Characteristics Of Gap In Pinus Massoniana And Its Effects On Soil Ecological Function

Pinus massoniana is a strong positive species with drought and barrenness resistant.It is a pioneer tree species in the process of ecological restoration of barren mountains,and is also an important tree species for greening and afforestation in mountainous and hilly areas in China.P.massoniana plays an important role in maintaining regional ecological environment and ensuring social and economic development.For a long time,the natural regeneration of P.massoniana forests is the research focus.However,the natural regeneration of the P.massoniana is impeded by the poor light condition under the forest canopy.Scientific management of P.massoniana forests will contribute to its growth and seedling renewal.Gap can be a key disturbance measure to promote the optimization of natural forest and secondary forest structure.The soil ecological functions can be changes by changing the microenvironmental elements,and ultimately affect the regeneration and growth of plants in gaps.In order to systematically and comprehensively reveal the influence of P.massoniana gaps on soil ecological function,remote sensing images and field surveys was combined to clarify the basic characteristics and microenvironment characteristics of P.massoniana gaps.Using the method of replacing time by space,4gaps in P.massoniana with age of 1 a(OM1),5 a(FM1),10 a(SM6)and 30 a(TM4)were selected as the research objects to explore the influence of the P.massoniana gaps on soil characteristics.The soil microbial community characteristics of the P.massoniana gaps and their effects on soil C and N conversion were explored deeply too.The findings revealed the function of P.massoniana gaps in soil moisture regulation and storage,and the soil ecological function of P.massoniana gaps was evaluated by extension.The main results were as follows:(1)There were 2250 gaps in the study area,mainly distributing in the surrounding areas of the main urban area.The gaps in the study area were classified into 3 types of gaps of different sizes:small gap,middle gap and big gap.The area of small gap was 16 to 100 m~2,the area of middle gap was 100 to 600 m~2,and the area of big gap was 600 to 2000 m~2.The density of P.massoniana gaps was 0.23/ha,and the void ratio of gaps was 0.32%.The frequency of forest gaps in the study area was not higher,and the interference degree was not higher.The smaller the P.massoniana gap was,the closer its shape was to a circle.The larger the P.massoniana gap was,the less circular its shape was.In the study,the margin trees of the P.massoniana gap were mainly composed of medium-diameter trees,with only a small amount of small-and large-diameter P.massoniana.There was no obvious correlation of the partial canopy rate of edge trees with the size of canopy gaps and extended gaps,which indicated that the partial canopy ratio of edge trees had nothing to do with the gap size.(2)When the forest gap area was greater than 1000 m~2,the larger the forest gap area was,the greater the illuminance of the forest gap and the higher the air temperature was.When the gap was less than 1000 m~2,the lager the area of the gap was,and the higher the heterogeneity of illuminance and air temperature was.The illuminance and air temperature under the forest gap were the highest in summer,followed by spring and autumn,and the lowest in winter.In contrast,the heterogeneity of air humidity under the forest gap was higher.The variability of air humidity at each monitoring point of the four forest gaps was relatively higher in the four solar terms.The daily range of air humidity under the canopy was greater on the vernal equinox than that of the other three solar terms.The daily ranges of the four gaps were between 38.8%and 61.1%.The maximum soil temperature under the forest gap was 10℃higher than the maximum air temperature under the forest gap.The average soil temperature was 1.32℃higher than the average air temperature.The diurnal range of soil temperature under the forest gap was smaller,only about 1.25℃.The heterogeneity of soil temperature under forest gaps was higher.(3)P.massoniana gaps at different developmental stages had significant effects on soil quality(p<0.05).Gaps at different developmental stages had obvious effects on soil properties such as total nitrogen,organic matter,malic acid 2,acetic acid,water content,bulk density,microbial biomass C,catalase,β-glucose and other soil properties.Among them,the forest gap with a developmental stage of 10 a was relatively unique in many indicators.There was no significant difference in soil quality among gaps of different sizes(p>0.05).In general,gaps of different sizes had little effect on soil characteristics,and there was no regularity in their relationship.However,when the gap area was larger than 250 m~2,some soil characteristics showed a certain regularity with the increase of the gap area.The effect of gap position on soil quality varied significantly(p<0.05),which showed that the position of the gap could significantly affect the soil quality.(4)With the development of gaps,the conversion rate of soil C and N under the gaps was improved.When the forest gap developed to 10 a,the conversion rate of soil C and N reached the maximum value.When the forest gap developed to 30 years,the soil C and N conversion rate decreased.Soil pH was an important factor controlling soil C conversion,which was significantly negatively correlated with soil C conversion.Moreover,when soil pH was the same or similar,soil respiration was significantly or extremely negatively correlated with other soil environmental factors.This indicated that pH should be the first soil environmental factor to be considered in order to promote soil C transformation under forest gaps.Nitrate was a crucial factor for nitrate conversion.Soil available phosphorus and organic matter were a pair of counterbalance factors for the conversion of ammonium nitrogen.Nitrate nitrogen and ammonium nitrogen were very important soil environmental factors for soil net mineralization rate.(5)Overall,with the development of forest gaps,the total number of OTUs in soil bacterial community and soil fungal community also increased.There were significant differences in the abundance of bacterial and fungal communities among gap SM and other developmental stages.The orders of simpson index and shannon index of soil bacterial community were:gap center>extended gap edge>under forest.The influence of soil environmental factors on bacterial community was greater than that of soil C and N transformation factors.Among them,soil moisture was the big environmental factor affecting the soil bacterial community under forest gaps.The top five genes with dominant function in soil bacterial community were chemoheterotrophy,aerobic＿chemoheterotrophy,cellulolysis,ureolysis,nitrate＿reduction.The effect of forest gap area on the total number of OTUs and Alpha diversity index of soil fungal community was not significantly different under the forest gap.The number of OTUs and diversity index of soil fungal community in different positions of gap were consistent with the distribution of soil bacteria.The effect of soil environmental factors on fungal community was greater than that of soil C and N transformation factors.Among them,soil pH was the big environmental factor affecting soil fungal communities under forest gaps.There were a variety of ecological functional types in the soil of P.massoniana gaps.Ten species,including Undefined Saprotroph,Unassigned,Ectomycorrhizal,Plant Pathogen-Soil Saprotroph-Wood Saprotroph,were dominant soil fungal functional genes.(6)The water storage function of forest gaps was highly heterogeneous.For forest gaps with a longer age,the larger the gap was,the richer the biodiversity was,and the more complex vertical structure was and the better the water conservation function was.The forest gap had a strong function of soil moisture regulation and storage.The soil moisture content in the center of the gap was higher,but the soil moisture changed smoothly after rainfall.The soil moisture content at the edge of the gap was lower,but the soil moisture changed dramatically after rainfall.For moderate rain in P.massoniana gaps,the size and position of the gaps had obvious effects on soil water content.There was a hysteresis effect on soil moisture in P.massoniana gaps.The strength of the hysteresis effect of gaps on soil moisture was directly related to the performance of the water source regulation and storage function of the gaps,and directly affected the regeneration of plants in the gaps.(7)Based on extension,,an index system of soil ecological function of P.massoniana gaps was established by combing the influence of P.massoniana gaps on soil characteristics and microorganisms.The classification standard of soil ecological function under P.massoniana gap was established,and the soil ecological function of gap was evaluated.In general,the soil ecological function under the P.massoniana gap was poor.The soil ecological function grade of P.massoniana gaps in different developmental stages was as follows:SM6>FM1>OM1>TM4.The soil ecological function grade of the largest gap SM2 was higher than that of other gaps.The soil ecological function grades of P.massoniana gaps in different positions were as follows:extended gap edge>gap center>under forest.(8)The gaps with a development age of 10 years were an inflection point of soil ecological function.Therefore,when the Pinus massoniana forest gaps developed to10 years old,forest managers should take corresponding measures to intervene,so as to promote the continuous improvement of soil ecological function and the better regeneration of Pinus massoniana seedlings.