The Microbial And Abiotic Mechanisms Of The Priming Effect Of Soil Organic Matter

The primary production of vegetation increases in the context of global climate change.Then,the input amount of fresh organic materials into soils increases.Fresh organic input can be decomposed by microbes and partially transformed into soil organic matter（SOM）through physical protection.In addition,fresh organic input may stimulate the decomposition of native SOM,called the priming effect（PE）,and the mineralization of soil nitrogen（N）.However,the underlying microbial mechanisms and the impacts of physical protection of input and mineralization of soil N remain unclear.We incubated four soils with distinct N content and microbial community composition following the addition of ¹³C-labelled glucose addition of 0,0.5,1.0 and 2.0 g C kg^-1 to achieve the following objectives:（1）to determine the PE responses of soils to glucose amounts;（2）to characterize the glucose-induced changes in microbial biomass and necromass,enzyme activities,the amount of soil mineral N and Physically protected glucose;（3）to reveal the mechanisms and key factors controlling the dynamic and intensity of PE and SOM change.The main results and conclusions are as follows:The PE dynamic and its controlling mechanisms.Glucose-induced PE rate peaked within Day 1,decreased till Day 20,and then stabilized at a low value thereafter.Physically protected glucose decreased from 67-92%at Day 1 to 4-37%at Day 48.Glucose addition increases the mineralization of soil N by 0.02-0.10%during the incubation in all soils compared to the unamended soils.The amount of soil mineralized N showed no significant change with increasing incubation time after Day1.The glucose-utilizing bacterial biomass increased during the incubation.The SOM-utilizing fungal biomass and bacterial necromass decreased between Day 1 and Day 20,and the activities of all measured enzymes increased at some days in the soils with glucose addition compared to the unamended soils.Before the PE peak,the peak rate was independent of the amount of added glucose in all soils,while the PE intensity was correlated with the percentage of fungal biomass and the ratio between soil mineral N and input C before incubation.After the PE peak,the PE rate depended on glucose amount,and the PE intensity was correlated with the amounts of physically protected glucose and mineralized soil N and increased biomass of glucose-utilizing bacteria and fungi.These findings provided evidence for the PE mechanisms including triggering effect,preferential substrate utilization,re-utilization,and co-metabolism.These results also suggested that the physical protection of added glucose controlled the PE dynamics through its impact on microbial growth and turnover.The PE rate peak was likely determined by activated fungi.The PE rate peak was also likely determined by fast turnover of glucose-utilizing microbes when glucose was not physically protected.The descending PE rate was controlled by the re-utilization of microbial residues.The long-lasting PE rate was affected by the physical protection of glucose.The PE intensity and its controlling mechanisms.Glucose addition induced the positive PE in soils(0.4-3.0 mg C g^-1 SOM-C).The PE intensity increased with increasing glucose amount and was lower in the high-N soils than low-N soils.The PE intensity at the greater additions increased less in three soils but increased more in Ultisol _HN soil.Glucose addition increased bacterial and fungal biomass（phospholipid fatty acids,PLFAs）,fungal necromass（amino sugar）in all soils,and decreased bacterial necromass in some soils.The biomass increase ratio between bacteria and fungi were lower in the low-N soils than the high-N soils.The PE intensity depended on fungal biomass increase and soil N mineralization after incubation.The fungal biomass increase was correlated with the ratio between input C and soil mineral N and the mineralized N before and after incubation,respectively.The dependence of biomass increase on the mineralized N was 40-fold for bacteria than for fungi.We concluded that microbial growth induced positive PE through co-metabolism;fungal growth induced positive PE and N mining through re-utilization of bacterial necromass;and bacterial growth induced negative PE via preferential utilization of added glucose and mineralized N.Glucose-induced microbial biomass increase led to the higher PE intensity at greater glucose addition.The higher PE intensity in the low-N soils than high-N soils was attributed to more fungal biomass increase.PE intensity increased less at the greater additions in three soils due to stimulated preferential glucose utilization with moderate bacterial growth,depending on availability of soil N.Another soil(Ultisol _HN)showed the greatest increase in PE intensity at the greater addition due to stimulated co-metabolism with bacterial overgrowth.Soil organic matter change and its controlling mechanisms.The relative proportion of O-alkyl was higher,while those of aromatic C-C and aromatic C-O were lower in the high-N soils than in the low-N soils.The PE was lower,while the physically-protected glucose was higher in the high-N soils than in the low-N soils.Glucose addition stimulated SOM increase(319.29-1515.10 mg C kg^-1)despite inducing the positive PE.The magnitude of SOM change was lower in the low-N(319.29-1406.67mg C kg^-1)soils than high-N soils(353.26-1515.20 mg C kg^-1).Structure equation modelling showed that metal oxide increased SOM through the interaction with sand-size mineral-associated organic matter and non-polar alkyl C.The increases in bacterial and fungal biomass depended on the mineralization of SOM and N,and glucose decomposition.Increased bacteria had a negative effect on the PE.Increased fungi had a positive effect on the PE.Metal oxides protected added glucose and SOM against microbial decomposition,leading to the SOM increase.The soils,which contained SOM with a higher proportion of recalcitrant chemical stucture,had the higher PE,and vice versa.These results suggested that metal oxides increase SOM by protecting added organic matter and native SOM,and revealed mineral protection of SOM and the regulation of microbial growth rather than SOM recalcitrance.