Effects Of Land Use Change On Soil Microbial Characteristics In Danjiangkou Reservoir Area

Land use change is a key component of global changes and largely impacts ecosystem structures,processes and functioning.However,how soil microbial community and soil carbon(C)-decomposition and nitrogen(N)-decomposition enzymes respond to soil C and N dynamics following land use change remains unclear.In this study,we conducted experiments in four different land use types including woodland,shrubland,cropland and uncultivated land without input of organic matter from trees and/or shrubs(i.e.,the control)in the Danjiangkou Reservoir region.We tried to explore the effect of land use change on soil C and N storage,microbial community composition,soil enzyme activities and soil basal microbial respiration,as well as their interactions and underlying mechanisms.The results are as follows:(1)Impacts of land use change on soil microbial community composition.Afforested soils(woodland and shrubland)averaged higher microbial PLFA biomass and higher basal microbial respiration compared with uncultivated soils with higher values in top soils than deep soils.Cultivation had no significant effect on microbial PLFA biomass as well as basal microbial respiration,but cultivation significantly reduced soil F: B ratios.The microbial PLFA biomass was strongly correlated with SON and LC.Higher SON,LN,moisture and lower pH in afforested soils also attributed to total variation of higher total PLFAs in afforested land.The basal microbial respiration was higher while the basal microbial respiration on a per-unitPLFA basis was lower in afforested land than adjacent cropland and uncultivated land,suggesting afforestation may increase soil C utilization efficiency and decrease respiration loss in afforested soils.(2)Impacts of land use change on soil C-decomposition enzyme activities.Afforested land and cropland averaged higher soil C-acquisition enzyme activities compared with uncultivated soils with higher values in top soils than deep soils.Higher hydrolytic enzyme activities in afforested soils were mainly attributed to higher DOC and LC contents,whereas higher hydrolase activities in cropland soils were possibly due to lower C: nitrogen(N)ratios and agricultural disturbance(i.e.fertilization and tillage).In contrast,two oxidases activities were the highest in the cropland might be attributed higher soil recalcitrant carbon index and lower C: N ratio.Cropland soils also averaged higher specific enzyme activities for all enzymes compared to the afforested land,revealing fast C turnover rates in cropland.Seasonal variations in soil enzyme activities with high levels in summer were primarily driven by soil temperature and pH.(3)Impacts of land use change on soil N-acquisition enzyme activities.Afforested land and cropland averaged higher soil N-acquisition enzyme activities compared with uncultivated soils with higher values in top soils than deep soils.Higher N-decomposition enzyme activities were mainly attributed to higher SON,RN,LN,SON and soil recalcitrant N index(RN/ SON).Soil temperature and pH also greatly affects soil N-acquisition enzyme activities.Variation in activity of N-acetyl-?-glucoasminidase(NAG)enzyme was mainly attributed to soil C: N ratios and pH.Variation in activity of leucine aminopeptidase(LAP)was mainly attributed to soil SON and pH.There was a significant positive correlation between soil Cdecomposition hydrolase and N-decomposition hydrolase activity,suggesting that soil N and C cycling are well-coupled in all land use types.(4)Impacts of land use change on soil C pool and N pool.Both afforestation(woodland or shrubland)and cultivation significantly enhanced soil organic C and N(SOC,SON),recaltricant C and N(RC and RN)contents and C: N ratios compared with uncultivated soil.Afforestation also averaged higher labile carbon(LC),labile nitrogen(LN).Afforestation improved soil carbon sequestration by reducing soil microbial respiration per unit PLFA,C-decomposition enzyme activity per unit SOC.Both afforestation and cultivation significantly improved the nitrogen ezyme activity of unit organic nitrogen,stimulated the rapid nitrogen cycle,and promoted the coupling cycle of carbon and nitrogen cycle.Overall,afforestation and cultivation increased SOC and SON pool,soil microbial biomass and C-decomposition as well as N-decomposition enzyme activities.They also impacted the microbial community composition and soil microclimate(soil moisture,temperature and pH).Afforested land also declined respiration consumption and reduced the rate of soil carbon turnover.While cropland increased respiration consumption and accelerated the rate of soil C turnover.Thus,our results suggest that afforestation improves the efficiency of soil C utilization and thus could be beneficial to soil C sequestration.