The Effect Of N Fertilization On Priming Of Soil Carbon Mineralization And Carbon Balance In Different Larch Plantations

Fertilization is one of key strategies in forest management,which would affect soil carbon（C）cycling by changing above-ground and below-ground C inputs,as well as microbial activities.Although recent studies have showed that nitrogen（N）fertilization may change priming effect of soil C mineralization,resulting in a powerful effect on soil C balance and C storage,its underlying mechanisms is not clear.Therefore,to investigate how N fertilization would affect priming of soil C mineralization in forest,and if N fertilization would shift the magnitude and direction of soil C balance via changing priming responses,we used soils from long-term N-fertilization field-experiments conducted in pure Larix olgensis and mixed Larix olgensis and Fraxinus mandshurica plantations.we investigated the priming of soil C and N mineralization,microbial growth and composition,and extracellular enzyme activities by adding ¹³C-labelled glucose semi-continuously to simulate rhizosphere inputs.During 4-week incubation,some main results were summarized as below.（1）In pure larch and mixed larch and manchurian ash plantations,semi-continuous additions of glucose induced a sustained and steady priming of soil C mineralization,with a repeating pattern of initial negative then increased priming of SOC mineralization following each addition cycle.The significant negative correlation between priming of soil C mineralization and glucose mineralization indicated that“preferential substrate utilization”was an explanation for the dynamics of C priming in our soils.Glucose additions induced a gradually increasing priming of soil N mineralization,which was greater than the priming of soil C mineralization.N fertilization reduced the priming of soil C mineralization and soil N mineralization,resulting in negative C priming responses of ca.-7%and-16%respectively in high N-fertilization field-treatment from pure and mixed plantations.These suggested that labile C inputs would stimulate microorganisms to prefer using N-rich soil organic matter（SOM）,resulting in soil C priming.This indicated that“selective microbial N-mining”was a main mechanism of priming effect in our soils.（2）In pure and mixed plantations,the priming of soil C mineralization induced by labile C was not linked to the shift of microbial composition,but instead related to sustained soil bacterial and fungal growth.Priming of soil bacterial growth was gradually increasing over time,which coincided with the pattern of soil N priming,suggesting a key role of bacterial growth in microbial N-mining.Compare to N-fertilized soils,in pure larch plantation soil without N fertilization,added glucose stimulated higher bacterial growth and resulted in higher priming of soil C mineralization.However,in the mixed plantation soil without N fertilization,added glucose stimulated fungal growth and resulted in higher priming of SOM mineralization.This indicated that N fertilization affected the priming of soil C mineralization controlled by different microorganisms,and that the control of microbial community on priming of soil C mineralization was linked to native microbial agents.Additionally,added glucose significantly stimulated LAP activity（P<0.05）,These findings suggested that the priming effect in our soils induced by semi-continuous added labile organic C was due to N-mining driven by microbial growth demands for N.（3）In pure and mixed plantations,N fertilization increased the ratio of soil C mineralization to soil N mineralization.Meanwhile,in the initial period of adding glucose,soil C mineralization transiently decoupled with soil N mineralization.However,sustained glucose additions subsequently led to new decoupling between soil C mineralization and soil N mineralization,reaching a new equilibrium with time.（4）In two plantations,additions of labile C increased soil C storage,which was gradually increasing over time.The establishment of mixed plantations decreased soil C loss,contributing to soil C sequestration.The effect of N fertilization on C balance depended on distinct forest types,with a higher C balance in N-fertilized soil from pure plantations,but a lower C balance in N-fertilized soil from mixed plantations.Compared to N-fertilized soils,added glucose stimulated soil C release in soils without N fertilization,but increased soil C content.This could be due to higher accumulation of microbial necromass or replenishment of glucose after adding glucose,contributing to soil C storage.Overall,in pure Larch and mixed Larch and Manchurian Ash plantations,semi-continuous labile C inputs could induce a sustained priming of soil C mineralization though stimulating microbial growth demands for N.However,N fertilization and establishment of mixed plantations would inhibit soil C loss by reducing microbial N-mining controlled priming effect.Additionally,although sustained labile C inputs would induce soil C priming in plantations,sustained accumulation in replenishment of C input and microbial necromass may contribute to soil C balance and result in soil C sequestration.