Effect Of Understory Removal On The Dynamics Of Soil Respiration,Fine Root And Microbes In A Fraxinus Mandschurica Rupr.Plantation

The studies on the response of soil respiration to understory removal are mostly conducted in subtropical forests,but few in temperate forests.There is a close interrelationship between soil respiration,fine root growth,and soil microbes dynamic,while how they respond to the understory removal are still remained unclear,which further limits our understanding in the linteraction between the understory removal and soil respiration,as well as carbon cycling in temperate forests.Therefore,the understory removal was implemented in a 28-year-old of Fraxinus mandschurica Rupr.plantation in Maoershan mountain,Heilongjiang Province.Soil temperature and water content at 5 cm and 10 cm depth were monitored during two growing seasons?from May 2015 to Octorber 2016?.Our overall objective was to investigate how understory remove affect the dynamics of soil respiration rate,fine root biomass,and production,contents of soil microbial biomass carbon?MBC?and nitrogen?MBN?,as well as soil microbial respiration rate.The results are as follows:?1?Fine root biomass of F.mandschurica increased?P>0.05?,whereas that of other species?mainly shrubs?was markedly decreased after understory removal in 2015 and 2016.In treatment plot,total fine root biomass in<0.5 mm,0.5-1 mm,1-2 mm diameter size at 0-30 cm soil depth were 193.07-313.66 g·m^-2,79.21-119.50 g·m^-2,134.20-190.30 g·m^-2,repectively,27.87%-84.28%,8.55%-45.52%,and 3.54%-34.52%higher than those of the control.Fine root productionin 0-1 mm diameter of F.mandschurica in the treatment plots were 81.83 g·m^-2 and105.85 g·m^-2 at 0-30 cm depth in 2015 and 2016 respectively,higher than the control plots(45.14 g·m^-2 and 58.23 g·m^-2,respectively).In contrast,the corresponding values of other trees species were 15.09 g·m^-2 and 18.05 g·m^-2 in two year,both lower than those of the control(25.81 g·m^-2 and 30.21 g·m^-2).Overall,the growth of fine root of F.mandschurica contribute to the increment of total fine root production following understory removal,increased by 45.35%and 40.11%in 2015 and 2016 respectively.However,there is no significant difference in total fine root production between understory removal and control due to their large variations at the plot level.?2?Following understory removal,MBC and MBN contents at surface soil?0-10 cm?and subsurface soil?10-20 cm?were both decreased during two growing seasons.Understory removal decreased MBC contents by 12.68%and 21.16 at surface soil?0-10 cm?and subsurface soil?10-20 cm?,and MBN contents 16.66%and 25.48%at 0-10 cm and 10-20 cm depth respectively in 2015;And decreased MBC contents by 12.73%and 21.39%,MBN contents 11.79%and 5.25%in 2016;The ratios of MBC/MBN were increased by 3.53%at 0-10 cm and 2.64%at 10-20 cm in 2015,but only increased by 4.35%at 0-10 cm depth in 2016,there were a little variations in total.Repeated measures ANOVA indicated that the effects of understory removal on soil variables at different soil depths were not significant in 2015 and2016,whereas sampling date did on MBC and MBN in 2015.The incubation of soil microbial respiration made no obvious difference in both control and treatment at 0-10 cm and 10-20 cm depths in July 2015 and 2016.The incubation of soil microbial respiration were 725.14?g·g^-1and 709.0?g·g^-1(370.74?g·g^-1 and 349.14?g·g^-1 in 2016),and were 592.14?g·g^-1 and 596.57?g·g^-1(267.04?g·g^-1 and 310.25?g·g^-1 in 2016)respectively.Repeated measures ANOVA indicated that the effects of understory removal on the incubation of soil microbial respiration were no significant,whereas incubation time did?P<0.001?.?3?Understory removal increased average soil respiration rates by 11.87%and 5.60%respectively during two growing seasons,and particularly in the late August,the early September in 2015,and the late April,the late June in 2016;Soil temperature and water content were both increased at 5 cm and 10 cm depths during two growing seasons.When consdering sampling date,repeated measures ANOVA indicated that the effects of understory removal on soil respiration rates,soil temperature and water content were not significant,whereas sampling date did during two growing seasons.In 2015 and 2016,soil respiration rates were only exponentially correlated with soil temperature at 5 cm and 10 cm depth,and the temperature sensitivity of soil respiration(i.e.,Q₁₀ value)at 5 cm depth also showed an increment of 15.26%and 4.05%in treatment plot.Overall,soil respiration rate was increased?only some individual sampling dates were significant?after understory removal during two growing seasons,but the effects of increaments were weaken with increasing time.Comprehensive results of total fine root biomass and production increased obviously,soil microbial biomass reduced and soil temperature,water content changed contributed to the weaken effects of understory removal on soil respiration.Our findings yield some insights into the interrelationship between understory removal application and soil respiration in temperate forests.