Study On Soil Fertility And Microbial Community Structure And Function Of Chinese Fir Plantation

Soil microorganisms are an integral part of forest ecosystems as they play central roles in most nutrient transformations in forest soils.The stability and sustainable development of forest ecosystems depend on these nutrient transformations and the interactions between above-and below-ground components drives forest ecosystem processes.The structure and function of forest ecosystems are deeply affected by the dominant tree species,as they determine the features of the soil and vegetation.Over the past few decades,a large number of natural forests have been converted to monoculture plantations for the increasing effort to obtain higher economic benefits and to meet the growing demand for commodities such as timber,paper,and fuel.The rapid expansion of plantations worldwide has increased the need for a better understanding of the relationship between nutrient cycling and sustainable production in intensively managed plantation systems.In this study,the natural broadleaved forest,species mixing plantations of Chinese fir and broadleaved trees,and Chinese fir monoculture plantations in the subtropical region of China were selected as the research objects.Using the methods of field investigation and data integration analysis,the soil fertility in the main production areas of Chinese fir plantations was evaluated,and the effects of conversion from natural forests to monoculture Chinese fir plantations on soil physicochemical properties,microbial characteristics,and soil enzyme activities were analyzed.The effects of mixing broadleaved trees with Chinese fir on maintaining or improving soil physical quality,nutrient availability,water conservation and soil organic carbon storage capacity,and soil nutrient cycling capacity of Chinese fir plantations were quantified.The key driving factors and mechanisms of the species mixing effects on soil physicochemical properties and enzymatic activities were discussed.The results show that:（1）The soil fertility in the main producing areas of Chinese fir plantation is grade II,which is on the medium level as a whole,and belongs to grade II acid soil.The individual fertility of soil organic matter（SOM）is grade V,rich;total nitrogen（TN）is grade V,rich;total phosphorus（TP）is grade III,poor;total potassium（TK）is grade IV,medium;available nitrogen（AN）is grade V,rich;available phosphorus（AP）content is grade II,very poor;and individual fertility of soil available potassium（AK）is grade III,poor.The degree of artificial influence of soil nutrients in Chinese fir plantations is as follows:AN>AK>OM>TK>p H>TN>TP=AP.Generally,the monoculture Chinese fir plantation converted from the natural forest had low soil fertility and insufficient nutrient supply capacity.（2）The conversion of natural broadleaved forests to Chinese fir plantations changed soil physicochemical properties and the abundance,composition,and structure of soil bacterial and archaeal communities.Soil p H and nutrient quantity（content）and quality（availability）,especially C,N,and P,were significantly associated with changes in microbial communities.Among numerous soil properties,we observed that soil organic carbon（SOC）,nitrate nitrogen（NO₃^-–N）,TN,and AP were essential soil factors closely linked to the structure and composition of bacterial and archaeal communities.The rare taxa of bacteria and archaea have a close relationship with soil physicochemical properties and abundant taxa;thus,their ecological functions need to be further determined.At the same time,the conversion of natural forests to plantations decreased the gene copies of ammonia-oxidizing archaea（AOA）,ammonia-oxidizing bacteria（AOB）,and nif H（nitrogen fixation function）but increased denitrification gene copies（i.e.,nir S,nir K,and nos Z）.（3）Soil physicochemical properties,microbial biomass,fungal alpha diversities,and enzymatic activities decreased with forest conversion,including p H,soil water content,soil organic carbon,available phosphorus,total nitrogen,nitrate nitrogen,microbial biomass carbon and nitrogen,urease,protease,and acid phosphatase.Fungal community composition and structure were also strongly affected by forest conversion.Ascomycota had a higher read abundance in the natural broadleaved forest,but Basidiomycota showed a higher read abundance in the Chinese fir plantation.The read abundance of saprotroph and arbuscular mycorrhizal fungi increased with forest conversion,while that of ectomycorrhizal fungi decreased.Some fungal guilds—dung saprotrophs,lichenized fungi,endophytes,and lichen parasites—were even nearly lost in the Chinese fir monoculture plantation.These changes in fungal communities are all closely correlated to soil physicochemical properties,microbial biomass,and enzymatic activities.（4）We found that species mixing had extensive positive effects on soil physicochemical properties and enzymatic activities in Chinese fir plantations,including total soil porosity（+10.09%）,soil water content（+5.01%）,maximum soil water holding capacity（+16.46%）,soil organic carbon（+19.38%）,total nitrogen（+19.63%）,total phosphorus（+16.37%）,total potassium（+8.23%）,available nitrogen（+22.31%）,available phosphorus（+32.38%）,and available potassium（+13.48%）.Compared with the monoculture plantations,the soil bulk density of the mixed plantations was 6.84%lower,the soil p H was 3.79%greater,and the microbial biomass carbon and microbial biomass nitrogen were 18.84%and 41.92%greater,respectively.Activity of C,N,and P cycling related enzymes was greater in multispecies plantations,including catalase（+27.43%）,invertase（+45.63%）,peroxidase（+35.74%）,protease（+24.35%）,urease（+41.85%）and acid phosphatase（+43.30%）.The positive effects of species mixing increased with tree species diversity and plantation age,and decreased slowly with the increase of mean annual temperature,mean annual precipitation,and forestland slope.In conclusion,soil microbial communities are susceptible to ecosystem conversion,and conversion from natural broadleaved forest to Chinese fir plantation alone will have long-term negative effects on soil microbial community diversity,richness,and microbial-mediated cycling of soil C,N,and P nutrients.Species mixing management with broadleaved trees can promote soil nutrient cycling and improve the ecosystem function of Chinese fir plantations.Therefore,in achieving wood production and maintaining the healthy and sustainable development of the plantation ecosystem,the mixture of Chinese fir and broadleaved trees is a better forest management option than the monoculture plantation.