The Effect Of Above/Belowground Carbon Inputs And Nutrient Inputs On Soil Microbial Community Structure And Function In A Coastal Dune Forest

Soil microbial communities play a principal role in providing ecosystem services in terrestrial ecosystem.As the primary driving forces for various biogeochemical cycles,soil microbes involve in a number of important ecological processes,e.g.,soil organic matter(SOM)transformation,energy exchanges,and nutrient cycles.In the past decades,ongoing global change such like nitrogen(N)deposition caused by anthropogenic activities have altered the above-and belowground carbon(C)allocation and possibly exacerbated phosphorus(P)limitation in subtropical forests.The stoichiometric imbalance in C and nutrient cycles in these forests would exert a strong impact on soil microbial community structure and function.Among these subtropical forests,the coastal dune plantation forests with simple habitat structure are generally recognized as being more susceptible to environmental changes and are different in various ecosystem processes when compared with montane forest.Yet,empirical evidence is almost lacking on several critical microbial-mediated processes in these forests.To better understand and predict the impact of climate change on regional forests,especially to examine mechanisms underlying the influences of C and nutrient availabilities on soil microbial community structure and function,we established a litter-manipulation experiment with four treatments(litter removal,root trenching,girdling and litter removal plus girdling)and a nutrient input experiment in a factorial design with N and P additions in a slash pine(Pinus elliottii)plantation grows on typical poor coastal sandy soils.The major findings are summarized as follows:(1)In the short-term,nitrogen addition(+50 kg N ha-1 year-1,equivalent to the level of local atmospheric N deposition)and P addition(25 kg P ha-1 year-1)significantly increased soil N and P availabilities in the coastal dune pine forest,respectively.Nitrogen addition also increased N concentrations but decreased P concentrations in plant tissues.Radial growths of pines at different tree sizes were not significantly affected by N addition,but the radial growths of pines with bigger tree sizes(original DBH>19 cm)was significantly increased when subjected to P addition and N+P addition for than control,indicating that the coastal dune forest is generally P-limited.(2)Litter removal significantly reduced soil total N content,while soil NO3-concentrations were significantly increased by root trenching and girdling treatment.Both of reduced above-and belowground C inputs inhibited soil enzyme activities,with litter removal exerting a stronger impact on hydrolytic enzyme activities than that in root trenching or girdling treatments.Decreases in belowground C inputs seemed to have a stronger negative impact on soil microbial biomass and community structure,within which the girdling treatment significantly increased the ratios of Gram-positive to Gram-negative(G+:G-)bacteria whereas reduced the fungi to bacteria ratios(caused by decreased ectomycorrhizal fungi).(3)Temporal variation overshadows the responses of soil microbial communities and soil enzyme activities to simulated short-term N and P nutrient additions.Soil microbial community structure is primarily determined by soil P availability and total N content,while soil enzyme activities are closely related to soil total N content,soil moisture and soil N:P ratios,indicating that the temporal variation in soil nutrient availability is the key determinant to soil microbial community structure and function in the coastal dune forest.In specific,the ratios of Gram-positive:Gram-negative bacteria significantly increased while the ratios of enzymes targeting labile SOM to enzymes targeting recalcitrant SOM decreased following nutrient additions.(4)Rhizosphere effect plays an important role that bulk and rhizosphere soils host distinct fungal communities and N cycling microbes that differentially vary in response to N and P additions.The fungal community at the phylum level in the bulk and rhizosphere soils showed a contrasting response to nutrient additions.In specific,the abundance of Ascomycota significantly decreased,while Basidiomycota abundance significantly increased in the rhizosphere soil.When assigned to different functional guilds,fungal community with a symbiotrophic lifestyle decreased whereas those with saprotrophic lifestyle increased in response to nutrient additions in the rhizosphere soil.Fungal community in bulk soil responded in direct contrast to the way in rhizosphere soil following nutrient additions.In similar,based on the results of qPCR,the abundances of ammonia-oxidising archaea(AOA)(nitrification)and denitrifying genes encoding nitrite(nirK)were decreased by N and P additions in the rhizosphere soil.By contrast,nitrogen and P additions significantly reduced the abundance of denitrifying genes(nirK and nirS)and the abundance of AOA genes in the bulk soil.Phosphorus addition also increased the abundance of nifH genes encoding nitrogenase reductase in both bulk and rhizosphere soils.These results indicated that incorporating the rhizosphere effect into future studies would largely improve our ability to predict the extent to which soil microbial-mediated belowground processes respond to ongoing climate or environmental changes.In summary,we demonstrate that soil microbial community structure in the coastal dune forest primarily depends on the availability of C and P,while variation in the total soil N supply is fundamental to the functional activity of soil microbial community.This study also highlights that,to ascertain a more comprehensive understanding when assessing the consequences of global change on regional forest ecological processes,temporal dynamism of soil microbial community and rhizosphere effect should be incorporated in future studies.