变对高寒草甸土壤氨基糖及微生物残体碳含量的影响OACHSSCD

The stabilization of soil organic carbon(SOC)constitutes a fundamental process in the global carbon cycle.Microbial necromass carbon(MNC),which comprises residual cell wall fragments derived from bacteria and fungi,is increasingly recognized as a major and persistent component of stable SOC.Understanding the responses of the MNC pool to global change drivers is therefore critical.Concurrent anthropogenic activities are elevating atmospheric nitrogen(N)deposition and altering precipitation regimes,both of which can disrupt the microbial physiological pathways governing MNC formation and stabilization.This is particularly critical for alpine meadow ecosystems,which store substantial SOC that is potentially vulnerable to climate change.However,a mechanistic understanding of how N availability and moisture change interactively regulate the quantity and composition of MNC across different soil depths remains incomplete.To address this knowledge gap,we established a long-term,full-factorial field experiment in 2017 on the northeastern Qinghai-Tibetan Plateau.The experiment included treatments of ambient conditions(CK),N addition(N+,10 g m-2 a-1),precipitation reduction(CK-50％),precipitation enrichment(CK+50％),and their combinations(N-50％and N+50％).After seven years,soil samples were collected from topsoil(0-10 cm)and subsoil(30-40 cm)layers.Bacterial necromass carbon(BNC)and fungal necromass carbon(FNC)were quantified via amino sugar biomarkers(muramic acid,MurA;glucosamine,GluN;galactosamine,GalN;mannosamine,ManN),alongside measurements of plant community attributes and soil physicochemical properties to identify key driving factors.The results showed pronounced and depth-specific interactions between N addition and precipitation manipulation.In the topsoil,their interaction significantly affected specific biomarkers like MurA.Effects were more systemic in the subsoil,where both the main effects of each factor and their interaction markedly regulated the dynamics of GalN and GluN.A key finding was the divergent impact of precipitation manipulation.Compared to the control,CK-50％treatment consistently exerted negative effects,depleting topsoil GalN,subsoil total amino sugars,and subsoil FNC.Conversely,CK+50％treatment enhanced topsoil MurA and BNC,and elevated the BNC/FNC ratio across both soil layers,indicating a systematic shift toward a greater relative contribution of bacterial necromass to the total MNC pool.N+50％treatment elevated subsoil FNC whereas N-50％treatment suppressed it.Critically,the overall response was depth-stratified:topsoil MNC components generally remained stable or increased,whereas the subsoil exhibited a consistent decline.Redundancy analysis indicated that alterations in soil physicochemical properties,particularly soil stoichiometry such as carbon-to-phosphorus(C/P)ratio,were the dominant drivers of these differential responses.This implies that drought-and N-induced stoichiometric imbalances critically constrain microbial carbon use efficiency,thereby reducing the conversion of microbial biomass to necromass.In conclusion,nitrogen addition and,more dominantly,precipitation reduction decreased MNC content in alpine meadow soils,mainly through the regulation of soil physicochemical properties and the disruption of elemental stoichiometric balances.These findings suggested that projected intensification of drought in the future could threaten the stability of the SOC pool in alpine meadows by diminishing the input of microbial necromass,particularly fungal-derived carbon in deeper soil layers.