Effects Of Forest Conversion On Microbial Community Structure And Enzyme Activities In Soil Aggregates In Mid-subtropical China

Forest conversion is an important way of land use change.It can change soil ecosystem by changing forest vegetation types.Soil aggregates are a micro-ecosystem.Soil microorganisms and enzymes are important components of the soil ecosystem,and their changes have a significant impact on the soil ecosystem.The research site is located in Wufang Forest Farm(26 38'-27 12'-N,117 29'-118 41'-E),Shunchang County,Fujian Province.Secondary forest and Cunninghamia lanceolata plantation are taken as the research objects.In March 2018,three 20m×20m standard plots were set up in secondary forest and Chinese fir plantation respectively.The main indicators determined were soil aggregate classification,soil physical and chemical properties,microbial community structure and enzyme activity.The main conclusions are as follows:(1)The soil aggregates of secondary forest and Chinese fir plantation were mainly macroaggregates(>2mm)(53.12-62.93%),followed by medium aggregates(0.25-2mm),accounting for 31.76-45.75%,and microaggregates(<0.25mm)were the least,accounting for only 2.69-5.39%.Soil microaggregates in Chinese fir plantations were significantly lower than those in secondary forests,with a significant decrease of 33.95% and no significant change in large and medium aggregates.It shows that forest conversion has a great influence on soil microaggregates.(2)Some soil nutrients in aggregates with different particle sizes of Chinese fir plantation were significantly lower than those in secondary forests.The results showed that forest conversion had a significant effect on nutrient content in soil aggregates.The nutrient content of soil aggregates in secondary forest and Chinese fir increased with the decrease of aggregate size.The results showed that forest conversion significantly affected soil nutrient content in soil aggregates.The soil fertility of secondary forests was better,and the protection of soil microaggregates on nutrients was more obvious.(3)Soil active carbon component I and refractory organic carbon in aggregates of Chinese fir plantations were significantly lower than those in secondary forests by sulfuric acid hydrolysis method.The results showed that the carbon sequestration capacity of soil aggregates in secondary forest was better than that in Chinese fir plantation.Active carbon components I and II in soil aggregates of secondary forest and Chinese fir plantation decreased with the decrease of particle size,while refractory organic carbon increased with the decrease of particle size.The results showed that active organic carbon was more easily aggregated in macroaggregates and refractory organic carbon was more easily aggregated in microaggregates.Pearson correlation analysis showed that soil active carbon component I was positively correlated with soil nutrients and negatively correlated with pH value.Activated carbon component II had no significant correlation with soil physical and chemical properties.The refractory organic carbon was only positively correlated with total carbon and available phosphorus.This indicated that the content of acid-hydrolyzed organic carbon and soil nutrient had significant and positive correlation.(4)By studying the microbial community structure and enzyme activities in aggregates with different particle sizes,it was found that the aggregates with different particle sizes in Chinese fir plantation were Gram-positive bacteria,Gram-negative bacteria,total PLFAs(total phospholipid fatty acid),GP/GN(Gram-positive bacteria/Gram-negative bacteria)significantly lower than those in secondary forests,and F/B(fungi-bacteria ratio)was significantly higher than that in secondary forests.Gram-positive bacteria,arbuscular mycorrhizal fungi,fungi and total PLFAs in soil aggregates of secondary forest and Chinese fir plantation showed a V-shaped distribution with the decrease of particle size.The results showed that forest conversion significantly affected the microbial community structure in soil aggregates,and the number of microorganisms in secondary forests was higher than that in Chinese fir plantations.The acid phosphatase(AP),glucosidase(?G),cellulose hydrolase(CBH)and Acetylglucosaminidase(NAG)in aggregates of Chinese fir plantation were significantly lower than those in secondary forests.AP,?G,CBH,NAG and PEO in soil aggregates of secondary forest increased with the decrease of particle size.Enzyme activities in soil aggregates of Chinese fir plantation increased with the decrease of particle size,but there was no significant difference.The results of principal component analysis showed that there were significant differences in microbial community structure and enzyme activity among the aggregates with different particle sizes,indicating that forest conversion had a significant impact on microbial community structure and enzyme activity in soil aggregates.Redundancy analysis showed that nitrate nitrogen played the most significant role in the structure of microbial community in macroaggregates and microaggregates,and pH played the most significant role in the structure of microbial community in macroaggregates and microaggregates.Total nitrogen and carbon had the most obvious effect on the activity of soil aggregates.