Response Mechanisms Of Soil Organic Carbon Fractions To Nitrogen Addition Gradient In An Alpine Meadow

Soil organic carbon（SOC）pool,the largest carbon（C）pool in terrestrial ecosystems,plays an important role in the global carbon cycle.Atmospheric nitrogen（N）deposition has remarkably increased,which changed the exogenous N input into soil and thus affected SOC sequestration.However,previous studies mostly focused on the bulk SOC pool in response to N addition,and relatively few studies have attempted to investigate different fractions of SOC in response to N addition.Therefore,the response of SOC and its fractions to N addition and its underlying mechanisms remain unclear.This study was conducted in an alpine meadow on the Qinghai-Tibetan Plateau.In 2014,a manipulative N addition experiment was established with six N addition levels of 0,2,4,8,16,and 32 g N m^-2 yr^-1,marked as N0（control）,N2,N4,N8,N16 and N32,respectively.We investigated the changes in soil aggregates carbon,particulate and mineral-associated carbon,and microbial necromass carbon,and explored the mechanisms driving the responses of these SOC fractions to N addition.The key findings are as follows:1.Soil aggregates were obtained using the wet sieving and particle-size fractionation approach,and the mean weight diameter（MWD）was used to characterize the stability of aggregates.Moreover,we determined the content of SOC and total nitrogen of all size aggregates.Our results showed that under ambient N additon,the mass proportion of macroaggregates（79%）was significantly higher than that of microaggregates（13%）and silt and clay（8%）,and the mean MWD is 0.9mm,indicating the well soil structure in this region.However,interestingly,both mass proportion and MWD of aggregates were not affected by N addition,which could attribute to the offset between the N-enhanced root biomass and decreased microbial activities under N addition.We also found that the contents of organic C and total N in aggregates from high to low were as follows:macroaggregates,silt and clay and microaggregates,which was related to the large specific surface area of smaller aggregates with the ability to adsorb more organic matters.The results also supported the hierarchical saturation model of SOC.Results in our study show that N addition did not affect aggregates carbon contents and reduced the C/N ratios,indicating the acceleration of SOC mineralization under increased N deposition.2.SOC can be operationally to separate into particulate organic C（>53μm）and mineral-associated organic C（<53μm）according to variations in SOC formation,turnover and function,using wet-sieving method based on soil particle size.We found that both SOC and its fractions were insensitive to N addition,but there were contrasting trends of POC and MAOC response to N addition.Specifically,SOC and POC decreased with N addtion,while MAOC had no significant change.The decreased POC was mainly attributed to the reduction of plant carbon inputs,the lower of microbial substrate utilization efficiency and the faster decomposition of plant residues under N addition.The unchanged respose in MAOC to N addition may be due to:（1）Lack of free oxides response to N addition;（2）The trade off of precursors of MAOC under N addition,that is the loss of dissolved organic C and microbial biomass C and the increase of microbial residues under N addition;（3）The turnover time of MAOC is about decades to hundred years.3.Amino sugar was used as a biomarker to quantitatively indicate microbial necromass,and the bacterial and fungal necromass C were calculated based on the content of amino sugars.After six years of N addition,compared with the control,the total amino sugar increased by an average of 4%,and glucosamine,galactose and muramic acid increased by an average of 5%,2%and 7%,respectively.Moreover,N addition increased the fungal and bacterial necromass by 5%and 7%on average,respectively.In the control,the contribution of microbial necromass C to SOC was 50%,and the contribution was up to 62%after N addition,indicating that microbial necromass C was the main source of SOC pool and N addition significantly promoted the accumulation of microbial source C.