Effects Of Carbon And Nitrogen Addition On Soil Carbon Mineralization And Sequestration In Forest Ecosystems Along Latitudinal Gradient

Soil is the largest carbon pool in the terrestrial ecosystems.Its small change would have a considerable important impact on global carbon(C)balance.Forest ecosystems are the most important component in the terrestrial ecosystems,and its soil C pool plays a key role in the global C balance.Labile C input could stimulate soil organic carbon(SOC)mineralization,causing priming effect(PE).At present,many studies on PE have been conducted,involving different forest ecosystems and types of labile C.However,there is still no consistent conclusion.This is mainly due to the lack of systematically uniform treatment for different studies.Moreover,the PE has been shown to be regulated by the nitrogen(N)availability,but the underlying mechanism is still elusive.The addition of labile C and N will not only change the mineralization of SOC,but also be utilized and transformed by microorganisms to form new organic C.The amount of newly sequestrated C and its stability is not well understood and require further study.Therefore,we intend to conduct field investigations and laboratory incubation in six forest ecosystems(Jianfengling,Dinghushan,Badagongshan,Taibaishan,Donglingshan,Genhe)along latitudinal gradient to explore the SOC mineralization and its response to C and N addition,so that we could gain understandings in the ecological process and mechanism of SOC dynamics.Meanwhile,the distribution of glucose C in the soil was tracked and its stability was evaluated with labeled glucose and N sources.The main conclusions are as follows:SOC has an obvious latitude gradient in topsoil.The SOC content increased with the increase of latitude.However,the SOC content of Badagongshan(BDGS)is comparable to that in Genhe for topsoil.The proportion of SOC in fine silt and clay is relatively high,which could contribute to more stable carbon.The deep soil has no obvious latitude gradient.The change of topsoil SOC content in the latitude gradient is mainly affected by climate and soil texture,while the change of deep SOC in latitude is mainly affected by microorganisms.According to the distribution of carbon fractions in the topsoil and deep soil,the proportion of C associated with minerals is higher in deep soil than that in topsoil,suggesting that the SOC in the deep soil is more stable.Along the latitude gradient,the addition of labile C promoted SOC mineralization both in topsoil and deep soil,producing positive priming effects.The observed increase of total microbial biomass carbon(MBC)in soil caused by the addition of labile carbon indicated that the positive priming effect might be caused by co-metabolism.The priming effect of topsoil has an obvious latitude gradient,and it is the highest in Genhe(GH)and Donglingshan(DLS).The priming effect in deep soil showed no obvious pattern along the latitude gradient.The priming effect of topsoil and deep soil is mainly affected by soil factors.The priming effect of topsoil was significantly lower than that in deep soil,indicating that lower C stability probably induced greater priming effect.After 30 da ys of incubation,45%-80% of glucose remained in the soil,which was much higher than the amount of primed C.The glucose remaining rate in the deep soil was higher than that in the topsoil,indicating that the deep soil had a higher C sequestration potential.The addition of labile C compensated for the loss of C caused by priming effect,resulting in a positive net C sequestration.High N availability reduced priming effect.However,the enzyme activity related to C cycle(?-glucosidase(BG),cellobiohydrolase(CBH))showed an increasing trend,suggesting that the change of enzyme activity was not the main factor for the reduction of priming effect.On the one hand,N addition reduces the relative change of microbial biomass C.On the other hand,it also improves the microbial C utilization efficiency,thus causing the reduction of PE.The addition of labile C caused positive priming effects in topsoil and deep soil,resulting in the loss of SOC.This study found that deep soil can retain a higher proportion of C than topsoil,leading to a higher net C sequestration.The soil C fraction distribution showed that the new C in deep soil was more distributed in clay fractions and less in unprotected C pool,indicating that the new C in deep soil was more stable than that in topsoil.Compared with the new C,the old C in topsoil and deep soil is more distributed in the biochemical C pool and less in the unprotected C pool,suggesting that the old C is also more stable than the new C.Therefore,deep soil not only has higher C sequestration potential but also higher stability,indicating that deep soil has a greater role in C sequestration and stabilization.The low new C stability suggests that short-term studies may overestimate soil C sequestration potential.In summary,the SOC content and priming effect induced by labile C addition had an obvious latitude gradient in topsoil,which increases with the increase of latitude.The deep SOC content and the priming effect have no obv ious latitude gradient.The topsoil and deep soil showed obvious priming effects.However,the deep soil was more sensitive to the addition of labile C and causing a stronger priming effect.The increase of microbial biomass after the addition of labile C indicated that the priming effect was probably mainly caused by the co-metabolism.Deep soil had higher C sequestration than topsoil.The addition of N increased the C utilization efficiency of microorganisms and decreased the relative change of microbial biomass C,which inhibited the priming effect and further promoted the net C sequestration of soil.The sequestrated labile C in deep soil has higher stability than that in topsoil.These findings could improve our understanding of C mineralization and sequestration in forest ecosystems and provided a theoretical basis for soil C sequestration in the context of global change.